by Leif Kullman
Panoramic imaging of the teeth and jaws.
The use of panoramic radiography is steadily increasing in the dental profession. The investigation is simple for the patient and an overview of the jaws including the teeth is obtained to a small radiation dosage. The procedure and equipment for obtaining the panoramic image have been steadily improving since the first images were taken about 60 years ago. However a panoramic radiograph is still a quite different image compared with our intraoral radiographs and a lot of problem are associated with the origin and interpretation of this image. Before even buying a panoramic equipment there is therefore a compulsory staff course to pass through in for example Sweden. During these courses the basic principles of panoramic imaging among other things are learned.
A special projection technique
The source of radiation and the film are both positioned extraorally in the most common panoramic technique. The image is created when the radiation beam and the film circles around the patient. The created image will then not be a summation image (all details lying between the radiation tube and film are visible in the radiograph) like an intraoral radiograph, but instead a layer of the jaws is shown and seen clearly in the image. Of course this layer should preferable be situated in line with the teeth. Details in the jaws outside this layer can also be portrayed in the image, however they will be severely distorted and sometimes magnified. Thus these details will make the image interpretation more difficult. It is therefore recognised that panoramic radiographs are useful as overviews of the teeth, jaws and even the temporomandibular joint but they cannot completely substitute our intraoral radiographs today. For making more accurate diagnosis possible, they usually must be supplemented with other radiographs.
Quality control
Due to the mentioned projectional backdrawings of panoramic radiographs and problems in image interpretation it is very important to make a careful quality control of panoramic images. The following is a summing up of a rationale for quality control and viewing that has been proposed by professor Lars Hollender, Seattle, USA in 1991:
check image sharpness, contrast and photographic density
test for symmetry in the image (i.e. between the quadrants)
the hard palate must be seen horizontal or slightly dipping
downward in the midline
nasal cavity should be symmetrical and of reasonable width
a complete view of the anterior teeth are seen and with an
acceptable appearance
tongue is positioned in contact with the palate
not too much overlapping in the premolar region
infraorbital margins are horizontally aligned and on the same
level
Different modes of applications for the panoramic unit
A lot of different programs are today available on panoramic equipment's from different manufacturers. Linear tomographic programs can produce tomographic slices of any part of the maxilla or mandible. Useful during planning for implants for example. Cephalometric measuring techniques can be used during orthodontic treatment if a cephalometric unit has been included and special paediatric programs usually allow for reducing the dose during panoramic imaging. Different programs for imaging of the TMJ have been produced. Other programs allow for placing the image layer in connection with sinus maxillaris, making different kind of pathology possible to diagnose in this area.
Today all manufacturers are also working on to convert their panoramic units into digital units and the first models have already been sold. Digital panoramic radiographs will bring a substantial reduction of the radiation dose besides the other pros always existing in digital imaging.
Next month I will tell you more about radiation doses and radiation risks.
Leif Kullman
