Section III. RADIATION HAZARDS AND PROTECTION
1-8. RADIATION HAZARDS
The dental x-ray technician should never receive primary radiation from a dental x-ray unit if safety precautions are observed. However, scattered and/or secondary radiation is more difficult to avoid and is a serious danger to the technician. This type of radiation is produced by a scattering of the primary x-ray beam. The x-ray photons and photo-electrons in the beam undergo a change of direction after interaction with atoms and molecules as they pass through a substance. (Photo-electrons will be discussed in Lesson 2.) Figure 1-6 depicts scattered/secondary radiation. Scattered/secondary radiation from a patient is depicted in figure 1-7.
Figure 1-6. Diagram of scattered/secondary radiation.
Figure 1-7. Scattered/secondary radiation from a patient.
1-9. RADIATION PROTECTION
- General. Filtration and collimation of the x-ray beam are very important safety measures. The filter and collimator (diaphragm) block the majority of the unwanted x-ray photons. As you progress through the next few paragraphs of this text, you will understand their importance. The following diagram will identify the location of these two devices (see figure 1-8).
Figure 1-8. Tube head assembly: filter, collimator (diaphragm), PID or cone or tube.
- Filter. The aluminum filter or disk is placed in the path of the x-ray beam. It is located at the base of the cone or position indicating device (PID) just inside the metal housing. Figure 1-8 shows the location of the PID. The filter completely covers the opening where the x-ray beam emerges from the x-ray tube. The reason for the aluminum filter is to absorb the low energy, long wavelength x-rays (photons) and allow the high energy, short wavelength x-rays (photons) to pass through the filter. Filters on dental x-ray machines with over 70 kVp have a minimum thickness of 2.5 mm of aluminum. Those machines below 70 kVp have a safety standard minimum of 1.5 mm aluminum.
NOTE: The terms cone, PID, or tube are used interchangeably throughout this text.
See figure 1-8.
- Collimator. The lead diaphragm is collocated with the aluminum filter. It restricts the x-ray beam to the desired size. The diaphragm or collimator is constructed of 1/16-inch lead. Without this collimator, x-ray photons would cover a wide area of the patient's head. With the lead diaphragm or collimator in place, only the area necessary for exposure receives the primary beam. This is depicted in figure 1-7. The diagram in figure 1-8 represents an x-ray tube, cone, or PID removed to show the location of the lead diaphragm or collimator and the aluminum filter.
1-10. PROTECTIVE MEASURES AND STANDARDS
- General. Every possible safety precaution must be utilized when exposing radiographs. Collimation and filtration are only two of the several measures used to protect the patient and the technician from ionizing radiation. If all safety rules are strictly adhered to, the technician should receive no radiation and the patient exposure will be minimal. Even with the numerous safety precautions, accidental exposure is still possible.
- Technician Protection and Standards.
- (1) Radiation monitoring. AR 40-14 prescribes monitoring practices for Army personnel. It requires all primary x-ray technicians to wear a dosimeter or film badge. The dosimeter monitors or measures radiation received by the technician. The results are recorded on DD Form 1141. This form is kept permanently and made a part of the individual's health record.
- (2) Radiation standards. For the technician operating a dental x-ray machine, the level of radiation must not ever exceed an accumulated whole body dose, in rems, of five times the number of years beyond age 18 and a maximum of three rems in any 3-month period.
NOTE: The term rem refers to "roentgen equivalent in man," a unit measuring the biological effect of radiation energy. For x-rays, 1 rem is equal to 1 rad, or "radiation absorbed dose" (rad).
- (3) Protective booth and shields. Standards for dental x-ray booths or rooms require a shielding thickness of 1/16-inch lead or equivalent. The timer switch used to activate the machine for exposures is permanently affixed to the outside wall. The timer switch is mounted outside the protective shielding to prevent the operator from standing inside the booth during exposures. The shield is so designed that the radiation must scatter at least twice before reaching the x-ray technician. Leaded glass on the booth or shield provides a continuous view of the patient during the exposure. Consequently, any holding of the film or tube head by the x-ray technician is strictly prohibited.
- Patient Protection. It is the responsibility of the x-ray technician to use all available protective measures to reduce exposure to the patient. Only those radiographs requested by the dental officer will be taken. Be sure that a good quality x-ray is produced each time a request is made. Wrong exposures, improper exposures, and faulty processing techniques must be avoided. These mistakes result in retakes and unnecessary patient exposure. Also, the lead apron must be used for every exposure. These safety devices significantly reduce patient exposure.
1-11. X-RAY BEAM QUANTITY AND QUALITY
The quality of the x-ray beam is controlled by the voltage, while the milliamperes control the quantity. An increase in the voltage and milliamperes reduces exposure time for the patient.
- X-ray Beam Quality. The quality of the x-ray beam is controlled by the amount of voltage. Voltage provides contrast to the film. The desired contrast appears as various shades of gray, black and white in the x-ray negative (radiograph). Increased voltage provides less contrast (or more shades of gray). However, the beam has more penetrating power. Decreased voltage, on the other hand, provides more contrast (fewer shades of gray and more black and white shades). However, there is less penetrating power in the low voltage exposure. The technique most commonly used to expose periapical and bite-wing X-rays is a 90 kilovolt peak and 15 milliamperes.
- X-ray Beam Quantity. The x-ray beam quantity is controlled by the milliamperes. The more x-rays (photons) in the x-ray beam, the more dense (dark) the x-ray negative (radiograph) becomes. By increasing the milliamperes, we increase the number of available electrons at the cathode filament. When electrical current (voltage) is applied to the x-ray tube, the electrons cross the gap. When they impact on the anode (tungsten target), a greater number of x-rays (photons) are also produced. The more x-rays that are available to penetrate an object, the more dense (dark) is the x-ray negative (radiograph).