Scatter radiation and the effects of air gaps in cephalometric radiography

Abstract

orthodontic patients are children and adolescents, who are more sensitive to ionizing radiation than adults. In many cases, several cephalometric radiographs are taken during treatment and follow-up. Therefore, it is important that these patients be exposed to as little radiation as possible. In many facilities, cephalometric radiography is still performed using a grid to prevent image degradation, such as the decreased contrast caused by scatter radiation. The use of a grid results in increased exposure to compensate for the reduction in the radiation that reaches the detector plane. Some authors have removed the grid, 1 and introduced the air-gap technique 2 in cephalometric radiography. However, the scatter fraction, the ratio of the scatter radiation reaching the film plane to the primary radiation, with and without a grid, have been studied in cephalometric radiography in only one case. 2 In 1976, Muntz et al. 3 suggested that scatter rejection resulting from air gaps could be described using an empirical model in which the scattered radiation behaved as if it originated from an effective scatter point source (ESPS) located between the focal spot of the X-ray tube and the exit surface of the phantom (Fig. 1). Their suggestion was based on chest phantom experiments. In a subsequent paper, 4 they showed a similar tendency on mammography. In 1985, Sorenson and Floch 5 reported the effectiveness of ESPS models for various field sizes, tube voltages, and phantom thicknesses. However, these experiments did not include the parameters used in cephalometric radiography. These parameters could be explored from their results, but it is better to experiment using cephalometric radiography conditions directly. If this model is applicable to cephalometric radiography, the effectiveness of the scatter reduction for any distance of air gap could be estimated.In this study, we evaluated the use of the ESPS model for the scatter property in cephalometric radiography and estimated the scatter rejection effect of the air gap to optimize the air-gap distance. AbstractObjectives. The scatter radiation and scatter rejection effect of air gaps in cephalometric radiography were evaluated using an effective scatter point source (ESPS) model. Methods.A 16-cm-thick water-equivalent phantom was used to measure the scatter fraction. The distance from the source to the center of the object (SOD) was 150, 200, or 300 cm. The air gap was varied from 0 to 96 cm for each SOD. A photostimulable phosphor plate was used as the X-ray sensor. The measured scatter fraction ESPS model was used to simulate the scatter rejection by the air gap, and the predictions were compared with the grid. Results. There was excellent agreement between the ESPS model and the scatter measurements. The air gap reduced the scatter radiation, especially for an SOD of 200 or 300 cm, while keeping an object magnification of 1.1 in view of the signal-to-noise ratio improvement factor. Conclusions. The results suggest that a grid should not be used in cephalometric radiogr...