February 1, 2000
by Martin J. Bourgeois DDS, M.Ed., Dip. Oral Rad.
A panoramic radiograph can provide a view of the maxilla, mandible, temporomandibular joints, teeth and their supporting structures on one film (Fig. 1). This film is referred to as a pantomograph or a panoramic radiograph. Sometimes, however, the film is incorrectly referred to as a “Panorex”, which is the brand name of the panoramic x-ray machine first introduced to North America by the S.S. White Company in 1959. Today, there are several types of panoramic machines available.
PRINCIPLES OF PANORAMIC RADIOGRAPHY
In panoramic radiography, the x-ray source and the film are connected to each other. These two components rotate simultaneously around the patient to produce an image. The three-dimensional, horseshoe-shaped zone where images are sharp is called the focal trough, or image layer. The panoramic radiograph is composed largely of the anatomic structures located within this focal trough.
The focal trough is the area in which structures will appear most sharply and clearly. Structures, which fall in front of or behind, the focal trough, can be distorted, magnified or reduced. The size and shape of the focal trough varies between manufacturers.
Patient positioning is important for obtaining a detailed panoramic radiograph. Patients must be properly prepared and positioned with their head carefully aligned in the focal trough. Since each panoramic machine is slightly different, the manufacturer’s instructions should be followed carefully.
ADVANTAGES AND DISADVANTAGES
Panoramic radiography offers several advantages over conventional intraoral radiography. Some of the main advantages are:
The broad anatomic region imaged, including additional visualization of the areas of the body of the mandible beyond the periapical region, the ramus, the temporomandibular joint, the maxillary sinus and the stylohyoid complex.
Relatively low patient radiation dose. One panoramic film generally delivers a radiation dose equivalent to about one set of four bitewing intraoral films.
Greater ease and less time necessary to produce a single image representing the patient’s entire dentition.
The disadvantages of panoramic radiographs relate to these weaknesses:
Panoramic radiography is an extraoral technique and the resultant image does not resolve the fine anatomic detail that may be seen on intraoral periapical radiographs.
Magnification, geometric distortion, and overlapped images of teeth sometimes occur.
Objects situated outside the focal trough will be distorted or obscured on the radiograph.
The cost of a panoramic machine is approximately two to four times that of an intraoral x-ray machine.
COMMON ERRORS DURING PANORAMIC RADIOGRAPHY
Anterior teeth positioned outside the focal trough will result in blurring of the anterior teeth. If teeth are positioned anterior to the focal trough, the occlusal plane will be increased, a generalized increase of overlap of tooth contacts occurs, and the anterior dentition will be demagnified (Fig. 2). However, if the anterior teeth are positioned posterior to the focal trough, they will be magnified and the occlusal plane will appear flattened (Fig. 3).
Midsagittal plane positioning error will occur if the patient’s head is shifted to the left or right side. This will result in an asymmetrical and distorted image. The image of the structure farthest from the film will be magnified, whereas, on the opposite side, the structures’ image closest to the film will be decreased (Fig. 4).
Occlusal plane positioning error will result if the patient’s head is tilted up or down incorrectly. When the patient’s head is tipped down, the resulting image will be shorter and the mandibular inferior border may be lost. The occlusal plane will be exaggerated and the anterior plane will be demagnified (Fig. 5). If, however, the patient’s head is tilted up, the occlusal plane will be flattened or even reversed (Fig. 6). There may be superimposition of the hard palate on the maxillary anterior tooth apices, a loss of density in the middle of the radiograph, and loss of either one or both temporomandibular joints.
Spinal column positioning error will occur if the patient is slumped. The resultant image will contain an unexposed area in the middle inferior portion of the film. If the spine is not kept erect, the spinal column, resulting in the low-density area near the lower centre of the film (Fig. 7) will excessively absorb the radiation.
Patient movement during panoramic radiograph exposure can result in a series of artifacts or distortion effects usually localized to one region of the radiograph, namely, the region the rotating beam was scanning when the patient moved (Fig. 8).
If placed improperly, lead apron shielding will produce an area on the film too dense to read due to the exposure of the lead apron.
Ghost images are reflected images of a structure situated between the x-ray source and the rotation centre. Earrings are a common source of ghost image (Fig. 9).
A fogged radiograph will appear gray or dark (Fig. 10). This is usually the result of old film, improper storage conditions, secondary exposure of film to x-rays, exposure to white light, exposure to wrong colour safelight or chemical fog.
If the image appears too light (thin), washed out or no detail is seen, this is usually due to underprocessing or underexposure. Some causes may be: x-ray beam energy level is too low and not producing enough radiation to properly expose the film; processing time in developer is too brief; chemicals are too cold or exhausted; or the processing temperature is too cold.
If the image appears too dark (dense) or no structures can be seen, this is usually due to the x-ray beam energy level producing too much radiation and overexposing the film; too much time spent in processing developer; high processing temperatures; or double exposure.
Tree-like static marks on the film are usually the result of crimped or creased film, localized overexposure, processing chemicals, or static electricity (Fig. 11).
Random white lines/artifacts appearing on the film may result from: lint or small pieces of debris between the film and screens; scratches or gouges on the intensifying screen; contact with fixer dust or solution (Fig. 12); or exposure error. Fingerprint artifacts may also result by contact with contaminated hands.
The article is not intended to promote or discourage the use of panoramic radiography. It is designed to assist dentists and dental auxiliaries to perfect their technical skills with panoramic radiography, and also dentists considering obtaining a panoramic machine by providing an overview of panoramic radiography’s application to dentistry, outlining its advantages and disadvantages, and pointing out some common positioning and technical errors made with this extraoral radiographic modality.
r. Bourgeois maintains a private practice in oral and maxillofacial radiology in Toronto.
Goaz, P.W., White, S.C. Oral Radiology-Principles and Interpretation. The C.V. Mosby Company, (1982). 1994 – third edition
Miles, VanDis, Jensen, Ferretti, Radiographic Imaging for Dental Auxiliaries. W.B. Saunders Company, (1989).
Successful Panoramic Radiographic. KODAK Dental Radiography Series, Eastman Kodak Company, (1991).