Nuclear Medicine Studies



Nuclear Medicine Studies






OVERVIEW OF NUCLEAR MEDICINE STUDIES

Nuclear medicine is a diagnostic modality that studies the physiology or function of any organ system in the body. Other diagnostic imaging modalities, such as ultrasound, magnetic resonance imaging (MRI), computed tomography (CT), and x-ray, generally visualize anatomic structures.

A pharmaceutical is labeled with a radioactive isotope to form a radiopharmaceutical. The radioisotope emits gamma and positron rays. Radioisotopes are reactor produced (iodine-131 [131I]), cyclotron produced (fluorine-18 [18F] for positron emission tomography [PET]), or generator produced (technetium-99m [99mTc]).

To visualize the function of an organ system, a radiopharmaceutical is administered. A time delay (in some cases, up to several hours) may be required for the radiopharmaceutical to reach its target site, and then the organ of interest is imaged with a gamma camera. Image formation technology involves the detection with very great density of a signal (gamma rays) emanating from the radioactive isotope. There is very little signal in the image that does not come from the radiopharmaceutical. The normal background level of radiation within the human body is minimal, with small amounts of radioactive potassium and some cesium. Routes of radiopharmaceutical administration vary with the specific study. Most commonly, a radiopharmaceutical is injected through a vein in the arm or hand. Other routes of administration include the oral, intramuscular, inhalation, intrathecal (within the subdural or subarachnoid space), subcutaneous, and intraperitoneal (within the peritoneal cavity) routes. See Table 9.1 for possible side effects of or adverse reactions to the administration of radiopharmaceuticals.

Nuclear medicine studies are performed by certified nuclear medicine technologists, interpreted by radiologists or nuclear medicine physicians, and performed in a hospital or clinic-based nuclear medicine department. The collaborative approach to care is evidenced by interventions from pharmacists, laboratory personnel, and nurses, among others.


Principles of Nuclear Medicine

The radiopharmaceutical is generally made up of two parts: the pharmaceutical, which is targeted to a specific organ, and the radionuclide, which emits gamma rays (high-energy electromagnetic radiation; short wavelength) and allows the organ to be visualized by the gamma camera. Nuclear medicine imaging can yield quantitative as well as qualitative data. A measurement of the ejection fraction of the heart is an example of quantitative data derived from a multigated acquisition (MUGA) or a myocardial stress procedure.

In general, nuclear medicine images visualize the distribution of a particular radiopharmaceutical, with hot, warm, or cold spots of activity indicating an abnormality. In a hot spot, there is an increased area of uptake of the radiopharmaceutical in diseased tissue compared with the distribution in normal tissue. Examples of this type of uptake can be seen on bone images. An example of a warm spot would be in a thyroid nodule. In a cold spot, there is an area of decreased uptake of the radiopharmaceutical compared with the distribution in normal tissue. Liver and lung imaging are examples of this type of uptake. Prompt uptake in transplanted organs correlates with (1) adequate perfusion, such as reperfusion of the transplanted lungs or pancreas; (2) excretory function, such as in kidney transplants; and (3) evidence of cardiac viability and reinnervation. Poor uptake and nonvisualization of the transplanted organ are evidence of rejection.









TABLE 9.1 Potential Side Effects in the Administration of Radiopharmaceuticals






































































Radiopharmaceutical (Trade Name)


Possible Side Effects


Iodine-131 [131I]


Chills, nausea, vomiting, headache, dizziness, diffuse rash, tachycardia


Fluorine-18 [18F]


None have been reported


Thallium-201 [201Tl]


Fever, flushing, diffuse rash, hypotension


Technetium-99m [99mTc] 99mTc-pertechnetate (Minitec, UltratecKow)


Chills, nausea, vomiting, headache, dizziness, diffuse rash, hypertension


99mTc-tetrofosmin (Myoview)


Angina, hypertension, hypotension, vomiting, dyspnea, dizziness, metallic taste, abdominal discomfort


99mTc-pyrophosphate [99mTc-PYP] (Pyrolite, TechneScan PYP, Phosphotec)


Chills, fever, nausea, vomiting, dizziness, diffuse rash, flushing, chest pain, syncope


99mTc-disofenin (Hepatolite)


None have been reported


99mTc-mebrofenin (Choletec)


Hives, urticaria


99mTc-sulfur colloid (AN-Sulfur Colloid, TechneColl, Tesuloid)


Chills, fever, nausea, vomiting, headache, dizziness, diffuse rash, flushing, chest pain, vertigo, hypertension, hypotension, dyspnea


99mTc-bicisate dihydrochloride (Neurolite)


Nausea, diffuse rash, dizziness, chest pain, seizures, syncope, vertigo


99mTc methylenediphosphonate (MDP) (Osteolite, TechneScan)


Chills, fever, nausea, vomiting, headache, dizziness, diffuse rash, flushing, chest pain, vertigo, hypertension, hypotension, syncope


99mTc-pentetate (diethylenetriaminepentaacetate [DTPA]) (TechneScan DTPA, Techneplex)


Chills, fever, nausea, flushing, vomiting, headache, dizziness, diffuse rash, syncope, hypertension, hypotension, dyspnea


99mTc-exametazime (Ceretec)


Fever, flushing, diffuse rash, hypertension, hypotension, seizures, dyspnea


111In-capromab pendetide (ProstaScint)


Increase in bilirubin, hypotension, hypertension, injection site reactions, fever, rash, headache, production of human antimouse antibody (HAMA)


Indium-111 [111In]-DTPA (MPI-DTPA)


Fever, nausea, vomiting, flushing, headache, hypertension


Indium oxine (111In)


Fever


123I metaiodobenzylguanidine (MIBG)


Nausea, flushing, hypertension, dizziness, vertigo, tachypnea


Gallium citrate (67Ga) (Neoscan)


Nausea, vomiting, flushing, diffuse rash, tachycardia, dizziness, vertigo, metallic or salty taste


Cobalt (57Co)


None have been reported


Chromium-51 (51Cr)


Flushing, hypertension, tachycardia


Note: Most adverse drug reactions (ADRs) include such symptoms as nausea, vomiting, hypotension, rash, dyspnea, tachycardia, fever, and headaches; however, it is difficult to determine whether these are due to administration of the radiopharmaceutical or other medications the patient is taking. The ADR rate has been estimated at about 0.003% (3 per 100,000). The half-life of radiopharmaceuticals ranges from a couple of minutes to several days.


Adapted from Silberstein EB, Ryan J, and Pharmacopeia Committee of the Society of Nuclear Medicine. Prevalence of Adverse Reactions in Nuclear Medicine. J Nucl Med. 1996;37:185-192.





Principles of Imaging

Gamma cameras all have basically the same components. The camera may have one, two, or three heads, with the capability of imaging in multiple configurations. The camera is networked with a multitasking computer capable of acquiring and processing the data.

Several methods of imaging are used: dynamic, static, whole-body, and single photon emission computed tomography (SPECT). These imaging capabilities are available on all current camera systems.

Dynamic imaging allows serial display of multiple frames of data, each frame lasting 1 to 3 seconds, to visualize the blood flow associated with a particular organ. Static imaging is also known as planar imaging. The camera acquires one image at a time, covering the field of view. This image is twodimensional. Whole-body imaging acquires both anterior and posterior sweeps of the patient’s body. This type of imaging also gives two-dimensional information.

SPECT imaging has revolutionized the field of nuclear medicine. SPECT imaging provides three dimensions of data. SPECT imaging increased the specificity and sensitivity of nuclear imaging through improved resolution and is often combined with CT scans. Recently, manufacturers have developed a combined gamma camera and CT scanner that allows both procedures to be performed without patient transfer. Therefore, positioning is not compromised, and both abnormal and normal areas are visualized without position change.


General Procedure



  • Alert the patient that he or she may be required to follow a study-specific preparation regimen before imaging determined by the type of nuclear medicine procedure (e.g., nothing by mouth [Latin: nil per os, NPO], no caffeine for 24 hours, hydration, bowel preparation).


  • Administer a radiopharmaceutical through one of several routes: oral, inhalation, intravenous, intramuscular, intrathecal, or intraperitoneal. On occasion, additional pharmaceuticals may be administered to enhance the function of the organ of interest.


  • A time delay may be necessary for the radiopharmaceutical to reach the organ of interest.


  • Imaging time depends on:



    • Specific study radiopharmaceutical used and the time that must be allowed for concentration in tissues


    • Type of imaging equipment used


    • Patient cooperation


    • Additional views based on patient history and nuclear medicine protocol


    • Patient’s physical size




Benefits and Risks

Benefits and risks should be explained before testing. Patients retain the radioisotope for a relatively short period. The radioactivity decays over time. Some of the radioisotope is eliminated in urine, feces, and other body fluids.

99mTc, the most commonly used radiopharmaceutical, has a radioactive half-life of 6 hours. This means that half of the dose decays in 6 hours. Other radioisotopes, such as iodine, indium, thallium, and gallium, take 13 hours to 8 days for half of the dose to decay.



  • Benefits



    • Nuclear medicine yields functional data that are not provided by other modalities.


    • Nuclear imaging is relatively safe, painless (except for intravenous administration), and noninvasive.


  • Risks



    • Radiation exposure is minimal; toxicity is nil.


    • Hematoma at intravenous injection site.


    • Reactions to the radiopharmaceutical (hives, rash, itching, constriction of throat, dyspnea, bronchospasm, anaphylaxis [rare]).


Clinical Considerations

The following information should be obtained before diagnostic nuclear imaging:



  • Pregnancy (confirmed or suspected). Pregnancy is a contraindication for most nuclear imaging.


  • Lactating women may be advised to stop nursing for a set period (e.g., 2 to 3 days with 99mTc). Most radiopharmaceuticals are excreted in the mother’s milk.


  • Radiopharmaceutical uptake from a recent nuclear medicine examination could interfere with interpretation of the current study.


  • The presence of any prostheses in the body must be recorded on the patient’s history because certain devices can shield the gamma rays from imaging.


  • Current medications, treatments, and diagnostic measures (e.g., telemetry, oxygen, urine collection, intravenous lines)


  • Age and current weight. This information is used to calculate the radiopharmaceutical dose to be administered. If the patient is younger than 18 years of age, notify the examining department before testing. The amount of radioactive substance administered is adjusted downward for anyone younger than 18 years of age.


  • Allergies. Past history of allergies, especially to contrast substances (e.g., iodine) used in diagnostic procedures.



Pediatric Nuclear Medicine Considerations

Many of the nuclear medicine procedures that are performed on adults may be indicated in children.




CARDIAC STUDIES


Myocardial Perfusion: Rest and Stress (Sestamibi/Tetrofosmin/ Thallium Stress Test)

99mTc sestamibi, thallium-201 (201Tl), and 99mTc tetrofosmin are the radioactive imaging agents available for myocardial perfusion imaging to diagnose ischemic heart disease and allow differentiation of ischemia and infarction. This test reveals myocardial wall defects and heart pump performance during increased oxygen demands. Nuclear medicine imaging may also be done before and after streptokinase treatment for coronary artery thrombosis, after surgery for great vessel translocation, and after transplantation to detect organ rejection and myocardial viability. Pediatric indications include evaluation for ventricular septal defects and congenital heart disease and postsurgical evaluation of congenital heart disease. Studies have shown the efficacy of performing SPECT imaging with 99mTc sestamibi when triaging diabetic patients arriving in the emergency department with symptoms suggestive of acute cardiac ischemia.

201Tl is a physiologic analogue of potassium. The myocardial cells extract potassium, as do other muscle cells. 99mTc sestamibi is taken up by the myocardium through passive diffusion, followed by active uptake within the mitochondria. Unlike thallium, technetium does not undergo significant redistribution. Therefore, there are some procedural differences. Myocardial activity also depends on blood flow. Consequently, when the patient is injected during peak exercise, the normal myocardium has much greater activity than the abnormal myocardium. Cold spots indicate a decrease or absence of flow.

A completely normal myocardial perfusion study may eliminate the need for cardiac catheterization in the evaluation of chest pain and nonspecific abnormalities of the electrocardiogram (ECG). SPECT imaging can accurately localize regions of ischemia.

Administration of dipyridamole (Persantine) or regadenoson (Lexiscan) is indicated in adults and children who are unable to exercise to achieve the desired cardiac stress level and maximum cardiac vasodilation. This medication has an effect similar to that of exercise on the heart. Physical stress testing may be initiated in children beginning at 4 to 5 years. Candidates for drug-induced stress testing are those with lung disease, peripheral vascular disease with claudication, amputation, spinal cord injury, multiple sclerosis, or morbid obesity. Dipyridamole stress testing is also valuable as a significant predictor of cardiovascular death, reinfarction, and risk for postoperative ischemic events and to reevaluate unstable angina.

Ejection fraction and wall motion can be assessed by computer analysis.


Reference Values


Normal

Normal stress test: ECG and blood pressure normal

Normal myocardial perfusion under both rest and stress conditions




Interfering Factors



  • Inadequate cardiac stress


  • Caffeine intake


  • Injection of dipyridamole in the upright or standing position or with isometric handgrip may increase myocardial uptake.



Myocardial Infarction (PYP) Imaging

99mTc pyrophosphate (99mTc-PYP) is the radioactive imaging agent used to evaluate the general location, size, and extent of myocardial infarction 24 to 96 hours after suspected myocardial infarction and as an indication of myocardial necrosis to differentiate between old and new infarcts. In some instances, the test is sensitive enough to detect an infarction 12 hours to 7 days after its occurrence. Acute infarction
is associated with an area of increased radioactivity (hot spot) on the myocardial image. This test is useful when ECG and enzyme studies are not definitive.


Reference Values


Normal

Normal distribution of the radiopharmaceutical in sternum, ribs, and other bone structures No myocardial uptake




Interfering Factors

False-positive infarct-avid PYP can occur in cases of chest wall trauma, recent cardioversion, and unstable angina.



Multigated Acquisition (MUGA) Imaging: Rest and Stress

The term gated refers to the synchronization of the imaging equipment and computer with the patient’s ECG to evaluate left ventricular function. The primary purpose of this test is to provide an ejection fraction (the amount of blood ejected from the ventricle during the cardiac cycle).

Once injected, the distribution of radiolabeled red blood cells (RBCs) is imaged by synchronization of the recording of cardiac images with the ECG. This technique provides a means of obtaining
information about cardiac output, end-systolic volume, end-diastolic volume, ejection fraction, ejection velocity, and regional wall motion of the ventricles. Computer-aided imaging of wall motion of the ventricles can be portrayed in the cinematic mode to visualize contraction and relaxation. This procedure may also be performed as a stress test. MUGA images are not often performed on children.


Reference Values


Normal

Normal myocardial wall motion and ejection fractions under conditions of stress and rest




Interfering Factors

If a reliable ECG cannot be obtained because of arrhythmias, the test cannot be performed.



Cardiac Flow Study (First-Pass Study; Shunt Imaging)

The cardiac flow study is performed to check for blood flow through the great vessels and after vessel surgery; it is useful in the determination of both right and left ventricular ejection fractions. Immediately after the injection, the camera traces the flow of the radiopharmaceutical in its “first
pass” through the cardiac chambers in multiple rapid images. The first-pass study uses a jugular or antecubital vein injection of the radiopharmaceutical. A large-bore needle is used.

This study is useful in examining heart chamber disorders, especially left-to-right and right-toleft shunts. Children are commonly candidates for this procedure. Indications for pediatric patients include evaluation for congenital heart disease, transposition of the great vessels, and atrial or ventricular septal defects and quantitative assessment of valvular regurgitation. In neonates, the cardiac flow study can be used in conjunction with computer software for quantitative assessments. These quantitative values are useful in determining the degree of cardiac shunting with septal defects in the atria or ventricles.


Reference Values


Normal

Normal wall motion and ejection fraction

Normal pulmonary transit times and normal sequence of chamber filling




Interfering Factors

Inability to obtain intravenous access to the jugular vein or large-bore antecubital access



ENDOCRINE STUDIES


Thyroid Imaging

The thyroid imaging test systematically measures the update of radioactive iodine (either 131I or 123I) by the thyroid. Iodine (and, consequently, radioiodine) is actively transported to the thyroid gland and is incorporated into the production of thyroid hormones. The test is required for the evaluation of thyroid size, position, and function. It is used in the differential diagnosis of masses in the neck, base of the tongue, or mediastinum. Thyroid tissue can be found in each of these three locations.

Benign adenomas may appear as nodules of increased uptake of iodine (“hot” nodules), or they may appear as nodules of decreased uptake (“cold” nodules). Malignant areas generally take the form of cold nodules. The most important use of thyroid imaging is the functional assessment of these thyroid nodules. Pediatric indications include evaluation of neonatal hypothyroidism or thyrocarcinoma (lower incidence than adults).

Thyroid imaging performed with iodine is usually acquired in conjunction with a radioactive iodine uptake study, which is usually performed 4 to 6 hours and 24 hours after dosing. For a complete thyroid workup, in both adults and children, thyroid hormone blood levels are usually measured. A thyroid ultrasound examination also may be performed.


Reference Values


Normal

Normal or evenly distributed concentration of radioactive iodine

Normal size, position, shape, site, weight, and function of the thyroid gland

Absence of nodules





Interfering Factors



  • Thyroid imaging needs to be completed before radiographic examinations using contrast media (e.g., intravenous pyelogram, cardiac catheterization, CT with contrast, myelogram) are performed.


  • Any medication containing iodine should not be given until the nuclear medicine thyroid procedures are concluded. Notify the attending physician if thyroid studies have been ordered or if there are interfering radiographs or medications.



Radioactive Iodine (RAI) Uptake Test

This direct test of the function of the thyroid gland measures the ability of the gland to concentrate and retain iodine. When radioactive iodine is administered, it is rapidly absorbed into the bloodstream.
This procedure measures the rate of accumulation, incorporation, and release of iodine by the thyroid. The rate of absorption of the radioactive iodine, which is determined by the increase in radioactivity of the thyroid gland, is a measure of the ability of the thyroid to concentrate iodine from blood plasma. The radioactive isotopes of iodine used are 131I and 123I.

This procedure is indicated in the evaluation of hypothyroidism, hyperthyroidism, thyroiditis, goiter, and pituitary failure and for posttreatment evaluation. The patient who is a candidate for this test may have a lumpy or swollen neck or complain of pain in the neck; the patient may be jittery and ultrasensitive to heat or sluggish and ultrasensitive to cold. The test is more useful in the diagnosis of hyperthyroidism than hypothyroidism.


Reference Values


Normal

Absorption (uptake) by the thyroid gland:

1% to 13% after 2 hours

5% to 20% after 6 hours

15% to 40% after 24 hours

Values are laboratory dependent.





Interfering Factors



  • The chemicals, drugs, and foods that interfere with the test by lowering the uptake are:



    • Iodized food and iodine-containing drugs such as Lugol solution, expectorants, cough medications, saturated solutions of potassium iodide, and vitamin preparations that contain minerals. The duration of the effects of these substances in the body is 1 to 3 weeks.


    • Radiographic contrast media such as iodopyracet (Diodrast), sodium diatrizoate (Hypaque, Renografin), poppy-seed oil (Lipiodol), ethiodized oil (Ethiodol), iophendylate (Pantopaque), and iopanoic acid (Telepaque). The duration of the effects of these substances is 1 week to 1 year or more; consult with the nuclear medicine laboratory for specific times.


    • Antithyroid drugs such as propylthiouracil (PTU) and related compounds. The duration of the effects of these drugs may last 2 to 10 days.


    • Thyroid medications such as liothyronine sodium (Cytomel), desiccated thyroid, thyroxine (Synthroid, levothyroxine sodium) (duration, 1 to 2 weeks)


    • Miscellaneous drugs such as thiocyanate, perchlorate, nitrates, sulfonamides, tolbutamide (Orinase), corticosteroids, para-aminosalicylate, isoniazid, phenylbutazone (Butazolidin), thiopental (Pentothal), antihistamines, adrenocorticotropic hormone, aminosalicylic acid, cobalt, and warfarin sodium (Coumadin) anticoagulants. Consult with the nuclear medicine department for duration of effects of these drugs as they vary widely.


  • The compounds and conditions that interfere by enhancing the uptake are:



    • Thyroid-stimulating hormone (thyrotropin)


    • Pregnancy


    • Cirrhosis


    • Barbiturates


    • Lithium carbonate


    • Phenothiazines (duration, 1 week)


    • Iodine-deficient diet


    • Renal failure



Adrenal Gland (MIBG) Imaging

The adrenal gland is divided into two different components: cortex and medulla. The scope of adrenal imaging is limited to the medulla. Testing can be performed in both adults and children.

The purpose of adrenal medulla imaging is to identify sites of certain tumors that produce excessive amounts of catecholamines. Pheochromocytomas develop in cells that make up the adrenergic portion of the autonomic nervous system. A large number of these well-differentiated cells are found in
adrenal medullas. Adrenergic tumors have been called paragangliomas when they are found outside the adrenal medulla, but many practitioners refer to all neoplasms that secrete norepinephrine and epinephrine as pheochromocytomas. Because the only definite and effective therapy is surgery to remove the tumor, identification of the site using adrenal gland imaging, CT, and ultrasound is an essential goal of treatment.


Reference Values


Normal

No evidence of tumors or hypersecreting hormone sites

Normal salivary glands, urinary bladder, and vague shape of liver and spleen can be seen.




Interfering Factors

Barium interferes with the test.



Parathyroid Imaging

Parathyroid imaging is done to localize parathyroid adenomas in clinically proven cases of primary hyperparathyroidism. It is helpful in demonstrating intrinsic or extrinsic parathyroid adenoma. 99mTc sestamibi, 123I capsules, or 201Tl, or a combination of these three, can be used for imaging. In children, nuclear medicine imaging is done to verify presence of the parathyroid gland after thyroidectomy.


Reference Values


Normal

No areas of increased perfusion or uptake in parathyroid or thyroid




Interfering Factors

Recent ingestion of iodine in food or medication and recent tests with iodine contrast are contraindications and reduce the effectiveness of the study.


Clinical Considerations

Pregnancy is a relative contraindication. However, if primary hyperparathyroidism is suspected and surgical exploration is essential before delivery, the study may be performed.




GENITOURINARY STUDIES


Renogram: Kidney Function and Renal Blood Flow Imaging (With Furosemide or Captopril/Enalapril)

The renogram is performed in both adult and pediatric patients to study the function of the kidneys and to detect renal parenchymal or vascular disease or defects in excretion. The radiopharmaceutical of choice, 99mTc mertiatide (MAG-3), permits visualization of renal clearance. In pediatric patients, this procedure is done to evaluate hydronephrosis, obstruction, reduced renal function (premature neonates), renal trauma, and urinary tract infections. The renogram is ideal for pediatric evaluation because of the nontoxic nature of the radiopharmaceuticals, compared with the contrast media used in radiology procedures. Post-kidney transplantation scans, which assess perfusion and excretory function as a reflection of glomerular filtration rate (GFR), are done when the serum creatinine level increases and determine kidney damage leading to acute tubular necrosis (ATN).


Reference Values


Normal

Equal blood flow in right and left kidneys

In 10 minutes, 50% of the radiopharmaceutical should be excreted.


Indications



  • To detect the presence or absence of unilateral kidney disease


  • For long-term follow-up of hydroureteronephrosis


  • To study the hypertensive patient to evaluate for renal artery stenosis. The captopril test is a firstline study to determine a renal basis for hypertension.


  • To study the azotemic (increase in urea in the blood) patient when urethral catheterization is contraindicated or impossible


  • To evaluate upper urinary tract obstruction


  • To assess renal transplant efficacy





Interfering Factors

Diuretics, ACE inhibitors, and β blockers are medications that may interfere with the test results.

Jun 11, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Nuclear Medicine Studies
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