Nuclear Medicine

Radionuclides and radiopharmaceuticals

Instrumentation

Treatment and nonimaging procedures

Recent developments in nuclear medicine

Resources

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 many 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 always 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, tissuemetabolism, 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, photonenergy, cost, and availability. Today,^99mTechnetium (^99m Tc [half-life = 6.0 hours]), a daughter product of⁹⁹ Molybdenum (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.

Instrumentation

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. 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 colortelevision monitor. Some common nuclear medicine imaging procedures include lung, thyroid, liver, spleen, biliary system, heart, kidney, brain, and bone scans.

Treatment and nonimaging procedures

Nonimaging 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 organifies, or “traps,” iodine a person’s thyroid cannot tell the difference between stable or radioactive 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 protonemission computerized tomography (SPECT) and positron emission tomography (PET). Like conventional images, tomographic images show how a radio–pharmaceutical is distributed within an organ. Areas of normal, increased, or decreased distribution can be seen,

KEY TERMS

Blood–brain barrier —A blockade of cells separating the circulating blood from elements of the central nervous system (CNS); it acts as a filter, preventing many substances from entering the central nervous system.

Disintegration –Spontaneous nuclear transformation characterized by the emission of energy and/or mass from the nucleus.

Gamma ray –Electromagnetic radiation originating from the nucleus of an atom.

Half—life –The time taken for a group of atoms to decay to half their original number.

Ionizing radiation –Any electromagnetic or particulate radiation capable of direct or indirect ion production in its passage through matter.

Monoclonal antibody –An antibody made in a laboratory, derived from a single clone, so that each hybrid cell produces the same antibody.

Nuclide –Any nucleus plus its orbital electrons.

Photon –The quantum or particle of light.

Positron –A type of beta particle with a positive charge.

Radionuclide –Radioactive or unstable nuclide.

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. The images are reconstructed by a special computer program 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^99m technetium and¹²³ Iodine. PET studies use only positron emitting radionculides such as¹¹ Carbon, and¹⁸ Fluorine. 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 neuro–chemical 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.

Resources

BOOKS

Carlisle, Rodney P. Encyclopedia of the Atomic Age. New York: Facts on File, 2001.

Gottschalk, Alexander, ed. 2002 Yearbook of Nuclear Medicine. Mosby, 2003.

Grigg, E.R.N. The Trail of Invisible Light for X–Strahlen to Radiobiology. Springfield, Il.: Charles C. Thomas, 1965.

Stimac, Gary K. Introduction to Diagnostic Imaging. W.B. Saunders Co. Harcourt, Brace, Jovanovich, 1992.

PERIODICALS

Kasner, Darcy L., and Michael E. Spieth. “The Day of Contamination.” Journal of Nuclear Medicine Technology 31 (2003): 21–24.

“Pretargeted Radioimmunotherapy of Cancer: Progress Step by Step.” Journal of Nuclear Medicine 44, no. 3. (March 2003).

OTHER

Society of Nuclear Medicine. <http://www.snm.org/> (accessed March 27, 2007).

Christine Miner Minderovic

Nuclear medicine

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 many out-patient 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 ultra-sonography. 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 always 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.

Instrumentation

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. 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 twodimensional 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.

Nonimaging 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 nonimaging 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 organifies, or "traps," iodine a person's thyroid cannot tell the difference between stable or radioactive 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, radio-pharmaceuticals, 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. The images are reconstructed by a special computer program 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.

Resources

books

Carlisle, Rodney P. Encyclopedia of the Atomic Age. New York: Facts on File, 2001.

Gottschalk, Alexander, ed. 2002 Yearbook of Nuclear Medicine. Mosby, 2003.

Grigg, E.R.N. The Trail of Invisible Light for X-Strahlen to Radiobiology. Springfield, IL: Charles C. Thomas, 1965.

Spencer, Richard P. New Procedures in Nuclear Medicine. Florida: CRC Press, Inc. 1989.

Stimac, Gary K. Introduction to Diagnostic Imaging. W.B. Saunders Co. Harcourt, Brace, Jovanovich, 1992.

periodicals

Kasner, Darcy L., and Michael E. Spieth. "The Day of Contamination." Journal of Nuclear Medicine Technology 31 (2003): 21-24.

"Pretargeted Radioimmunotherapy of Cancer: Progress Step by Step." Journal of Nuclear Medicine 44, no. 3. (March 2003).

organizations

Society of Nuclear Medicine [cited March 2003] <http://www.snm.org/>.

Christine Miner Minderovic

KEY TERMS

Blood-brain barrier

—A blockade of cells separating the circulating blood from elements of the central nervous system (CNS); it acts as a filter, preventing many substances from entering the central nervous system.

Disintegration

—Spontaneous nuclear transformation characterized by the emission of energy and/or mass from the nucleus.

Gamma ray

—Electromagnetic radiation originating from the nucleus of an atom.

Half-life

—The time taken for a group of atoms to decay to half their original number.

Ionizing radiation

—Any electromagnetic or particulate radiation capable of direct or indirect ion production in its passage through matter.

Monoclonal antibody

—An antibody made in a laboratory, derived from a single clone, so that each hybrid cell produces the same antibody.

Nuclide

—Any nucleus plus its orbital electrons.

Photon

—The quantum or particle of light.

Positron

—A type of beta particle with a positive charge.

Radionuclide

—Radioactive or unstable nuclide.

Nuclear Medicine

Nuclear medicine involves the injection of a radiopharmaceutical (radioactive drug) into a patient for either the diagnosis or treatment of disease. The history of nuclear medicine began with the discovery of radioactivity from uranium by the French physicist Antoine-Henri Becquerel in 1896, followed shortly thereafter by the discovery of radium and polonium by the renowned French chemists Marie and Pierre Curie. During the 1920s and 1930s radioactive phosphorus was administered to animals, and for the first time it was determined that a metabolic process could be studied in a living animal. The presence of phosphorus in the bones had been proven using radioactive material. Soon ³²P was employed for the first time to treat a patient with leukemia. Using radioactive iodine, thyroid physiology was studied in the late 1930s. Strontium-89, another compound that localizes in the bones and is currently used to treat pain in patients whose cancer has spread to their bones, was first evaluated in 1939.

A nuclide consists of any configuration of protons and neutrons. There are approximately 1,500 nuclides, most of which are unstable and spontaneously release energy or subatomic particles in an attempt to reach a more stable state. This nuclear instability is the basis for the process of radioactive decay , and unstable nuclides are termed radionuclides. During the 1940s and 1950s nuclear reactors, accelerators, and cyclotrons began to be widely used for medical radionuclide production. Reactor-produced radionuclides are generally electron-rich and therefore decay by β ⁻-emission. The main application of β ⁻-emitters is for cancer therapy, although some reactor-produced radionuclides are used for nuclear medicine imaging. Cyclotron-produced radionuclides are generally prepared by bombarding a stable target (either a solid, liquid, or gas) with protons and are therefore proton-rich, decaying by β ⁺-emission. These radionuclides have applications for diagnostic imaging by positron-emission tomography (PET). One of the most convenient methods for producing a radionuclide is by a generator. Certain parent–daughter systems involve a long-lived parent radionuclide that decays to a short-lived daughter. Since the parent and daughter nuclides are not isotopes of the same element, chemical separation is possible. The long-lived parent produces a continuous supply of the relatively short-lived daughter radionuclide and is therefore called a generator.

Currently, the majority of radiopharmaceuticals are used for diagnostic purposes. These involve the determination of a particular tissue's function, shape, or position from an image of the radioactivity distribution within that tissue or at a specific location within the body. The radiopharmaceutical localizes within certain tissues due to its biological or physiological characteristics. The diagnosis of disease states involves two imaging modalities: Gamma (γ ) scintigraphy and PET. In the 1950s γ scintigraphy was developed by Hal O. Anger, an electrical engineer at Lawrence Berkeley Laboratory. It requires a radiopharmaceutical containing a radionuclide that emits γ radiation and a γ camera or single photon emission computed tomography (SPECT) camera capable of imaging the patient injected with the γ -emitting radiopharmaceutical. The energy of the γ -photons is of great importance, since most cameras are designed for particular windows of energy, generally in the range of 100 to 250 kilo-electron volts (keV). The most widely used radionuclide for imaging by γ scintigraphy is ^99mTc (T _½ = 6 hours), which is produced from the decay of ⁹⁹Mo (T _½ = 66 hours). In 1959 the Brookhaven National Laboratory (BNL) developed the ⁹⁹Mo/^99mTc generator, and in 1964 the first ^99mTc radiotracers were developed at the University of Chicago. The low cost and convenience of the ⁹⁹Mo/^99mTc generator, as well as the ideal photon energy of ^99mTc (140 keV), are the key reasons for its widespread use. A wide variety of ^99mTc radiopharmaceuticals have been developed during the last forty years, most of them coordination complexes. Many of these are currently used every day in hospitals throughout the United States to aid in the diagnosis of heart disease, cancer, and an assortment of other medical conditions.

PET was developed during the early 1970s by Michel Ter-Pogossian and his team of researchers at Washington University. It requires a radio-pharmaceutical labeled with a positron-emitting radionuclide (β ⁺) and a PET camera for imaging the patient. Positron-decay results in the emission of two 511 keV photons 180° apart. PET scanners contain a circular array of detectors with coincidence circuits designed to specifically detect the 511 keV photons emitted in opposite directions. The positron-emitting radionuclides most frequently used for PET imaging are ¹⁵O (T _½ = 2 minutes), ¹³N (T _½ = 10 minutes), ¹¹C (T _½ = 20 minutes), and ¹⁸F (T _½ = 110 minutes). Of these, ¹⁸F is most widely used for producing PET radiopharmaceuticals. The most frequently used ¹⁸F-labeled radiopharmaceutical is 2-deoxy-2 [¹⁸F]fluoro-D-glucose (FDG). This agent was approved by the Food and Drug Administration (FDA) in the United States in 1999 and is now routinely used to image various types of cancer as well as heart disease.

The use of radiopharmaceuticals for therapeutic applications (α - or β ⁻-emitters) is increasing. The first FDA-approved radiopharmaceutical in the United States was, in fact, for therapeutic use. Sodium [¹³¹I] iodide was approved in 1951 for treating thyroid patients. There are currently FDA-approved radiopharmaceuticals for alleviating pain in patients whose cancer has metastasized to their bones. These include sodium ³²P-phosphate, ⁸⁹Sr-chloride, and ¹⁵³Sm-EDTMP (where EDTMP stands for ethylenediaminetetramethylphosphate). In February 2002 the first radiolabeled monoclonal antibody was approved by the FDA for the radioimmunotherapy treatment of cancer. Yttrium-90-labeled anti-CD20 monoclonal antibody is used to treat patients with non-Hodgkin's lymphoma.

Many branches of chemistry are involved in nuclear medicine. Nuclear chemistry has developed accelerators and reactors for radionuclide production. Inorganic chemistry has provided the expertise for the development of metal -based radiopharmaceuticals, in particular, ^99mTc radiopharmaceuticals, whereas organic chemistry has provided the knowledge base for the development of PET radiopharmaceuticals labeled with ¹⁸F, ¹³N, ¹¹C, and ¹⁵O. Biochemistry is involved in understanding the biological behavior of radiopharmaceuticals, while medical doctors and pharmacists are involved in clinical studies. Nuclear medicine, which benefits the lives of millions of people every day, is truly a multidisciplinary effort, one in which chemistry plays a significant role.

see also Becquerel, Antoine-Henri; Curie, Marie Sklodowska; Nuclear Chemistry; Nuclear Fission; Radiation Exposure; Radioactivity.

Carolyn J. Anderson

Bibliography

McCarthy, T. J.; Schwarz, S. W.; and Welch, M. J. (1994). "Nuclear Medicine and Positron Emission Tomography: An Overview." Journal of Chemical Education 71: 830–836.

Schwarz, S. W.; Anderson, C. J.; and Downer, J. B. (1997). "Radiochemistry and Radiopharmacology." In Nuclear Medicine Technology and Techniques, 4th edition, ed. D. R. Bernier, P. Christian, and J. K. Langan. St. Louis, MO: Mosby Year Book. utes),

Internet Resources

"A Brief History of Nuclear Medicine." UNM, Ltd. Available from <http://www.nucmednet.com/history.htm>.

"The History of Nuclear Medicine." Society of Nuclear Medicine. Available from <http://www.snm.org/nuclear/history.html>.

Nuclear medicine

Nuclear medicine is a special field of medicine in which radioactive materials are used to conduct medical research and to diagnose (detect) and treat medical disorders. The radioactive materials used are generally called radionuclides, meaning a form of an element that is radioactive.

Diagnosis

Radionuclides are powerful tools for diagnosing medical disorders for three reasons. First, many chemical elements tend to concentrate in one part of the body or another. As an example, nearly all of the iodine that humans consume in their diets goes to the thyroid gland. There it is used to produce hormones that control the rate at which the body functions.

Second, the radioactive form of an element behaves biologically in exactly the same way that a nonradioactive form of the element behaves. When a person ingests (takes into the body) the element iodine, for example, it makes no difference whether the iodine occurs in a radioactive or nonradioactive form. In either case, it tends to concentrate in the thyroid gland.

Third, any radioactive material spontaneously decays, breaking down into some other form with the emission of radiation. That radiation can be detected by simple, well-known means. When radioactive iodine enters the body, for example, its progress through the body can be followed with a Geiger counter or some other detection instrument. Such instruments pick up the radiation given off by the radionuclide and make a sound, cause a light to flash, or record the radiation in some other way.

If a physician suspects that a patient may have a disease of the thyroid gland, that patient may be given a solution to drink that contains radioactive iodine. The radioactive iodine passes through the body and into the thyroid gland. Its presence in the gland can be detected by means of a special device. The physician knows what the behavior of a normal thyroid gland is from previous studies; the behavior of this particular patient's thyroid gland can then be compared to that of a normal gland. The test therefore allows the physician to determine whether the patient's thyroid is functioning normally.

Treatment

Radionuclides can also be used to treat medical disorders because of the radiation they emit. Radiation has a tendency to kill cells. Under many circumstances, that tendency can be a dangerous side effect: anyone exposed to high levels of radiation may become ill and can even die. But the cell-killing potential of radiation also has its advantages. A major difference between cancer cells and normal cells, for example, is that the former grow much more rapidly than the latter. For this reason, radiation can be used to destroy the cells responsible for a patient's cancer.

A radionuclide frequently used for this purpose is cobalt-60. It can be used as follows. A patient with cancer lies on a bed surrounded by a large machine that contains a sample of cobalt-60. The machine is then rotated in such a way around the patient's body that the radiation released by the sample is focused directly on the cancer. That radiation kills cancer cells and, to a lesser extent, some healthy cells too. If the treatment is successful, the cancer may be destroyed, producing only modest harm to the patient's healthy cells. That "modest harm" may occur in the form of nausea, vomiting, loss of hair, and other symptoms of radiation sickness that accompany radiation treatment.

Words to Know

Diagnosis: Any attempt to identify a disease or other medical disorder.

Isotopes: Two or more forms of an element that have the same chemical properties but that differ in mass because of differences in the number of neutrons in their nuclei.

Radioactivity: The property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.

Radioactive decay: The process by which an isotope breaks down to form a different isotope, with the release of radiation.

Radioactive isotope: A form of an element that gives off radiation and changes into another isotope.

Radionuclide: A radioactive isotope.

Radioactive isotopes can be used in other ways for the treatment of medical disorders. For example, suppose that a patient has a tumor on his or her thyroid. One way of treating that tumor might be to give the patient a dose of radioactive iodine. In this case, the purpose of the iodine is not to diagnose a disorder, but to treat it. When the iodine travels to the thyroid, the radiation it gives off may attack the tumor cells present there, killing those cells and thereby destroying the patient's tumor.

Some Diagnostic Radionuclides Used in Medicine

[See also Isotope ]

Nuclear medicine scans

Definition

A nuclear medicine scan is a test in which radioactive material is taken into the body and is used to create an image of a specific organ or bone.

Purpose

The purpose of a nuclear medicine scan is to locate areas of impaired function in the organ or bone being scanned. Nuclear medicine scans are widely used for diagnosis and monitoring of many different conditions. In the diagnosis and treatment of cancer, nuclear medicine scans are used to identify cancerous sites, for tumor localization and staging, and to judge response to therapy.

Precautions

Women who are pregnant or breast feeding should not undergo this test. A patient who is unable to remain still for an extended period of time may require sedation for a nuclear medicine scan.

Description

A nuclear medicine scan is an extremely sensitive test that can provide information about the structure and function of specific parts of the body. Types of nuclear scans include bone scans, heart scans, lung scans, kidney and bladder scans, thyroid scans, liver and spleen scans, and gallbladder scans. Brain scans are done to detect malignancy.

In a nuclear medicine scan, a small amount of radioactive material, or tracer, is injected or taken orally by the patient. After a period of time during which the radioactive material accumulates in one area of the body, a scan is taken by a special radiation detector, called a radionuclide scanner. This machine produces an image of the area for analysis by the medical team.

This test is performed in a radiology facility, either in a hospital department or an outpatient x-ray center. During the scan, the patient lies on his or her back on a table, but may be repositioned to the stomach or side during the study. The radionuclide scanner is positioned against the body part to be examined. Either the camera, the table, or both, may change position during the study. Depending on the type of scan, the procedure may take anywhere from 15 to 60 minutes. It is important for the patient not to move except when directed to do so by the technologist.

Preparation

The required preparation for nuclear medicine scans ranges from slight to none. The doctor may advise that certain prescription medications be discontinued before the test or that the patient not eat for three to four hours before the test. Depending on the type of test, a reference scan or specialized blood studies may be done before the scan is taken. Jewelry or metallic objects should be removed.

The patient should advise the doctor of any previously administered nuclear medicine scans, recent surgeries, sensitivities to drugs, allergies, prescription medications, and if there is a chance that she is pregnant.

Aftercare

No special care is required after the test. Fluids are encouraged after the scan to aid in the excretion of the radioactive material. It should be almost completely eliminated from the body within 24 hours.

Risks

The risks of nuclear medicine scans are very low. Most scans use the same or less amount of radiation as a conventional x ray and the radioactive material is quickly passed through the body. Side effects or negative reactions to the test are very rare.

Normal results

A normal result is a scan that shows the expected distribution of the tracer and no unusual shape, size, or function of the scanned organ.

Abnormal results

Depending on the tracer and technique used, the scan can identify and image particular types of tumors or certain cancers. Too much tracer in the spleen and bones, compared to the liver, can indicate potential hypertension or cirrhosis. Liver diseases such as hepatitis may also cause an abnormal scan, but are rarely diagnosed from the information revealed by this study alone.

In a bone scan, a high concentration of tracer occurs in areas of increased bone activity. These regions appear brighter and may be referred to as "hot spots." They may indicate healing fractures, tumors, infections, or other processes that trigger new bone formation. Lower concentrations of tracer may be called "cold spots." Poor blood flow to an area of bone, or bone destruction from a tumor, may produce a cold spot.

See Also Imaging studies; Magnetic resonance imaging

Resources

BOOKS

Wilson, Michael, ed. Textbook of Nuclear Medicine. New York:h5Raven Press, 1998.

ORGANIZATIONS

Society of Nuclear Medicine. 1850 Samuel Morse Dr., Reston, Virginia 20190. (703) 708-9000. Fax (703) 708-9015. <http://www.snm.org>.

OTHER

Virtual Hospital: Iowa Health Book: Diagnostic Radiology: Patient's Guide to Nuclear Medicine. 25 Mar. 2001. 27 June 2001. <http://www.vh.org/Patients/IHB/Rad/NucMed/PatGuideNucMed/PatGuideNucMed.html>.

Ellen S. Weber, M.S.N.

Paul A. Johnson, Ed.M.

KEY TERMS

Tracer

—A radioactive, or radiation-emitting, substance used in a nuclear medicine scan.

QUESTIONS TO ASK THE DOCTOR

• How long will my scan take?
• How long will the tracer stay in my body?
• Will repeat scans be necessary?