[Quantitative analysis of the complete image layer in rotational panoramic radiography].

Radiography is a fundamental method of examination in clinical dentistry. Rotational panoramic radiography provides comprehensive information on the maxilla and mandible, and is thus a very valuable procedure for dental practitioners. It is impossible for the image layer to be altered to perfectly suit each individual patient, so the closest corresponding image layer is selected from the several layers in the current panoramic x-ray units. Therefore, it is essential for the two-dimensional characteristics of the layers to be accurately understood when performing radiography and diagnosis. There are a number of subjective and objective methods available to evaluate the image layers, but they all have certain disadvantages. Accordingly, an experimental model was used to quantitatively analyze the complete image layer from the anterior to the TMJ region. At first the central plane of the image layer was obtained by analyzing the x-ray beam. Subsequently, the modulation transfer function (MTF) values of the complete image layer from the anterior to the TMJ region were measured using the slit method with digital Fourier transformation. In order to define the image layer, the information transfer factor (ITF) was calculated by integrating the squared MTF from 0 to 10 cycles/mm at each position. The measured image layer was then compared with the layer calculated by the method of McDavid et al. A Veraview apparatus (J. Morita Corp., Japan) with a 1.1 mm primary slit width was used as the panoramic x-ray unit, and DuPont HiPlus/Kodax X-Omat RP was used as the screen/film combination. The results are summarized below. 1. The central plane of the layer, which was obtained by analyzing the x-ray beam, showed a palabolic curve on the right side and a sigmoid curve on the left side, and the forms of the curves differed between the premolar and the molar regions. 2. The average horizontal angle of the beam to the median line on the right and left sides was analyzed using panoramic radiograms. It was 24.2 degrees for the mandibular canine, 58.4 degrees for the surface between the 1st and 2nd molars, and 95.1 degrees for the condyle. 3. When the limit of the image layer was defined as an ITF of 75, the measured image layer thickness was 5.74 mm in the anterior region, 7.4 mm in the canine region, 15.1 mm in the molar region, and 20.7 mm in the TMJ region. The calculated values were 6.16 mm, 8.9 mm, 15.7 mm, and 20.0 mm, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)