RDA Radiographic Techniques and Errors 2 — Questions and Answers
Question 1: Which radiographic technique uses a film/sensor holder with an aiming ring to align the central ray perpendicular to the film?
- Bisecting angle technique
- Paralleling technique (Correct answer)
- Occlusal technique
- Panoramic technique
Correct answer: Paralleling technique
The paralleling technique uses XCP (Extension Cone Paralleling) instruments or similar film holders with indicator rings to ensure the film/sensor is parallel to the long axis of the tooth and the central ray is perpendicular.
In the paralleling technique, the film/sensor is positioned parallel to the long axis of the teeth and the central ray is directed at right angles (perpendicular) to both the film and the tooth. This reduces geometric distortion and produces the most accurate image of true tooth length and shape. Film holders (XCP, Rinn, or similar) with aiming rings guide the cone. The bisecting angle technique places the film close to the tooth and aims the beam at the bisector of the angle between tooth and film — easier but more prone to distortion.
Question 2: What radiographic error appears as a cone-shaped radiopaque shadow on the periapical image?
- Overlapping
- Cone cutting (Correct answer)
- Elongation
- Film/sensor bending
Correct answer: Cone cutting
Cone cutting occurs when the PID (cone) is not aligned correctly and the primary beam misses part of the film/sensor, resulting in a clear (unexposed) area with a cone/circular shape.
Cone cutting is a positioning error where the X-ray beam (PID) is not centered over the film, leaving a portion of the film unexposed. This appears as a clear (white on processed film; no exposure digitally) semicircular or cone-shaped area. To correct: center the PID over the film holder ring and ensure the entire film/sensor is within the beam. This error usually requires retaking the image. It is more common with the bisecting angle technique or when using rectangular collimation without proper alignment.
Question 3: What is the purpose of the lead apron and thyroid collar during dental radiography?
- To prevent scatter radiation from exposing the patient's gonads and thyroid gland (Correct answer)
- To improve image quality
- To reduce the radiation dose to the dentist
- To prevent electrical shock from the x-ray unit
Correct answer: To prevent scatter radiation from exposing the patient's gonads and thyroid gland
The lead apron protects the patient's gonads and torso from scatter radiation, while the thyroid collar specifically protects the thyroid gland, which is particularly sensitive to radiation.
Dental X-ray exposure involves primary beam (directed X-rays) and scatter radiation (secondary radiation that changes direction). The lead apron (0.25–0.5 mm lead equivalent) absorbs scatter radiation protecting the gonads and abdominal/thoracic organs. The thyroid collar protects the thyroid gland, which is highly radiosensitive and located near the treatment area. According to NCRP and ADA recommendations, lead aprons should be used routinely; thyroid collars should be used especially for children, pregnant patients, and patients with thyroid disease.
Question 4: Which error causes radiographic images to appear lighter (less dense) than normal?
- Overexposure
- Overdevelopment
- Underexposure (Correct answer)
- Double exposure
Correct answer: Underexposure
Underexposure (too little radiation reaching the film/sensor) produces a light, low-density image that appears washed out and lacks proper contrast for diagnostic use.
Image density on conventional film is determined by the amount of radiation reaching the film. Underexposure results from: insufficient mA (milliamperage), inadequate kVp, too short an exposure time, or too great a film-to-source distance. The resulting image is light/pale with poor contrast. Overexposure produces a dark image. Overdevelopment also darkens the image. Double exposure produces a double image. In digital radiography, the software adjusts for some exposure variation, but extremely underexposed images will still appear noisy and diagnostically inadequate.
Question 5: What technique error causes the teeth to appear shorter than their actual length on a periapical radiograph?
- Elongation
- Foreshortening (Correct answer)
- Overlapping
- Cone cutting
Correct answer: Foreshortening
Foreshortening occurs when the vertical angulation of the X-ray beam is excessive (too steep), causing the teeth to appear shorter than their actual length.
Vertical angulation errors cause elongation or foreshortening: Foreshortening (too much positive/steep vertical angulation) — the teeth appear shorter than actual. Correction: decrease vertical angulation. Elongation (insufficient vertical angulation) — the teeth appear longer than actual, which is common with the bisecting angle technique if the beam angle is too flat. These errors require reexposure. With the paralleling technique and proper film holder alignment, vertical angulation errors are minimized because the aiming ring guides the beam perpendicular to the film.
Question 6: In digital radiography, what replaces the conventional X-ray film?
- Photostimulable phosphor plates or solid-state sensors (Correct answer)
- Intensifying screens
- Nitrocellulose-based film
- Emulsion-coated polyester
Correct answer: Photostimulable phosphor plates or solid-state sensors
Digital radiography uses either photostimulable phosphor (PSP) plates (indirect digital) or solid-state CCD/CMOS sensors (direct digital) to capture the X-ray image instead of conventional silver halide film.
Digital dental radiography systems: Direct digital (CCD/CMOS sensors) — solid-state sensors connected by wire or wirelessly to a computer; images appear immediately on screen. Indirect digital (PSP plates/phosphor plates) — flexible plates similar to film in handling; after exposure, they are scanned by a laser reader to release stored energy as visible light, which is digitized. Both eliminate chemical processing (no darkroom, no fixer/developer), reduce radiation dose by 50–80% compared to D-speed film, and allow image enhancement. Disadvantages: initial cost, sensor size/rigidity with direct sensors.
RDA Radiographic Techniques and Errors 2
Which radiographic technique uses a film/sensor holder with an aiming ring to align the central ray perpendicular to the film?