Panoramic tomography in dental imaging.

Abstract

Most general dental practitioners routinely conduct radiological evaluation of asymptomatic patients who require dental assessments, in addition to those patients who manifest signs or symptoms of oral disease. Panoramic dental radiography is one of the most frequently used radiographic methods for dental treatment planning. The basic principle of panoramic radiography is identical to conventional tomography. The x-ray tube and cassette holder are linked at a fixed distance of 50 to 70 cm and both rotate around the head during the exposure. The cassette moves in the same direction as the X-ray beam but at a slower speed. Structures outside the image layer are blurred and distorted. Panoramic radiography is a unique extra oral film technique that allows the dentist to view the entire dentition and related structures, from condyle to condyle, on one film. Obtaining a high diagnostic panoramic radiograph is possible by paying attention to special details unique to this type of equipment. The procedure and equipment for obtaining a panoramic radiograph have been steadily improved since the first one taken in 1934. A panoramic radiograph is taken with patient sitting, standing or lying down. In all situations, the manufacturer's instructions must be carefully followed and the patient must remain perfectly still while the x-ray beam and image receptor (film screen combination) rotate together around the patient's head. Because the machine moves in a set path, the patient has to be positioned very carefully. And, because the beam and the film are both moving, any movement from the patient will blur the image on the screen. That's why such care is taken to keep the patient's head absolutely still in exactly the right position. The machines may have chin rests, forehead rests, and side head positioners, plus bite-blocks that patients will be asked to close their teeth around. All this may look and feel intimidating, but the process is very safe and not only does it provide an enormous amount of information about the jaws, panoramic radiography delivers only a small dose of radiation to the patient when compared to intra oral films. They are a good screening tool and used extensively for pre-orthodontic assessment of bone, to visualize impacted wisdom teeth (It is difficult to take an intra oral X-ray of this area) as well as in pre-implant surgery bone height assessment. They are able to show any abnormal growths or cysts in the jaw bone. The quality of the radiographic image is dependent upon many factors and to prevent errors occurring, dental staff must have a thorough understanding of each stage of image production, and sufficient training to recognise film faults.