Machine Preparation Errors
The second category of errors involves factors associated with the panoramic equipment that may lead to poor image formation.
– The most common machine variable error occurs with selection of the incorrect exposure setting – usually kVp. Kilovoltage settings that are too high produce dark images while low kVp settings result in light images. Correction of this type of error usually involves a retake at a kVp that is either 5% above, in the case of an underexposed radiograph, or 5% below, in the case of an overexposed radiograph, the initial setting. In the case of digital radiography, only overexposures can be corrected with use of image software. A severely underexposed image does not contain all of the information in the first place and density manipulation will not improve the image quality. The two examples here show the effect of overexposure (Figure 44) and underexposure (Figure 45) on the panoramic image.
Figure 44. High Density Image.
Figure 45. Low Density Image. The two examples here show the effect of overexposure (top) and underexposure (bottom) on the panoramic image.
– Another error occurs principally with panoramic units that use a flexible cassette attached to a rotating drum. If the drum is not re-aligned at the correct start position after attachment of the cassette, only a partial image will be obtained. In addition, a partial image can be produced if the operator lets go of the exposure button prior to completion of the entire exposure cycle or if shoulder contact stops machine rotation.
Figure 46. Incomplete Exposure
Incorrect program selection – An error that may occur principally with panoramic units that allow various program options, is failure to change the program after the previous patient. This results in exposure of the patient using the previous program choice.
Figure 47. The two examples above show the effect of incorrect program selection. The image on the left image resulted from using a child exposure program, limiting the field of exposure posteriorly. The image on the right is due to the use of an orthogonal program segmenting the anterior and posterior dentition without coverage of the ramus and TMJ.