Nuclear medicine is a medical
subspecialty that is most often associated with
radiology, as the examinations that are performed result in images that are interpreted by a trained specialist. Nuclear medicine studies involve the administration of
radioactive isotopes, either alone or bound to a molecular substance, that are taken up by the target
organ; these "tracers" emit
gamma rays that can be measured by gamma cameras or
PET (
positron emission tomography) cameras. The camera delivers
data to a
computer and an image is produced that reflects the
physiological functioning of the target organ.
Examples of Common Procedures and Their Applications
Thyroid scan: Iodine-131 is administered intravenously and the thyroid is imaged to detect hyperthyroidism and tumors. Increased iodine uptake reflects hyperthyroidism; a measurement of activity will be high and the entire organ will appear "hot" (darker than normal). Benign nodules usually produce "cold" spots due to less uptake of iodine in the region of the abnormal tissue. Thyroid carcinoma may produce an abnormal contour of the organ and may produce either cold or hot spots.
Bone scan: Technetium-99m bound to a phosphate is injected intravenously and taken up by the skeletal system. Full-body and spot images are taken to detect cancers that metastasize to bone (e.g., breast, lung, and prostate cancer) - metastatic carcinoma induces a high level of new bone formation and will appear as hot spots. Suspected fractures that cannot be demonstrated on an x-ray may show up as hot spots on a bone scan. "Shin splints" (stress fractures) are a result of repetitive stress on the bones of the lower leg and will result in multiple striations in the tibias.
Lung ventilation/perfusion scan: Also known as a "V/Q scan", this exam involves two phases - the inhalation of radioactive gas and the injection of radioactive albumin. This test is most commonly performed to detect a pulmonary embolus, which is a blood clot that has been trapped in the pulmonary circulation. Decreased uptake of the tracer on the perfusion scan with normal aeration on the ventilation scan may be indicative of this potentially fatal condition.
Thallium stress test: This exam is used to visualize cardiac perfusion. After exercising to the point of near exhaustion, the patient is injected with thallium (a radioactive isotope that is taken up by myocardial cells). The patient is imaged immediately thereafter and 2-3 hours later and the two scans are compared. If they are the same, the heart is normal. If there is perfusion at rest but not during exercise, there may be a narrowing of one or more of the coronary arteries. If there is no perfusion during both tests, this is indicative of a prior myocardial infarction.
There are numerous other nuclear medicine studies, and several isotopes are used therapeutically for the treatment of various types of cancer.
Radiation safety note: the amount of radiation that a patient receives in a diagnostic nuclear medicine study is about the same as that of a chest x-ray (which is not that much and has not been shown to cause any untoward effects).