☢️ Nuclear Scanning

No tracer escapes the vascular space. Activity stays inside the great vessels, liver, spleen, and kidneys with a smooth blood-pool pattern, and no focus of activity appears inside the bowel lumen or moves along the course of the gut.

Ordered to find the site of active gastrointestinal bleeding when the patient has melena, hematochezia, or an unexplained falling hemoglobin and endoscopy has not located the source. It is especially useful for small bowel and intermittent bleeds that endoscopy cannot reach, and it is often used to decide whether the patient should go to angiography or to surgery, and which segment to target.

The patient's own red blood cells are labeled with technetium-99m and reinjected, so the tracer stays in the circulation. Wherever blood is leaking out of a vessel into the bowel lumen, tracer pools there and then moves along with peristalsis, producing a focus of activity that grows and migrates on serial images. A gamma camera records continuous or repeated images over the abdomen; the study can detect bleeding rates far lower than angiography can, on the order of a tenth of a milliliter per minute.

Pregnancy is a contraindication unless the hemorrhage is life-threatening and no alternative exists; in that case the decision is made jointly by the physician and radiation safety and the fetal dose is documented. Breastfeeding requires a planned interruption. The test is not appropriate for a hemodynamically unstable patient who needs to go directly to angiography or the operating room, because the scan takes time the patient does not have.

Radiation exposure is low and comparable to other routine technetium imaging; the practical risk is not the radiation but delay, since an unstable bleeding patient should not be held in nuclear medicine. Because blood is withdrawn, labeled, and reinfused, there is a small but real risk of a labeling or identification error, so two-patient-identifier verification at both draw and reinjection is mandatory. Minor venipuncture bruising and rare tracer reactions are possible.

Bleeding is often intermittent, so a patient who is not actively bleeding at the moment of imaging will scan negative even with significant disease. Retained barium in the bowel attenuates the signal and can hide a bleed, so barium studies should not precede this test. Poor red cell labeling, recent transfusion, heparin, and some medications reduce label efficiency and degrade image quality. Free technetium from an imperfect label collects in the stomach and thyroid and mimics an upper GI bleed, and a prior nuclear scan still in the body confuses interpretation.

Verify the order and the signed consent for the blood labeling. Screen every patient of childbearing potential for pregnancy and lactation and document the result before the tracer is drawn up. Confirm no barium study has been done recently; if one has, notify nuclear medicine. Establish reliable IV access, draw the baseline hemoglobin, hematocrit, and coagulation studies the provider wants, and make sure type and crossmatch is current. Label the blood tube for tagging with two identifiers and stay with the specimen chain. Tell the patient the blood is drawn, tagged, and given back, and that imaging begins right after reinjection and may be repeated for up to twenty-four hours if the first images are negative.

The patient lies supine on the imaging table with the camera over the abdomen. Early dynamic images run continuously for roughly the first sixty to ninety minutes, which is the highest-yield window, and delayed images may be taken at intervals for up to twenty-four hours. The patient must lie still during each acquisition because motion smears the images and mimics or masks migrating activity. Monitor vital signs throughout; this is an actively bleeding patient, and deterioration during the scan is the event you are watching for. Nothing is felt during imaging.

Encourage fluids unless contraindicated, and have the patient void frequently to clear tracer through the kidneys. Continue close hemodynamic monitoring and serial hemoglobin. Staff should use standard precautions with urine and stool for the first day and should minimize unnecessary time close to the patient, though routine care is entirely safe. Breastfeeding mothers pump and discard milk for the interval nuclear medicine specifies for labeled red cells, usually about twelve to twenty-four hours; milk pumped before the injection may be given. Assess the venipuncture site.

A focus of activity that appears inside the bowel and then migrates along the gut identifies active bleeding and localizes it to a region, most usefully as upper versus lower and as small bowel versus colon. Increased activity that stays fixed and does not migrate suggests a vascular lesion such as an angiodysplasia, a hemangioma, or a varix rather than free luminal bleeding. A negative scan means the patient was not bleeding fast enough at that moment; it does not rule out a lesion.

Never let the scan take priority over resuscitation: if the patient becomes hemodynamically unstable, stop the study, notify the provider, and move toward angiography or surgery.

Tracer distributes evenly and symmetrically through the whole skeleton, with normally brighter uptake at growth plates in children, at the sacroiliac joints, sternum, and scapular tips, and normal visualization of the kidneys and bladder because the tracer is cleared renally. No focal hot or cold areas.

Ordered to look for bone metastases in cancers that spread to bone such as breast, prostate, lung, kidney, and thyroid; to find an occult or stress fracture that plain films miss; to evaluate osteomyelitis; to work up unexplained bone pain, prosthetic loosening or infection, Paget disease, avascular necrosis, and suspected child abuse; and to survey the whole skeleton at once when disease could be anywhere.

A phosphate compound labeled with technetium-99m is injected intravenously and is picked up by bone in proportion to blood flow and osteoblastic activity, so any place where bone is actively remodeling concentrates the tracer. After a two to three hour uptake and clearance period the gamma camera images the entire skeleton. A three-phase study adds an immediate flow phase and a blood-pool phase, which is what distinguishes soft tissue infection from true bone infection.

Pregnancy is a contraindication; the tracer crosses the placenta and concentrates in fetal bone and in the maternal bladder next to the fetus. Screen and document before injection. Breastfeeding requires temporary interruption. Patients who cannot lie still for the imaging period may need sedation arranged in advance, and a patient with a full bladder who cannot void may need a catheter, since bladder activity obscures the pelvis.

Radiation exposure is low and is in the same general range as a CT of the abdomen; the tracer is not a contrast dye, so it does not cause kidney injury or allergic contrast reactions. Rare mild tracer hypersensitivity, discomfort at the IV site, and infiltration of the injection are the practical concerns. There is no sedative effect and no post-procedure recovery period.

A distended bladder hides pelvic and sacral lesions. Retained barium in the abdomen attenuates the signal. Any other radionuclide still in the body from a recent nuclear study confuses the picture, so scans are sequenced deliberately. Tracer infiltrated at the IV site creates a false hot spot in that arm and can cause spurious axillary node uptake. Bisphosphonates, recent chemotherapy, and high-dose steroids alter uptake. Degenerative arthritis, recent surgery, healing fractures, and injection sites all light up and can be mistaken for metastases if the history is unknown.

Confirm the order and screen for pregnancy and lactation, documenting the result. No fasting and no sedation are required, and routine medications are continued unless specifically held. Explain the timeline clearly, because patients are surprised by it: the injection takes a moment, they then leave or wait for two to three hours, and only then are the pictures taken. Push oral fluids during the waiting period, typically several glasses of water, so unbound tracer clears through the kidneys and background activity drops. Have the patient void immediately before imaging. Remove metal objects, jewelry, belts, and coins from the field. Ask about recent trauma, surgery, injections, and prior nuclear scans, and pass that history to the technologist.

The patient lies supine on a narrow imaging table, usually with arms at the sides, while the camera moves slowly over the body from head to feet. Whole-body imaging takes roughly thirty to sixty minutes, and additional spot views or SPECT of a specific area add time. The patient must lie still; motion is the single most common cause of a repeat. The table is hard and the position can be uncomfortable for a patient in pain, so time analgesia so it is working during the scan. Nothing is felt from the tracer itself.

Continue generous fluids for the rest of the day and have the patient void often, flushing the toilet twice and washing hands, to clear tracer and reduce bladder and gonadal dose. Resume normal diet and activity. Staff use standard precautions with urine; the patient is not a hazard to family, but it is reasonable to avoid prolonged close contact with infants and pregnant women for the first day. A breastfeeding mother interrupts nursing for the period nuclear medicine specifies for technetium phosphate agents, commonly on the order of several hours to a day, and can pump and discard in the meantime. Check the IV site for infiltration.

Focal hot spots mean increased osteoblastic activity: metastasis, fracture, infection, arthritis, Paget disease, or a benign bone lesion. Multiple randomly scattered hot spots in a patient with a known primary strongly suggest metastatic disease, while hot spots that follow a rib in a line suggest trauma. Cold or photopenic areas mean absent blood flow or bone destruction that outpaces repair, seen in avascular necrosis, radiation fields, multiple myeloma, and some very aggressive lytic metastases. A superscan, with intense skeletal uptake and no visible kidneys, indicates diffuse metastatic or metabolic bone disease.

Push fluids and make the patient empty the bladder right before imaging: hydration and voiding are what make the images readable and what lower the radiation dose to the bladder and gonads.

Blood flow and tracer distribution are symmetric between the two hemispheres, gray matter takes up more tracer than white matter, and the cerebellum and basal ganglia are well visualized. The blood-brain barrier is intact, so tracer does not leak into brain tissue. In a brain death study, normal means intracranial flow is present.

Ordered to evaluate regional cerebral blood flow in stroke and transient ischemic attack, to localize a seizure focus before epilepsy surgery, to help differentiate types of dementia, to assess vasospasm and perfusion reserve, and to evaluate suspected brain death as a confirmatory test when the clinical exam cannot be completed or is confounded. Older barrier-based studies were used for tumor and abscess, a role now largely filled by MRI.

A lipophilic technetium-99m tracer is injected intravenously and crosses the intact blood-brain barrier in proportion to regional blood flow, then converts to a form that is trapped inside the neuron, so the images freeze a snapshot of perfusion at the moment of injection. A rotating gamma camera acquires SPECT images that are reconstructed into slices. In a brain death study the point is simpler: an immediate flow sequence shows whether any tracer enters the intracranial circulation at all.

Pregnancy is a contraindication except in an emergency such as a brain death determination, where the decision is individualized. Breastfeeding requires interruption. Patients who cannot lie still, are agitated, or are claustrophobic may not tolerate SPECT and need sedation planned in advance, but be aware sedatives themselves alter cerebral perfusion and should be given only per protocol, ideally after tracer injection.

Radiation exposure is low. There is no contrast dye and no nephrotoxicity. Occasional transient dizziness or a metallic taste at injection, rare hypersensitivity, and IV site discomfort are the practical risks. If sedation is used, the risks of sedation apply, including respiratory depression.

Because the tracer captures perfusion at the instant of injection, the environment at that moment changes the result: noise, light, talking, and anxiety alter regional flow, which is why the injection is done in a quiet dim room with the patient resting. Caffeine, nicotine, alcohol, sedatives, anticonvulsants, stimulants, and vasoactive drugs all shift cerebral blood flow. Recent seizure activity changes the pattern dramatically. Hypoglycemia, hypercapnia, and recent nuclear studies also interfere.

Confirm the order and consent per facility policy, and screen for pregnancy and lactation. Review medications with the provider; caffeine, alcohol, nicotine, and drugs affecting cerebral flow are commonly held for roughly twenty-four hours, and sedatives are held before injection unless ordered. Insert the IV well before the tracer is given so the venipuncture itself does not alter perfusion at injection. Place the patient in a quiet, dimly lit room, eyes open or closed per protocol, with no talking, reading, or music for several minutes before and after injection. Then allow the uptake period, typically about thirty to sixty minutes, before imaging. Explain that the head will be inside a camera that rotates close to the face.

The patient lies supine with the head immobilized in a holder, which is essential because head motion ruins SPECT reconstruction. The camera rotates slowly around the head, close but not enclosing like an MRI. Acquisition typically takes thirty to sixty minutes. The patient stays awake unless sedated and must not move or talk. A brain death flow study is much shorter, performed at the bedside or in the department immediately after injection, and takes only minutes for the flow images plus brief static views.

Encourage fluids and frequent voiding to clear tracer, and use standard precautions with urine. Resume normal medications and diet. If sedation was given, monitor airway, level of consciousness, and vital signs until the patient returns to baseline and keep the patient NPO until fully awake. A breastfeeding mother interrupts nursing for the interval nuclear medicine specifies for the technetium brain agent used, generally a matter of hours. If the study was done for brain death, support the family, keep the explanation clear and gentle, and do not interpret the result to them yourself.

Areas of decreased uptake mean reduced regional perfusion: infarct, ischemia, vasospasm, or the hypoperfusion patterns of dementia, such as posterior parietotemporal reduction in Alzheimer disease or patchy multifocal defects in vascular dementia. Increased uptake occurs in a seizure focus during the ictal phase, while the same focus is hypoperfused between seizures. Complete absence of intracranial tracer, the hollow skull or empty light bulb appearance, supports cerebral circulatory arrest and is consistent with brain death.

Control the environment at the moment of injection: keep the room quiet and dim and the patient still and unstimulated, because the tracer permanently fixes whatever perfusion pattern exists at that instant.

Tracer normally appears in the liver, spleen, bone and bone marrow, nasopharynx, lacrimal and salivary glands, breast tissue, and the bowel as it is excreted, and in the kidneys in the first day. No abnormal focal accumulation in soft tissue, lung, or lymph nodes.

Ordered to localize occult infection or inflammation when the source of a fever of unknown origin cannot be found, and to evaluate chronic infections such as osteomyelitis, abscess, and spinal infection. It is also used for inflammatory lung disease, notably sarcoidosis and interstitial pneumonitis, for opportunistic infection in immunocompromised patients, and historically for staging and following lymphoma and some other tumors, a role now largely taken over by PET.

Gallium-67 citrate is injected intravenously and binds to transferrin in the blood, then leaks into areas of increased vascular permeability and is bound by lactoferrin in leukocytes and by siderophores in bacteria, so it concentrates wherever there is active inflammation, infection, or certain tumors. Because that accumulation is slow, imaging is delayed. Whole-body and spot gamma camera images are typically taken at twenty-four, forty-eight, and often seventy-two hours after injection, sometimes with a six-hour early image.

Pregnancy is a contraindication; gallium crosses the placenta. Breastfeeding must be stopped for a prolonged period and, given the seventy-eight hour physical half-life, is often discontinued entirely for that infant, so this decision requires nuclear medicine input. The study is impractical for a patient who cannot return for multiple imaging sessions across several days, and for an unstable patient who needs an answer today.

The radiation dose is higher than most technetium studies because gallium has a long half-life and stays in the body for days; still, the exposure is justified when the diagnostic question matters. Rare hypersensitivity and IV site discomfort occur. The main practical burden is the multi-day protocol and the number of return trips.

Normal bowel excretion is the classic pitfall, since colonic activity can mimic an abdominal abscess, which is why laxatives or enemas are often given between imaging sessions. Barium in the bowel attenuates and obscures. Recent chemotherapy, corticosteroids, and antibiotics suppress the inflammatory uptake and cause false negatives. Recent surgical sites, healing wounds, and gadolinium or iodinated contrast studies interfere. Any other radionuclide still in the body must be accounted for, and gallium's own long persistence interferes with subsequent nuclear studies for days.

Confirm the order and consent, and screen for pregnancy and lactation with documentation. No fasting is needed for the injection itself. Bowel preparation is the key nursing detail: laxatives, an enema, or both are frequently ordered before the delayed images to clear colonic tracer, so know the protocol and carry it out. Explain the multi-day schedule so the patient understands they will leave and come back at twenty-four, forty-eight, and possibly seventy-two hours, and confirm they have transportation and can commit. Ask about recent antibiotics, steroids, chemotherapy, barium, and prior nuclear scans, and report them.

The injection itself takes only moments. Each imaging session has the patient lying supine and still on the table while the camera scans the whole body or targeted regions, typically thirty to sixty minutes per session, and SPECT of a region adds time. The patient may eat, drink, and move about between sessions. Nothing is felt during imaging.

Encourage fluids and frequent voiding throughout the several days of the study. Carry out the ordered bowel prep between images. Because gallium is excreted in both urine and stool and persists for days, staff should use standard precautions with all body fluids for about a week, flush the toilet twice, and wash hands. Advise the patient to avoid prolonged close contact with infants and pregnant women for several days. Breastfeeding is interrupted for a prolonged interval or stopped, as determined by nuclear medicine and radiation safety, not by the nurse alone.

Focal increased uptake outside the normal distribution indicates active infection, an abscess, or inflammation, and in the right clinical setting a gallium-avid tumor such as lymphoma. Diffuse bilateral lung uptake suggests interstitial inflammation, sarcoidosis, or opportunistic pneumonia. Symmetric hilar and mediastinal nodal uptake with lacrimal and parotid uptake is a recognized sarcoidosis pattern. In a treated lymphoma, loss of previously gallium-avid uptake indicates response, while persistent uptake suggests residual active disease.

Make sure the patient understands and can complete the multi-day return schedule and any ordered bowel prep, because a single missed delayed image can waste the entire study and the radiation already given.

The stomach empties progressively over four hours. Using the standard solid meal protocol, roughly no more than about ninety percent of the meal remains at one hour, about sixty percent at two hours, and about ten percent at four hours, meaning at least ninety percent has emptied by four hours. Values are read against the specific protocol used.

Ordered to evaluate suspected gastroparesis in patients with early satiety, postprandial fullness, bloating, nausea, and vomiting, most often in diabetes, after gastric or vagal surgery, or in idiopathic disease. It is also used to look for abnormally rapid emptying, such as dumping syndrome after gastric surgery, and to follow response to prokinetic therapy. It gives an objective number where symptoms alone are unreliable.

A standardized meal, classically egg whites with toast and jam, is labeled with technetium-99m sulfur colloid, which binds to the solid phase and stays with the food rather than dissolving into liquid. The patient eats the whole meal, and the gamma camera counts how much tracer remains in the stomach at set time points. The percentage retained over time is the measurement; a dual-isotope study can measure liquid emptying at the same time with a second tracer.

Pregnancy is a contraindication. Breastfeeding requires brief interruption. The study cannot be done in a patient who cannot eat the meal, and egg or wheat allergy requires an alternative meal, so ask about food allergies specifically. Marked hyperglycemia is a functional contraindication because it slows emptying and invalidates the result.

Radiation exposure is low. The realistic problems are nausea or vomiting during or after the meal, which may make the study uninterpretable and require repeating, and hypoglycemia in a diabetic patient who has fasted and held insulin. Rare tracer sensitivity.

Hyperglycemia is the single biggest confounder and slows emptying markedly, so blood glucose is checked before the meal and the study is usually deferred if it is high, commonly above roughly 200 to 275 mg/dL depending on protocol. Opioids, anticholinergics, and other antimotility drugs slow emptying, while prokinetics such as metoclopramide and erythromycin speed it, and all are typically held for about forty-eight hours before the study. Smoking, alcohol, cannabis, and recent nuclear scans also interfere. Menstrual cycle phase can shift results modestly. Vomiting any part of the meal invalidates the count.

Confirm the order and screen for pregnancy and lactation. Keep the patient NPO after midnight or for at least six to eight hours. Review medications with the provider and hold prokinetics, opioids, and anticholinergics for roughly forty-eight hours as ordered. Ask about egg, wheat, and other food allergies before the meal is prepared. In a diabetic patient, check a fingerstick glucose before the meal, report a high value, and clarify insulin dosing with the provider, since the patient is fasting but will eat a measured meal. Instruct no smoking on the day of the study and for the duration, because nicotine delays emptying. Tell the patient the meal must be eaten completely and fairly quickly, usually within about ten minutes, and that the timer starts when the last bite is swallowed.

The patient eats the entire labeled meal under observation while the technologist notes the start and finish times. Imaging is done with the patient upright or standing at the camera in most protocols, with images taken immediately after the meal and at one, two, three, and four hours. The patient is free to move between images but must return promptly and must not eat or drink anything else during the study. Report any vomiting immediately, since it changes the interpretation. The total study lasts about four hours.

The patient resumes normal diet and held medications when the last image is complete and the provider clears it. Encourage fluids and frequent voiding to clear tracer. Recheck blood glucose in the diabetic patient and give insulin per orders. Standard precautions with urine for about a day; the patient is safe to be around family, though prolonged close contact with infants can reasonably be limited for the first day. A breastfeeding mother interrupts nursing for the short interval nuclear medicine specifies for technetium sulfur colloid.

Retention above the protocol threshold at two hours and especially at four hours indicates delayed gastric emptying, that is gastroparesis, commonly from diabetic autonomic neuropathy, prior vagotomy or gastric surgery, medications, or idiopathic causes. Emptying that is much faster than expected indicates rapid gastric emptying or dumping syndrome, typical after gastric bypass or pyloroplasty. Normal emptying in a symptomatic patient shifts the diagnosis toward functional dyspepsia or another cause.

Check the blood glucose before the meal and hold the study if it is high, because hyperglycemia by itself slows emptying and will produce a false diagnosis of gastroparesis.

The labeled meal passes down the esophagus and stays in the stomach. No tracer moves retrograde back up into the esophagus during the imaging period, and no tracer appears in the lungs on delayed images.

Ordered to document and quantify gastroesophageal reflux, particularly in infants and children with recurrent vomiting, failure to thrive, apnea, or recurrent pneumonia, and in adults when reflux is suspected but other testing is equivocal. Its distinctive value is that it can also demonstrate pulmonary aspiration of gastric contents and can estimate gastric emptying at the same time, which pH probes cannot do.

A small amount of technetium-99m sulfur colloid is mixed into milk, formula, orange juice, or another liquid the patient will drink, and the patient swallows it. The gamma camera then watches the esophagus and stomach continuously, so any tracer that moves backward from stomach to esophagus is seen directly and can be counted as reflux episodes and quantified as a percentage. Delayed images over the chest hours later look for tracer in the lung fields, which indicates aspiration.

Pregnancy is a contraindication. Breastfeeding requires brief interruption. The study cannot be done in a patient who cannot swallow the labeled liquid, and it should be deferred in a patient with an unprotected airway at high risk of frank aspiration during the feed unless that is the specific question and the team is prepared. Milk allergy or lactose intolerance requires an alternative vehicle.

Radiation exposure is low, which is an important part of why this study is used in children rather than repeated fluoroscopy. The real risk during the study is aspiration of the feed itself, so suction should be available. Rare tracer sensitivity.

Reflux is intermittent, so a short imaging window can miss it and produce a false negative. Crying, straining, and agitation in an infant increase reflux and can exaggerate results, while sedation suppresses it. Abdominal compression with a binder, if used, artificially provokes reflux. Recent feeding, position, and any prokinetic or acid-suppressing medication change the result. Retained tracer in the mouth or esophagus from swallowing technique can be mistaken for reflux, and any recent nuclear study interferes.

Confirm the order and screen for pregnancy and lactation in adolescents and adults. Keep the patient NPO for the interval ordered, which is usually about four to six hours in adults and shorter and age-based in infants, often one feeding cycle, so the stomach is empty and the child is hungry enough to take the whole feed. Ask about milk, formula, and citrus allergies. Explain to parents that the tracer is mixed into an ordinary feed, that the amount of radiation is small, and that the child must be positioned under the camera for the duration. Hold antireflux and prokinetic medications only if ordered. Have suction available.

The patient drinks or is fed the entire labeled liquid, sometimes followed by a small unlabeled chaser to clear tracer from the esophagus and mouth. The patient then lies supine under the camera and is imaged continuously, commonly for about sixty minutes, sometimes longer. Infants are swaddled and immobilized and a parent is usually allowed to stay, which helps keep the child calm and still. Delayed chest images are obtained hours later, often at two to twenty-four hours, if aspiration is a question. Movement, crying, and repositioning must be reported because they affect the trace.

Resume normal feeding and medications when cleared. Encourage fluids and, in a toilet-trained patient, frequent voiding. Diapers should be handled with gloves and disposed of per facility radiation policy for the first day, and caregivers should wash hands after each change. Standard precautions with urine and emesis. A breastfeeding mother interrupts nursing for the short interval nuclear medicine specifies for technetium sulfur colloid; when the infant is the patient, feeding the infant is not restricted.

Retrograde movement of tracer from stomach into esophagus confirms gastroesophageal reflux, and the number, height, and duration of episodes grade its severity. Tracer that reaches the upper esophagus or pharynx indicates high-level reflux with aspiration risk. Tracer seen in the lung fields on delayed images confirms pulmonary aspiration and is a significant finding that often changes management toward feeding modification or surgery. Delayed gastric emptying seen at the same time helps explain the reflux.

Keep suction at the bedside and watch the airway during the feed, because the same physiology being measured can cause a real aspiration event during the study.

The liver and spleen are normal in size, shape, and position, with homogeneous tracer distribution and no focal defects. Most of the tracer is in the liver, a smaller portion in the spleen, and only a small fraction in the bone marrow.

Ordered to evaluate liver and spleen size, shape, and position, to detect focal defects such as tumors, cysts, abscesses, and hematomas, to assess diffuse liver disease including cirrhosis and fatty infiltration, to confirm the presence and function of splenic tissue after trauma or splenectomy or in suspected asplenia, and to identify accessory spleens and splenosis. Cross-sectional imaging has replaced it for most focal lesion work, but it remains uniquely good at answering functional questions such as whether splenic tissue is present and working.

Technetium-99m sulfur colloid is injected intravenously and is phagocytosed by the reticuloendothelial cells of the body, the Kupffer cells of the liver, the macrophages of the spleen, and to a small extent the bone marrow, so the images map functioning reticuloendothelial tissue rather than anatomy. Anything that displaces or destroys those cells shows as a cold defect. Imaging begins roughly fifteen to thirty minutes after injection with multiple views around the abdomen; a variant using heat-damaged labeled red cells is highly specific for splenic tissue.

Pregnancy is a contraindication. Breastfeeding requires brief interruption. There are few other absolute contraindications; a patient who cannot lie still or cannot lie flat may not tolerate the multiple views.

Radiation exposure is low. No contrast dye is used, so there is no risk of contrast nephropathy or iodine reaction. Rare hypersensitivity to the colloid and IV site discomfort are the practical risks.

Barium in the bowel and retained iodinated contrast attenuate and obscure the liver. Recent nuclear scans leave activity that confounds. The resolution limit matters: lesions smaller than roughly one to two centimeters are usually not seen, so a normal scan does not exclude small disease. Severe hepatic dysfunction shifts colloid away from the liver toward spleen and marrow and can be mistaken for diffuse disease, and ascites, obesity, and overlying structures degrade the images.

Confirm the order and screen for pregnancy and lactation. No fasting is required and medications are continued. Confirm that no barium study is pending or recent; if barium has been given, notify nuclear medicine, as this study is normally scheduled before or well after barium. Explain the sequence: injection, a short wait of roughly fifteen to thirty minutes for the colloid to be taken up, then about thirty to sixty minutes of imaging in several positions. Remove metal from the abdomen and chest field. Ask about prior splenectomy, trauma, or liver disease and pass it along.

The patient lies on the imaging table and is repositioned for anterior, posterior, lateral, and oblique views, so expect to be asked to roll and turn. Each view takes several minutes and the patient must lie still and may be asked to hold quietly or breathe shallowly. The whole imaging portion usually runs about thirty to sixty minutes. Nothing is felt.

Encourage fluids and frequent voiding. Standard precautions with urine for about a day. Resume normal diet and activity. The patient poses no meaningful hazard to family, but limiting prolonged close contact with infants and pregnant women for the first day is a reasonable precaution. A breastfeeding mother interrupts nursing for the brief interval nuclear medicine specifies for sulfur colloid. Check the IV site.

A cold or photopenic focal defect means tissue that is not phagocytic: tumor, metastasis, cyst, abscess, hematoma, or laceration. Hepatomegaly with patchy uneven uptake and a colloid shift, meaning increased splenic and bone marrow uptake with reduced liver uptake, indicates cirrhosis or portal hypertension. Splenomegaly points to portal hypertension, hematologic disease, or infiltration. Absent splenic uptake indicates functional asplenia or prior splenectomy, while a small focus of splenic-type uptake away from the normal location indicates an accessory spleen or splenosis. A hot spot in the caudate lobe or a superior vena cava obstruction pattern can appear in Budd-Chiari syndrome.

Schedule this study before any barium contrast study, because retained barium blocks the gamma signal and can force the scan to be repeated after the radiation has already been given.

Both ventilation and perfusion are uniform throughout both lungs and match each other, with normal gravity-dependent gradients. No segmental defects and no mismatch between the two phases.

Ordered primarily to diagnose or exclude pulmonary embolism, especially in patients who should not receive iodinated contrast, such as those with renal impairment or contrast allergy, in pregnancy where it delivers less breast radiation than CT angiography, and in younger patients. It is also used to quantify regional lung function before lung resection or transplant, to evaluate chronic thromboembolic pulmonary hypertension, and to assess right-to-left shunt.

For the perfusion phase, technetium-99m macroaggregated albumin is injected intravenously; the particles are slightly larger than capillaries and lodge temporarily in the pulmonary capillary bed in proportion to blood flow, so any region with no blood flow shows no tracer. For the ventilation phase, the patient inhales a radioactive gas or a technetium-labeled aerosol through a mouthpiece, mapping where air actually reaches. The gamma camera images both phases in the same projections, and the interpretation rests on comparing them: a region that ventilates but does not perfuse is the signature of embolism.

Pregnancy is a relative rather than absolute contraindication here, because untreated pulmonary embolism is more dangerous than the scan; when it is done in pregnancy the tracer dose is reduced, the perfusion phase may be done alone, and hydration and frequent voiding are emphasized to protect the fetus from bladder activity. Breastfeeding requires interruption. Known severe pulmonary hypertension and known right-to-left cardiac shunt are cautions for the perfusion phase because the albumin particles can bypass the lung and lodge in the brain or kidneys, so the particle number is reduced. Albumin hypersensitivity is a contraindication to the perfusion agent. The ventilation phase requires a patient who can follow breathing instructions and hold a mouthpiece seal.

Radiation exposure is low and, for the breast in particular, lower than CT pulmonary angiography, which is one reason it is favored in young women and pregnancy. The macroaggregated albumin particles are blood products in origin and can rarely cause hypersensitivity. Transient dyspnea or coughing can occur during the ventilation phase, and a patient with severe respiratory distress may not tolerate lying flat or holding the mouthpiece. There is no contrast nephrotoxicity.

Any condition that reduces blood flow or air movement regionally can mimic or mask embolism: COPD, emphysema, pneumonia, atelectasis, effusion, tumor, and prior embolism all create defects. This is why the scan is often reported as a probability rather than a yes or no, and why a current chest radiograph, usually within about twenty-four hours, is required for comparison. Patient position during injection matters because gravity redistributes the particles. Recent nuclear studies and retained tracer interfere, and a poorly mixed or clumped albumin syringe produces artifactual hot spots.

Confirm the order and screen for pregnancy and lactation; if the patient is pregnant, notify nuclear medicine so the dose and protocol are adjusted rather than cancelling the study outright. Verify a recent chest radiograph is available. Ask about albumin allergy, known right-to-left shunt, and pulmonary hypertension. No fasting is required. Assess respiratory status and oxygen requirement and make sure oxygen and the means to continue it are available in the department. Explain both phases, especially that the ventilation phase requires breathing through a mouthpiece with a nose clip and following instructions to breathe normally and then hold the breath briefly. Remove metal, jewelry, and radiopaque objects from the chest. Encourage hydration and voiding, especially in pregnancy.

The perfusion injection is given with the patient supine so the particles distribute evenly; this positioning detail is deliberate, not incidental. Imaging is done in multiple projections, anterior, posterior, lateral, and obliques, with the patient lying still, and the ventilation phase has the patient breathe the gas or aerosol through a mouthpiece with a nose clip while seated or supine. The combined study usually takes about thirty to sixty minutes. The patient must lie still and follow breathing commands; monitor for increasing dyspnea, desaturation, or anxiety throughout, and keep oxygen on if the patient requires it.

Encourage fluids and frequent voiding to clear tracer through the kidneys, which is especially emphasized in a pregnant patient to reduce fetal bladder dose. Standard precautions with urine and respiratory secretions for about a day. Monitor respiratory status and oxygenation and resume routine care. Advise limiting prolonged close contact with infants and pregnant women for the first day. A breastfeeding mother interrupts nursing for the interval nuclear medicine specifies, which is short for the perfusion agent and can be longer for some ventilation agents. Do not let the patient leave without knowing the plan for anticoagulation while results are pending.

A wedge-shaped or segmental defect that is perfused poorly but ventilated normally, a mismatch, indicates pulmonary embolism, and multiple such mismatched segmental defects mean high probability. Matched defects, where both ventilation and perfusion are reduced in the same area, indicate parenchymal disease such as pneumonia, COPD, atelectasis, or effusion rather than embolism. Diffuse patchy matched abnormalities are typical of emphysema. Normal perfusion essentially excludes clinically significant pulmonary embolism. Extrapulmonary tracer in the brain and kidneys on a perfusion scan indicates a right-to-left shunt.

Screen for and report pregnancy, but do not assume it cancels the study: pulmonary embolism kills, and the protocol is modified with a reduced dose and aggressive hydration and voiding rather than withheld.

Tracer appears normally in the gastric mucosa, the thyroid, the salivary glands, the kidneys, and the bladder. No abnormal focus of activity appears in the right lower quadrant or elsewhere in the abdomen.

Ordered to find a Meckel diverticulum as the cause of painless rectal bleeding, most often in children and young adults, and sometimes in unexplained abdominal pain, obstruction, or intussusception. It is the standard noninvasive test for this diagnosis because the diverticulum is in a segment of small bowel that endoscopy usually cannot reach.

Technetium-99m pertechnetate given intravenously is taken up and secreted by gastric mucous-secreting cells wherever they are in the body. A Meckel diverticulum that bleeds does so because it contains ectopic gastric mucosa, so it concentrates the tracer at roughly the same time the stomach does. The gamma camera images the abdomen continuously, and a focus that appears in the right lower abdomen simultaneously with the stomach and persists is the diagnostic finding.

Pregnancy is a contraindication. Breastfeeding requires interruption. The study should not be done soon after a barium study, since barium blocks the signal. It is not the right test for a hemodynamically unstable, briskly bleeding patient, who needs resuscitation and angiography or surgery.

Radiation exposure is low and appropriate for the pediatric population in which the test is most used, though the thyroid, stomach, and bladder receive the highest tracer concentration. If pretreatment medications are used, their own effects apply: histamine-2 blockers are generally well tolerated, while pentagastrin can cause cramping, nausea, and flushing and glucagon can cause nausea and transient hyperglycemia. Rare tracer sensitivity and IV site discomfort.

Barium in the bowel is the classic interfering factor and is the reason this scan is scheduled before any contrast study. Recent nuclear scans leave residual activity. Normal activity in the bladder, kidneys, ureters, and bowel loops can be mistaken for a diverticulum, which is why the bladder is emptied before and during imaging. Free tracer in the bowel from swallowed gastric secretions moves and can mislead. Recent laxatives, enemas, and endoscopy irritate the mucosa and increase nonspecific uptake. Not all Meckel diverticula contain gastric mucosa, so a negative scan does not exclude the diagnosis.

Confirm the order and consent, and screen for pregnancy and lactation in adolescents. Keep the patient NPO for the ordered interval, commonly about four to six hours, so bowel activity is reduced. Confirm and document that no barium study has been done recently; if one has, the scan is postponed. Give pretreatment medication exactly as ordered, since it is what makes the test sensitive: a histamine-2 blocker such as ranitidine or famotidine is commonly given for a day or two before or intravenously about an hour before the scan to keep the tracer in the ectopic mucosa rather than letting it wash into the lumen. Have the patient void immediately before imaging and reinforce that they will be asked to void again during the study. Explain that the child must lie very still and that a parent may usually stay.

The patient lies supine under the camera with the abdomen and pelvis in the field. Imaging begins immediately at injection and continues as rapid sequential images for roughly thirty to sixty minutes, sometimes with additional lateral, oblique, or post-void views. The patient must lie still for the whole acquisition, which is the hardest part in a young child; distraction, swaddling, or occasionally sedation is used. The bladder is emptied partway through if it obscures the pelvis. Nothing is felt.

Resume diet and normal activity. Encourage fluids and frequent voiding to clear tracer from the bladder and reduce dose. Handle diapers with gloves and dispose of them per facility policy for the first day, and wash hands after changes. Standard precautions with urine and stool. Continue to monitor for ongoing bleeding, checking stools and hemoglobin. A breastfeeding mother who is the patient interrupts nursing for the interval nuclear medicine specifies for pertechnetate, commonly around a day. Advise limiting prolonged close contact with pregnant women and infants for the first day.

A focus of increased uptake in the abdomen, classically in the right lower quadrant, that appears at the same time as the stomach, increases in parallel with it, and does not move with peristalsis, indicates a Meckel diverticulum containing ectopic gastric mucosa. A negative scan does not fully exclude a Meckel diverticulum, because a diverticulum without gastric mucosa will not concentrate the tracer, so a strongly suspicious clinical picture may still go to further imaging or laparoscopy. Diffuse or moving intraluminal activity usually reflects secreted tracer rather than a true lesion.

Give the ordered acid-suppressing pretreatment on schedule and confirm no recent barium, because both determine whether the scan can detect the lesion at all.

Tracer appears normally in the spleen, liver, kidneys, bladder, thyroid, and pituitary, and some bowel activity is expected. No abnormal focal uptake in soft tissue, nodes, or elsewhere.

Ordered to locate and stage neuroendocrine tumors that express somatostatin receptors, including carcinoid tumors, gastrinoma and other pancreatic islet cell tumors, pheochromocytoma and paraganglioma, neuroblastoma, medullary thyroid carcinoma, and small cell lung cancer. It is used to find an occult primary in a patient with a hormonal syndrome, to detect metastases, and to determine whether a tumor expresses the receptors that make somatostatin-analogue therapy likely to work.

Indium-111 pentetreotide is a radiolabeled analogue of somatostatin that binds to somatostatin receptors on tumor cell surfaces, so receptor-rich tumors concentrate the tracer and stand out against background. Because binding and background clearance take time, imaging is delayed and repeated, typically at about four hours and again at twenty-four hours, sometimes with additional images at forty-eight hours, often with SPECT or SPECT/CT for localization. Newer gallium-68 labeled somatostatin analogue PET studies work on the same receptor principle with better resolution and a same-day protocol.

Pregnancy is a contraindication. Breastfeeding requires a prolonged interruption because of the multi-day half-life of indium-111, so nuclear medicine determines the interval. Severe renal impairment is a caution since the tracer is renally cleared. A patient who cannot return for delayed imaging over one to two days cannot complete the study.

Radiation exposure is moderate and higher than a single-day technetium study because of the longer half-life and multi-day imaging. Rare hypersensitivity to the peptide, flushing, nausea, dizziness, and injection site discomfort can occur. In a patient with an insulinoma, the peptide can provoke hypoglycemia, so glucose monitoring may be ordered.

Therapeutic octreotide or lanreotide saturates the receptors and blocks tracer binding, which is the most important interference; short-acting octreotide is typically held for roughly one to two days and long-acting depot formulations for weeks, exactly as the ordering team directs. Bowel activity is a major source of false positives, which is why laxatives are usually ordered between imaging sessions. Recent surgery, inflammation, granulomas, accessory spleens, and the gallbladder can show uptake and mimic tumor. Barium, recent nuclear scans, and corticosteroids interfere.

Confirm the order and consent, and screen for pregnancy and lactation. The key medication question is somatostatin analogue therapy: verify with the provider exactly when the last dose was and whether it must be held, since giving the tracer during active therapy can produce a falsely negative study. Ensure good hydration before and after injection because the tracer is renally excreted. Carry out ordered bowel preparation, usually a laxative on the evening of injection and before the twenty-four hour images, to clear colonic activity. Explain the two-day schedule clearly and confirm the patient can return. In a patient with a functioning tumor, know the syndrome: have glucose available for a suspected insulinoma and be alert for carcinoid symptoms.

The injection takes a few minutes. Each imaging session has the patient lying supine and still while whole-body images and often SPECT are acquired, typically about sixty to ninety minutes per session, longer than a routine scan because of the low count rate. The patient goes home between sessions and returns the next day. Nothing is felt during imaging.

Encourage generous fluids and frequent voiding for the full one to two days of the study to clear tracer and protect the kidneys and bladder. Complete the ordered bowel prep. Because indium-111 persists for days, staff use standard precautions with urine and stool for several days, flush twice, and wash hands. Advise the patient to avoid prolonged close contact with infants and pregnant women for a few days. Breastfeeding is interrupted for the extended interval nuclear medicine specifies. Restart held somatostatin analogue therapy when the imaging is complete and the provider approves.

Focal uptake outside the normal distribution indicates somatostatin receptor-positive tumor, identifying the primary site, nodal spread, or distant metastases, especially in liver and bone. The intensity of uptake predicts response to somatostatin analogue treatment and eligibility for peptide receptor radionuclide therapy, so a positive scan is both a diagnostic and a treatment-selection result. A negative scan means either no tumor or a tumor that has lost receptor expression, which in a known neuroendocrine tumor often signals a more aggressive, poorly differentiated phenotype.

Verify with the provider whether therapeutic octreotide or lanreotide needs to be held before the scan, because active analogue therapy occupies the receptors and can make an obvious tumor invisible.

Normal parathyroid glands are too small to be seen. Tracer is taken up by the thyroid on early images and washes out uniformly by the delayed images, leaving no residual focal activity in the neck or mediastinum.

Ordered to localize a hyperfunctioning parathyroid gland in a patient who already has a biochemical diagnosis of hyperparathyroidism, meaning elevated calcium with an inappropriately elevated parathyroid hormone. Its purpose is surgical planning, not diagnosis: it tells the surgeon whether there is a single adenoma that can be removed through a small targeted incision, whether there are multiple abnormal glands, and whether an ectopic gland sits in the mediastinum or elsewhere.

Technetium-99m sestamibi is injected intravenously and is taken up by both thyroid and parathyroid tissue because it concentrates in mitochondria-rich cells. Hyperfunctioning parathyroid tissue is packed with mitochondria and holds onto the tracer, while normal thyroid tissue washes it out, so a dual-phase protocol images early, at about ten to fifteen minutes, and again at delayed time, about two to three hours later, and the focus that persists on the delayed image is the abnormal gland. SPECT or SPECT/CT is usually added to give the surgeon three-dimensional localization, and some protocols subtract a separate thyroid image to isolate the parathyroid signal.

Pregnancy is a contraindication. Breastfeeding requires temporary interruption. There are no other absolute contraindications; a patient who cannot lie still with the neck extended for repeated imaging sessions will have degraded images.

Radiation exposure is low. Sestamibi occasionally causes a transient metallic or bitter taste and rarely a hypersensitivity reaction. IV site discomfort and infiltration are the practical concerns. The clinical risk of a falsely negative or misleading localization is that the surgeon plans the wrong operation, which is why the scan is always read alongside ultrasound and the biochemical data.

Thyroid nodules, multinodular goiter, thyroiditis, and thyroid cancer also retain sestamibi and are the leading cause of false localization. Recent iodinated contrast and iodine-containing medications, including amiodarone, suppress thyroid uptake and interfere with subtraction protocols. Thyroid hormone and antithyroid drugs alter the thyroid image. Small adenomas, hyperplasia of all four glands, and glands with low mitochondrial content are commonly missed, so a negative scan does not exclude hyperparathyroidism. Recent nuclear scans and patient motion between the early and delayed images also degrade the study.

Confirm the order and screen for pregnancy and lactation. No fasting is required and most medications continue, but review thyroid medications, amiodarone, and any recent iodinated contrast with the provider and report them, since they change the thyroid background. Verify the calcium and parathyroid hormone results are on the chart, because the scan is interpreted in that context. Explain the timing so the patient plans the day: injection, early images within about fifteen minutes, then a wait of two to three hours, then delayed images. Remove necklaces, dentures if requested, and anything metallic from the neck and chest. Encourage fluids during the wait.

The patient lies supine with the neck extended and a support under the shoulders, holding still while planar images of the neck and upper chest are taken; each acquisition lasts several minutes and SPECT adds fifteen to thirty minutes. Positioning must be reproduced between the early and delayed sets, so the patient is asked to hold the same posture. The neck extension is uncomfortable for patients with cervical arthritis, so anticipate it and support the head. Nothing is felt from the tracer.

Encourage fluids and frequent voiding. Standard precautions with urine for about a day. Resume normal diet and activity. Advise limiting prolonged close contact with infants and pregnant women for the first day. A breastfeeding mother interrupts nursing for the interval nuclear medicine specifies for sestamibi, typically several hours. Continue to monitor for symptoms of hypercalcemia while the workup proceeds, including confusion, constipation, polyuria, bone pain, and dysrhythmias, and keep the patient hydrated.

A focus of tracer that persists on the delayed images after the thyroid has washed out indicates a hyperfunctioning parathyroid gland, most often a single adenoma, and its position tells the surgeon where to operate. Two or more persistent foci suggest multigland disease or hyperplasia, which changes the operation to a bilateral exploration. A focus below the thyroid bed or in the mediastinum indicates an ectopic gland and may require a different surgical approach. A negative scan in a patient with clear biochemical hyperparathyroidism means the gland is small, ectopic, or not sestamibi-avid, not that disease is absent.

Keep the patient hydrated and monitor for hypercalcemia throughout the workup, because the underlying disease, not the scan, is what can harm this patient.

Tracer distribution follows normal metabolism, with expected uptake in the brain, myocardium to a variable degree, liver, spleen, kidneys, ureters, and bladder from renal excretion, and mild uptake in bowel, muscle, and marrow. No focal hypermetabolic activity outside these expected sites, and standardized uptake values within the normal range for the tissue.

Ordered most often in oncology to stage and restage cancer, distinguish benign from malignant lesions, evaluate treatment response, detect recurrence, and identify an unknown primary. It is also used in cardiology to assess myocardial viability and perfusion, and in neurology to evaluate dementia, seizure focus localization, and, with specialized tracers, amyloid and tau. Its value is that it shows metabolic and molecular activity, so it can identify disease before anatomic imaging shows a structural change, and PET is nearly always combined with CT so function and anatomy are read together.

The standard tracer, fluorine-18 fluorodeoxyglucose, is a glucose analogue: cells take it up through the same transporters they use for glucose, phosphorylate it, and then cannot metabolize it further, so it accumulates inside metabolically active cells. Because most malignancies consume glucose avidly, tumor lights up against background. The positron emitted by the tracer annihilates with a nearby electron and produces two photons traveling in opposite directions, which the ring detector registers as coincident events and reconstructs into tomographic images, with the paired CT providing attenuation correction and anatomic localization.

Pregnancy is a contraindication except in rare situations where the clinical need is overwhelming, and screening must be explicit. Breastfeeding requires interruption of close contact with the infant even though the milk itself clears quickly, because the mother's body is the radiation source. Uncontrolled hyperglycemia is a functional contraindication for FDG-PET because glucose competes with the tracer, and a patient who cannot fast, cannot lie still for the acquisition, or is severely claustrophobic may not be able to complete the study. If IV iodinated contrast is used with the CT portion, contrast allergy and renal function become relevant.

Radiation exposure is meaningful and higher than most single-photon studies, because the patient receives both the PET tracer and a CT, though it remains justified for the diagnostic yield. The realistic risks are hypoglycemia in a fasting diabetic patient, anxiety and claustrophobia in the scanner, and, if contrast is given, contrast reaction and nephropathy. Rare tracer hypersensitivity and infiltration at the IV site occur.

Blood glucose is the dominant factor: hyperglycemia competes with FDG and reduces tumor uptake, so glucose is checked before injection and the study is usually deferred above a threshold, commonly around 150 to 200 mg/dL depending on the facility. Recent insulin drives FDG into muscle and away from tumor. Physical activity, talking, chewing, and shivering before or during the uptake period cause muscle and brown fat uptake that mimics disease, so the patient rests quietly in a warm room. Caffeine and nicotine alter distribution. Inflammation, infection, healing surgical sites, granulomatous disease, and recent radiation or chemotherapy all cause FDG-avid uptake that can be mistaken for tumor, and recent chemotherapy can also transiently suppress tumor uptake and cause false negatives. Recent colony-stimulating factors cause diffuse marrow uptake. Retained barium and metal artifacts distort the CT attenuation correction.

Confirm the order, verify insurance authorization where required, and obtain consent per policy. Screen for pregnancy and lactation and document. Fasting is mandatory for FDG-PET, generally four to six hours, with only plain water allowed and absolutely no sugar-containing drinks, gum, mints, candy, or IV dextrose. Check a blood glucose before injection and report it; know your facility's threshold. In a diabetic patient, clarify insulin and oral agent timing with the provider well in advance, since short-acting insulin given close to injection ruins the study. Instruct no strenuous exercise for about twenty-four hours before. Cardiac viability protocols reverse some of this and may use a glucose load, so follow the specific protocol rather than a general rule. Hold the patient warm and quiet, since cold triggers brown fat uptake. Remove all metal. After injection, the patient rests quietly in a dim warm room for roughly sixty minutes without talking, reading, walking, or using a phone, and voids immediately before imaging.

The patient lies supine on a narrow table with arms usually raised overhead for body imaging, and the table moves slowly through a ring-shaped scanner that is shorter and more open than an MRI bore but still enclosing. Acquisition typically takes about twenty to forty-five minutes for a whole-body study, with the CT portion taking only a few minutes. Absolute stillness is required, and the patient must not talk. Breath-hold instructions may be given for the CT. Assess for claustrophobia in advance and arrange anxiolysis before the tracer is given if needed, since sedation given during the uptake period alters the images.

Encourage generous fluids and frequent voiding for several hours to clear tracer, especially from the bladder. Resume diet and medications, and recheck glucose and give insulin in the diabetic patient. Fluorine-18 decays quickly, with a physical half-life under two hours, so the patient's activity falls off fast; even so, advise avoiding prolonged close contact with infants, small children, and pregnant women for the rest of the day, generally about six to twelve hours. A breastfeeding mother may typically resume feeding after a short interval as specified by nuclear medicine, but should avoid prolonged holding of the infant against her body for several hours, expressing milk in the meantime if needed. Check the IV site for infiltration, since infiltrated FDG creates an artifact and reduces the delivered dose.

Focal hypermetabolic uptake with a raised standardized uptake value suggests malignancy, whether primary, nodal, or metastatic, and is graded against normal tissue and prior studies. Decreasing uptake after treatment indicates response, while new or increasing uptake indicates progression or recurrence. In the heart, a region with reduced perfusion but preserved FDG uptake indicates hibernating but viable myocardium that will benefit from revascularization, while matched reduction in both indicates scar. In the brain, hypometabolic patterns support specific dementias, such as posterior temporoparietal reduction in Alzheimer disease and frontotemporal reduction in frontotemporal dementia, and an interictal seizure focus is typically hypometabolic. Remember that inflammation and infection are also hypermetabolic, so a positive finding is not automatically cancer.

Enforce the fast and check the blood glucose before the tracer is injected, because a hyperglycemic or recently insulin-dosed patient will produce an uninterpretable scan that cannot simply be repeated the same day.

Tracer appears in the blood pool on early images and then in the liver, spleen, bone marrow, kidneys, bladder, and bowel. No abnormal focal uptake in the prostate bed, pelvic or retroperitoneal nodes, or elsewhere.

Historically ordered to detect soft tissue and nodal spread of prostate cancer, either at initial staging in high-risk disease or, more commonly, in a man with a rising prostate-specific antigen after prostatectomy or radiation whose conventional imaging is negative, to distinguish local recurrence in the prostate bed from distant nodal disease. This distinction mattered because it decided between salvage radiation to the bed and systemic therapy. In current practice this agent has largely been replaced by prostate-specific membrane antigen PET tracers, which are far more sensitive and complete in a single day, so a student should recognize the study and its principle but expect to see PSMA PET ordered instead.

Capromab pendetide is a murine monoclonal antibody directed against an intracellular portion of prostate-specific membrane antigen, labeled with indium-111 and given intravenously. It circulates and binds to prostate cancer cells, but because the target epitope is inside the cell it binds best to cells with disrupted membranes, which is one reason its sensitivity is limited. Imaging is done early for blood pool and then at long delay, typically about seventy-two to one hundred twenty hours, to allow background blood activity to clear, usually with SPECT and often with fused or co-registered CT.

This is a study for men, so pregnancy is not applicable, but the general rule that nuclear tracers are withheld in pregnancy still governs any female staff and household exposure planning. It should be avoided in patients with known hypersensitivity to murine proteins or to the antibody, and used cautiously in patients who have previously received mouse-derived monoclonal antibodies because of sensitization. Patients who cannot return for delayed imaging several days later cannot complete the study.

Because this is a murine antibody, hypersensitivity and anaphylaxis are genuine risks, and emergency medications and monitoring must be available at injection. Repeat exposure can generate human anti-mouse antibodies, which can cause reactions on re-exposure and can also interfere with future laboratory immunoassays, including some tumor marker tests, a point worth flagging in the chart. Radiation exposure is moderate and higher than a technetium study because of the long half-life and multi-day protocol. Hypotension, fever, chills, rash, and injection site reactions have been reported.

Bowel and bladder activity are major sources of false positives in the pelvis, which is exactly the region of interest, so bowel prep and bladder emptying are essential. Blood pool activity in pelvic vessels can mimic nodal disease, which is why early and delayed images are compared. Recent surgery, inflammation, and hematoma cause uptake. Barium interferes. Prior mouse antibody exposure alters biodistribution. Recent nuclear studies confound. Overall sensitivity is limited, so a negative scan carries little reassurance.

Confirm the order and obtain informed consent, which for a monoclonal antibody is more substantial than for a routine tracer. Ask specifically about prior monoclonal antibody exposure and any allergy to mouse protein, and report positives. Have emergency equipment, epinephrine, antihistamines, and steroids immediately available and keep the patient in a monitored setting during and after the injection. Carry out ordered bowel preparation, typically laxatives and sometimes enemas before the delayed imaging, and have the patient void before each acquisition, often with a bladder catheter or bladder irrigation in some protocols. Explain that images are taken today and again in three to five days and confirm the patient can return. Document a baseline PSA and prior imaging on the chart.

The antibody is infused slowly with vital signs monitored, and the nurse stays with the patient and watches for flushing, dyspnea, urticaria, hypotension, or chest tightness. Early blood pool images follow shortly after infusion. The patient returns days later for the diagnostic images, lying supine and still for planar and SPECT acquisition of the abdomen and pelvis, which takes roughly sixty to ninety minutes. Nothing is felt during imaging itself.

Continue monitoring for delayed hypersensitivity for a period after infusion and instruct the patient to report rash, fever, chills, or wheezing at home. Encourage generous fluids and frequent voiding throughout the multi-day study. Because indium-111 persists for days, staff use standard precautions with urine and stool for several days, flush twice, and wash hands, and the patient should avoid prolonged close contact with infants and pregnant women for a few days. Complete the ordered bowel prep before returning. Document the antibody exposure permanently in the chart so future immunoassay results are interpreted with the possibility of human anti-mouse antibody interference in mind.

Focal uptake confined to the prostate bed suggests local recurrence and supports salvage radiation to that field. Uptake in pelvic, retroperitoneal, or more distant lymph nodes or soft tissue indicates metastatic spread beyond the surgical field and shifts management toward systemic therapy. A negative scan in a man with a rising PSA does not exclude disease, because the sensitivity of this agent is limited and small-volume recurrence is often missed, which is precisely why PSMA PET has displaced it.

Treat this as a monoclonal antibody infusion, not a simple tracer injection: stay with the patient, monitor vital signs, and have anaphylaxis medications at hand.

All four major salivary glands, both parotids and both submandibular glands, take up tracer promptly and symmetrically, and then empty rapidly into the mouth after a sour stimulus, producing a clear washout on the time-activity curve.

Ordered to evaluate dry mouth and suspected Sjögren syndrome by measuring how well the glands take up and secrete, to assess salivary gland function after radioactive iodine therapy or head and neck radiation, to investigate ductal obstruction from stones or stricture, to evaluate salivary gland masses, and to look for salivary tissue in a suspected fistula. Its strength is that it measures function over time rather than just showing anatomy.

Technetium-99m pertechnetate given intravenously is actively taken up from the blood by the ductal epithelial cells of the salivary glands, the same transport mechanism the thyroid uses, and is then secreted into saliva. The gamma camera records the head continuously so uptake, concentration, and emptying can be plotted as curves for each gland. Partway through, a sour stimulus such as lemon juice or a sour candy is given to trigger secretion, and the resulting drop in gland activity measures how well each gland can actually empty.

Pregnancy is a contraindication. Breastfeeding requires interruption. There is no absolute contraindication otherwise; a patient who cannot tolerate the sour stimulus or cannot keep the head still for the acquisition will produce a limited study.

Radiation exposure is low, though the thyroid, stomach, and bladder concentrate pertechnetate and receive the highest dose. The sour stimulus can be unpleasant and may cause transient discomfort in a patient with mucositis or oral ulceration. Rare tracer sensitivity and IV site discomfort.

Recent iodinated contrast, iodine-containing medications, and thyroid medications alter pertechnetate handling. Anticholinergics, antihistamines, tricyclics, and many other drugs that cause dry mouth suppress secretion and can mimic disease, so the medication list must be reviewed. Recent sialography, recent nuclear studies, eating or chewing before the study, and smoking change gland activity. Head motion between the baseline and post-stimulus images invalidates the curves. Prior radioactive iodine therapy permanently reduces uptake and must be known to interpret the study.

Confirm the order and screen for pregnancy and lactation. The patient is usually kept NPO for a few hours before, and specifically instructed not to eat, chew gum, or smoke beforehand, since chewing empties the glands and blunts the response. Review medications with the provider and report anticholinergics, antihistamines, and other drugs causing xerostomia. Ask about recent iodinated contrast and prior radioactive iodine treatment and document it. Remove dentures, jewelry, and metal from the head and neck. Explain the sour stimulus so it is not a surprise and confirm the patient can tolerate lemon juice or candy; check for oral ulcers.

The patient lies or sits with the head immobilized under the camera and must hold still for the whole study, which typically runs about thirty to sixty minutes. Imaging begins at injection and continues as a dynamic series. At a set point, usually about twenty to thirty minutes in, the sour stimulus is given by mouth and imaging continues to capture the washout. The patient should avoid swallowing excessively or moving the head, and should not chew during the baseline phase.

Encourage fluids and frequent voiding to clear tracer, and encourage oral fluids and good oral hygiene, which also helps a patient with dry mouth. Standard precautions with urine and saliva for about a day; because pertechnetate is secreted in saliva, remind the patient to avoid sharing utensils and drinking glasses and to flush the toilet twice. Resume normal diet and medications. Advise limiting prolonged close contact with infants and pregnant women for the first day, and no kissing an infant for that period. A breastfeeding mother interrupts nursing for the interval nuclear medicine specifies for pertechnetate, commonly around a day.

Reduced uptake and reduced or absent washout in all glands indicates diffuse glandular dysfunction, characteristic of Sjögren syndrome, radiation-induced injury, or chronic sialadenitis, and the degree of loss correlates with severity. Normal uptake with impaired washout after the sour stimulus indicates ductal obstruction, as from a stone or stricture. A focal cold area within a gland indicates a mass such as a cyst, abscess, or most salivary tumors, while a focal hot area that retains tracer and does not wash out is characteristic of a Warthin tumor or an oncocytoma. Absent function on one side alone suggests unilateral duct obstruction or prior surgery.

Keep the patient from eating, chewing, or smoking before the study, because any of these empties the glands and destroys the baseline the whole functional measurement depends on.

Blood flow to both hemiscrota is symmetric and equal, with faint uniform activity in the scrotum on the static images and no focal area of increased or decreased perfusion.

Ordered urgently in acute scrotal pain to distinguish testicular torsion, which requires immediate surgery to save the testis, from epididymitis or orchitis, which is treated medically. It is also used to evaluate a suspected testicular abscess, hydrocele, hematoma, or infarct. In most centers color Doppler ultrasound is now the first-line test because it is faster and requires no radiation, so the nuclear study is used when ultrasound is unavailable or equivocal.

Technetium-99m pertechnetate is injected intravenously as a rapid bolus and the gamma camera records a fast dynamic sequence of arrival and flow through the scrotal vessels, followed by static blood-pool images. Because torsion cuts off arterial supply, the twisted testis shows as an area with no perfusion, while inflammation increases blood flow and shows as a hot area. The comparison between the two sides is what makes the study interpretable.

Pregnancy does not apply to the patient, but the study is not appropriate as a delay tactic: if torsion is strongly suspected clinically, surgical exploration should not wait for imaging. There are no other true contraindications; hypersensitivity to the tracer is exceedingly rare.

Radiation exposure is low but is delivered directly to the gonads, which matters in a young male, and this is part of why ultrasound is preferred when available. The far greater risk is time: every hour of delay reduces testicular salvage, and salvage rates fall sharply after roughly six hours of torsion. Rare tracer sensitivity and IV site discomfort.

A large hydrocele, scrotal edema, or hematoma attenuates the signal and can mimic decreased flow. Late or missed torsion develops surrounding inflammatory hyperemia that produces a rim of increased activity around a cold center and can be misread as epididymitis. Very small testes in young children may not be resolvable. Recent nuclear studies, retained tracer, motion, and improper lead shielding of the thighs and penis degrade the images. Torsion that has spontaneously detorsed can look hyperemic and normal.

This is time-critical, so move quickly. Confirm the order, notify urology, and keep the patient NPO in case he goes directly to the operating room. Establish IV access and give analgesia as ordered, since pain control does not obscure the imaging. Document the time of symptom onset precisely, because it drives the surgical decision. Explain the study briefly and provide privacy and draping; adolescents in particular need reassurance and a chaperone per policy. Position the scrotum on a supportive sling or towel with the penis taped up out of the field, and shield the thighs. Do not delay surgical consultation while arranging the scan.

The patient lies supine with the scrotum supported and the legs slightly abducted, penis taped superiorly out of the imaging field. A rapid flow sequence is acquired over the first minute after the bolus injection, followed by static images over the next several minutes. The whole study is short, typically fifteen to thirty minutes, which is appropriate for an emergency. The patient must lie still despite the pain, so pre-medicate. Monitor pain, nausea, and vital signs.

Keep the patient NPO and prepared for surgery until the result is known and the surgeon has decided. Continue analgesia, apply scrotal support and, if ordered, ice. Encourage fluids and voiding once cleared, and use standard precautions with urine for about a day. If the finding is torsion, the priority shifts entirely to expediting the operating room. If epididymitis, begin antibiotics as ordered and teach scrotal elevation, rest, and follow-up. Provide age-appropriate education about fertility concerns and, in adolescents, involve the family per consent rules.

A photopenic, cold area with absent flow in one hemiscrotum indicates acute testicular torsion and demands immediate surgical exploration. A cold center surrounded by a rim of increased activity indicates late or missed torsion with surrounding inflammation, and the testis may already be nonviable. Diffusely increased flow and blood pool on one side indicates epididymitis or orchitis. A focal cold area with a hyperemic rim can also represent an abscess. Symmetric normal flow argues against torsion but does not exclude intermittent torsion that has spontaneously resolved.

Never let imaging delay surgery: if torsion is clinically suspected, the surgeon is called first and the scan is arranged around that call, because the testis is lost in a matter of hours.

One or a few sentinel nodes are identified in the expected drainage basin and, on pathologic examination, contain no tumor cells. A negative sentinel node means the rest of the basin is very likely free of disease.

Performed to determine whether a cancer has spread to the regional lymph nodes without removing the entire nodal basin, most commonly in breast cancer and melanoma and increasingly in other solid tumors. Nodal status is the single strongest prognostic factor and drives adjuvant treatment decisions. Because it removes only the first-draining node or nodes, it spares most patients the lymphedema, numbness, and shoulder dysfunction that come with a full axillary or groin dissection.

A small amount of technetium-99m sulfur colloid is injected around the tumor or into the skin over it, and the colloid travels through the lymphatic channels to the first node that drains that area, the sentinel node. A gamma camera can map the drainage pathway before surgery, and in the operating room a handheld gamma probe locates the radioactive node through the skin. A blue vital dye is usually injected as well so the surgeon can see the stained lymphatic channel and node visually, and the node identified by both radioactivity and blue color is excised and sent to pathology.

Pregnancy is a relative contraindication; the radiocolloid dose to the fetus is very small and the procedure has been performed in pregnancy when cancer treatment cannot wait, but blue dye is avoided in pregnancy because of teratogenic concern, and the decision is individualized with the surgical and obstetric team. Breastfeeding is interrupted, and breastfeeding on the operative side is typically discontinued. Known allergy to the blue dye is a contraindication to the dye component. Prior extensive surgery or radiation to the nodal basin can disrupt lymphatic drainage and make the technique unreliable, and clinically obvious node-positive disease usually goes straight to formal dissection rather than sentinel biopsy.

The radiation dose is very small, well below that of most diagnostic scans, and poses no meaningful risk to the patient or the operating room staff. The most significant risk is anaphylaxis to the blue dye, which is uncommon but can be severe, so the patient is monitored closely after the dye is given. Blue dye also causes transient blue-green discoloration of the urine and stool for one to two days, bluish skin staining at the injection site that can persist for weeks to months, and a falsely low pulse oximetry reading for several hours because the dye absorbs light at the wavelengths the probe uses. Injection of the tracer around the tumor is painful, particularly intradermal breast injections. Surgical risks include bleeding, infection, seroma, numbness, and a small residual risk of lymphedema.

Prior breast or axillary surgery, prior radiation, and prior biopsy alter lymphatic drainage and can send tracer to an unexpected basin or nowhere at all. Injecting too much volume or injecting into the tumor mass rather than around it obscures the axilla with a shine-through effect that makes the nearby node hard to isolate. Excessive delay between injection and surgery lets tracer pass beyond the sentinel node to second-echelon nodes. Obesity and older age reduce the migration rate. Neoadjuvant chemotherapy can fibrose lymphatics and change the accuracy of the technique.

Confirm the order, verify the surgical consent, and confirm site marking and the correct laterality with the surgeon, which is essential in a bilateral organ. Screen for pregnancy and lactation and document. Ask specifically about allergy to blue dye and about prior surgery or radiation in the drainage basin, and report positives. Explain the two-part timing: the tracer is injected in nuclear medicine, sometimes the afternoon before surgery and sometimes the morning of, and imaging may follow within about thirty minutes to a few hours. Warn the patient honestly that the injections sting and that ice or topical anesthetic may be offered. Warn about blue urine and skin staining so it does not frighten them afterward. Follow standard preoperative preparation including NPO status.

For the injection and mapping, the patient lies still while several small injections are placed around the tumor or in the skin, then remains for imaging if lymphoscintigraphy is ordered, typically thirty to sixty minutes, and the location of the sentinel node may be marked on the skin. In the operating room under anesthesia, the blue dye is injected, the surgeon uses the gamma probe to find the hot node, excises the node or nodes that are hot, blue, or both, and sends them for pathology, sometimes with an intraoperative frozen section that determines whether a full dissection proceeds in the same operation. Monitor closely for hypotension, bronchospasm, and rash after the dye, and expect the pulse oximeter to read falsely low.

Provide routine postoperative care: monitor the incision for bleeding, hematoma, and infection, manage pain, and assess sensation and arm or leg function. Do not be alarmed by a low pulse oximetry reading in the first hours if blue dye was used; correlate with the patient's actual respiratory status and, if needed, an arterial blood gas, rather than escalating oxygen therapy for an artifact. Reassure the patient about blue-green urine for a day or two and skin staining that fades over weeks. Radiation precautions are minimal given the tiny dose; standard precautions suffice, and specimen handling follows facility policy. Teach lymphedema precautions for the affected limb, including avoiding blood pressure cuffs, venipuncture, and injections on that side, and to report swelling, redness, or heaviness. A breastfeeding mother interrupts nursing per nuclear medicine guidance and typically stops feeding on the operative breast.

A sentinel node containing tumor cells means regional metastatic spread, upstages the cancer, and generally triggers further axillary or basin treatment and adjuvant systemic therapy or radiation. A negative sentinel node means the basin is very likely uninvolved and spares the patient a full dissection. Isolated tumor cells and micrometastases are reported separately and are managed differently from frank macrometastasis. Drainage to an unexpected basin, such as internal mammary or contralateral nodes in breast cancer, changes the surgical and radiation plan. Failure to identify any sentinel node means the technique failed and the surgeon may proceed to formal dissection.

If blue dye is used, expect and correctly interpret the falsely low pulse oximetry reading, and watch for anaphylaxis, which is the real danger of this procedure rather than the radiation.

The gland is normal in size, shape, and position, with both lobes and the isthmus visualized and homogeneous, symmetric tracer distribution. No focal hot or cold areas. When uptake is measured, the percentage of the administered radioiodine taken up by the gland falls within the reference range for the facility, typically a low single-digit to low double-digit percentage at a few hours and a somewhat higher percentage at twenty-four hours.

Ordered to determine the functional status of a thyroid nodule, that is whether it is hot and autonomously functioning or cold and therefore possibly malignant; to determine the cause of hyperthyroidism by distinguishing Graves disease from toxic nodular goiter from thyroiditis, since these are treated very differently; to evaluate goiter, substernal thyroid tissue, and congenital thyroid abnormalities; to detect residual or metastatic thyroid cancer after thyroidectomy with whole-body iodine imaging; and to calculate the therapeutic dose before radioactive iodine treatment.

The thyroid is the only tissue that actively traps and organifies iodine, so a radioactive isotope of iodine given by mouth, usually iodine-123 as a capsule or liquid, is concentrated by functioning thyroid tissue in proportion to how hard the gland is working. A probe or gamma camera measures the percentage taken up at set times, commonly at four to six hours and again at twenty-four hours, and images the distribution. Technetium-99m pertechnetate is an alternative given intravenously with imaging about twenty to thirty minutes later; it is trapped but not organified, so it images distribution quickly but does not measure true iodine handling.

Pregnancy is an absolute contraindication because radioiodine crosses the placenta and is concentrated by the fetal thyroid after about ten to twelve weeks, where it can destroy fetal thyroid tissue. A pregnancy test is mandatory in any patient of childbearing potential before radioiodine, no exceptions. Breastfeeding must be interrupted for iodine-123 and must be stopped permanently for that child if iodine-131 is used, so lactation status must be established before dosing. Recent iodinated contrast, iodine-containing medications, and iodine-rich diets block uptake and make the study invalid, so the study is deferred. Known iodine allergy is not a true barrier to the tiny tracer dose but should be reported.

The tracer dose of radioiodine is small and carries low risk; iodine-123 is preferred for diagnostic imaging because it delivers a much lower thyroid dose than iodine-131. The gland itself receives the highest dose of any organ. Rare hypersensitivity occurs. If the study proceeds to therapeutic radioiodine, the risks change substantially and include sialadenitis, dry mouth, taste change, transient neck tenderness, nausea, and marrow suppression at higher doses, along with extensive radiation precautions.

This test is unusually easy to invalidate. Iodinated contrast from a recent CT blocks uptake for weeks to months, and amiodarone blocks it for many months, sometimes over a year. Kelp, seaweed, shellfish, iodized salt, and iodine-containing supplements, cough preparations, and topical antiseptics all suppress uptake. Thyroid hormone replacement and antithyroid drugs such as methimazole and propylthiouracil alter uptake and are held for a specified period as ordered. Corticosteroids, lithium, phenytoin, and some anticoagulants also change results. Renal failure alters clearance and the calculated uptake. Recent thyroid surgery and any recent nuclear scan interfere. Diarrhea or vomiting after an oral capsule means the dose was not absorbed.

Confirm the order and, in any patient who could be pregnant, obtain and document a pregnancy test before the dose is given; this is the non-negotiable step. Establish lactation status. Take a careful iodine history: recent CT with contrast, amiodarone, kelp or seaweed supplements, multivitamins with iodine, cough syrups, and topical povidone-iodine, and report all of them. Review and hold thyroid and antithyroid medications exactly as ordered, which commonly means weeks off thyroid hormone and days off antithyroid drugs, and never hold them on your own judgment. A low-iodine diet may be ordered for one to two weeks beforehand for uptake studies and especially before therapy. Keep the patient NPO for a few hours before and after the oral capsule so it is absorbed, generally about two hours on either side, and confirm the patient can swallow a capsule. Verify thyroid function tests are on the chart. Remove necklaces, dentures, and metal from the neck. Explain the return schedule for delayed uptake measurements.

For radioiodine the patient swallows a capsule or liquid, then leaves and returns at the specified times. At each measurement the patient sits or lies with the neck extended while a probe is positioned over the thyroid or the gamma camera images the neck; each acquisition takes only a few minutes to about twenty minutes, and the patient must hold still with the neck extended. For a technetium study the tracer is given intravenously and imaging follows in about twenty to thirty minutes. Support the head, since neck extension is uncomfortable. Nothing is felt from the tracer.

Resume diet and held medications when the provider clears it. Encourage fluids and frequent voiding to clear unbound tracer. With a diagnostic dose of iodine-123 or technetium, precautions are modest: standard precautions with urine and saliva for about a day, flush the toilet twice, wash hands, do not share utensils, and avoid prolonged close contact with infants and pregnant women for the first day or two. A breastfeeding mother interrupts nursing for the interval nuclear medicine specifies for iodine-123, typically several days, and permanently discontinues nursing that child if iodine-131 was given. If therapeutic radioiodine is given, the precautions become extensive and are dictated by radiation safety, including sleeping alone, limiting close contact for days, separate laundry and utensils, and avoiding pregnancy for a defined period, typically six to twelve months for women.

A hot nodule, one that takes up more tracer than surrounding tissue, is almost always benign and functions autonomously, often causing hyperthyroidism. A cold nodule, one that takes up less, requires further evaluation with ultrasound and fine-needle aspiration because a minority of cold nodules are malignant. Diffusely increased uptake throughout an enlarged gland indicates Graves disease. Patchy uptake with multiple hot and cold areas indicates toxic multinodular goiter. Low uptake in a hyperthyroid patient indicates thyroiditis, factitious thyroid hormone ingestion, or iodine loading, because the gland is leaking stored hormone rather than making more, and this distinction matters because radioiodine therapy will not help those patients. On a post-thyroidectomy whole-body scan, any focus of uptake outside the thyroid bed indicates residual or metastatic thyroid cancer.

Confirm and document a negative pregnancy test and the patient's lactation status before any radioiodine is given, because the fetal and infant thyroid concentrate iodine and the damage is permanent.

Measured total blood volume, red cell mass, and plasma volume fall within a narrow range of the value predicted for the patient's height, weight, sex, and body composition, conventionally within roughly plus or minus eight to ten percent of the predicted normal. Adult total blood volume is approximately sixty-five to seventy-five milliliters per kilogram, with the exact reference determined by the calculation method used.

Ordered to determine whether an elevated hematocrit reflects a true increase in red cell mass, as in polycythemia vera, or simply a contracted plasma volume, as in dehydration or relative erythrocytosis, since the hematocrit alone cannot distinguish these and the treatments differ entirely. It is also used to quantify volume status in complex heart failure, shock, syncope, and hypertension, to guide transfusion and phlebotomy decisions, to assess preoperative volume, and to evaluate anemia when the cause is unclear.

This is a dilution measurement rather than an image. A precisely known amount of a radioactive label is injected into the circulation, either chromium-51 attached to a sample of the patient's own red cells or iodine-125 attached to albumin that stays in the plasma. After the label has had time to mix evenly through the circulation, blood samples are drawn at timed intervals and counted; the more the tracer has been diluted, the larger the volume it distributed into. From the measured compartment plus the hematocrit, the red cell mass, plasma volume, and total blood volume are calculated. No camera is used.

Pregnancy is a contraindication for the radioisotope method except in unusual circumstances. Breastfeeding requires interruption, and for the longer-lived isotopes the interval is prolonged. The red cell labeling method requires withdrawing, labeling, and reinfusing the patient's own blood, so it cannot be done in a patient who refuses blood handling on religious grounds without discussion. Active brisk hemorrhage or ongoing rapid fluid administration invalidates the measurement because the volume is changing during the test.

Radiation exposure is very low because the amount of isotope is tiny and no imaging dose is involved. The genuine risk is the blood handling: mislabeling or misidentification during withdrawal and reinfusion of autologous blood can be catastrophic, so two-identifier verification at every step is mandatory. Venipuncture bruising, discomfort, and rare tracer reaction are the other concerns. If iodinated albumin is used, thyroid blocking with an iodine preparation is sometimes ordered to protect the thyroid from free iodine.

Anything that changes volume during the mixing and sampling period distorts the result: ongoing bleeding, active diuresis, dialysis, large-volume IV fluids or blood transfusion, and dehydration. Splenomegaly delays mixing and can require extended sampling. Extreme obesity, edema, ascites, amputation, and pregnancy make the predicted normal value inaccurate, so the comparison to normal must be interpreted carefully. Poor red cell labeling, hemolysis of the sample, an incomplete injection, or tracer infiltrated at the IV site all falsely alter the calculated volume. Recent nuclear studies leave background counts that interfere with the sample counting.

Confirm the order and consent for blood withdrawal and reinfusion. Screen for pregnancy and lactation. The patient should be at rest and euvolemic relative to their baseline, so coordinate with the provider about holding diuretics, transfusion, and large-volume fluids for the period specified, and note the timing of recent dialysis. No fasting is generally required unless the facility specifies it. Establish reliable IV access; ideally use one site for injection and a different site for sampling so the injection site does not contaminate the samples, and document which arm is which. Verify patient identity with two identifiers at withdrawal and again at reinfusion, and never leave labeled blood unattended. Record an accurate current height and weight, since the predicted normal value is calculated from them. Explain that this is a series of blood draws over a period of time and there are no pictures.

A baseline sample is drawn, the measured tracer dose is injected, and then timed samples are drawn at specified intervals, commonly over roughly thirty to sixty minutes and sometimes longer if mixing is delayed. The patient rests quietly, usually supine or seated, and should not exercise or become dehydrated during the sampling period. Accuracy depends on the exact timing of each draw and on the exact injected volume, so document times precisely and report any partial injection, infiltration, or missed draw immediately. The total time is usually one to two hours.

Encourage fluids unless volume restriction is ordered. Standard precautions with urine and blood for about a day; the isotope amount is small, but if a longer-lived isotope such as chromium-51 or iodine-125 was used, radiation safety may specify precautions for several days. Assess the venipuncture sites for hematoma and apply pressure. Resume held medications and fluids per the provider. Advise limiting prolonged close contact with infants and pregnant women per department guidance, and interrupt breastfeeding for the interval specified, which can be prolonged with these isotopes. Continue monitoring the underlying condition, whether that is polycythemia, heart failure, or anemia.

An increased red cell mass with a normal or increased plasma volume indicates true, or absolute, polycythemia, as in polycythemia vera or a secondary cause such as chronic hypoxia, high altitude, smoking, or erythropoietin excess. A normal red cell mass with a decreased plasma volume indicates relative polycythemia from dehydration or plasma contraction, which is treated with fluids rather than phlebotomy. A decreased red cell mass confirms true anemia and, when combined with plasma volume, distinguishes true anemia from dilutional pseudoanemia, which is common in pregnancy, heart failure, and fluid overload. An expanded total blood volume supports hypervolemia and guides diuresis, while a contracted volume supports hypovolemia and guides fluid resuscitation.

Verify patient identity with two identifiers at every step of withdrawing, labeling, and reinfusing the blood, because a labeling error here is a transfusion error.

Labeled leukocytes distribute to the spleen, which shows the most intense uptake, along with the liver and bone marrow. No abnormal focal accumulation in soft tissue, bowel, bone outside the normal marrow pattern, or elsewhere.

Ordered to find and localize occult infection, particularly in fever of unknown origin, suspected abdominal or pelvic abscess, osteomyelitis including diabetic foot infection, infected prosthetic joints and vascular grafts, and to assess the extent and activity of inflammatory bowel disease. Its particular value is specificity for infection: because the tracer is the patient's own white cells, uptake indicates where leukocytes are actually migrating rather than simply where blood flow or metabolism is increased.

A sample of the patient's blood, usually forty to sixty milliliters, is withdrawn and the white cells are separated and labeled in the laboratory, then reinfused. The labeled leukocytes then do what leukocytes do and migrate to sites of active infection, where they accumulate and can be imaged. With indium-111 oxine the cells are imaged at about eighteen to twenty-four hours, which allows good background clearance and is preferred for chronic and abdominal infection; with technetium-99m hexamethylpropyleneamine oxime the images are obtained much sooner, around one to four hours, which is better for acute situations and for lower radiation dose, though bowel and urinary excretion complicate abdominal reading.

Pregnancy is a contraindication. Breastfeeding requires prolonged interruption with indium-111 and a shorter interruption with the technetium label, as determined by nuclear medicine. The study cannot be performed reliably in a severely leukopenic patient because there are not enough cells to label, so check the white count before ordering. Patients who cannot return for delayed imaging cannot complete an indium study.

The dominant risk is not radiation but the handling of blood: this is autologous blood that leaves the patient, is manipulated in a laboratory, and is returned, so misidentification is a real and serious hazard and two-identifier verification at both draw and reinfusion is mandatory. Radiation exposure is moderate with indium-111 because of the multi-day half-life and lower with the technetium label. Venipuncture bruising, rare reinfusion reaction, and a transient chill or fever after reinfusion can occur.

Recent antibiotics, corticosteroids, and immunosuppressants blunt leukocyte migration and cause false negatives, as does hyperglycemia, which impairs neutrophil function. Leukopenia limits labeling. Normal marrow uptake is intense and can mask osteomyelitis, which is why a complementary bone marrow scan with sulfur colloid is often done and compared: infection is where white cells go but marrow tracer does not. Swallowed labeled cells from a pulmonary or sinus source appear in the bowel and mimic abdominal infection. Gastrointestinal bleeding, recent surgery, healing wounds, drains, stomas, catheter sites, hematomas, and accessory spleens all produce uptake. Barium interferes, and any recent nuclear study confounds.

Confirm the order and obtain consent for the blood withdrawal and reinfusion. Screen for pregnancy and lactation and document. Check the white blood cell count and report leukopenia to the provider before the blood is drawn. Review antibiotics and steroids with the provider; they are not always held, but the interpreting physician must know about them, so document doses and timing. Verify patient identity with two identifiers when the blood is drawn, label the specimen at the bedside, and follow the chain of custody to the laboratory. Explain the timeline: blood is drawn, it takes a few hours to label, the cells are reinfused, and images follow either the same day or the next morning depending on the label used. Confirm no recent barium study.

The reinfusion of labeled cells is done slowly with the patient monitored for chills, fever, or reaction, exactly as you would monitor any blood product. Imaging then has the patient lying supine and still while whole-body and spot images are acquired, typically forty-five to ninety minutes per session, with SPECT or SPECT/CT added for precise localization. With indium-111 the patient goes home and returns the next morning; with the technetium label imaging is the same day. Multiple sessions may be needed. Nothing is felt during imaging.

Monitor for reinfusion reaction for a period after the cells are returned. Encourage fluids and frequent voiding. Standard precautions with urine and stool for about a day with the technetium label and for several days with indium-111, flushing twice and washing hands. Advise limiting prolonged close contact with infants and pregnant women for the corresponding interval. Interrupt breastfeeding for the period nuclear medicine specifies, which is short for the technetium agent and prolonged for indium-111. Continue to monitor the patient's temperature, white count, and clinical signs of infection while results are pending, and do not stop the search for a source based on a single negative scan.

Focal accumulation of labeled leukocytes outside the normal liver, spleen, and marrow distribution indicates active infection or inflammation at that site: an abscess, osteomyelitis, an infected prosthesis or graft, or active inflammatory bowel disease, where the pattern follows the affected segments of bowel. Uptake in bone that exceeds the corresponding marrow scan indicates osteomyelitis rather than normal marrow. Absence of abnormal uptake makes active bacterial infection unlikely but does not exclude it, particularly in a patient on antibiotics or steroids, in chronic low-grade or granulomatous infection, or in spinal infection, where this scan is notably insensitive.

Treat the labeled blood like a transfusion: verify identity with two identifiers at withdrawal and at reinfusion, and monitor the patient during reinfusion for a reaction.