Nuclear medicine is a medical specialty that uses radioactive materials, called radionuclides, to help diagnose and treat a wide variety of diseases, and for biomedical research. The development of nuclear medicine reflects the advances in the fields of nuclear physics, nuclear chemistry, and, later, molecular biology. While there was considerable research in the nuclear sciences during the first part of the twentieth century, it was not until the 1930s and 1940s, when radioactive substances were made readily available by nuclear reactors and cyclotrons, that nuclear medicine evolved into a separate specialty.
Nuclear medicine procedures are an important diagnostic tool, and are performed in hospitals and outpatient facilities all over the world. A nuclear medicine team commonly consists of a nuclear medicine physician, a nuclear medicine technologist, a nuclear medicine physicist, and a radiopharmacist. Nuclear medicine procedures sometimes detect the presence of disease rather than provide a specific diagnosis, and are frequently performed together with other medical imaging modalities such as x-ray, CT (computerized tomography), MRI (magnetic resonance imaging), and ultrasonography. In some cases, a disease may be detected before an organ function is altered or symptoms appear. Early detection prompts early treatment.
Radionuclides and radiopharmaceuticals
A nuclear medicine procedure requires the use of a radionuclide. Radionuclides, by virtue of their natural tendency to achieve stability, decay or disintegrate at a constant rate. Each radionuclide has its own distinct method of decay and rate of decay, or half-life. During disintegration, radionuclides emit electromagnetic radiation (photons), which can be detected, localized, and quantitated by sophisticated radiation detectors. Most frequently, the radionuclide is chemically bound to a stable molecule or compound chosen for its ability to localize in a specific organ system. The combination of the radionuclide bound to a molecule or compound is known as a radiopharmaceutical. The foundation of radionuclide or radiopharmaceutical use is based on the tracer principle, invented by the Hungarian chemist Georg von Hevesy (1885–1966) in 1912. Hevesy demonstrated that radioactive nuclides had chemical properties that were identical to those of their nonradioactive, or stable, form, and could therefore be used to “trace” various biochemical and physiological behaviors in the body and obtain diagnostic information.
Typically, the radiopharmaceutical is injected intravenously (in a vein), but some studies require inhalation (as a radioactive gas), or ingestion. The distribution of the radiopharmaceutical in the body or organ can reveal the normal or altered state of blood flow, capillary permeability, tissue metabolism, or specific function of an organ system. For example, if the physiology of an organ system or area of an organ is changed for reasons such as a tumor, absence of blood flow, duct blockage, or disease process, the way in which the radiopharmaceutical is incorporated will reflect any alteration. Nuclear medicine procedures can show structural as well as functional changes.
Radiopharmaceuticals are also chosen for their particular radioactive properties such as half-life, type of radiation emitted during decay, photon energy, cost, and availability. Today, 99mTechnetium (99mTc [half-life = 6.0 hours]), a daughter product of 99Molybdenum (99Mo), is the most commonly used radionuclide for nuclear medicine procedures and for making radiopharmaceuticals. Technetium is considered ideal because it gives a low radiation dose to the patient, has a low energy (140keV), most of its decay emissions are gamma-rays, it has a short half-life (six hours), is inexpensive and readily obtained, and combines easily with many compounds.
Unlike an x-ray procedure, where an image is obtained by an x-ray beam (generated by a machine) that passes through the body, a nuclear medicine image occurs when the radioactive decay occurring within the body is detected and recorded externally. Nuclear medicine images are most often obtained by a machine called a scintillation camera or gamma camera, invented in 1958 by the American physicist Hal Anger (1920–2005). The images or pictures are often called scans, which is a word left over from the time when nuclear medicine images were obtained by scintillating detector machines called rectilinear scanners. A scintillation or gamma camera is made of many components. This machine is capable of detecting radiation and converting the detected events into electrical impulses. Most gamma cameras are equipped with computers to process the information collected, to store the information, and produce an image of the organ of interest. The resulting picture is usually seen as a two-dimensional image on a black and white or color television monitor. Some common nuclear medicine imaging procedures include lung, thyroid, liver, spleen, biliary system, heart, kidney, brain, and bone scans.
Treatment and nonimaging procedures.
Non-imaging nuclear medicine exams such as radioimmunoassay studies require mixing serum with radioactive tracers to detect the presence of a certain hormone, chemical, or therapeutic drug. In other non-imaging studies the patient is given a radiopharmaceutical, and after a certain amount of time, samples of blood or urine are obtained and tested. Occasionally, a large amount of a radioactive substance is given to a patient to produce a biologic effect. For example, the therapeutic treatment for Grave’s disease, a hyperactive condition of the thyroid gland, requires a high dose of radioactive iodine (131I)—enough to destroy thyroid tissue. Radioactive iodine is often used to treat or detect thyroid conditions because the thyroid naturally traps iodine. When radioactive iodine is ingested, the thyroid, depending on its physiological state, absorbs a certain amount, temporarily making the thyroid radioactive.
Recent developments in nuclear medicine
Advances in monoclonal antibody research, radiopharmaceuticals, and computer technology have allowed nuclear medicine practitioners to probe deeper into the workings of the human body. Tumor-specific antibodies have been labeled or mixed with radiopharmaceuticals and administered to patients for both localizing and treating various types of tumors.
Conventional planar studies do not give detailed information about the depth of an abnormality seen on an image. The tomographic (tomos is the Greek word for slice) principle has been applied to nuclear medicine procedures enabling the physician to see regions of an organ in slices or layers. Two tomographic methods in nuclear medicine are single proton emission computerized tomography (SPECT) and positron emission tomography (PET). Like conventional images, tomographic images show how a radiopharmaceutical is distributed within an organ. Areas of normal, increased, or decreased distribution can be seen, thus revealing areas of altered biochemical and physiological function. When a tomographic study is obtained, the gamma camera detector circles the body and obtains multiple two dimensional images at various angles. A special computer program reconstructs the images and an organ can be visualized, in slices or layers, from top to bottom, front to back, and left to right. Viewing organs in slices eliminates interference from areas overlying a possible abnormality.
Single photon emission computed tomography (SPECT) studies are most often used for cardiac imaging and brain imaging, although the tomographic technology can be helpful for viewing other organs as well. SPECT studies use conventional radionuclides such as 99mtechnetium and 123Iodine. PET studies use only positron emitting radionculides such as 11Carbon, and 18Fluorine. The radionuclides used for PET are very short lived and therefore a cyclotron must be on site. Cyclotrons and PET equipment is very expensive, so there are few institutions that perform these tests. Their clinical use is consequently very limited. The focus of PET is biochemical rather than structural and is used most often for exploring neurochemical phenomena in the brain. PET can help distinguish one form of dementia from another, test for psychiatric drug effectiveness, and demonstrate regional metabolic differences between certain psychiatric disorders. PET and SPECT imaging procedures are used to study the areas of the brain affected by strokes, epilepsy, and Parkinson’s disease. Newer SPECT radiopharmaceuticals, because of their ability to cross the blood-brain barrier, have made it possible to study brain function and metabolism. Since assessing brain function is important to both physical medicine and behavioral medicine, SPECT may very well move these studies into the clinical setting.