4.4 Radiographs, CBCT, Positioning, and Safety

Technique: paralleling, bisecting, and exposure factors

Dental radiographs are diagnostic records, not pictures. A useful image shows the correct anatomy with enough clarity for the dentist to interpret. The two intraoral techniques are the paralleling technique (also called the long-cone, extension-cone, or right-angle technique) and the bisecting-angle technique. In paralleling, the receptor is placed parallel to the long axis of the tooth and the central ray is directed perpendicular to both the receptor and the tooth's long axis, producing the most dimensionally accurate image with the least distortion. It is the preferred method.

The bisecting-angle technique is used when anatomy (a shallow palate, tori, a strong gag reflex) prevents parallel placement; the central ray is aimed perpendicular to an imaginary line that bisects the angle between the tooth's long axis and the receptor.

Exposure factors control image density and contrast:

On modern intraoral units, kVp and mA are often preset, so exposure time is the operator's main control. Increasing kVp lightens contrast and increases penetration; increasing mA or time increases density (a darker image).

Recognizing technique errors

The exam loves to show an image defect and ask for the cause. Learn these pairings cold:

A memory aid: vertical angulation errors change the length (elongation/foreshortening); horizontal angulation errors cause overlap; beam centering errors cause the cone cut. The RDA recognizes the defect and follows the office retake policy under dentist direction; the RDA does not over-read the image diagnostically or proceed with a nondiagnostic record.

CBCT preparation and radiation safety (ALARA)

Cone-beam computed tomography (CBCT) produces three-dimensional volumes and is prescribed by the dentist for specific needs (implant planning, impacted teeth, pathology). RDA duties center on preparation, not interpretation: ask the patient to remove removable metal (jewelry, eyeglasses, hairpins, removable partials, certain piercings) per protocol, position the chin, forehead, and bite guide as the unit requires, and stress stillness, because motion ruins the volume. Confirm the correct patient and scan purpose before exposure. Never decide independently that a CBCT is needed.

Radiation safety follows ALARA — As Low As Reasonably Achievable. Practical RDA steps:

• Patient protection: California's radiation rules (17 CCR 30311(b)(6)) require each patient undergoing dental radiography to be draped with a protective apron of at least 0.25 mm lead equivalent covering the gonadal area. That rule does not require a thyroid collar, and since February 2024 the ADA no longer recommends routine thyroid collars or abdominal aprons; follow California law and office protocol.
• Dose reduction: use fast digital receptors, rectangular collimation, the long open-ended PID, correct exposure settings, and proper technique to avoid retakes (the single biggest avoidable dose source).
• Operator protection: stand at least 6 feet away and 90-135 degrees to the primary beam, or behind a barrier; never hold the receptor or the tube head during exposure.
• Infection control: barrier sensors, holders, and switches; sensors are semicritical and require barriers plus disinfection.

In California, anyone in a dentist's office who operates dental radiographic equipment must have passed a Board-approved radiation safety course (B&P 1656, in effect since January 1, 1985; the only alternative is a Board radiation safety exam passed before that date). RDA applicants complete the 32-hour course before licensure, and an unlicensed assistant may not expose radiographs until completing it (B&P 1750(f)(3)).

Three more safety ideas appear on the exam. First, follow the principle of an ordered, justified exposure: imaging is prescribed for a clinical reason, never "routine" without dentist direction, and frequency follows the dentist's selection guidelines for the individual patient rather than a fixed calendar. Second, digital sensors reduce dose compared with older film speeds and allow lower exposure settings, which is itself an ALARA measure; they also remove chemical processing errors as a retake cause.

Third, the patient's lead-equivalent apron protects the patient, while distance, position, and barriers protect the operator, two different protections that the exam may try to swap.

Use a final checklist: confirm patient and order, prepare receptor/holder/barrier, drape the patient's lead-equivalent apron, set or confirm exposure factors, position patient and receptor, instruct on stillness/bite/tongue, expose from a safe position, then verify image quality and labeling and route to the dentist. The best exam answer protects diagnostic value, patient and operator safety, and the legal record simultaneously, and when those goals seem to conflict, the safe, in-scope, retake-avoiding choice is correct.