Nuclear Radiology

Nuclear Radiology or Nuclear Medicine involves the use of radioactive materials in the human body for diagnosis or treatment. In diagnostic nuclear medicine, small quantities of gamma-emitting radioactive materials are ingested or inhaled by, or injected into patients. Gamma radiation is identical to x-ray radiation from the perspective of radiation detection and imaging. As the gamma radiation penetrates out from the body, it can be detected and used to make an image. In most cases, the radioactive material is attached to a chemical that is targeted at a specific organ, such as the heart, kidneys or liver. When an image is taken from a nuclear medicine study, the function of a given organ can be inferred from the distribution of the radioactive material.

Figure 7. Nuclear Radiology Bone Scan

Radioactive materials are composed of atoms that are unstable. These unstable atoms become stable by releasing subatomic particles (usually electrons) and energy. This process is called radioactive decay. The quantity of radioactive material is measured in terms of activity. Activity is the number of decay transformations that occur in one second. The most commonly used unit of radioactivity is the curie (Ci), which is equal to 37 billion transformations per second. In diagnostic nuclear medicine, radioactive materials are most frequently used in amounts much less than one curie. Usually, quantities are measured in millicuries (mCi), or thousandths of a curie.

Radioactive materials continually emit radiation until all of the unstable atoms are transformed. Each different radioactive material (or isotope) decays at a different rate. This rate is described in terms of half-life. The half-life of a radioactive isotope is the amount of time it takes for one-half of the unstable atoms to decay. Figure 3 shows the decay of radioactive material as time passes.

Figure 8. Decay of Radioactive Material

This graph shows the decay of a radioactive isotope with a half-life of one day. After one half-life (one day) has past, 50% of the original activity is remaining. Note that after two half-lives have past, 25% of the original activity still remains. It takes approximately ten half-lives for a radioactive isotope to decay away to essentially zero.

The radioactive half-life of an isotope is called the physical half-life. The physical half-lives of some common radioactive isotopes used in nuclear medicine are listed in Table 2.

Table 2. Nuclear Radiology Isotopes and Half-lives

A concept of equal importance in nuclear medicine is the biological half-life. Any material that is introduced into the human body will be eliminated at a gradual rate. A few of the processes of elimination are excretion, perspiration, and exhalation. The combination of biological elimination and radioactive decay result in relatively short residence times within the human body.

Exposure to personnel from patients undergoing diagnostic nuclear radiology is not a major concern. The external exposure rates are low and the necessity of close contact is limited. When caring for such patients, distance often provides the most effective means of protection from radiation exposure. As distance is increased from most sources of radiation, the dose rate decreases according to the inverse square law. The intensity of radiation present may be calculated for any distance using the following formula:

where:		Known radiation intensity
		Known distance
		Intensity at new location
		New distance

The simplest way of using this formula is by doubling the distance. When the distance is multiplied by 2, the intensity is divided by 4 (22 = 4). For example, a radiation intensity of 8 mrem per hour at 3 feet will result in a dose rate of 2 mrem per hour at 6 feet. Remember, this law works in reverse as well. In the same example, the dose rate at 1.5 feet would be 32 mrem per hour! Table 3 outlines typical exposure rates from a diagnostic nuclear radiology patient.

Table 3. External Dose Rate from Diagnostic Nuclear Radiology
(Based on 10 mCi Tc-99m, assuming point source six inches beneath skin surface.)

The other important factor in reducing external exposure from diagnostic nuclear radiology patients is time minimization. In the care of such patients, it is not critical for personnel to be at the bedside at all times. Patients should be given the care required, but excessive time within three feet of the patient is discouraged. In some cases, pediatric patients must be held by nursing personnel for extended periods. The Office of Radiation Safety has done monitoring of personnel in these instances, and exposures are seen to be less than 20 mrem per month for nurses routinely performing this duty. Pregnant personnel may request reassignment to other duties that do not involve radiation exposure.

One of the main concerns for personnel exposure with nuclear radiology patients is through internal exposure. This type of exposure involves the inhalation or ingestion of radioactive materials from patient contamination. Although most of the internal exposure concerns involve iodine therapy patients, precautions should be taken with diagnostic patients as well.

The best method of protection from internal exposure is through the use of standard precautions. The protective methods employed against blood-borne pathogens are sufficient to protect against radioactive material contamination as well.

The sources of radioactive material contamination in nuclear radiology patients are directly related to patient body fluids. During the first 24 hours after a nuclear medicine procedure, all body fluids are considered to be contaminated. This includes blood, sputum, sweat and excreta. Several rules should be followed:

• Excreta may be disposed down the sanitary sewer. Any biological material that is flushed down the toilet is excluded from radioactive waste requirements.
• All materials contaminated with biological material from nuclear radiology patients must be disposed of in an approved radioactive waste container. This includes: diapers, needles, syringes and lines from blood draws, or any material contaminated by the patient.
• The Nuclear Radiology Department personnel must monitor rooms for patients undergoing therapy.

Proper disposal of radioactive waste is important. The State of California requires strict monitoring of medical waste, to prevent radioactive contamination from entering the environment. Any contaminated materials from patients who have received more than one millicurie of a radioactive isotope should be handled as radioactive waste. Table 4 outlines some of the common nuclear radiology procedures and the activity for each. The Office of Radiation Safety is responsible for controlling the disposal of all radioactive waste and provides proper disposal containers. Linens and trash are monitored before processing to prevent accidental release.

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