Chapter 3 of 425% of exam
Radiographic Technique
A diagnostic radiograph depends on where the receptor sits, where the beam points, and how the image is processed. This chapter covers the paralleling and bisecting techniques, receptor placement by region, angulation, the classic errors and what causes each one, and quality assurance.
Paralleling and Bisecting Techniques
Intraoral periapical images are taken with one of two geometries. Paralleling places the receptor parallel to the long axis of the tooth and directs the beam perpendicular to both, while bisecting angles the beam to an imaginary line that splits the angle formed by the tooth and the receptor. Paralleling is the preferred method because it produces the least dimensional distortion.
Paralleling geometry
The receptor is placed parallel to the long axis of the tooth and away from the teeth toward the middle of the mouth, and the central ray is aimed perpendicular to both the tooth and the receptor.
Long cone and holders
Paralleling requires a receptor holder and a longer position indicating device, usually 12 to 16 inches, so the increased distance offsets the magnification caused by moving the receptor away from the tooth.
Bisecting geometry
The receptor rests against the tooth and palate or floor of the mouth, and the beam is directed perpendicular to an imaginary bisector of the angle between the long axis of the tooth and the receptor.
Rule of IsometryWhen bisecting is used
Bisecting may be chosen for a shallow palate, a small mouth, tori, a strong gag reflex, or an endodontic procedure where a rubber dam clamp is in place, but it distorts length more easily.
Preferred method
Paralleling with a beam-alignment device is preferred because it gives the most accurate image dimensions and allows rectangular collimation, which lowers patient dose.
Receptor Placement and Angulation
Each region of the mouth has a standard receptor orientation and a standard vertical angulation. Vertical angulation is the up-and-down tilt of the tube head and controls image length, while horizontal angulation is the side-to-side aim through the contacts and controls overlap.
Vertical angulation
Vertical angulation is positive when the tube head points down toward the maxilla and negative when it points up toward the mandible, and it controls whether the image is the correct length.
Horizontal angulation
Direct the central ray through the interproximal contacts, perpendicular to the curve of the arch, so the contacts open on the image.
Anterior versus posterior
Position the receptor vertically for anterior projections and horizontally for posterior projections, always with the identification dot toward the occlusal surface and toward the beam.
Cover the apices
A periapical image must show the entire tooth plus at least two to three millimeters of bone beyond the apex, so place the receptor deep enough to capture the root ends.
Bitewing placement
Bitewings show the crowns and crestal bone of both arches on one image and are angled about plus 10 degrees vertically, which is the standard caries and bone-level projection.
Patient head position
For most intraoral projections seat the patient upright with the occlusal plane parallel to the floor and the midsagittal plane perpendicular to it.
Exposure and Placement Errors
Most retakes come from a small set of predictable mistakes, and each error has a specific geometric cause. Learning to read the error backward to its cause is the fastest way to fix a technique problem without simply re-exposing the patient.
Cone cut
A clear unexposed area with a straight or curved border means the beam was not centered on the receptor; recenter the position indicating device on the receptor, ideally with an aiming ring.
Elongation
Teeth appear stretched and too long because vertical angulation was too flat, so increase the vertical angulation.
Foreshortening
Teeth appear squat and too short because vertical angulation was too steep, so decrease the vertical angulation.
Overlapping contacts
Proximal surfaces superimpose because horizontal angulation was incorrect and the central ray did not pass through the contacts; adjust horizontal angulation, not vertical.
Missing apices
Root ends cut off at the edge means the receptor was not placed deep enough or was positioned too far occlusally, not that the angulation was wrong.
Blurred image and double exposure
Blurring comes from patient or tube head movement during exposure, while a double image with two superimposed sets of teeth means the same receptor was exposed twice.
Processing and Handling Errors
Film-based imaging adds a second family of errors that occur after the exposure, in the darkroom and in handling. Digital systems remove the chemistry but introduce their own artifacts from sensor handling and software settings.
Developer and fixer faults
Film that is too dark was left in developer too long or the solution was too warm, and film that is too light was underdeveloped, in depleted solution, or in solution that was too cold.
Fogged film
A gray, low-contrast image with no clear detail is caused by light leaks, an improper safelight, outdated film, radiation exposure in storage, or contaminated solutions.
Clear or blank film
A completely clear film means it was never exposed or was placed in fixer before developer, while a completely black film means it was exposed to white light or the machine was fired without a receptor in the mouth.
Spots and marks
White spots come from fixer splashed before developing, dark spots from developer contact before processing, and thin dark or white lines from bent film, fingernail pressure, or static electricity.
Reversed film
A light image with a herringbone or tire-track pattern means the film packet was backward in the mouth and the beam passed through the lead foil.
Digital artifacts
Handle sensors and photostimulable plates carefully, since scratches, bending, and residual images from incomplete erasure appear as permanent artifacts.
Extraoral Projections and Quality Assurance
Panoramic and occlusal images cover areas a periapical cannot, and each has its own positioning rules. Quality assurance is the routine testing that keeps equipment, chemistry, and technique producing consistent images.
Occlusal technique
The patient bites on a large receptor placed on the occlusal plane to show a broad area of the maxilla or mandible, useful for locating impacted teeth, salivary stones, and fractures.
Panoramic positioning
Align the midsagittal plane vertically and the Frankfort plane parallel to the floor, have the patient bite in the notch, place the tongue on the palate, and remove all metal jewelry and removable appliances.
Panoramic errors
Chin too high flattens the occlusal plane into a reverse smile, chin too low exaggerates the smile curve, forward position narrows the anterior teeth, and back position widens and blurs them.
Ghost images
A blurred, magnified shadow on the opposite side and higher than the real object is a ghost image, caused by dense objects such as earrings or a lead apron placed in the beam path.
Equipment and chemistry testing
Test processing solutions daily with a reference film or step wedge, check the darkroom for light leaks, and have x-ray output and timers checked on the schedule set by state regulation.
Retake analysis
Track why images are retaken so recurring technique faults are corrected at the source, which reduces both wasted time and total patient dose.
Test your knowledge
Practice questions on Radiographic Technique