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Radiographic Selection Criteria

Course Number: 584

Patient Dose Reduction

It is well-established that x-radiation is detrimental and, when delivered with enough intensity, a known carcinogen.4,5,24 However, the degree of biologic effects from low-dose diagnostic imaging is less certain.4,5,27-29 Therefore, it is prudent to keep patient exposure to ionizing radiation low, especially considering that the effects are cumulative.

Attending to the ALARA (As Low As Reasonably Achievable) Principle is the ethical and professional responsibility and obligation of dental health care providers. There are many ways to minimize radiation exposure to dental patients. First and foremost is the determination of whether, or not, a radiographic examination is indicated, and if so, what type and with what frequency. The previous discussion has addressed these questions.

There are several other methods that, collectively, can serve to minimize exposure to dental patients. These radiation dose reduction measures are outlined in Table 7.

Table 7. Methods to Limit Patient Radiation Exposure.1

  • Limit the number of acquired images to the minimum necessary
  • Use the fastest receptor compatible with the diagnostic task
  • Use receptor holders designed to align the collimated beam for intraoral imaging
  • Collimate the beam to the size of the receptor when possible
  • Use protective aprons and thyroid collars as appropriate
  • Use proper receptor exposure and processing (when applicable) techniques

Image Receptors

Digital receptors for intraoral and extraoral radiographic imaging including the charge-coupled device (CCD), complementary metal oxide semiconductor (CMOS) or photostimulable phosphor plate detectors. These devices can be used in place of traditional film to reduce radiation dose to patients. If film is used for intraoral radiographic imaging, F speed film should be used instead of D speed film to achieve a greater dose reduction. For film-based panoramic or cephalometric extraoral radiography, rare-earth intensifying screens with matching high-speed film systems should be used.29

Receptor Holders

For intraoral imaging regardless of receptor type, a holding device is recommended to help facilitate placement and alignment of the collimated x-ray beam. Disposable bite blocks and/or bitewing tabs as well as heat-sterilizable bite blocks and instrument pieces are available. Dental professionals should not hold the receptor or patient during exposure. In unusual situations in which patient restraint is necessary, the parent, guardian or caregiver who is provided an appropriate protective shield, can restrain the patient or maintain the holder in position during exposure.29

X-ray Beam Collimation

X-ray beam collimation serves to limit the amount of primary and scatter radiation delivered to the patient during intraoral imaging. Rectangular collimation is preferred over round collimation because it reduces radiation dose to the patient significantly, approximately fivefold.29,30 Receptor holding devices with rings have insets to facilitate rectangular collimation alignment. Several commercial devices are available to convert from round to rectangular collimation (Figure 3). In addition, rectangular collimation has the added benefits of improving image geometry and reducing scatter radiation which degrades the resultant image.4 This is especially applicable to intraoral digital receptors because they are more sensitive to scatter radiation than film.31

Photo showing examples of rectangular collimators

Figure 3. Rectangular Collimators.

Patient Shielding

The thyroid gland, given its anatomic location, is often in the path of the x-ray beam during intraoral imaging. The thyroid is one of the most sensitive organs to radiation-induced tumors and is particularly sensitive in children.29 A thyroid collar (Figure 4) should be used on all patients during intraoral radiography which significantly reduces exposure to the gland.

Photo showing an example of a thyroid collar

Figure 4. Thyroid collar.

An AAOMR report indicates that the gonadal dose from dental radiography is considered to be negligible such that the use of the lead apron can be considered optional unless required by law.32 The National Council on Radiation Protection and Measurements (NCRP) states that lap apron shielding is not necessary if all of the safety recommendations outlined in Report 145 are employed.29 This recommendation includes the use of rectangular collimation of the primary beam which is less common than round collimation in dentistry.33,34 Given that caveat, a lap apron should be used when the NCRP recommendations are not fully implemented.

When not in use, patient shields should be hung on hangers or other devices designed for proper storage or laid flat. Periodically, these devices should be inspected for damage and replaced as needed.

Exposure and Processing Techniques

The optimal operating kilovoltage range for intraoral x-ray machines is 60 to 70 kVp.29 Consult the manufacturer’s operating manual to determine the appropriate exposure time for each area of the mouth per the type of receptor being used to image the patient. Technique charts should be used to indicate proper exposure settings for intraoral and extraoral radiographic imaging systems with adjustable settings.1 The clinician should make appropriate exposure adjustments when imaging children versus adult patients. X-ray machines should be evaluated regularly at intervals as mandated by state regulation.

If film-based imaging systems are used, it is important that optimal processing techniques are utilized to avoid retakes related to darkroom and/or processing issues. There are several quality assurance measures that can be utilized to test solution chemistry, darkroom conditions, film storage and cassette integrity to ensure that quality film-based images are produced.29,32