Nuclear medicine scan

ALSO KNOWN AS: Radionuclide scan, positron emission tomography (PET) scan, single photon emission computed tomography (SPECT) imaging

DEFINITION: A nuclear medicine scan detects electromagnetic radiation, usually gamma rays, emitted from an injected radioactive tracer taken up by an organ in the body to be studied to produce an image.

The most common radioisotope used is technetium-99m, the workhorse of nuclear medicine whose gamma rays are absorbed by a sodium iodide crystal detector. Interaction of the gamma rays with the crystals produces a pulse of fluorescent light proportional in intensity to the gamma ray's energy. The light is amplified and converted into an electrical signal by the photomultiplier tubes. The electrical signal is fed to a computer, which analyzes the pulse height and generates an image of the radiotracer distribution in the body or organ under study. An array of these crystal detectors attached to a collimator, along with the photomultipliers and computer, is called a gamma camera. The gamma camera scans the patient. The pattern of uptake of radiotracers in the organs or whole body under study varies depending on the disease process.

Multiple gamma cameras are sometimes used to generate a three-dimensional view of an organ. This type of nuclear medicine scan is called single photon emission computed tomography (SPECT) imaging.

The exceptions to the use of technetium in nuclear medicine scanning include gallium scanning, which uses Gallium-67 as the radionuclide; some thyroid imaging that uses radioactive iodine; indium-labeled white cell studies and cerebral perfusion scans; some cardiac imaging that uses thallium; the Schilling test, which uses cobalt-labeled vitamin B¹²; and positron emission tomography (PET) scanning, which uses antimatter or positron emission instead of a gamma emitter.

PET imaging utilizes fluorodeoxyglucose (FDG) labeled with F-18, a positron emitter. The F-18-labeled FDG is preferentially taken up by cancer cells because of their increased metabolic rate and, therefore, increased need for sugar compared to normal cells. The scanner used in PET imaging is not the gamma camera but a separate scanner based on coincidence detection of the annihilation photons resulting from positron decay. PET scanning is often combined with computed tomography (CT), which is performed simultaneously.

Cancers diagnosed: All types of cancers, both primary and secondary (metastatic), can be diagnosed using nuclear medicine scans; for PET scans, the cancers more commonly diagnosed are lymphoma (Hodgkin and non-Hodgkin), esophageal cancer, lung cancer, head and neck cancers, colorectal cancer, pancreatic cancer, renal cancer, breast cancer, thyroid cancer, and melanoma.

Most cancers are identified due to their increased uptake of the radiotracer. Others are identified as a result of their lack of uptake, called photopenia, as can be seen in some liver tumors evaluated with HIDA or sulfur colloid. For HIDA scans, differentiation among primary hepatic tumors is performed because of increased uptake seen in focal nodular hyperplasia but not hepatic adenoma, which appears photopenic. In sulfur colloid liver-spleen scans, hepatic adenoma is again seen as a cold defect because of the lack of Kupffer cells (the phagocytic or “sweeper” cells in the liver that form part of the reticuloendothelial system) compared with focal nodular hyperplasia, which has normal or increased colloid uptake as a result of the presence of Kupffer cells. Macrophages in the spleen function similarly. A type of benign hepatic tumor known as a cavernous hemangioma is often diagnosed by increased uptake of radiotracer on red blood cell scans in a virtually pathognomonic pattern. For bone scans, metastatic disease from the prostate and breast, as well as osteosarcoma and osteoid osteoma, are most commonly diagnosed as a result of increased radiotracer activity from the osteoblastic activity of the cancer cells (that is, increased bone turnover caused by the cancer cells). Multiple myeloma, on the other hand, is photopenic on bone scans. For iodine scans, thyroid cancer of both primary and secondary types is identified because of the increased uptake of radioiodine (iodine-123 or iodine-131). In some cases, such as thyroid cancer, the scan and the treatment can be combined using radioactive iodine. Carcinoid tumors and medullary carcinoma of the thyroid are diagnosed with somastatin receptor imaging called indium DTPA-labeled octreotide, and pheochromocytomas are diagnosed with iodine-labeled metaiodobenzylguanidine (MIBG).

Why performed: Nuclear medicine scans (bone scan, iodine scan, HIDA scan, red blood cell scan, sulfur colloid scan, octreotide scan, PET scan) are used to diagnose primary and secondary cancer. They are also used to diagnose various ailments depending on the type of scan, including but not limited to the following—acute and chronic cholecystitis and evaluation for postoperative leaks following cholecystectomy (HIDA or DISIDA scan); gastrointestinal bleeding (red blood cell pertechnetate scan); goiter, hypothyroidism versus hyperthyroidism, and evaluation for ectopic thyroid tissue (thyroid scan); hyperparathyroidism caused by parathyroid adenoma (parathyroid scan); Meckel’s diverticulum (Meckel’s pertechnetate scan); pernicious anemia and malabsorption as a result of sprue (Schilling test); testicular torsion, testicular trauma, orchitis, and epididymitis (testicular pertechnetate scan); bone fractures, Paget disease, evaluation, reflex sympathetic dystrophy, bone infarction, and bone infection (bone scan); kidney obstruction, renal transplant rejection, and renal artery stenosis (renal scan); infectious and inflammatory disorders of the lungs, abdomen, pelvis, genitourinary tract, and bone, including acquired immunodeficiency syndrome (AIDS), sarcoidosis, and fever of unknown origin, among others (gallium scan); infection (indium-labeled white blood cells); cardiomyopathy-myocarditis and ejection fraction of the heart, often performed after doxorubicin for breast cancer (MUGA scan); coronary artery disease, coronary artery bypass graft surgery evaluation, valvular heart disease, and risk stratification following myocardial infarction (myocardial perfusion imaging, radionuclide ventriculography, cardiac SPECT, cardiac PET); pulmonary embolus (lung or V/Q scan); gastrointestinal bleeding, portal hypertension, and gastric emptying (sulfur colloid scan); normal pressure hydrocephalus, cerebral spinal fluid leaks, and surgical patency (indium-labeled DTPA cisternography); evaluation for brain death (DTPA cerebral blood flow study); stroke (HMPAO cerebral perfusion imaging with SPECT); identification of seizure focus, evaluation for Alzheimer’s disease versus other forms of dementia and depression, Parkinson’s disease, and drug addiction (brain SPECT and brain PET).

Patient preparation: Patients should fast at least four hours before most scans, especially DISIDA and PET scans. For thyroid imaging, patients should stop thyroid medication like Synthroid (levothyroxine sodium) and avoid CT contrast intravenous dye for one month before the scan, as both Synthroid and CT contrast dye will interfere with the scan results. Cisternography requires the injection of indium-labeled DTPA radiopharmaceutical into the lumbar subarachnoid space by lumbar puncture, also called spinal tap, prior to scanning. The Schilling test requires the patient to collect urine for twenty-four hours after injection of the radiotracer labeled vitamin B¹², and the urine is evaluated. The patient also receives an intramuscular injection of nonlabeled vitamin B¹² as part of the test before the urine collection.

Steps of the procedure: The technologist prepares the radioscope, and the radiologist or nuclear medicine physician injects it into a peripheral vein. For white and some red blood cell scans, a small amount of blood is withdrawn from the patient and labeled with the radioisotope, which is reinjected before scanning. The patient is placed on their back under a gamma camera connected to a computer. Scan time varies depending on the procedure but usually takes about one hour.

After the procedure: The scan is generated by the computer attached to the detector and read by the radiologist the same day. The patient must contact their doctor for the radiology report and follow-up treatment.

Risks: Minor pain or bruising at the injection site may occur. If the patient is pregnant, the scan should be avoided, if possible, since the radiation dose, although small in most cases, is not negligible. Radioactive iodine should not be administered to a pregnant patient because of the risk to the fetus as the radioiodine crosses the placenta with significant exposure to the fetal thyroid, causing cretinism. Radioiodine is also excreted in human breast milk, and nursing should be stopped following diagnostic or therapeutic studies performed using radioiodine.

Results: The results of a nuclear medicine scan depend on the type of scan performed and the reason for the study. Their use in detecting tumors helps the cancer care team stage the cancer and develop a treatment plan.

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