Uses In Medicine
Looking Inside the Human Body
Radioisotopes provide doctors with powerful tools to peer inside the human body, giving them a new vision. Instead of relying on x-rays, doctors can use the radiation given off by certain radioisotopes to take pictures of the internal workings of the body. To do this, a radioisotope is concentrated in an organ, such as the heart or the brain, and an external camera, sensitive to the emitted radiation, produces an image of the organ. One method of imaging an organ with a radioisotope is called single photon emission computed tomography (SPECT), and the process is called a SPECT scan.
Some elements concentrate naturally in certain parts of the body-iodine in the thyroid, phosphorus in the bones, potassium in the muscles, and so one. For this reason, doctors can use radioisotopes of these elements for certain types of procedures. If the required radioisotope does not concentrate naturally in the organ under study, a radioisotope can be chemically attached to a compound that will carry it to the organ so that a picture can be taken. This process is called labeling, and the procedure has greatly expanded the diagnostic applications of radioisotopes.
With technetium-99m, now the most widely used radioisotope for diagnosis, doctors examine the brain, heart, blood, lungs, liver, kidneys, thyroid, spleen, and bone. For example, the loss of calcium in older people-especially women-can lead to weakened bones and a condition called osteoporosis. To diagnose the condition, doctors can view a patient's skeleton by injecting technetium-99m into the blood in a chemical form that concentrates in the bones. From pictures taken by a sensitive camera, doctors diagnose the condition of the bones and prescribe treatment.
Heart Disease and Other Diagnoses
Doctors currently use two radioisotopes to diagnose heart disease: thallium-201 and technetium-99m. These radioisotopes are used as blood flow markers, allowing doctors to detect the risk of heart disease.
Doctors inject one of the radioisotopes into the patient's blood during exercise on a treadmill. The radioisotope concentrates in the heart, allowing doctors to follow the blood flow. Looking at an image of the heart, doctors can detect the reduced blood flow through the arteries that feed it, which can signal heart disease. Technetium-99m is being used increasingly because it has a short half-life than thallium-201 (six hours as opposed to 72), thus reducing the amount of radiation a patient receives.
Radioisotopes such as carbon-11 or fluoroine-18 provide another method of looking at internal organs. These radioisotopes emit positively charged beta particles-called positrons-and pairs of gamma rays traveling in opposite directions. Two moveable cameras, placed opposite each other, detect the rays and permit a three-dimensional image to be constructed by a computer. The technique is called positron emission tomography (PET).
Because most positron emitters have short half-lives and must be prepared on site, PET scans require ready access to cyclotron, a special machine for producing these short-lived radioisotopes. Positron emitters are prepared by irradiating light elements-carbon, nitrogen, oxygen, fluorine-using the cyclotron. With the help of a computer to translate the images captured by the cameras, both SPECT and PET are especially valuable in medical diagnosis.
Doctors also use these scanning methods to study the circulatory system and probe the activities of the brain, adding to our understanding of epilepsy, schizophrenia, Parkinson's disease, strokes, Down's syndrome, and Huntington's chorea.
Cancer Treatment and Other Therapies
The property of radiation that makes it dangerous also makes it useful in healing. When radiation's energy is deposited in living tissue, cells can be damaged or destroyed. For this very reason, radioisotopes play an important role in cancer therapy. Large doses of radiation focused directly on a cancer can destroy it with little damage to surrounding tissue.
Doctors rely on three different techniques to destroy cancer cells with radioisotopes. One approach involves an external machine that destroys the tumor with gamma radiation from a radioisotope such as cobalt-60. Another approach is to place the radioisotope directly into the tumor itself. An innovative example of this approach is the treatment of liver cancer with tiny spheres containing the radioisotope yttrium-90, which lodge in the capillaries of the cancerous tissue, providing a localized dose of radiation. A third approach is to use a radioisotope that naturally concentrates in the diseased organ. This technique is so widely used to treat many thyroid conditions that it has almost replaced thyroid surgery.
An important advance in the radiation treatment of cancer involves radioisotope-labeled, monoclonal antibodies. Our bodies naturally produce antibodies that attack foreign matter to eliminate it from the body. Recent advances in biotechnology have enabled scientists to manufacture artificial antibodies, called monoclonal antibodies. Artificial antibodies can be genetically engineered to bind to a type of molecule found primarily in a cancerous tumor.
Labeled with a radioisotope, the antibody can identify or destroy the cancerous tissue. Monoclonal antibodies labeled with iodine-131, yttrium-90, or bismuth-213 are beginning to show promising results for the treatment of cancer.
Surgical Applications
In addition to their importance in the diagnosis and treatment of diseases, radioisotopes play an important role in the sterilization of medical supplies. Syringes, surgical gloves, and pharmaceuticals like ointments and powders, which might be damaged by conventional methods of sterilization, are routinely treated by radiation from radioisotopes. Other supplies, such as petri dishes, test tubes, and surgical instruments, are also sterilized with radiation.
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