Tracer

Pronounced TRAY-ser, a tracer is a substance such as a radioisotope used in imaging procedures such as CT scan, PET scan, etc.

Radioisotope production

• Radioisotopes are isotopes that are unstable, or radioactive, and give off radiation spontaneously.
• Many radioisotopes are produced by bombarding suitable targets with neutrons now readily available inside atomic reactors.
• Some of them, however, are more satisfactorily created by the action of protons, deuterons, or other subatomic particles that have been given high velocities in a cyclotron or similar accelerator.

Half-lives

Most artificially made radioisotopes have relatively short half-lives.

• This makes them useful in two ways. First, it means that very little material is needed to obtain a significant number of disintegrations.
• It should be evident that, with any given number of radioactive atoms, the number of disintegrations per second will be inversely proportional to the half-life.
• Second, by the time 10 halflives have elapsed, the number of disintegrations per sec - ond will have dwindled to V-[02i th^ original number, and the amount of radioactive material is so small it is usually no longer significant.

Uses

A radioisotope may be used either as a source of radiation energy (energy is always released during decay), or as a tracer: an identifying and readily detectable marker material. The location of this material during a given treatment can be determined with a suitable instrument even though an unweighably small amount of it is present in a mixture with other materials.

In general, tracers ar e used for analysis and diagnosis, and radiant-energy emitters are used for treatment (therapy).

Advantages

Radioisotopes offer two advantages. First, they can be used in extremely small amounts. As little as one-billionth of a gram can be measured with suitable apparatus. Secondly, they can be directed to various definitely known parts of the body. For example, radioactive sodium iodide behaves in the body just the same as normal sodium iodide found in the iodized salt used in many homes. The iodine concentrates in the thyroid gland where it is converted to the hormone thyroxin. Other radioactive, or "tagged", atoms can be routed to bone marrow, red blood cells, the liver, the kidneys, or made to remain in the blood stream, where they ar e measured using suitable instruments.

Types of radiation

Of the three types of radiation, alpha particles (helium nuclei) are of such low penetrating power that they cannot be used for measurement from outside the body.

Beta particles (electrons) have a moderate penetrating power, therefore they produce useful therapeutic results in the vicinity of their release, and they can be detected by sensitive counting devices.

Gamma rays ar e highly energetic, and they can be readily detected by counters — radiation measurement devices—used outside the body. Particle or ray Alpha I Beta Gamma Paper Wood y Concrete Relative penetration of alpha, beta, and gamma radiation.

For comparison, a sheet of paper stops alpha particles, a block of wood stops beta particles, and a thick concrete wall stops gamma rays.

Use in medicine

Radioisotopes are used both in diagnosis and therapy.

Diagnostic uses

In order to diagnose certain conditions, a small harmless quantities of many isotopes serve as tools to aid him in gaining information about normal and abnormal life processes. The usefulness of this information depends upon his ingenuity in devising questions to be answered, apparatus to measure the results, and explanations for the results.

Therapeutic uses

For therapeutic uses, on the other hand, the important thing to remember is that radiation damages many kinds of cells, especially while they ar e in the process of division (reproduction)."*^ Cancer cells are self-reproducing cells, but do so in an uncontrolled manner. Hence cancer cells are particularly vulnerable to radiation. This treatment re - quires potent sources and correspondingly increases the hazards of use.

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