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The gamma camera is a key diagnostic infinite number of projections through carbon dioxide. Thus, a PET scan can be
imaging tool used in nuclear medicine. Its the body. Unlike CAT, gamma, or SPECT used to study processes in such organs
use requires a radioactive material, called a scans, MRI does not use any form of ion- as the brain and heart where glucose is
radiopharmaceutical, to be injected into or izing radiation. MRI uses magnetic fields, being broken down or oxygen is being
swallowed by the patient. A given radio- radio waves, and a computer to produce consumed. This diagnostic method can be
pharmaceutical tends to go to a specific organ detailed images. As the patient enters an used to detect epilepsy or brain tumors,
of the body; for example, radioactive iodine MRI scanner, his or her body is surrounded among other problems.
tends to go to the thyroid gland, and others by a large magnet. The technique requires a Radiopharmaceuticals used for diagnos-
go to other organs. Gamma-emitting radio- very strong magnetic field, a field so strong tic examinations are selected for their affinity
pharmaceuticals are used with the gamma that it aligns the nuclei of the person’s for certain organs, if they emit sufficient radi-
camera, and the gamma camera collects atoms. The scanner sends a strong radio ation to be easily detectable in the body and if
and processes these gamma rays to produce signal, temporarily knocking the nuclei out they have a rather short half-life, preferably
images. These images provide a way of study- of alignment. When the radio signal stops, no longer than a few hours. Useful radioiso-
ing the structure as well as measuring the the nuclei return to the aligned position, topes that meet these criteria for diagnostic
function of the selected organ. Together, the releasing their own faint radio frequencies. purposes are technetium-99, gallium-67,
structure and function provide a way of iden- These radio signals are read by the scanner, indium-111, iodine-123, iodine-131, thal-
tifying tumors, areas of infection, or other which uses them in a computer program to lium-201, and krypton-81.
problems. The patient experiences little or no produce very detailed images of the human The goal of therapy in nuclear medi-
discomfort, and the radiation dose is small. anatomy (Box Figure 13.2). cine is to use radiation to destroy diseased
A SPECT scan (single photon emission The PET scan (positron emission or cancerous tissue while sparing adja-
computerized tomography) is an imaging tomography) produces 3D images superior cent healthy tissue. Few radioactive thera-
technique employing a gamma camera that to gamma camera images. This technique peutic agents are injected or swallowed,
rotates about the patient, measuring gamma is built around a radiopharmaceutical that with the exception of radioactive iodine—
rays and computing their point of origin. emits positrons (like an electron with a mentioned earlier as a treatment for can-
Cross-sectional images of a three- dimensional positive charge). Positrons collide with cer of the thyroid. Useful radioisotopes
internal organ can be obtained from such electrons, releasing a burst of energy in for therapeutical purposes are iodine-131,
data, resulting in images that have higher the form of photons. Detectors track the phosphorus-32, iridium-192, and gold-
resolution and thus more diagnostic infor- emissions and feed the information into a 198. The radioactive source placed in the
mation than a simple gamma camera image. computer. The computer has a program to body for local irradiation of a tumor is
A gallium radio pharmaceutical is often plot the source of radiation and translates normally iridium-192. A nuclear pharma-
used in a scan to diagnose and follow the the data into an image. Positron-emitting ceutical is a physiologically active carrier
progression of tumors or infections. Gal- radiopharmaceuticals used in a PET scan to which a radioisotope is attached. Today,
lium scans also can be used to evaluate the can be low atomic weight elements such it is possible to manufacture chemical or
heart, lungs, or any other organ that may be as carbon, nitrogen, and oxygen. This is biological carriers that migrate to a par-
involved with inflammatory disease. important for certain purposes since these ticular part of the human body, and this
Use of MRI (magnetic resonance are the same elements found in many bio- is the subject of much ongoing medical
imaging) also produces images as an logical substances such as sugar, urea, or research.

among the fission products of uranium-235. Some of these A chain reaction is self-sustaining until all the uranium-235
products are fission fragments, and others are produced by nuclei have fissioned or until the neutrons fail to strike a
unstable fragments that undergo radioactive decay. Selected fis- uranium-235 nucleus (Figure 13.15).
sion fragments are listed in Table 13.6, together with their major You might wonder why all the uranium in the universe
modes of radioactive decay and half-lives. Some of the isotopes does not fission in a chain reaction. Natural uranium is mostly
are the focus of concern about nuclear wastes, the topic of A uranium-238, an isotope that does not fission easily. Only
Closer Look reading at the end of this chapter. about 0.7 percent of natural uranium is the highly fissionable
The fission of a uranium-235 nucleus produces two or uranium-235. This low ratio of readily fissionable uranium-235
three neutrons along with other products. These neutrons nuclei makes it unlikely that a stray neutron would be able to
can each move to other uranium-235 nuclei where they are achieve a chain reaction.
absorbed, causing fission with the release of more neutrons, To achieve a chain reaction, there must be (1) a sufficient
which move to other uranium-235 nuclei to continue the pro- mass with (2) a sufficient concentration of fissionable nuclei.
cess. A reaction where the products are able to produce more When the mass and concentration are sufficient to sustain a
reactions in a self-sustaining series is called a chain reaction. chain reaction, the amount is called a critical mass. Likewise, a

13-15 CHAPTER 13 Nuclear Reactions 337