Tissue Projection Errors
The second category of technical errors in panoramic radiography are tissue projection errors or those due to tissue superimposition. There are two errors possible:
Hard Tissue Projection Errors
1. Spine – The first hard tissue error can occur when the patient assumes the “head extended” position. One of the final instructions to patients before panoramic exposure is that they should move their feet slightly forward into the machine – a sort of “panoramic shuffle.” This has the effect of straightening the cervical spine of the patient. If the patient is not instructed to do this, they will often assume a head projected position. This is because of the natural tendency of patient’s is to put their head forward into the panoramic head holding apparatus rather than physically moving their head and their body. The positioning effect of this is that the spine assumes a more oblique position with respect to the x-ray beam and attenuates (absorbs) more x-rays.
Figure 60. Head Projected.
Figure 61. Head Straightened.
The most obvious anatomic feature of head projection is the superimposition of the spine on the ramus of the mandible and the production of a more ghost images. The most characteristic ghost image is of the spine, and it appears as a central midline radiopacity that broadens inferiorly. In addition, because of poor contrast, the dentition may be difficult to visualize because of this superimposed ghost image radiopacity.
Figure 62. Panoramic radiograph (schematic on right) showing the effect of the spine on the anterior of the image with slumping of the patient.
2. Patient Movement – The second hard tissue error can occur when the patient moves during an exposure. Because the panoramic exposure is of the order of between 16 to 20 seconds, the possibility exists that a patient may move during the exposure. Prevention of this error can be aided by continuing to inform the patient, throughout the exposure, of the progress of the unit around their head while instructing the patient to remain still. Movement produces characteristic anatomic effects by stretching or breaking the image in the zone of movement. This may lead to the appearance of a “false” fracture. Appreciation of this artifact is important in the assessment of patients reporting with maxillofacial injury.21Additional presentations may result from double imaging or loss of segments. Another characteristic effect is the appearance of a dent in the lower border of the mandible.
Figure 63. Panoramic radiograph (schematic on right) showing the effect patient movement on the image. Note the discontinuity of the left lower border of the mandible and distortion of dentition and palate immediately superiorly.
The effect of patient movement on the dentition can be subtle – and contribute to what could be called “motion microdontia.”
Figure 64. In this image (cropped and zoomed on right) notice that in the midline anterior region, tooth # 8 in the maxilla is extremely narrow and suggests that the patient may have a central incisor that is a microdont. On further examination it can be see that tooth # 26 in the mandibular arc, directly below # 8 is also a microdont. Further clues to this appearance being due to a motion artifact are revealed by comparing the anatomy above and below the teeth with the opposite side. This is especially apparent in the relative width of the ala or soft tissue shadow of the nose.
Soft Tissue Projection Errors
1. Tongue Drop – The second tissue projection error can result from soft tissue artifacts associated with either the tongue not being placed on the palate during the exposure, or the lips not being closed. These errors occur when the patient is not instructed to place their tongue on the roof of their mouth during the entire exposure and to keep their lips together. The principal effect of this is to produce areas of relative radiolucency associated with the oro and naso pharyngeal airspaces, which can obscure visualization. This occurs because panoramic x-ray beam exposure is designed to be sufficient to transmit through both the hard and soft tissues of the maxillofacial region. This is particularly evident with underexposed radiographs where the features of the maxilla in particular are obscured by the radiolucency created by the incorrect position of the tongue and radiopacity of the palatal hard and soft tissue. Prevention of this error can be aided by continuing to instruct the patient, throughout the entire exposure, to keep their tongue on the roof of their mouth. Failure to place the tongue on the roof of the palate throughout the exposure does not produce any anatomic effects but does have some consequences to the visibility of the dentition.
Figure 65. Panoramic radiograph (schematic on right) showing the effect of not placing the tongue on the roof of the mouth on the image.
While failure to position the tongue on the roof of the mouth is one of the most common technique errors, it is rarely the cause for a re-take. However, the effect can be accentuated in two situations when the patient is edentulous and when the patient is positioned too far forward. Failure to place the tongue on the roof of the mouth may also create apparent “apical pathology.”
Figure 66. This image demonstrates the effect that a relaxation in tongue position during panoramic exposure can produce. Notice that in the maxillary midline there appears to be a radiolucent, cyst-like radiolucency that has expanded inferiorly below the alveolar bone. Closer examination reveals that the uppermost part of this lesion is shaped in a gradual curve – this curve represents the top of the palate and is the tell tail sign that this "lesion" has been created because the patient’s tongue dropped during the exposure.
2. Lips not closed – The second soft tissue error results from the lips not being closed throughout the exposure. While there are no anatomic effects of this error, it can also contribute to “burnt out” of the crowns of the anterior teeth or be responsible for increased radiolucency over the maxillary anterior region that could be interpreted as apparent anterior bone loss.
Figure 67. Panoramic radiograph demonstrating the effect that occurs when the lips are not closed during exposure resulting in an area of increased radiodensity in anterior crown regions.