- Continuing Education
Receptor Placement Errors
Correctly exposing intraoral receptors includes four basic steps: receptor placement, vertical PID (cone) angulation alignment, horizontal PID (cone) angulation alignment, and central ray centering. Technique errors can occur if any of these steps are completed improperly. Placement errors will be discussed first as they are the most common of all errors.
Inadequate Coverage/Missing Apices
A common receptor placement error is inadequate coverage of the area to be examined radiographically. This typically occurs in molar projections when the patient has difficulty maintaining or tolerating proper receptor placement. Each periapical and bitewing in a complete survey has established placement criteria which describes the structures of interest that should be recorded on each view. Consistent application of these criteria will minimize this error.
Bitewing – Mandibular Bone Margin Cut Off
Periapical – Maxillary Apices Cut Off
Missing apices can be caused by a receptor placement error. When using receptor holders, the bite block should be placed on the teeth to be imaged and not on the opposing teeth. If the bite block is placed on the opposing teeth and the patient is required to bite the receptor into place, a placement error is likely to result. Placing the receptor more lingual to the teeth where the palate and floor are deeper will make positioning easier and more comfortable for the patient. Placement of the bite block and receptor in the correct position first and then having the patient slowly bite to maintain the placement is the preferred and most effective approach.
Tilted Occlusal Plane
When the receptor is not placed perpendicular to the occlusal plane, the occlusal plane will appear slanted or diagonal on the recorded image. When exposing bitewing radiographs, the top edge of the receptor may come in contact with the palatal gingiva or curvature of the palate or the lingual aspect of the mandible. When this occurs, the occlusal plane will appear crooked. The receptor must be placed straight or perpendicular with the occlusal plane or placed farther away from the teeth to avoid this error. For periapicals, always place the bite block in contact with the occlusal or incisal surfaces of the teeth you are imaging rather than on the opposing teeth.
Tilted Occlusal Plane
Tilted Occlusal Plane
Placement on the opposing teeth or too close to the teeth will cause the receptor to displace when it contacts bony anatomy. Preferably, the receptor orientation dot or plate marker should be placed toward the crowns of the teeth for periapical images and toward the mandible for bitewing images to reduce interference with viewing the structures of interest on the recorded image.
The exposure side of any receptor must be directed toward the x-ray source to produce an acceptable image. Backwards placement is unlikely with rigid digital receptors because of the wire attachment on the non-exposure side of the sensor.
Backward placement of a film in the mouth causes the lead foil inside the packet to face the radiation source instead of the film directly. The x-ray beam is attenuated by the lead foil before striking the film. This causes the embossed pattern on the foil, a herringbone or diamond effect, to appear on the processed film.
Rear Side of Film Packet
Backwards Film Image
This error also results in a lighter image and reversal of the image. This can lead to confusion about the correct anatomical area recorded when mounting the processed film.
Rear Side of Plate
Exposure Side of Plate
Plate or film bending may occur due to contact with the curvature of the palate or lingual arch and/or mishandling of the receptors. These receptors can be flexed but should never be bent. If the receptor is too large for the area, bending or curving can occur. Select a receptor size that will adequately cover the area without producing excessive discomfort to the patient. Crimping, creasing, or folding a plate or film receptor damages the emulsion and compromises the quality of the image. Careful handling, use of a smaller receptor, and correct placement will address the problems of bending and other receptor distortions that produce image artifacts.
Rigid digital receptors cannot be bent but as previously indicated phosphor plate receptors can be creased, bent, scratched, or folded. These alterations result in permanent damage of the plate and produce artifacts on the current and any subsequent image taken with the marred plate.2
Film Receptor Creasing
Plate Receptor Scratches
Damaged plates should be replaced when the artifacts interfere with the production of diagnostic images. Tissue cushions are better alternatives than bending or creasing a plate or film receptor.
Vertical Alignment Errors
Vertical angulation controls the length of the recorded image. When using the paralleling technique and receptor holders, the vertical angulation is dictated by the holding device to direct the x-ray beam perpendicular to both the receptor and teeth. Therefore, it is important that the clinician place the receptor parallel to the teeth to ensure proper vertical angulation. In contrast, when using the bisecting angle technique, the beam is perpendicular to the plane that bisects or divides the angle formed by the teeth and the receptor. Vertical alignment errors often occur with the bisecting angle technique and can result in elongation or foreshortening of the teeth. Other errors that can occur which cause the teeth to appear elongated or foreshortened include:
It is important to determine the cause of the error in order to correct it.
Elongation or lengthening of the teeth and surrounding structures results from underangulation of the x-ray beam (not enough vertical angle). When elongation occurs using the paralleling technique, the angulation of the x-ray beam is less than the long axis plane of the teeth. To correct this error the clinician must increase the vertical angulation. In other words, for the maxillary arch, the positive vertical angulation must be increased (PID pointing down); for the mandibular arch, the negative vertical angulation must be increased (PID pointing up).
This error can also occur when using the bisecting angle technique. Often the error is caused by the x-ray beam being perpendicular to the long axis of the teeth, rather than bisecting the angle between the teeth and the receptor.
Receptor AngularBeam Under-Angulated
Foreshortening or shortening of the teeth and the surrounding structures can also result from improper vertical angulation. Foreshortening is the result of overangulation of the x-ray beam (too much vertical angle). While using the paralleling technique, foreshortening can occur when the angulation of the x-ray beam is greater than the long axes plane of the teeth. To correct foreshortening when using the paralleling technique, the operator should decrease the positive vertical angulation for maxillary projections and, decrease the negative vertical for mandibular projections. This error can also occur if the receptor is not placed parallel to the long axis of the teeth.
Horizontal Alignment Errors
Proper horizontal alignment of the x-ray beam will open interproximal contacts and facilitate a thorough radiographic caries evaluation and assessment of alveolar bone levels, both important components of a thorough clinical and radiographic examination. The x-ray beam should be aimed directly between the targeted teeth in order to open the interproximal surfaces. Horizontal alignment errors cause the image to shift anteriorly or posteriorly, resulting in the overlapping of the proximal contacts.
Correct Horizontal Angulation Entry
To determine if the horizontal angulation is positioned incorrectly, evaluate the extent of the overlap. The Buccal Object Rule can be used to determine the movement of the buccal and lingual cusps when trying to understand the error. The Buccal Object Rule states: Buccal objects move in the opposite direction compared to the direction of the x-ray tubehead, while lingual objects move in the same direction as the movement of the x-ray tubehead.19 Application of the Buccal Object Rule to determine the cause of interproximal overlapping requires evaluation of the position of the x-ray tubehead and the direction of the overlapping on the bitewing image. For example, if the x-ray head is placed too posterior in position, the buccal cusps will overlap in an anterior direction as demonstrated in the molar bitewing illustration. By way of comparison, if the x-ray head is placed too anterior in position, the buccal cusps will overlap in a posterior direction. To avoid this error, the central ray must pass through the proximal surfaces of the teeth where the contacts need to be open. To aid in the determination of the correct horizontal angle, the clinician can place the end of a cotton-tip applicator into the contact zone. This method will help visualize the direction the x-rays should be directed to open the teeth contacts.
When using receptor holding devices, horizontal errors can occur by improper horizontal alignment of the receptor. These errors can be avoided by placing the receptor in the same horizontal plane as the teeth so that the x-ray beam travels directly through the contact areas. This will position the receptor parallel to the buccal plane of the teeth as well as parallel to the instrument indicator ring.
Beam Centering Errors
The central ray should be aligned over the center of the receptor with the x-ray beam directed perpendicular to the receptor. When this alignment is not observed, a cone-cut occurs. Cone-cuts appear as a clear zone on traditional radiographs after processing, due to the lack of x-ray exposure of the emulsion. When using digital imaging, the cone-cut appears as an opaque or white zone. The shape of the cone-cut depends on the type of collimator used when exposing the receptor.
Round Cone Cut
Rectangular Cone Cut
For example, if a round collimator is used, a curved cone-cut will appear. Square cone-cuts occur when using a rectangular collimator. To correct a cone-cut error, the beam should be re-centered toward the area of non-exposure. Improper assembly of receptor holding devices can also cause cone-cuts. When assembling these devices, make certain that the entire receptor can be seen when looking through the indicator ring.
The technical errors previously discussed are briefly summarized in Table 2.
Table 1. Principles of Accurate Image Projection Summary.
|1. X-rays should be emitted from the smallest source of radiation as possible||Improves image resolution|
Reduces geometric unsharpness
|2. X-ray source-to-object distance should be as long as possible||Improves image sharpness|
|3. Object-to-receptor distance should be as short as possible||Improves image sharpness|
|4. Receptor and long axis of the tooth should be parallel to each other||Improves anatomic accuracy|
Reduces shape distortion
|5. X-ray beam should be directed perpendicular to the tooth and the receptor||Improves anatomic accuracy|
Reduces shape distortion