Nuclear medicine

Nuclear medicine, also known as radionuclide imaging or nuclear scintigraphy, is a radiological modality primarily used diagnostically to investigate physiological function. Nuclear medicine has applications in neurology, cardiology, oncology, endocrinology, lymphatics, urinary function, gastric function, respiratory function and osteotic (bone) function. Nuclear medicine also has radiation therapy applications in the treatment of thyroid cancer, metastatic bone lesions arising from prostate cancer, and joint diseases.

A typical nuclear medicine study involves introduction of a radionuclide into the body via injection in liquid or aggregate form, inhalation in gaseous/aerosol form or, rarely, injection of a radionuclide that has undergone microencapsulation. Most diagnostic radionuclides emit gamma rays, while the cell-damaging properties of beta particles are used in therapeutic applications. Refined radionuclides for use in nuclear medicine are derived from fission processes in nuclear reactors or cyclotrons. The most commonly used liquid radionuclides are:

technetium-99m
iodine-123
iodine-131
thallium-201
gallium-67

The most commonly used gaseous/aerosol radionuclides are:

xenon-133
krypton-81m
technetium-99m Technegas®
technetium-99m DTPA

The subsequent radioactivity emitted from the body is usually detected using a gamma-camera and reconstucted on computer into an image or dynamic series of images (ie. cine or movie). SPECT (single photon emission computed tomography) is the process by which images acquired from a rotating gamma-camera can be reconstructed into a three-dimensional (3D) representation.

The radionuclide introduced into the body is often chemically bound to a complex that acts characterisically within the body; this is known as a tracer. For example, the ligand methylene-diphosphonate (MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to bone for imaging. Any increased physiological function will usually mean increased concentration of the tracer. This often results in the appearance of a 'hot-spot' (focal increase in radio-accumulation), or generally increased radio-accumulation. Some disease processes result in the exclusion of a tracer, resulting in the appearance of a 'cold-spot'. Many tracer complexes have been developed in order to image or treat many different organss and glands.

Radiation dose to a patient from a nuclear medicine study depends on the physical half-life of the radionuclide (the rate of radioactive decay) in conjunction with the initial level of radioactivity introduced, and the biological half-life of the radionuclide (the rate of excretion from the body). The level of radioactivity introduced to the body depends on the type of study being performed, but is typically within the range of 37MBq-1110MBq (1mCi-30mCi).