Radiation and radioisotopes are extensively used medications to allow physicians to image internal structures and processes in vivo (in the living body) with a minimum of invasion to the patient. Higher doses of radiation are also used as means to kill cancerous cells.
Radiation is actually a term that includes a variety of different physical phenomena. However, in essence, all these phenomena can be divided into two classes: phenomena connected with nuclear radioactive processes are one class, the so-called radioactive radiation (RR); electromagnetic radiation (EMR) may be considered as the second class.
Both classes of radiation are used in diagnoses and treatment of neurological disorders.
Description
There are three kinds of radiation useful to medical personnel: alpha, beta, and gamma radiation. Alpha radiation is a flow of alpha particles, beta radiation is a flow of electrons, and gamma radiation is electromagnetic radiation.
Radioisotopes, containing unstable combinations of protons and neutrons, are created by neutron activation. This involves the capture of a neutron by the nucleus of an atom, resulting in an excess of neutrons (neutron rich). Proton-rich radioisotopes are manufactured in cyclotrons. During radioactive decay, the nucleus of a radioisotope seeks energetic stability by emitting particles (alpha, beta, or positron) and photons (including gamma rays).
Radiation—produced by radioisotopes—allows accurate imaging of internal organs and structures. Radioactive tracers are formed from the bonding of short-lived radioisotopes with chemical compounds that, when in the body, allow the targeting of specific body regions or physiologic processes. Emitted gamma rays (photons) can be detected by gamma cameras and computer enhancement of the resulting images and allows quick and relatively noninvasive (compared to surgery) assessments of trauma or physiological impairments.
Because the density of tissues is unequal, x rays (a high frequency and energetic form of electromagnetic radiation) pass through tissues in an unequal manner. The beam passed through the body layer is recorded on special film to produce an image of internal structures. However, conventional x rays produce only a two-dimensional picture of the body structure under investigation.
Tomography (from the Greek tomos, meaning "to slice") is a method developed to allow the detailed construction of images of the target object. Initially using the x rays to scan layers of the area in question, with computer assisted tomography a computer then analyzes data of all layers to construct a 3D image of the object.
Computed tomography (also known as CT, CT scan) and computerized axial tomography (CAT) scans use x rays to produce images of anatomical structures.
Single proton (or photon) emission computed tomography (SPECT) produces three-dimensional images of an organ or body system. SPECT detects the presence and course of a radioactive substance that is injected, ingested, or inhaled. In neurology, a SPECT scan can allow physicians to examine and observe the cerebral circulation. SPECT produces images of the target region by detecting the presence and location of a radioactive isotope. The photon emissions of the radioactive compound containing the isotope can be detected in a manner that is similar to the detection of x rays in computed tomography (CT). At the end of the SPECT scan, the stored information can be integrated to produce a computer-generated composite image.
Positron emission tomography (PET) scans utilize isotopes produced in a cyclotron. Positron-emitting radionuclides are injected and allowed to accumulate in the target tissue or organ. As the radionuclide decays, it emits a positron that collides with nearby electrons to result in the emission of two identifiable gamma photons. PET scans use rings of detectors that surround the patient to track the movements and concentrations of radioactive tracers. PET scans have attracted the interest of physicians because of their potential use in research into metabolic changes associated with mental diseases such as schizophrenia and depression. PET scans are used in the diagnosis and characterizations of certain cancers and heart disease, as well as clinical studies of the brain. PET uses radio-labeled tracers, including deoxyglucose, which is chemically similar to glucose and is used to assess metabolic rate in tissues and to image tumors, and dopa, within the brain.
Electromagnetic radiation
In contrast to imaging produced through the emission and collection of nuclear radiation (e.g., x rays, CT scans), magnetic resonance imaging (MRI) scanners rely on the emission and detection of electromagnetic radiation.
Electromagnetic radiation results from oscillations of components of electric and magnetic fields. In the simplest cases, these oscillations occur with definite frequency (the unit of frequency measurement is 1 Hertz (Hz), which is one oscillation per second). Arising in some point (under the action of the radiation source), electromagnetic radiation travels with the velocity that is equal to the velocity of the light, and this velocity is equal for all frequencies. Another quantity, wavelength, is often used for the description of electromagnetic radiation (this quantity is similar to the distance between two neighbor crests of waves spreading on a water surface, which appear after dropping a stone on the surface). Because the product of the wavelength and frequency must equal the velocity of light, the greater the wave frequency, the less its wavelength.
MRI scanners rely on the principles of atomic nuclear-spin resonance. Using strong magnetic fields and radio waves, MRIs collect and correlate deflections caused by atoms into images. MRIs allow physicians to see internal structures with great detail and also allow earlier and more accurate diagnosis of disorders.
MRI technology was developed from nuclear magnetic resonance (NMR) technology. Groups of nuclei brought into resonance, that is, nuclei absorbing and emitting photons of similar electromagnetic radiation such as radio waves, make subtle yet distinguishable changes when the resonance is forced to change by altering the energy of impacting photons. The speed and extent of the resonance changes permit a non-destructive (because of the use of low-energy photons) determination of anatomical structures.
MRI images do not utilize potentially harmful ionizing radiation generated by three-dimensional x-ray CT scans, but rely on the atomic properties (nuclear resonance) of protons in tissues when they are scanned with radio frequency radiation. The protons in the tissues, which resonate at slightly different frequencies, produce a signal that a computer uses to tell one tissue from another. MRI provides detailed three-dimensional soft tissue images.
These methods are used successfully for brain investigations.
Radiation therapy (radiotherapy)
Radiotherapy requires the use of radioisotopes and higher doses of radiation that are used diagnostically to treat some cancers (including brain cancer) and other medical conditions that require destruction of harmful cells.
Radiation therapy is delivered via external radiation or via internal radiation therapy (the implantation/injection of radioactive substances).
Cancer, tumors, and other rapidly dividing cells are usually sensitive to damage by radiation. The goal of radiation therapy is to deliver the minimally sufficient dosage to kill cancerous cells or to keep them from dividing. Cancer cells divide and grow at rates more rapid than normal cells and so are particularly susceptible to radiation. Accordingly, some cancerous growths can be restricted or eliminated by radioisotope irradiation. The most common forms of external radiation therapy use gamma and x rays. During the last half of the twentieth century, the radioisotope cobalt-60 was the frequently used source of radiation used in such treatments. More modern methods of irradiation include the production of x rays from linear accelerators.
Iodine-131, phosphorus-32 are commonly used in radiotherapy. More radical uses of radioisotopes include the use of boron-10 to specifically attack tumor cells. Boron-10 concentrates in tumor cells and is then subjected to neutron beams that result in highly energetic alpha particles that are lethal to the tumor tissue.
Precautions
Radiation therapy is not without risk to healthy tissue and to persons on the health care team, and precautions (shielding and limiting exposure) are taken to minimize exposure to other areas of the patient's body and to personnel on the treatment team.
Therapeutic radiologists, radiation oncologists, and a number of technical specialists use radiation and other methods to treat patients who have cancer or other tumors.
Care is taken in the selection of the appropriate radioactive isotope. Ideally, the radioactive compound loses its radioactive potency rapidly (this is expressed as the half-life of a compound). For example, gamma-emitting compounds used in SPECT scans can have a half-life of just a few hours. This is beneficial for the patients, as it limits the contact time with the potentially damaging radioisotope.
The selection of radioisotopes for medical use is governed by several important considerations involving dosage and half-life. Radioisotopes must be administered in sufficient dosages so that emitted radiation is present in sufficient quantity to be measured. Ideally the radioisotope has a short enough half-life that, at the delivered dosage, there is insignificant residual radiation following the desired length of exposure.
New areas of radiation therapy that may prove more effective in treating brain tumors (and other forms of cancers) include three-dimensional conformal radiation therapy (a process where multiple beans are shaped to match the contour of the tumor) and stereotactic radiosurgery (used to irradiate certain brain tumors and obstructions of the cerebral circulation). Gamma knives use focused beams (with the patient often wearing a special helmet to help focus the beams), while cyberknifes use hundreds of precise pinpoint beams emanating from a source of irradiation that moves around the patient's head.
BOOKS
Saha, Gopal B. Fundamentals of Nuclear Pharmacy. New York: Springer-Verlag, 1999.
