Nuclear radiology
Nuclear radiology, also known as nuclear medicine, is a specialized medical field that utilizes radioactive substances to visualize and assess both the structure and function of various organs within the body. Unlike conventional radiology, which primarily focuses on anatomical imaging through X-rays, nuclear radiology involves the administration of radiopharmaceuticals that emit radiation as they decay. This technique allows clinicians to gather functional information about organs such as the heart, brain, kidneys, and bones, making it invaluable for diagnosing a wide range of conditions.
Central to nuclear radiology are isotopes like technetium, thallium, and iodine, each chosen for their specific properties and half-lives, which determine the duration and effectiveness of their use in medical imaging. For instance, technetium-99m is frequently utilized due to its convenient half-life and optimal energy for detection. Procedures can take significantly longer than traditional X-ray imaging, often requiring anywhere from 15 to 30 minutes or more to acquire sufficient data for diagnosis.
Additionally, advancements in technology have led to enhanced imaging techniques such as PET/CT and PET/MRI, further expanding the capabilities of nuclear radiology. This field plays a crucial role in understanding physiological processes and disease states, offering insights that can guide effective treatment plans. Overall, nuclear radiology is an essential tool in contemporary medicine, benefiting patients through improved diagnostic accuracy and tailored healthcare solutions.
Subject Terms
Nuclear radiology
Also known as: Nuclear medicine
Anatomy or system affected: Bones, brain, glands, kidneys, musculoskeletal system, nervous system
Definition: The use of radiopharmaceuticals for the diagnosis of disease and the assessment of organ function
Indications and Procedures
Nuclear radiology , also known as nuclear medicine, is similar to conventional radiology in that radiation is used to look inside the patient’s body. Unlike conventional radiology, however, nuclear radiology looks not only at the anatomy of the patient but also at the functioning of the organ of interest. In conventional radiology (X-rays), the radiation or X-ray photon is produced by accelerating electrons, elemental negative charges, up to 50,000 to 125,000 volts and then ramming them into a metal anode. The physical act of stopping the electrons causes about 0.2 percent of the electrons to give off the accelerating energy as packets of energy called photons. This radiation is then directed by the lead housing of the X-ray tube toward the patient. The radiation transmitted through the patient is then recorded on either film or an image-intensifying tube. In nuclear radiology, radiation is supplied by pharmaceuticals instead of X-rays.
Nuclear radiology differs from conventional radiology because there is no X-ray tube generating the radiation. The radiation comes from the pharmaceuticals injected into the patient. The source of the radiation is a radioactive atom attached to a pharmaceutical (therapeutic drug). The physiological action and distribution of the pharmaceutical determine the diagnostic ability of the radiation given to the patient. The radiation can be emitted in any direction from the atom. The radiation, which travels in the direction of a piece of cesium-iodine crystal and through a collimator, or set of lead holes, is detected. The cesium-iodine crystal with the accompanying computer is known as a gamma camera.
The radioactive atom used in most nuclear radiology departments is an isotope of technetium. The isotope is in a semistable state known as a metastable state. When it becomes unstable and decays, it emits a single photon with the energy equivalent of an electron accelerated in a 140,000-volt potential (140 keV). After the emission of this single photon, the technetium atom is nonradioactive. The number of photons given off is dependent on the number of technetium atoms present in the metastable state. The time required for half of those present to emit the photons is called its half-life. The half-life of technetium is 6.02 hours. Because of its ease of production and reasonable half-life, technetium is used in many pharmaceuticals.
Specialty isotopes are also used. A gaseous isotope of xenon 133 is used for some lung studies. Xenon has a more complicated decay than does technetium. Xenon decays by the release of an energetic electron to unstable states of cesium 133. Cesium 133 gives off six gamma rays, with the predominant one being 80.9 keV. The half-life of xenon is 5.31 days. Xenon, being a noble gas, is chemically inert.
An isotope that can be used without being attached to a pharmaceutical is thallium 201. Thallium can be used in cardiac studies since it is readily taken up in the cardiac tissue. When thallium 201 decays, it becomes mercury 201. As mercury 201 becomes stable, it gives off high-energy X-rays and gamma rays, with the predominant energy being between 68.8 and 80 keV. Iodine is another isotope that does not necessarily need to be attached to a pharmaceutical. The three isotopes of iodine used are iodine 123, 125, or 131. All give off gamma rays that can be detected, the predominant being 159.1, 35.4, and 364.4 keV, respectively. Iodine 131 also gives off energetic electrons when it decays and, as such, is used when energetic electrons are desired for therapy purposes. The emission of energetic electrons can damage surrounding tissues. Iodine 123 and iodine 125 decay by absorbing an electron from the atom and only emit gamma rays. The use of one over the other depends on the cost, with iodine 123 more costly because of its thirteen-hour half-life. Iodine 125 has a half-life of 60.2 days. Iodine 131 has a half-life of 8.06 days. Other isotopes are used for specialty purposes, such as chromium 51, which can be used to attach to red blood cells. Chromium has a half-life of 27.7 days, with a predominant gamma emission of 320 keV.
A collimator is employed to force only the radiation from the front of the crystal through a known path to be detected. A collimator can consist of a piece of metal, usually lead, with one or multiple holes. The size and length of the hole determine the number of photons that will reach the crystal. The collimator works because it absorbs the photons that are not directed along the axis of the hole. The higher the energy of the photon being directed, the thicker the sides of the holes, known as septa, must be. The smaller the hole, the better the spatial resolution and ease of detecting small concentrations of the isotope of interest. The longer the hole, the better defined the path will be from the crystal face out through the hole to the patient. The limitations of the size and length are dictated by the need to detect a sufficient number of photons to give a diagnostic result. Unlike conventional radiology, in which the films are acquired in a short time (usually a fraction of a second), nuclear radiology can require fifteen to thirty minutes or more to acquire enough photons for a clinician to make a diagnostic determination.
Perspective and Prospects
The main structures that nuclear medicine studies are blood, brain, heart, thyroid gland, parathyroid glands, liver, kidneys, lungs, and bones. Blood studies use several different pharmaceuticals and radioisotopes depending on what is being measured. These studies involve the measurement of the blood volume or blood filtration. Brain studies use technetium with several different pharmaceuticals. Some pharmaceuticals will not pass the blood-brain barrier and can be used to detect bleeding in the cranial compartment. Others pass easily through the blood-brain barrier and can be used to detect sections of the brain that are either hyperactive or hypoactive. Heart studies are involved in determining the health of the cardiac muscles. By the use of thallium and new, additional technetium-labeled pharmaceuticals, the viability of heart tissue after a heart attack can be assessed, along with the thickness of cardiac structures such as the septa between the right and left ventricles. Other properties, such as the filling and ejection fractions, can be determined in this study. Thyroid studies look at the size and location and are easiest to use with iodine isotopes. A different thallium-labeled radiopharmaceutical can be used to look at the same properties of the parathyroid glands. Liver studies involve the determination of areas that are not functioning and that are revealed as voids on a scan. These pharmaceuticals use technetium as the labeled isotope. Kidney studies, which determine whether the kidneys are filtering the blood properly and in sufficient quantities, use technetium as the labeled isotope. Lung studies determine if all the lobes of the lungs are filling properly by using an inhalation isotope of xenon or aerosol compounds labeled with technetium. The health of the alveoli can be determined by the introduction of a radiopharmaceutical that congregates in the alveolar space. Bone studies are involved in determining whether new bone is being formed. Advances continued to be made in nuclear radiology in the early twenty-first century, including the use of pharmaceuticals during whole-body PET/CT scans, simultaneous PET/MRI, and multimodal molecular imaging systems.
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