Nuclear Medicine

A subspecialty of medicine based on the use of radioactive substances in medical diagnosis, treatment, and research. Cyclotron-produced radioactive materials were introduced in the early 1930s, but the invention of the nuclear reactor during World War II made carbon-14, hydrogen-3, iodine-131, and later technetium-99m available in large quantities. Today most biomedical research and the care of many patients depend on the use of radioactive materials.

The most widely used radionuclides are technetium-99m, iodine-123, carbon-11, and fluorine-18. The latter two require a cyclotron near the site of radiotracer production because of their rapid radioactive decay (carbon-11 decays with a half-life of 20 min, fluorine-18 with a half-life of 110 min). The short half-life of the radioactive tracers makes it possible to administer the radiotracers to individuals without the harmful effects of radiation.
Tracer principle. The most fundamental principle in nuclear medicine is the tracer principle, invented in 1912 by Nobel laureate G. Hevesy, who found that radioactive elements had identical chemical properties to the nonradioactive form and therefore could be used to trace chemical behavior in solutions or in the body. One of the important consequences of the use of tracers was to establish the principle of the dynamic state of body constituents.

Prior to the development of radioactive tracer methods, the only way to study biochemical processes within the body wastomeasure the input and output of dietary constituents and examine concentrations of molecules at autopsy. With radioactive tracers, the movement of labeled molecules could be followed from the processes within organs, to excretion. In essence, the use of radioactive tracers makes it possible to study the biochemistry within the various organs of the human body.

According to the principle of the constancy of the internal environment, the concentration of chemical constituents in body fluids is usually kept within a very narrow range, and disturbances of these values result in disease. This concept has been one of the foundations of modern biochemistry. Nuclear medicine makes it possible to examine regional physiology and biochemistry in ways that at times surpass the perception of surgeons during an operation or pathologists during an autopsy. Imaging methods make it possible to measure regional as well as overall organ function, and to portray the results in the form of functional or biochemical pictures of the body in health and in disease. Such pictures enhance the information about structure that is obtained by other imaging methods, such as computerized tomography (CT) or magnetic resonance imaging (MRI), often providing unique, objective evidence of disease long before structural changes are seen.

The physiological and biochemical orientation of nuclear medicine provides a better approach to understanding disease. The body is viewed as a complex array of coordinated chemical and physical processes that can become impaired before signs of disease develop. This has led to the concept of chemical reserve. It is now known that chemical abnormalities, such as a reduced rate of glucose metabolism in Huntington's disease or a marked deficiency of a neurotransmitter such as dopamine in Parkinson's disease, can occur long before the onset of symptoms. This makes possible the detection of disease far earlier than symptoms or structural abnormalities can. In focal epilepsy, for example, chemical abnormalities are often detectable before structural changes occur.

Diagnosis. In a typical examination, a radioactive molecule is injected into an arm vein, and its distribution at specific time periods afterward is imaged in certain organs of the body or in the entire body. The images are created by measuring the gamma-ray photons emitted from the organs or regions of interest within the body. Nuclear medicine imaging procedures differ from ordinary x-rays in that the gamma rays are emitted from the body rather than transmitted across the body, as in the case of x-rays. As in most modern imaging, the principle of tomography is used, that is, the person is viewed by radiation detectors surrounding the body, or by rotation of a gamma camera around the body.

Such procedures include single-photon emission computed tomography (SPECT), based on the use of iodine-123 or technetium-99m, and positron emission tomography (PET), based on the use of carbon-11 and fluorine-18. The latter two elements are short-lived (carbon-11 half-life is 20 min; fluorine-18 half-life is 110 min) and therefore must be produced near the site where the studies are performed.
The nature of the injected material, called a radiopharmaceutical, determines the information that will be obtained. In most cases, either blood flow or biochemical processes within an organ or part of an organ are examined. The essence of a nuclear medicine examination is measurement of the regional chemistry of a living human body.

Examples of commonly used procedures in nuclear medicine using the tracer principle are examination of the blood flow to regional heart muscle with thallium-201- or technetium-99m-labeled radio pharmaceuticals, imaging the regional movements of the ventricles of the heart, detection of blood clots in the lung or impaired lung function, detection of breast and prostate tumors, detection of acute inflammation of the gallbladder, and examination of practically all organs of the body.
Positron emission tomography and single-photon emission computed tomography are used to study regional blood flow, substrate metabolism, and chemical information transfer. In the last category, positron emission tomography has been used to establish the biological basis of neurological and psychiatric disorders, and may help improve the drug treatment of depression, Parkinson's disease, epilepsy, tardive dyskinesia, Alzheimer's disease, and substance abuse. Advances in PET and SPECT and the use of simple detector systems may help in the monitoring of the response of an individual to drug treatment, and perhaps reduce the incidence of side effects. These methods can also provide information on the physiologic severity of coronary stenosis and myocardial viability, especially after thrombolytic therapy or other forms of treatment.

One of the most important areas of research in nuclear medicine is the study of recognition sites, that is, the mechanisms by which cells communicate with each other. For example, some tumors possess recognition sites, such as estrogen receptors.
Another area is in assessment of the availability of receptors that are the primary site of action of many medications. Specific effects of a drug begin by the binding of the drug to specific chemical receptors on specific cells of the body. For example, the finding that Parkinson's disease involves the neurotransmitter dopamine led to the development of L-DOPA treatment, which relieves many of the symptoms of the disease. Measurement of abnormalities of predopaminergic neurons makes it possible to characterize the abnormalities of pre-synaptic neurons in individuals with Parkinson's disease early in their illness at a time when the progress of the disease might be halted.

Treatment. In some diseases, radiation can be used to produce a biological effect. An example is the use of radioactive iodine to treat hyperthyroidism or cancer of the thyroid. The effects of treatment can be assessed with nuclear medicine techniques as well. For example, the metabolism of pituitary tumors can be used as an index of the effectiveness of chemotherapy with drugs that stimulate dopamine receptors.
Reference : McGraw - Hill Encyclopedia of Science and Technology