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Intraoral Radiography: Principles, Techniques and Error Correction

Gail F. Williamson, RDH, MS

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Receptor Placement Errors

Inadequate Coverage / Missing Apices
A common receptor placement error is inadequate coverage of the area to be viewed. This commonly occurs in molar projections when the patient cannot tolerate proper receptor placement. The stated criteria should be based on the structures of interest to the person viewing the image. Using criteria that establish which teeth and surrounding structures must be present in a given view will minimize this error.

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 exposed 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 placement easier and more comfortable for the patient.

Backward receptor image
Backward Placement

Placing the film backwards in the mouth causes the lead foil inside the packet to face the radiation source instead of the film. The x-ray beam is attenuated by the lead foil before striking the film, an embossed pattern on the foil, a herringbone or diamond effect, will appear on the processed film. This error also results in a lighter image and confusion when mounting the processed film.

Backwards placement is less likely with digital receptors particularly the rigid sensors. The exposure side of any receptor must be directed toward the x-ray source to produce an acceptable image.

Receptor bending
Bending

Receptor bending may occur due to the curvature of the palate or lingual arch and/or mishandled receptors. Receptors can be flexed but should never be bent. When using holders, flex the receptor into place to decrease the chances of bending. If the receptor is too large, bending can occur. Choose a receptor that will adequately record the image needed while producing little discomfort to the patient. Bending a receptor causes the emulsion to crease, which in turn compromises the quality of the image. Careful handling, use of a smaller receptor, and correct placement will address the problems of bending.

Rigid digital receptors cannot be bent but phosphor plate receptors can be creased, bent or folded. This produces a permanent artifact on the plate and the image. Tissue cushions are better alternative than bending or creasing a plate or film receptor.


Tilted Occlusal Plane
Tilted Occlusal Plane

When the receptor is not placed perpendicular to the occlusal plane, the occlusal plane will appear slanted or diagonal. When exposing bite-wing radiographs, the top edge of the receptor may come in contact with the palatal gingiva or curvature of the palate. When this occurs, the occlusal plane will appear slanted. The receptor must be placed straight or perpendicular with the occlusal plane, or the receptor should be farther away from the teeth to avoid this error. Always place the biteblock in contact with the occlusal or incisal surfaces of the teeth you are imaging not on the opposing teeth. Placement on the opposing teeth will cause the receptor to displace when it contacts bony anatomy. In addition, note that the film identification dot was placed apically on this radiograph. It is preferable to place the identification dots in the coronal part of the image so that it does not interfere with interpretation of apical structures.

Vertical Alignment Errors


When using the paralleling technique and film holders, the vertical angulation is dictated by the film-holding device to enable the x-ray beam to be 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 which can occur causing teeth to appear elongated or foreshortened include:

  • receptor position
  • patient position

It is important to determine the cause of the error.

Elongation

Elongation or lengthening of the teeth and surrounding structures results from under-angulation. When a vertical angulation error is made using the paralleling technique, the image moves in a vertical dimension (up or down) on the receptor. To correct this error the operator must increase the vertical angulation. In other words, for the maxillary arch, the positive angulation must be increased (PID pointing down); for the mandibular arch, the negative 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.


Foreshortening

Foreshortening or shortening of the teeth and surrounding structures can also result from improper vertical angulation. Foreshortening is the result of over-angulation of the x-ray beam. 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 spaces and facilitate a thorough caries evaluation and assessment of bone levels, which are all components of a thorough clinical and radiographic examination. The x-ray beam should be aimed directly between the targeted teeth in order to open interproximal surfaces. Horizontal alignment errors cause the image to shift right or left, resulting in the overlapping of the interproximal surfaces.

To determine if the horizontal angulation is positioned incorrectly, evaluate the extent of the overlap. The buccal object rule can be used to designate the facial and lingual cusp when trying to define the error. (The buccal object rule states: Objects closer to the buccal surface will move in the opposite direction from the tube head movement or shift when two periapical or bitewings radiographs are exposed and the horizontal angulation is altered approximately 20º mesially or distally in the second exposure.) To avoid this error, the central ray must pass through the teeth where the contacts need to be open.

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 relationship in alignment with the teeth so that the x-ray beam travels directly through the contact area. This will place the receptor parallel to the buccal plane of the teeth of interest as well as the parallel to the instrument indicator ring.

Cone Cutting
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 clear zone on traditional radiographs after processing, due to the lack of x-ray exposure in the area of the cut. When using digital imaging, the cone-cut appears as an opaque area. The shape of the cone-cut depends on the type of collimator used when exposing the receptor. For example, if a round collimator or PID is used, a curved cone cut will appear. Square cone cuts occur when using a rectangular collimation. To correct this error, the beam should be recentered toward the area of nonexposure. Improper assembly of receptor holding devices can also cause cone cuts. When assembling these devices, make certain that the entire receptor can be seen while looking through the indicator ring.

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