Nuclear Imaging Flashcards

Question

F-18 FDG

Answer 1

F-18 FDG, the same radiotracer used for oncologic imaging, is a positron emitting PET viability agent with a half-life of 110 minutes. Unlike rubidium-82 and N-13 ammonia, FDG cannot be used for perfusion. F-18 FDG PET images are correlated with a sestamibi perfusion study to evaluate viability. A defect on sestamibi rest perfusion with discordant FDG uptake represents viable hibernating myocardium that could potentially be revascularized. In contrast, a sestamibi perfusion defect correlating to lack of F-18 FDG uptake is a scar.

Answer 2

Prior to undergoing a myocardial perfusion study, the patient should be NPO for 6 hours to decrease splanchnic blood flow and therefore reduce liver and bowel uptake. Calcium channel blockers and B-blockers should be held to allow patient to reach target heart rate. Exercise is performed with a multistage treadmill (Bruce of modified Bruce) protocol. The target heart rate, which is calculated as 85% of maximal heart rate, must be achieved for the study to be diagnostic. The maximal calculated heart rate is 220 bpm - age.

Answer 3

Dipyridamole is an adenosine deaminase inhibitor that allows endogenous adenosine to accumulate. Adenosine is a potent vasodilator, increasing coronary blood flow by 3-5 times. A critical coronary artery stenosis cannot further dilate in response to adenosine. That coronary artery territory will appear as a relative perfusion defect on stress imaging. Unlike a physical stress, a dipyridamole stress does not increase cardiac defect on stress imaging. Unlike a physical stress, a dipyridamole stress does not increase cardiac work or O2 demand. Caffeine and theophylline reverse the effects of dipyridamole and must be held for 24 hours. The antidote is aminophylline (100-200 mg), which has a shorter half-life than dipyridamole, so the patient must be continuously monitored.

Answer 4

Adenosine has identical physiologic effects to dipyridamole but a more rapid effect. Adenosine half-life is approximately 30 seconds and thus does not require a reversal agent.

Answer 5

Regadenoson is an adenosine receptor agonist with a 2-3 minute half-life. It is easier to administer than adenosine with a convenient universal-dose intravenous injection.

Answer 6

Dobutamine is a B1 agonist that increases myocardial oxygen demand. Dobutamine is usually reserved for when adenosine is contraindicated (severe asthma, COPD, or recent caffeine).

Answer 7

A single-day Tc-99m sestamibi perfusion study is the most common myocardial perfusion exam performed. Rest images are first obtained after 8-10 mCi Tc-99m sestamibi. Stress images are obtained after an additional 20-30 mCi Tc-99m sestamibi is administered during peak exercise, or after administration of pharmacologic stress. Imaging is performed approximately 30 minutes after injection, to allow liver activity to clear. Because there is no redistribution, imaging can be delayed after tracer administration. Gated SPECT images show wall motion at time of imaging, while perfusion images show perfusion at time of injection.

Answer 8

PET rest-stress myocardial perfusion has greater sensitivity, specificity, and accuracy for diagnosis of coronary artery disease compared to SPECT imaging. Attenuation-correction CT improves diagnostic accuracy by eliminating attenuation artifact. Rubidium-82 and N-13 ammonia are perfusion agents and are imaged on a PET system using coincidence detection. The shorter half-life of these tracers allows higher activities to be administered with lower overall radiation exposure. For quantification of myocardial blood flow, N-13 ammonia is preferred as rubidium has a lower extraction fraction.

Answer 9

Because thallium undergoes redistribution, imaging is performed immediately post-exercise and approximately 3-4 hours later once redistribution has occured. Thallium is uncommonly used because of the long 73 hour half-life and resultant high patient dose.

Answer 10

The heart is reconstructed into short axis (SA, the traditional "donut" view from apex of heart through the base), vertical long axis (VLA, a "U-shaped" view pointing to the left), and the horizontal long axis (HLA, a "U-shaped" view pointing down). The polar plot represents the entire three-dimensional left ventricle unfolded onto a two-dimensional map.

Answer 11

For evaluation of perfusion defect size, there are 17 standard left ventricular segments, evaluated on SA ("donut") views. Each segment is usually supplied by the color-coded artery indicated above, although vascular supply is variable between patients.

Answer 12

I-131 emits both beta particles and 364 keV gamma photons (only the gamma photons are used for imaging). The half-life is 8 days. I-131 is generator produced. I-131 is only used for therapy. Indications include treatment of thyroid cancer status post thyroidectomy, and hyperparathyroidism from Graves disease or multinodular gland.

Answer 13

I-123 decays by electron capture and produces 159 keV gamma photons. It has a half-life of 13 hours. I-123 is expensive as it is produced by cyclotron. It is administered orally. I-123 is an excellent radioisotope for thyroid imaging, as it can image in high detail and obtain thyroid uptake values.

Answer 14

Technetium-99m emits a 140 KeV gamma photon and has a half-life of 6 hours. Unlike iodine, pertechnetate is not trapped by the thyroid. After initial uptake it is released into the blood pool. Thyroid uptake is not routinely quantified with Tc-99m (due to its rapid washout), but pertechnetate does provide excellent iamges of the thyroid gland. Because pertechnetate does not specifically localize to the thyroid, high background counts are typical. Only 1-5% of administered activity is taken up by the thryoid. In contrast to I-123, the salivary glands are clearly seen with pertechnetate. Unlike I-123, Tc-99m is administered intravenously. Tc-99m pertechnetate is preferred over I-123 when the patient has received recent intravenous iodinated contrast (iodine in contrast blocks thyroid uptake of additional iodine), when IV medication is necessary, or when a quick study is required.

Answer 15

All thyrotropic agents cross the placenta and I-131 is contraindicated in pregnancy. Fetal iodine is taken up beginning at 12 weeks gestation. A breast feeding mother who medically requires an I-131 ablative dose must stop breast feeding permanently for the current child. For I-123, breast feeding can be resumed 2-3 days after administration. For Tc-99m, breast feeding can be resumed 12-24 hours after administration.

Answer 16

Patients undergoing I-123 or I-131 imaging/therapy must have non-suppressed TSH, which can be achieved by stopping exogenous thyroid hormone for 4 weeks, or by two intramuscular injections of recombinant TSH (rTSH).

Answer 17

Either I-123 or Tc-99m can be used to localize ectopic thyroid tissue. Lingual thyroid is ectopic thyroid tissue at the base of the tongue. Functional thyroid tissue must rarely be seen in an ovarian teratoma (struma ovarii). Retrosternal thyroid tissue is most often due to a substernal goiter.

Answer 18

Thyroid nodules are typically only imaged if the cytology is indeterminate. Hyperfunctioning nodules are almost always benign adenomas. Cold nodules have approximately 20% risk of malignancy, although the most common cold nodule (~70%-75%) is a benign colloid cyst. A warm nodule usually represents a cold nodule with overlapping thyroid tissue. A warm nodule requires further investigation such as biopsy if oblique views are indeterminate. A discordant thyroid nodule is "hot" on Tc-99m and "cold" on I-123 as it has maintained the ability to uptake technetium but is unable to trap iodine. Biopsy is usually recommended as a discordant nodule may be malignant. Malignancy is relatively uncommon in a multinodular goiter. While a dominant cold nodule should undergo further investigation, smaller cold nodules are unlikely to be malignant.

Answer 19

Graves disease is an autoimmune disorder characterized by hyperthyroidism, thyromegaly, homogenously increased thyroid activity, and often a prominent pyramidal lobe. Both 6-hour and 24-hour iodine uptake are elevated. Normal 6-hour uptake is 6-18% and normal 24 hour uptake is 10-30%. Although usually an I-123 and a Tc-99m scan can be differentiated by the presence of salivary uptake with Tc-99m, in Graves disease this distinction is often not possible. In Graves disease, thyroid uptake can be so strong taht the salivary glands are often not seen, causing a similar appearance with either radiotracer. Definitive treatment of Graves disease is I-131 radiotherapy or (less commonly) surgery. Antithyroid drugs (e.g., methimazole or propylthiouracil) are another option and may achieve a remission after 1-2 years of use.

Answer 20

Hashimoto thyroiditis is the most common inflammatory disease of the thyroid. Like Graves disease, Hashimoto thyroiditis also clinically presents with thyromegaly. In Hashimoto thyroiditis, however, thyroid hormone levels are variable depending on the disease stage. Most patients with Hashimoto thyroiditis are hypothyroid. Appearance on thyroid scan is variable, ranging from diffusely increased activity that resembles Graves disease to patchy uptake similar to a multinodular goiter. The patchiness is thought to be due to cold areas from infiltration by lymphocytes and lymphoid follicles.

Answer 21

The classical clinical presentation of subacute thyroditis is a painful swollen gland, although many patients present with silent hyperthyroidism. Imaging shows decreased radiotracer uptake and a low 24-hour uptake. Subacute thyroiditis is typically a self-limited condition. Treatment is directed towards symptom control with nonsteroidal anti-inflammatory drugs or steroid in severe cases.

Answer 22

Approximately 1-2 months after thyroidectomy, I-131 is administered to treat and simultaneously image residual and potential metastatic disease. Following thyroidectomy, thyroid replacement therapy is withheld to allow endogenous therapy is withheld to allow endogenous uptake of therapeutic I-131 by any residual or metastatic thyroid tissue. The goal TSH is 30-50 uIU/mL. The dosing of I-131 is dependent on the oncologic risk: Low-risk patient (tumor \<1.5 cm, no invasion of thyroid capsule): \<30 mCi I-131 administered. High-risk patient: 100-200 mCi I-131 administered. A new approach is a standard dose of 30 mCi for treatment of all T1, T2 and N2 cancers.

Answer 23

After ablation with I-131, patients with thyroid carcinoma are monitored by following thyroglobulin levels. If thyroglobulin levels rise, an I-123 scan is performed to evalute for disease recurrence or metastasis. If the I-123 scan is positive, repeat I-131 radioiodine is adminstered for ablation. Note that the presence of anti-thyroglobulin antibodies precludes the ability to monitor the thyroglobulin levels.

Answer 24

I-131 is administered in a single oral dose to treat Graves disease. Contraindications to I-131 include pregnancy, lactation, and inability ot comply with radiation safety guidelines. The dosing of I-131 for treatment of Graves varies by institution. Many endocrinologists advocate a calculated dose based on the estimated thyroid weight and 24-hour uptake, while another study has shown one of three fixed doses (up to 15 mCi) to be equally effective. Adequate dosing of I-131 can treat greater than 90% of patients with Graves disease

Answer 25

I-131 can be used to reduce goiter size, especially in patients who choose not to undergo surgery. The dose of I-131 to treat toxic multinodular goieter may be higher than that for Graves disease, and multiple treatments may be required. 25 mCi is a typical dose to treat toxic multinodular goiter.

Answer 26

Tc-99m sestamibi is taken up by parathyroids and the thyroid; however, thyroid activity decreases over time. The same agent is used for nuclear cardiology.

Answer 27

Dual-phase (early and delayed) imaging with Tc-99m sestamibi can localize a suspected parathyroid adenoma in a patient with elevated parathyroid hormone. A parathyroid adenoma shows increased uptake on early images and persistent retained acitivity on delayed images. In contrast, a thyroid adenoma will also initially show inreased uptake but will then wash out on delayed images. Parathyroid tissue does not take up Tc-99m pertechnetate, which can be administered in indeterminatne cases.

Answer 28

Sulfur colloid is taken up by reticuloendothelial cells, which are found in the liver, spleen, and bone marrow. Sulfur colloid is also take up by Kupffer cells in the liver. Hepatic Kupffer cells make up only approximately 10% of the liver mass. 80-90% of sulfur colloid particles are taken up by the liver. Most of the remainder is taken up by the spleen, and a small amount taken up in the bone marrow. The bone marrow is not normally seen at typical windowing levels. Although Tc-99m has a physical half-life of 6 hours, sulfur colloid is rapidly cleared with a biologic half-life of 2-3 minutes.

Answer 29

The most common cause of a photopenic defect (complete absence of radiotracer) is a hepatic cyst. It may be difficult to distinguish focal decreased uptake from a photopenic defect if the lesion is small. Most hepatic masses cause focal decreased radiocolloid uptake, including hepatocellular carcinoma (HCC), adenoma, and abscess. Focal decreased uptake should raise concern for HCC in a patient with any risk factors for HCC such as cirrhosis or chronic hepatitis.

Answer 30

Focal nodular hyperplasia may hyperconcentrate radiocolloid. A regenerating nodule in a cirrhotic liver can cause focal increased sulfur colloid uptake. Budd-Chiari syndrome (hepatic vein thrombosis) can lead to increased uptake in the caudate lobe in the later stage of disease.

Answer 31

Colloid shift is increased sulfur colloid accumulation within the spleen and bone marrow. It suggests liver dysfunction, most commonly due to cirrhosis.

Answer 32

Diffuse pulmonary uptake on a sulfur colloid scan is nonspecific, and can be seen in: Cirrhosis; COPD with superimposed infection; Langerhans cell histiocytosis; High serum aluminum (either due to antacids or excess aluminum in the colloid preparation)

Answer 33

Focal nodular hyperplasia (FNH) is a benign liver mass that can have a variable appearance on a sulfur colloid scan. Most commonly, FNH will be indistinguishable from background liver on a sulfur colloid scan due to Kupffer cells within the FNH. FNH may also appear to ahve increased uptake due to a combination of hypervascularity and Kupffer sulfur colloid uptake. In approximately one third of cases there is insufficient colloid concentration and FNH may appear as a photopenic defect. In contrast to FNH, a hepatic adenoma does not contain Kupffer cells and will consistently cause a cold defect. An adjunct or alternative to sulfur colloid scan is a HIDA scan. FNH contains biliary ductules so it should be positive on a HIDA scan.

Answer 34

The presence of an intrapancreatic spleen can be confirmed by sulfur colloid scan in indeterminate cases, where the suspected intrapancreatic spleen will show uptake. A Tc-99m damaged red cell study can also be performed.

Answer 35

Technetium-99m labeled red blood cells are prepared in vitro by mixing 1-3 mL of anticoagulated blood with stannous chloride and an oxidizing agent. Tc-99m is added and has a labeling efficiency of 95%. The labeling procedure takes more than 20 minutes. An in-vivo technique provides much noisier images due to worse labeling efficiency and resultant free pertechnetate, and is therefore uncommonly performed. Alternatively, Tc-99m sulfur colloid can be used. Sulfur colloid does not require significant preparation time, but has rapid blood clearance and a vascular half life of 2-3 minutes.

Answer 36

A tagged red cell study can be used to identify patients who may be suitable for angiography versus those who are not. Bleeding rates as low as 0.2 mL/min can be detected with a tagged red-blood cell study compared to 1mL/min for angiography. A positive study shows activity that changes shape and position over time due to peristalsis of intraluminal blood, with a pattern suggestive of either small or large bowel source.

Answer 37

Tecnetium-99m pertechnetate localizes to gastric mucosa

Answer 38

Meckel diverticulum is a remnant of the embryological omphalomesenteric duct, most commonly located in the distal ileum. Approximately 10-60% of Meckel diverticula contain ectopic gastric mucosa, which may result in mucosal damage and GI bleeding. A positive Meckel scan demonstrates a focal area of increased activity, typically in the right lower quadrant. A lateral view is often helpful to ensure that activity is anterior and not associated with the ureter. Other causes of right lower quadrant pain include appendicitis and intussesception, which can cause more diffuse, regional increased uptake due to hyperemia.

Answer 39

Tc-99m-iminodiacetic acid (IDA) analogs are used to image the biliary system. Regardless of the tracer, the test is typically called a "HIDA" scan. Disofein allows visualization of biliary system with bilirubin levels as high as 20 mg/dL and has 90% hepatic uptake. Mebrofenin allows visualization of biliary system with bilirubin levels as high as 30 mg/dL and has an even higher 98% hepatic uptake. Both are actively transported into hepatocytes but are not conjugated.

Answer 40

Patient must be NPO for 6 hours prior, but must have eaten within 24 hours. If the patient has been NPO for \>24 hours, then cholecystokinin (CCK; dose 0.02 ug/kg, given as slow infusion) can be given to empty the bladder before radiotracer is administered. CCK must be administered slowly or the patient may experience an exacerbation of their symptoms. In general, one should wait at least two hours after adminsitering CCK before beginning the exam. Dynamic imaging of the right upper quadrant begins immediately after injection of radiotracer. As soon as the gallbladder is visualized, acute cholecystitis is ruled out. If the gallbladder is not visualized by one hour, then morphine is given (0.04 mg/kg, up to a maximum dose of 4 mg) and imaging continues for 30 more minutes. Morphine contracts the sphincter of Oddi, redirecting bile into the cystic duct. Morphine should only be given if tracer is visualized in the small bowel, otherwise there is the theoretical risk of worsening a potential common bile duct obstruction. However, nonvisualization of tracer in the small bowel is not specific for common bile duct obstruction. If the patient has a morphine allergy, an alternative is to image for a total of 4 hours.

Answer 41

In a normal HIDA scan, the liver is visible by 5 minutes. The gallbladder is typically seen by 15 minutes due to radiotracer flow into the cystic duct. Tracer should be seen in the small bowel to ensure a patent common bile duct.

Answer 42

Almost all patients with acute cholecystitis have a cystic duct obstruction. Nonvisualization of the gallbladder after 90 minutes of imaging and morphine augmentation is approximately 86-98% sensitive for the diagnosis of acute cholecystitis, with a false-positive rate of approximately 6%, most commonly due to pancreatitis and biliary stasis. The rim sign describes increased hepatic activity surrounding the gallbladder fossa, thought to be due to hyperemia and possibly gangrenous cholecystitis. False postiive HIDA (gallbladder non-visualization without acute cholecystitis) can be due to: Recent meal (within 4 hours) or prolonged fasting (greater than 24 hours). Administration of CCK immediately prior to the exam, which can cause persistent sphincter of Oddi relaxation. Total parental nutrition. Pancreatitis. Severe illness. Chronic cholecystitis. Cholangiocarcinoma of the cystic duct (very rare). False negative HIDA (gallbladder visualization with acute cholecysitis) is very rare and can be due to: Acalculous cholecystitis with a patent cystic duct. Duodenal diverticulum simulating the gallbladder; however, a lateral view would differentiate. Biliary cyst simulating the gallbladder.

Answer 43

Chronic cholecystitis is long-standing gallbladder inflammation causing loss of normal gallbladder function and predisposing to formation of stones. Chronic cholecystitis may be a cause of chronic recurrent abdominal pain. Diagnosis can be difficult as there is no single pattern that is pathognomonic of chronic cholecystitis based on HIDA. In fact, most cases of chronic cholecystitis have a normal HIDA scan. A low gallbladder ejection fraction (GBEF) is thought to be suggestive of chronic cholecystitis. To measure the ejection fraction, pre- and post-CCK injection gallbladder counts are compared. A GBEF \<35% suggests chronic cholecystitis, although the reliability of this finding is controversial as ejection fraction is dependent on the rate of CCK injectin and standard infusion protocols have not been determined.

Answer 44

A HIDA scan is an accurate method to assess for the presence of a biliary leak. Imaging can be performed in the right lateral decubitus position to promote dependent pooling of bile.

Answer 45

Since the IDA tracers are actively transported into hepatocytes and then secreted into the bile, severe hepatic dysfunction will cause very poor hepatic uptake and blood pool clearance will be delayed. Since functioning hepatocytes are necessary to extract the tracer, a hepatic mass will appear as a focal photopenic defect.

Answer 46

Technetium-99m macro-aggregated albumin (MAA) is a particulate radiopharmaceutical that lodges int he pulmonary capillary bed and can therefore be used to evaluate pulmonary perfusion. Most particles are between 10 and 30 um in size. Typically 3 to 5 mCi Tc-99m MAA are administered, comprising between 200,000 and 600,000 particles. Particle fragments begin to break down in approximately 30 minutes, In children, pregnant patients, patients with mild pulmonary hypertension, and patients with known right-to-left shunt, the dose can be halved to approximately 100,000 particles. A relative contraindication to MAA is severe pulmonary hypertension, as obstruction of even a few pulmonary capillaries can cause clinical worsening. A right-to-left shutn causes immediate renal and brain uptake after intravenous injection. A Tc-99m MAA study may be able to quantify the shunt fraction in these patients. Note that renal uptake can also be seen if free pertechnetate is present, but evaluation of the head and neck can differentiate if free pertechnetate from a right-to-left shutn. Free pertechnetate is taken up by the thyroid, but not the brain. A right-to-left shunt, in contrast, demonstrates immediate brain uptake and no significant uptake in the neck. Clumping of MAA can be seen when MAA is inadvertently drawn back into the injection syringe, causing coagulation with the patient's blood.

Answer 47

Xenon-133 is an inhaled gas with a physical half-life of 5.3 days, which emits 81 keV gamma photons and is also a beta-emitter. The critical organ is the trachea. The biologic half-life is very short because the vast majority of the gas is exhaled. Although 10-20 mCi is administered, there is very little radiation exposure. Xenon-133 is imaged posteriorly to avoid breast artifacts, as teh relativley low keV is easily attenuated by soft tissue. Washin-washout imaging can be performed to evaluate for air trapping, which is seen in COPD. Xenon-133 requires good patient cooperation as the patient must breathe on a closed spirometer for several minutes. Exhaled Xenon-133 must be carefully disposed of, either exhausted to the atmosphere or trapped in a charcoal trap until it decays. Xenon-133 must be adminstered in a negative pressure room to prevent accidental leakage.

Answer 48

Tc-99m DTPA is a technetium-labeled aerosol. Unlike Xenon-133, DTPA does not allow for dynamic washin-washout imaging. Once inhaled, the particles remain in place for approximately 20-60 minutes. 30mCi is typically administered. Compared to Xenon, DTPA has greater ease of use. Specific advantages include the ability to image in multiple projections, no need for exhaust systems, and ability to use portably.

Answer 49

Pulmonary embolism is the leading cause of death in pregnancy in the developed world, and diagnosis must be weighed against radiation exposure to the mother and the fetus. Pulmonary embolism CT angiogram: Maternal breast dose approximately 10-70 mGy (much greater than ACR mammography guidelines of 3 mGy/breast). Fetal dose 0.1-0.66 mGy. Perfusion scanning: Maternal breast dose \<0.31 mGy. Fetal dose 0.1 - 0.37 mGy. Technique: In most cases, only perfusion (Q) scanning is necessary and ventilation scanning is not commonly performed. The administered dose of Tc-99m MAA is typically one half of the standard dose, or approximately 100,000 particles. Diagnostic accuracy for PE has been shown to be equivalent between PE-CT and Q scanning. The primary advantage of PE-CT is the demonstration of an alternative diagnosis not seen on plain radiographs approximately 5% of the time, at the expense of 100x increase in breast radiation exposure and slightly higher fetal dose. Nuclear medicine perfusion scanning is recommended in suspected pulmonary embolism in a pregnant patient with a normal chest radiograph. In clinical practice, PE-CT is often more readily available, which may influence the choice of exam performed.

Answer 50

Two or more large (\>75% of a segment) mismatched segmental defects without associated radiographic abnormality is high probability for pulmonary embolism.

Answer 51

One large segmental mismatched perfusion defect is intermediate probability. A triple match in the lower lung is intermediate probability. A triple match describes a defect on perfusion, a matched defect on ventilation, and a corresponding abnormality on the chest radiograph.

Answer 52

A single large or moderate matched VQ defect is low probabliity of pulmonary embolism. Other low probability findings include absent perfusion of an entire lung, or more than three small segmental lesions.

Answer 53

Nonsegmental lesions are very low probablity The stripe sign peripheral to a perfusion defect is very low probability for pulmonary embolism. The stripe sign represents a thin line of MAA uptake between a perfusion defect and the adjacent pleural surface, representing intervening perfused imaging. A solitary triple-matched defect in the mid-upper lung is very low probability. In contrast, the previously mentioned lower lung triple-match is more likely to represent PE and is intermediate probability.

Answer 54

Tc-99m MDP is a technetium-labeled diphosphonate. 20 mCi is typically adminstered, with imaging performed 2-4 hours later. Rapid renal excretion is normal. Diffuse soft tissue uptake can be seen in renal failure. Patients should void before imaging to prevent the bladder from obscuring a pelvic lesion. A three-phase study includes: 1) Radionuclide angiogram (flow) evaluates blood flow, with images taken every few seconds. Increased flow suggests hyperemia. 2) Blood pool evaluates the extracellular distribution immediately following the blood flow phase. 3) Standard delayed (skeletal) images are performed approximately 3 hours after injection.

Answer 55

A signle focus of uptake in a rib is thought to represent malignancy only ~10% of the time. Uptake in similar locations in two adjacent ribs is almost always due to trauma. Multiple adjacent photopenic bony lesions are unlikely to be metastases, but may represent infarction, avascular necrosis, or sequela of radiation therapy.

Answer 56

A single sternal lesion in a patient with breast cancer is due to metastasis ~80% of the time. Multifocal areas of increased activity in nonadjacent ribs are suspicious for metastases. A single photopenic lesion in patients with a known malignancy (especially neuroblastoma, renal cell carcinoma, and thyroid cancer) is due to metastasis 80% of the time.

Answer 57

Increased uptake in the brain or heart may be due to recent infarction. Malignant pleural effusion or ascites can cause increased uptake. Soft-tissue metastases containing calcium, including osteosarcoma, neuroblastoma, or mucin-producing tumors (gastrointestinal and ovarian) can cause soft tissue uptake. In the breasts, faint uptake can be normal, but focal intense uptake suggests breast carcinoma. Tracer uptake can be seen at site of recent breast procedure (e.g. biopsy). Inflammatory disease, such as myositis ossificans, dermatomyositis, and rhabdomyolysis, can cause radiotracer uptake in the soft tissues and muscles.

Answer 58

A superscan represents diffusely increased osseous uptake. Sometimes the uptake can be so diffuse that it may look normal on first glance. The clue to the presence of a superscan will be nonvisualization (or very faint visualization) of the kidneys. Renal failure may also cause lack of visualization of the kidneys, although the typical difuse soft tissue uptake seen in renal failure helps distinguish renal failurefrom a superscan. A superscan is most commonly due to metastatic prostate cancer. Other malignancies producing a superscan include breast cancer and lymphoma. A superscan can also be caused by metabolic bone disease in primary and secondary hyperparathyroidism. If the cause for the superscan is metabolic, then the long bones tend to be seen very well. In contrast, with metastatic disease the axial skeleton and proximal humeri and femora are primarily affected.

Answer 59

Osteosarcoma typically causes markedly increased uptake, often with increased uptake in the entire affected limb. Ewing sarcoma featrues intense and homogenous activity. It may be positive on all three phases of a three-phase bone scan, mimicking osteomyelitis. Osteoid osteoma features a vascular central nidus, which demonstrates intense activity. The double density sign describes an intense focus of uptake corresponding to the nidus, surrounded by relatively increased uptake representing hyperemia. The differential of the double density sign includes osteoid osteoma, Brodie abscess, and less likely stress fracture.

Answer 60

A stress fracture due to abnormal stress in a normal bone. Common sites of stress fractures include the tibial diaphysis, the femoral neck, and the metatarsals. Metatarsal stress fractures are the most common stress injury in the foot/ankle, with 90% occurring in the 2nd or 3rd metatarsals. The bone scan is typically positive by the time the patient has pain. A stress fracture is positive on all three phases of the bone scan.

Answer 61

Shin splints cause pain from periostitis at the tibial insertions of the anterior tibialis and soleus muscles. A three-phase bone scan shows normal blood flow and blood pool images, with linearly increased uptake in the posteromedial tibia on delayed (skeletal) phase.

Answer 62

An insufficiency fracture represents a fracture in response to normal stress in an abnormal bone due to underlying osteoporosis. A sacral insufficiency fracture typically shows "H" shaped uptake in the sacrum, often referred to as the Honda sign.

Answer 63

Evaluation of a painful hip or knee prosthesis for loosening or infection is a common indication for a bone scan. In a cemented prosthesis, it is normal to see activity surrounding the prosthesis up to 12 months. In a non-cemented prosthesis, activity may remain increased up to 2 years as bony ingrowth continues. In evaluation of a hip prosthesis, focal activity at the lesser trochanter (which acts as a fulcrum site) and distal femoral prosthetic tip seen \>1 year for cemented or \>2 years for non-cemented prosthesis suggests loosening. In contrast, generalized increase in radiotracer activity surrounding the prosthesis may suggest osteomyelitis. Mild to moderate activity that is limited to the greater trochanter/intertrochanteric region is often due to heterotopic ossification.

Answer 64

Osteomyelitis is positive on all three phases of a bone scan. A positive three-phase bone scan is very specific for osteomyelitis in the presence of a normal radiograph (confirming that no fracture is present). To evaluate for osteomyelitis in the presence of an underlying abnormality, such as a fracture or prosthesis, WBC imaging (indium-111 or Tc-99m labeled WBCs) combined with a Tc-99m sulfur colloid marrow scan can increase specificity for osteomyelitis. Focal WBC activity in a region devoid of colloid marrow activity is suggestive of osteomyelitis. Although rarely performed, gallium-67 scan can also increase specificity for osteomyelitis if the gallium uptake exceeds the bone scan uptake in the area of concern. Cellulitis shows increased MDP activity during the blood flow and soft tissue phases, but the skeletal phase is negative. Septic arthritis is positive on all three phases of the bone scan on both sides of the joint. Discitis shows increased skeletal activity in two adjacent vertebral bodies.

Answer 65

Hypertrophic pulmonary osteoarthropathy is long bone diaphyseal periosteal reaction due to pulmonary disease, most common caused by lung cancer. Bone scan shows increased parallel lines of activity along the cortex of long bones.

Answer 66

Avascular necrosis (AVN) initially shows decreased radiotracer activity in the affected region, followed by a hyperemic phase with increased uptake. SPECT imaging of AVN (especially of the hips) will often show a rim of increased uptake with central photopenia, thought to represent revascularization progressing from outside in. Spontaneous osteonecrosis of the knee (SONK) is a cause of a traumatic knee pain in the elderly. It typically appears as intensely increased radiotracer activty in the medial femoral condyle.

Answer 67

Paget disease is an idiopathic disturbance of osteoclastic and osteoblastic regulation. Paget has three discrete phaes: lytic, mixed, and sclerotic. The lytic (early) phase is typically positive on bone scan and often negative on radiography. The mixed phase is abnormal on bone scan and radiography. The sclerotic phase shows persistent radiographic changes, but the bone scan activity may subside. One complication of Paget disease is malignant degeneration to osteosarcoma. A focal cold lesion should raise concern for necrosis in a region of malignant degeneration, although this may be best appreciated by evaluating serial studies.

Answer 68

Complex regional pain syndrome, previously called reflex sympathetic dystrophy, causes persistent pain, tenderness, and swelling often due to minor trauma. Bone scan shows diffusely increased juxta-articular activity in multiple small joints of the hand or foot on delayed (skeletal) images. Blood pool and soft tissue phase uptake is variable, but most commonly both phases are increased.

Answer 69

DTPA can measure glomerular filtration rate (GFR) and evaluate renal perfusion. DTPA is excreted by glomerular filtration. Approximately 20% is extracted by the glomerulus into the tubules with each pass. This is the identical extraction fraction as inulin, which is used to measure GFR exactly.

Answer 70

MAG3 can estimate renal plasma flow and evaluate renal perfusion; however, MAG3 cannot measure GFR. MAG3 is filtered and excreted by the tubules. Greater than 50% is extracted by the glomerulus into the tubules with each pass. MAG3 is cleared predominantly by the proximal tubules with minimal filtration. It has a higher extraction fraction than DTPA, which provides better images in patients with renal insufficiency or obstruction.

Answer 71

DMSA scanning is performed in children to evaluate for the presence of renal scarring associated with pyelonephritis. The collecting system is not imaged. DMSA is a specialized cortical agent. It is bound in the renal tubules, which allows anatomic imaging of the cortex. It is the only renal tracer where SPECT imaging is performed.

Answer 72

A nuclear renogram is a time-activity curve that provides a graphical representation of renal uptake and excretion. Approximately 10 mCi Tc-99m MAG3 (most commonly) or DTPA is administered. A normal renogram has three phases: The flow phase (sharp upslope), cortical function phase (defined peak), and clearance phase (rapid excretion).

Answer 73

A positive ACE renogram is relatively specific for renal artery stenosis, which may be a cause of hypertension. The radiotracer is Tc-99m MAG3, much less commonly Tc-99m DTPA. In compensated renal artery stenosis, decreased blood to the glomerulus stimulates renin -\> angiotensin I -\> angiotensin II. Angiotensin II constricts the efferent (outgoing) arterioles to restore GFR. Angiotensin I is converted to angiotensin II by angiotensin converting enzyme. After administration of an ACE inhibitor (typically captopril, 25-50 mg), the efferent arterioles will relax and GFR will decrease. If a patient is already on an ACE inhibitor, it should be stopped for 48 hours to 1 week prior to the exam. Intravenous access should be maintained in case fluid resuscitation is needed to treat hypothyroidism. A positive study depends on radiotracer (MAG3 or DTPA), but the hallmark is a post-ACE inhibitor renogram that becomes abnormal or more abnormal.

Answer 74

If hydronephrosis is present, a diuretic renogram can distinguish between obstruction and a nonobstructive cause of collecting system dilation. Both renal function and urodynamics are evaluated. The radiotracer is Tc-99m MAG3, much less commonly Tc-99m DTPA. After approximately 20 minutes of imaging, 40 mg IV lasix is administered. A higher dose of lasix may be needed in patients with renal insufficiency. After administration of lasix a fixed mechanical obstruction will show no change in the renogram curve. However, in causes of non-obstructive hydronephosis, the additional pressure of the diuresed kidney will open up the collecting system and allow drainage of radiotracer from the kidney. After administration of lasix, a clearance half-time \<10 minutes is normal, 10-20 minutes is borderline, and \>20 minutes suggests obstruction. False positives (diuretic renogram is positive for obstruction in the absence of a true obstruction) can be seen in dehydration, distended bladder, or renal failure (decreased response to diuretics).

Answer 75

Renal cortical imaging is limited to renal imaging in children to evaluate for pyelonephritis or scarring. The radiotracer is Tc-99m DMSA. A normal DMSA study excludes the diagnosis of acute pyelonephritis. A positive study for pyelonephritis can have three patterns: Focal cortical defect, multifocal cortical defects, or diffusely decreased radiotracer.

Answer 76

Radionuclide cystography (RNC) is the most sensitive test for evaluation of pediatric reflux, with less radiation exposure compared to voiding cystourethrogram. The radiotracer chosen is variable. Tc-99m pertechnetate, Tc-99m DTPA, and Tc-99m sulfur colloid work well. Regardless of radiotracer chosen, the radionuclide is injected retrograde into the catheterized bladder. Grading of reflux: Minimal (RNC grade I): Reflux is confined to the ureter. Moderate (RNC grade II): Reflux extends superiorly to the pelvicalyceal system. Severe (RNC grade III): Severe reflux causing a tortuous ureter and/or dilated intrarenal system.

Answer 77

I-123 MIBG is used to image pheochromocytoma in adults and neuroblastoma in children. To a lesser extent, MIBG is taken up by carcinoid, medullary thyroid carcinoma, and paraganglioma. I-131 MIBG can be used for therapy of neuroblastoma in children. Normal distribution is areas of sympathetic innervation, including salivary glands, heart, thyroid (although this is typicaly intentionally blocked with Lugol's solution to reduce thyroid dosimetry), liver, kidney, and bladder.

Answer 78

An Octreoscan study is primarily used to image carcinoid. Indium-111 is cyclotron-produced and decays by electron capture, emitting two photons at 173 and 247 keV. Its half-life is 67 hours. Pentreotide is an analog of octreotide used to detect tumors with somatostatin receptors, including amine precursor uptake and decarboxylation (APUD) tumors. In clinical use, the most common applications of Indium-111 pentreotide are evaluation of carcinoid or islet cell tumors. Pentreotide is quite good for imaigng gastrinoma, with relatively reduced sensitivity for insulinoma. Although uncommonly performed, glomus tumors of the head and neck (extra-adrenal pheochromocytomas) are seen better with Indium-111 pentreotide compared to MIBG. Normal distribution is intense renal and splenic uptake, with slightly less hepatic uptake.

Answer 79

MIBG is generally considered the first line agent for adrenal pheochromocytoma, as the normal renal uptake of pentreotide may obscure an adrenal lesion. Pentreotide can be considered as a second line agent for pheochromocytoma if the MIBG scan is negative. For evaluation of extra-adrenal pheochromocytoma (i.e. paraganglioma or glomus tumor), MIBG and pentreotide are roughly equivalent.

Answer 80

Gallium-67 is cyclotron-produced and decays by electron capture, with a half-life of 78 hours. It emits multiple gamma rays at 93, 184, 296, and 388 keV (easier to remember as 90, 190, 290, and 390 keV). Gallium binds to transferrin, which is found in infection, inflammation, and neoplasm. Normal gallium distribution is high activity in bowel and colon, and less in liver, skull, bone marrow, and salivary glands. Use is limited in the abdomen due to high bowel and liver activity. Persistent gallium in the kidneys is never normal after 24 hours and signifies renal disease. Diffuse pulmonary uptake is also not normal, with a wide differential of infectious and inflammatory conditions: Pneumocystis pneumonia, idiopathic pulmonary fibrosis, sarcoidosis, lymphangitic carcinomatosis, miliary tuberculosis, and fungal infection. The panda sign represents increased uptake of gallium in the nasopharynx, parotid glands, and lacrimal glands due to inflammation, resembline the dark markings on a panda's face. Classically the panda sign is due to sarcoidosis. The panda sign is not specific for sarcoidosis, although it strongly suggests sarcoid when the lambda sign is also seen (due to bilateral hilar and right paratracheal adnopathy). Sarcoid is the most common disorder to affect the salivary and lacrimal glands symmetrically. The differential diagnosis of the panda sign includes Sjogren syndrome, lymphoma after irradiation and AIDS. Gallium scans have been largely replaced by PET-CT. A residual first-line application of gallium scan is the evaluation of spinal osteomyelitis.

Answer 81

Thallium-201 is cyclotron-produced, decays by electron capture, and produces relatively low-energy characteristic X-rays of 69-81 keV. Its half-life is 73 hours. The normal distribution of thallium demonstrates prominent uptake in the kidneys, heart, liver, thyroid, and bowel. Thallium imaging is infrequently used due to its long half-life and resultant high radiation exposure. Historically, thallium has been used in combination with a gallium scan to distinguish between Kaposi sarcoma, lymphoma, and tuberculosis, in immunocompromised patients. Thallium was also historically used as a myocardial agent.

Answer 82

The normal distribution of indium-111 labeled WBCs is spleen \> liver \>\> bone marrow. Infection imaging with an indium-111 WBC scan is performed at 24 hours. The key advantage of indium-111 oxine WBCs compared to gallium is teh lack of physiologic bowel accumulation, which allows evaluation of abdominal or bowel infection/inflammation. Disadvantages compared to gallium include a tedious labeling procedure, higher radiation dose, and less accuracy in diagnosing spinal osteomyelitis. Advantages of indium-111 white blood cell scan compared to Tc-99m HMPAO include absence of interfering bowel and renal activity, ability to perform delayed imaging, and ability to perform simultaneous Tc-99mm sulfur colloid or Tc-99m MDP bone scan. These combined approaches are very helpful for evaluation of osteomyelitis in teh setting of baseline abnormal bone scan (e.g., prosthesis evaluation)

Answer 83

The advantages of a white blood cell scan labeled with Tc-99m HMPAO compared to indium-111 are related to the shorter half-life of Tc-99m, which allows a higher administered activity, better counts, a lower absorbed dose, and ability to perform earlier imaigng. For these reason, Tc-99m is often preferred in children in appropriate cases. However, a major disadvantage is physiologic uptake within the gastrointestinal and genitourinary tracts due to unbound Tc-99m HMPAO complexes which limits bowel evaluation. Renal activity occurs early, while bowel activity is seen after 1-2 hours. Additionally, delayed imaging is less practical with Tc-99m due to its shorter half-life.

Answer 84

Historically, increased focal gallium uptake in the liver suggests hepatocellular carcinoma (HCC). Conversely, HCC is extremely unlikely if gallium uptake is diminished.

Answer 85

Kaposi sarcoma is thallium avid, but does not take up gallium. Mnemonic: KaT (Kaposi is Thallium avid) Tuberculosis and atypical mycobacteria take up gallium but not thallium. This is the opposite of Kaposi sarcoma. Mnemonic: TuG (Tuberculosis is Gallium avid). Lymphoma takes up both gallium and thallium. Mnemonic: Lymphoma likes both.

Answer 86

A positive triple-phase Tc-99m MDP bone is only specific for osteomyelitis if the radiograph is normal. If there is an underlying abnormality, gallium or WBC scan can increase specificity. Gallium imaging can increase specificity of a positive bone scan, especially for vertebral osteomyelitis and discitis. A WBC scan (typically with indium-111) can increase specificity in evaluation of an infected orthopedic prosthesis, where scanning is typically performed in conjunction with Tc-99m sulfur colloid. When comparing the two scans (WBC and sulfur colloid), a discordant region with increased WBC tracer uptake and decreased sulfur colloid uptake is suggestive of marrow replacement by WBCs, consistent with osteomyelitis. Conversely, a region of normal marrow would be expected to demonstrate uptake both by WBCs and sulfur colloid.

Answer 87

The sensitivity of I-123 MIBG for detection of extra-adrenal pheochromocytoma ranges from 63-100%, with potential loss of MIBG uptake seen in tumor cell dedifferentiation, altered membrane transport proteins, or interference by medications. Indium-111 pentetreotide is also an option. The role of F-18 FDG PET in the evaluation of metastatic extra-adrenal pheochromocytoam is evolving. Most tumors are FDG avid.

Answer 88

Indium-111 pentetreotide (Octreoscan) is the tracer of choice for evaluation of carcinoid tumor. SPECT is almost always performed. The sensitivity for detecting neuroendocrine tumors is 82-95%; however, specificity is only around 50%, especially when a "hot spot" is found near a region of physiologic uptake in the pituitary, thyroid, spleen, urinary tract, or bowel. False positives can also occur in benign processes such as inflammation, Graves disease, and sarcoidosis. These inflammatory false positives are thought to be due to octreotide receptors expressed in activated lymphocytes.

Answer 89

Tc-99m DTPA is a transient perfusion agent. There is no uptake within the brain parenchyma as it does not cross the blood brain barrier. DTPA is used for planar imaging only and is uncommonly used.

Answer 90

Both HMPAO and ECD are perfusion agents that cross the blood brain barrier. To be retained in the cell, ECD is enzymaticaly modiefied. In contrast, HMPAO simply needs to be protonated to be trapped. Thus, ECD is only taken up by living cells while HMPAO uptake is a marker of perfusion. In the evaluation of subacute infarct, the phenomenon of luxury perfusion can cause HMPAO uptake to increase (due to increased perfusion), while ECD will show the true defect representing the infarct core. Of these two agents, ECD is generally preferred for brain imaging. Compared to HMPAO, ECD has more rapid blood pool clearance, better shelf life, more accurate characterization of perfusion, and is only taken up by living cells. Both tracers are used for SPECT imaging.

Answer 91

In brain death, intravenously injected radiotracer is unable to enter the cranial cavity due to increased intracranial pressure. Planar imaging is typically performed with either Tc-99m pertechnetate or Tc-99m DTPA SPECT imaging may also be performed, either with HMPAO or ECD-labelled Tc-99m. Imaging shows no tracer perfusing the brain. The hot-nose sign represents increased collateral flow seen in brain death, but is not specific and may be seen in other abnormalities of cerebral perfusion.

Answer 92

Evaluation of cerebral perfusion reserve can be performed with an acetazolamide challenge. Normally, cerebral blood flow increases after administration of 1,000 mg acetazolamide, a carbonic anhydrase inhibitor. Areas of brain that already have maximized their autoregulatory mechanisms will not show an increase in perfusion after acetazolamide administration. These areas will have relatively reduced activity compared to the rest of the brain.

Answer 93

In general, seizure foci are hypermetabolic during a seizure (ictal imaging), and hypometabolic between seizures (inter-ictal imaging). Neither Tc-99m HMPAO or ECD undergo redistribution. Either radiotracer can be injected during the seizure or up to 30 seconds after the end of the seizure.

Answer 94

Alzheimer disease typically shows symmetrically decreased SPECT tracer in the posterior temporal and parietal lobes. Lewy body dementia appears similar to Alzheimer but also involves the occipital calcarine cortex. Multi-infarct dementia shows multiple asymmetric foci of decreased metabolism. Pick disease is characterized by decreased uptake in the frontal lobes and anterior portion of the temporal lobes.

Answer 95

Nuclear medicine can aid in the differentiation of tumor recurrence versus radiation necrosis. Several protocols have been described. Regardless of the nuclear medicine tracer involved, image fusion with MRI is usually performed. Thallium-201 generally accumulates in malignant gliomas and not in post-treatment granulation tissue (i.e., thallium is not taken up by radiation necrosis). Thallium-201 uptake requires a living cell and blood brain barrier disruption. The degree of thalium-201 uptake can be grade in comparison to the scalp activity: Uptake \< scalp is low; uptake btween scalp and 2x scalp is modera; and uptake \>2x scalp is high. Single-isotope thallium scan suggestive of tumor recurrence, with 4.4 mCi thallium-201 administered. Dual-phase F-18 FDG PET employs and delayed imaging to evaluate a region of suspected tumor recurrence versus radiation necrosis. PET scanning is performed at 1 hour and 4 hours. Dual-phase PET has been shown to have increased accuracy for the assessment of recurrence versus post-treatment changes in metastatic disease compared to single-phase PET.

Answer 96

Crossed cerebellar diaschisis is a commonly encountered phenomenon in the presence of a supratentorial lesion (seen in tumors, stroke, and truma), where the cerebellar hemisphere contralateral to the lesion shows decreased radiotracer uptake. This phenomenon is thought to be due to interruption of corticopontine-cerebellar pathways.