Medical Uses of Isotopes
Radioisotopes are widely used in the medical research arena, perhaps more so than in any other. It is estimated that one in every three people treated at a hospital makes use of a radioisotope in laboratory tests, diagnoses, or therapy. Each day, over 40,000 patients benefit from medical imaging technologies and more than 100 million laboratory tests each year. It is estimated that there are over 13 million nuclear medicine procedures performed each year in more than 4,000 nuclear medicine facilities in the United States.
Medical uses for radioisotopes generally fall into one of two categories: diagnostics or treatment. The first tries to understand a patient's medical condition, and the second attempts to treat it. Both uses, however, rely on the detection of emitted radiation that can be used for either imaging (diagnostics) or treatment.
Radioisotopes Used in Imaging
First, let us look at some imaging techniques and how they work. One of the most widely used imaging techniques is known as Positron Emission Tomography or PET for short. A positron is a positively charged electron emitted in a decay mode known as beta decay. The range of the positron in human tissue is about one millimeter mm before it encounters a negatively charged electron. The positron is the antiparticle to the electron and they soon annihilate each other with the emission of two gamma rays (called annihilation radiation) of the same energy (511 keV) that travel in opposite directions.
The PET scanner is a ring of detectors that surrounds the patient who has been administered a radioactive dose. The scanner is used to detect both gamma rays from the same decay. Based on the physics of annihilation radiation, detection of one 511 keV gamma ray means the other is traveling in the opposite direction. That is, their trajectories fall on the same straight line. But where on the line did they originate? (Hopefully from the tumor or tissue one is trying to image.) Since both gamma rays are traveling at the same speed, and the scanner records their relative arrival times (one is usually detected earlier than the other and therefore has less distance to travel), one can calculate where on the line of their trajectories they originated. The detection of these gamma rays can then be used to produce a computer-assisted image where dark areas represent regions of high activity and light areas less. In this manner, a PET image differs from an x-ray where light and dark areas represent transmission of x-rays (light areas) through soft tissue, and scattering or absorption of x-rays (dark areas) due to denser material such as bone.
Isotopes produced by Department of Energy (DOE) that are used in PET imaging are:
- Germanium-68, a long-lived PET calibration source.
- Strontium-82/Rubidium-82, used in cardiac imaging.
Other widely used isotopes are fluorine-18, oxygen-15, and carbon-11.
These isotopes all have rather short half-lives and can be made in small cyclotrons available to the private sector.
Radioisotopes used in imaging only work well if a significant fraction of the dose goes to the targeted tissue such as the brain, liver, or tumor. Rubidium-82 is widely used in cardiac imaging because it is a chemical analog to potassium. It is found in the same column as potassium in the Periodic Table and therefore follows similar chemistry. Potassium ions are used by muscle tissue, and the heart of course is a large, hard-working muscle. The ingestion of rubidium-82 therefore has an excellent chance of reaching the targeted organ, namely the heart. the heart, whereupon it can beta decay and help produce a PET image.
The half-life of rubidium-82 is 1.273 minutes. A short half-life assures that the isotope will decay away quickly. In other words, within a day, rubidium-82 has completely decayed away. This is a desirable feature as no one wants residual–and possibly harmful-radiation lingering about once it has served its useful purpose.
However, it poses a storage problem. One cannot simply purchase pure rubidium-82 and put it away for later use as it will decay away in a short period of time and not be available for the patient when needed. To handle this problem, one employs a rubidium-82 generator. The parent isotope of rubidium-82 is strontium-82; it beta decays to rubidium-82. It is an isotope that can readily be made in an accelerator and has an half-life of 25.5 days–much longer than that of rubidium-82. One makes the rubidium-82 generator by loading the relatively long-lived strontium-82 in the form of a solution onto a column composed of a resin or other material that will retain both the strontium and any rubidium-82 that grows in. Then the task is to remove the rubidium-82 from the column while leaving the strontium-82 behind. This is easily done by using a solvent that selectively removes the rubidium from the column. As the strontium-82 is continually decaying and producing rubidium-82 , one can allow the rubidium to grow in, then "milk" the generator when the rubidium is needed.
Another imaging technique widely used is known as SPECT, short for Single Photon Emission Computed Tomography. It is similar to PET in that the SPECT scanner detects gamma rays. However, no coincidence is required. That is, the radioisotope need not be a positron emitter as only one gamma ray is required instead of two. The scanner is rotated around the patient thus picking up the radiation from different angles. To pinpoint the source of the radiation (i.e., obtain positional information), the scanner is highly collimated, that is, has a small acceptance angle or window that will allow the gamma rays through. Because only a few gamma rays pass through such a small opening (the rest being undetected), resolution is not as good as PET. The resulting resolution is approximately 7 mm, whereas for PET it is approximately 1 to 2 mm. This loss of resolution, however, is compensated by the availability of radioisotopes that can be targeted for the brain, where SPECT is widely used, at about one-third the cost of a PET scan.
Two widely used SPECT isotopes are described below. All are attached to biologically active ligands that target the brain.
- Iodine-123: This is a radionuclide with an half-life of approximately 13 hours. Its emitted gamma photon has an energy of 159 keV. One ligand that it is attached to is known as isopropyliodoamphetamine (or IMP) so that the compound with the iodine-123 attached is I-123-IMP for short. Approximately six to nine percent of the dose reaches the brain within 30 minutes and remains active for 20 to 60 minutes after injection depending on the size of the dose. That is, during this time period it can be used for SPECT imaging.
- Technetium-99m: This isotope emits a gamma ray with an energy of 140 KeV and has a half-life of six hours. Molybdenum-99, the parent of technetium-99m, is used to make a generator from which technetium-99m is milked for use. Radiopharmaceuticals can be prepared on the spot using commercially available kits. One form that technetium-99m is available in is technetium-99m-bicisate (bicisate ethyl cysteibate dimer) or Tc-99m-ECD. It is taken up by the brain rather quickly, with six percent of the dose reaching the brain in five minutes.
Radioisotopes Used for Therapeutic Purposes
Radioisotopes are widely used in the treatment of disease, most notably cancer. Selected isotopes produced by DOE are discussed here.
- Actinium-225: This isotope decays through a series of alpha emissions to bismuth-213, also an alpha emitter. An alpha particle (same as a helium nucleus) compared to a beta particle, in general, is about 7,200 times heavier, is more energetic (8.4 MeV), and has a much shorter range (100 microns compared with 1 mm for electron). This means it delivers a lot of energy in a small volume of human tissue and therefore is effective in the destruction of cancerous cells. A monoclonal antibody is labeled with bismuth-213 (half-life equals 45 minutes) then delivered to the patient. Types of cancer fought with bismuth-213 are lung cancer and leukemia. Bismuth-213 is usually sold in the form of a generator. Actinium-225 is derived from the decay of thorium-229, which comes from the alpha decay of uranium-233.
- Tungsten-188/rhenium-188: The isotope of interest here is rhenium-188, a beta emitter. It is used to prevent restenosis in patients recovering from cardiac surgery. The technique involves filling the blood vessel with an inflatable, low-pressure balloon filled with a solution of rhenium-188 that delivers a uniform dose to the vessel wall. It is also used to treat liver cancer. Usually sold as a generator. Tungsten-188 is produced in a reactor.
- Arsenic-73: Arsenic is a carcinogen. Arsenic-73 is a radioactive tracer used to track biological uptake of this element.
- Tin-117m: A gamma emitter, tin-117m is used to reduce bone pain in cancer patients. Bone cancer results when tumor cells spread through the bloodstream to the bones. It affects 60 to 80 percent of all prostate and breast cancer patients, causing severe pain in more than 75 percent of patients. Preliminary trials that involved 47 patients demonstrated that at least 75 percent of these had their pain reduced or eliminated for up to one year. The ligand that tin-117m is attached to is diethylenetriaminepentaacetic acid (or DTPA). It helps target the tin to the bone without being taken up by the blood or soft tissue.
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