A nuclear study is a diagnostic procedure that is part of nuclear medicine. It uses radioactive isotopes that bind to receptors or organs in the body. These isotopes can then be detected with the use of a gamma ray camera which detects the radioactive decay in the body. Unlike radiology, which applies ionizing radiation to the patient to image it (such as in an x-ray), nuclear medicine uses radioactivity placed in the body to provide the image to a passive detector.
For example, the thyroid gland absorbs iodine far more readily than any other organ in the body. There are several radioactive isotopes of iodine. Small amounts of the isotope are administered to the patient (orally or intravenously), which then migrate to the thyroid. Using the gamma ray detector, the iodine in the thyroid outlines the thyroid in more detail than can a CT scan or MRI.
A nuclear study is preferred when it is believed a disease is in its early stages, as it can often reveal disease well before any other diagnostic imaging technique. However, it does expose the patient to a fair amount of radiation, and also tends to be more expensive than other techniques due to the cost of the isotopes themselves.