32-2. Medical Imaging and Diagnostics
A host of medical imaging techniques employ nuclear radiation. What makes nuclear radiation so useful? First, radiation can easily penetrate tissue; hence, it is a useful probe to monitor conditions inside the body. Second, nuclear radiation depends on the nuclide and not on the chemical compound it is in, so that a radioactive nuclide can be put into a compound designed for specific purposes. The compound is said to be
Table 1 lists certain medical diagnostic uses of radiopharmaceuticals, including isotopes and activities that are typically administered. Many organs can be imaged with a variety of nuclear isotopes replacing a stable element by a radioactive isotope. One common diagnostic employs iodine to image the thyroid, since iodine is concentrated in that organ. The most active thyroid cells, including cancerous cells, concentrate the most iodine and, therefore, emit the most radiation. Conversely, hypothyroidism is indicated by lack of iodine uptake. Note that there is more than one isotope that can be used for several types of scans. Another common nuclear diagnostic is the thallium scan for the cardiovascular system, particularly used to evaluate blockages in the coronary arteries and examine heart activity. The salt TlCl can be used, because it acts like NaCl and follows the blood. Gallium-67 accumulates where there is rapid cell growth, such as in tumors and sites of infection. Hence, it is useful in cancer imaging. Usually, the patient receives the injection one day and has a whole body scan 3 or 4 days later because it can take several days for the gallium to build up.
Table 1: Diagnostic Uses of Radiopharmaceuticals
Note that Table 1 lists many diagnostic uses for , where “m” stands for a metastable state of the technetium nucleus. Perhaps 80 percent of all radiopharmaceutical procedures employ because of its many advantages. One is that the decay of its metastable state produces a single, easily identified 0.142-MeV ray. Additionally, the radiation dose to the patient is limited by the short 6.0-h half-life of . And, although its half-life is short, it is easily and continuously produced on site. The basic process for production is neutron activation of molybdenum, which quickly decays into . Technetium-99m can be attached to many compounds to allow the imaging of the skeleton, heart, lungs, kidneys, etc.
Figure 2 shows one of the simpler methods of imaging the concentration of nuclear activity, employing a device called an
Imaging techniques much like those in x-ray computed tomography (CT) scans use nuclear activity in patients to form three-dimensional images. Figure 3 shows a patient in a circular array of detectors that may be stationary or rotated, with detector output used by a computer to construct a detailed image. This technique is called
Images produced by
emitters have become important in recent years. When the emitted positron (
) encounters an electron, mutual annihilation occurs, producing two rays. These rays have identical 0.511-MeV energies (the energy comes from the destruction of an electron or positron mass) and they move directly away from one another, allowing detectors to determine their point of origin accurately, as shown in
Figure 4. The system is called
PhET Explorations: Simplified MRI
Is it a tumor? Magnetic Resonance Imaging (MRI) can tell. Your head is full of tiny radio transmitters (the nuclear spins of the hydrogen nuclei of your water molecules). In an MRI unit, these little radios can be made to broadcast their positions, giving a detailed picture of the inside of your head.
In terms of radiation dose, what is the major difference between medical diagnostic uses of radiation and medical therapeutic uses?
One of the methods used to limit radiation dose to the patient in medical imaging is to employ isotopes with short half-lives. How would this limit the dose?
Problems & Exercises
A neutron generator uses an source, such as radium, to bombard beryllium, inducing the reaction . Such neutron sources are called RaBe sources, or PuBe sources if they use plutonium to get the s. Calculate the energy output of the reaction in MeV.
Neutrons from a source (perhaps the one discussed in the preceding problem) bombard natural molybdenum, which is 24 percent . What is the energy output of the reaction ? The mass of is given in Appendix A: Atomic Masses, and that of is 98.907711 u.
The purpose of producing (usually by neutron activation of natural molybdenum, as in the preceding problem) is to produce Using the rules, verify that the decay of produces . (Most nuclei produced in this decay are left in a metastable excited state denoted .)
(a) Two annihilation rays in a PET scan originate at the same point and travel to detectors on either side of the patient. If the point of origin is 9.00 cm closer to one of the detectors, what is the difference in arrival times of the photons? (This could be used to give position information, but the time difference is small enough to make it difficult.)
(b) How accurately would you need to be able to measure arrival time differences to get a position resolution of 1.00 mm?
Table 1 indicates that 7.50 mCi of is used in a brain scan. What is the mass of technetium?
The activities of and used in thyroid scans are given in Table 1 to be 50 and , respectively. Find and compare the masses of and in such scans, given their respective half-lives are 8.04 d and 13.2 h. The masses are so small that the radioiodine is usually mixed with stable iodine as a carrier to ensure normal chemistry and distribution in the body.
(a) Neutron activation of sodium, which is 100%, produces , which is used in some heart scans, as seen in Table 1. The equation for the reaction is . Find its energy output, given the mass of is 23.990962 u.
(b) What mass of produces the needed 5.0-mCi activity, given its half-life is 15.0 h?
(a) 6.958 MeV