Now let’s develop a stepwise approach to analyze and identify panoramic errors. Panoramic errors result in image presentations that may be due to either screen/film handling errors, technique errors or a combination of both.
- Screen/film handling errors
- Film or cassette loading – Incorrectly loading film in the cassette or the cassette into the cassette carrier results in a generalized or localized change in image density.
- Reversed cassette loading is a relatively common artifact associated with rigid cassettes resulting in the appearance of radiopaque “springs” on the image. While this error in and of itself does not require a retake, this error has the potential for serious misinterpretation as often left and right radiopaque markers are built within the cassette. This results in the L marker appearing on the right side and vice versa. Therefore care must be taken to re-label the radiograph.
Spring artifacts only result from reversed cassette loading of rigid cassettes. Carefully examination may also reveal the outline of the plastic hinge or locking device. In this particular example there is also a conical shaped artifact on the lower left side of the image that is due to the imaging of the lead apron.
Inadequate cassette sealing or a split cassette seam results in the presence of areas of darkness extending from the periphery. This appearance can be either a linear streak, multiple areas along the edge of the radiograph or almost the entire image and are due to exposure or light fogging of the film.
Inadequate cassette sealing
Intensifying screen reversal produces an image with overall greatly reduced radiodensity (very light). The image may have a peripheral margin that corresponds to the peripheral binding of the intensifying screen. As intensifying screens only function on one side, if positioned backwards (i.e., flipped) and inserted into the cassette they will not function to expose the panoramic film. This error is more apt to occur using the flexible cassette system after screens are removed.
Example of intensifying screen reversal artifact (left). A static electric artifact is also observable superimposed on the left mandibular ramus. The intensifying screen (right) in the flexible cassette clearly indicates the correct orientation of the screen.
Processing of the film and associated darkroom errors
The whole gamut of processing errors is possible as with intraoral radiography and includes inadequate development, chemical contamination and film overlap.
Example of a panoramic radiograph showing multiple radiolucent areas due to contamination with developer solution, which appears black.
Example of a panoramic radiograph showing multiple radiopaque areas due to contamination with fixer solution, which appears white.
Transport errors are associated with the physical handling of the film and include:
- Film crimping – Crimping of the film can occur prior to exposure as the film is removed from the box or after exposure, when it is removed from the cassette.
- Scratches in intensifying screens – Intensifying screens do not last indefinitely and as they are made of plastic are prone to warp and crack with time. This cracking is usually peripherally and presents as peripheral opaque crazing lines.
Panoramic radiograph demonstrating imaging effects due to scratches in the intensifying screen and film crimping (shown on right).
Static electricity – A common effect seen on panoramic radiographs is static electricity. This usually presents as a divergent, branch-like or “lightning” pattern. This effect is due to the physical action (friction) and resultant transfer of electrons, between the film and intensifying screen on insertion or removal from the cassette or on removal of the film from the film box. While this effect occurs most often on cold damp days, it is more likely to occur with panoramic systems that use a flexible plastic cassette and necessitate the insertion of a film between the intensifying screens.
Characteristic static electricity panoramic artifact.
2. Technique Errors
Technique errors can result from difficulties encountered from each of the three stages in panoramic technique:
- Patient Preparation
The features of these errors on radiographs are characteristic and are therefore usually readily self-diagnostic.
- Metallic artifacts – The most common patient preparation error is failure to remove metallic or radiodense objects – This causes two problems:
- First they produce an opaque outline of themselves, usually providing a telltale indication of the error.
This image illustrates the telltale signs of leaving a number of metallic objects on the patient – earrings, glasses and a neck chain.
Second they may produce so-called “ghost images” of themselves. Ghost imaging is a normal component of the panoramic projection and occurs when an object is penetrated twice by the x-ray beam. Structures or objects that are located posterior to (behind) the center of rotation and the x-ray source tend to be ghosted onto the contralateral side of the image. Ghost images have particular, recognizable characteristics that are viewable on the panoramic film.
- The ghost image has the same general shape of the original but does not produce a mirror image.
- The ghost image appears on the opposite side of the radiograph compared to the original.
- The ghost image appears in a higher position than the original.
- The ghost image appears magnified and unsharp (blurred) more in the vertical plane than the horizontal plane compared to the original.
An example of this occurring to normal anatomic structures is the angle of the mandible projected to the other side or the two lines of the palate. Ghosting of metallic objects however may produce unacceptable coverage of structures. To prevent such artifacts the patient must always be instructed to remove all items in the head and neck region prior to positioning of the patient.
Figure 87. Glasses
Figure 89. Hairclip
Figure 88. Earrings
Figure 90. Necklace or Napkin Chain
Examples of images produced when metallic or radiodense objects are not removed prior to patient exposure.
Lead apron – The second most common patient preparation error is incorrect placement of the lead apron. This produces a characteristic radiopaque domed or wedge-shaped artifact that also obscures diagnostic information associated with the mandible. Correct positioning of the lead apron involves ensuring that the apron is not placed too high up the patients back prior to exposure.
Example of lead apron artifact.
Thyroid collar – Because of the nature of the projection beam in panoramic radiography, in effect coming up and over the shoulder of the patient, thyroid collars are not to be used. If it is placed on the patient it produces a characteristic appearance bilaterally – a radiopaque cone shaped artifact centrally located which obscures diagnostic information particularly in the mandible.
Examples of thyroid collar artifact.
The second category of errors involves factors associated with the panoramic equipment that may lead to poor image formation.
Incorrect Exposure – 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 (left) and underexposure (right) on the panoramic image.
Incomplete exposure – 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.
The two examples above show incomplete exposure due to the effect of incorrect drum alignment.
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.
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.
The second component of technique error analysis is the determination of patient positioning errors. This is of critical importance because even small patient positioning errors can produce effects that can obscure areas of the image and result in the loss of diagnostic information.
Technique errors can be easily recognized by careful analysis of the position, relationship of structures, unsharpness and distortion in a panoramic radiograph. The visual effects of incorrect patient positioning on the image may be described according to whether they affect anatomic representation or features of the dentition.
- Dentition features
- Occlusal plane slope – A convex curve of the smile line indicates that the patient’s head is tilted too far down whereas a concave line indicates that the head is tilted too far up.
Convex occlusal plane
Concave occlusal plane
Tooth crown and root visibility – All the crowns and root tips should be in focus. This indicates that the dentition is within the focal trough. The most important area to assess is the anterior region, where the focal trough is inherently narrow.
If the teeth in the anterior region are in focus, the enamel of the teeth is demonstrated and the pulp canals within the teeth can be identified.
- Condylar position – The condyle should be positioned such that it is positioned in the center of the right and left outer regions of the panoramic image.
Spine position – The spine should not overlap the ramus of the mandible, which is indicative of the patient not assuming a correct upright standing or sitting position.