Reduction of radiation dose at HRCT of the temporal bone in children

PURPOSE: The purpose of this study was to reduce the radiation exposure of the eye lens in high resolution computed tomography (HRCT) of the temporal bone using an experimental phantom. MATERIALS AND METHODS: The HRCT image that was used for analysis was obtained by changing parameters including effective-mAs (E-mAs), distance coverage, and height of object in the Y-axis. Radiation exposure was measured to calculate equivalent doses by glass rod dosimeters that were fixed above the right orbit parallel to the body axis. Deterioration in image quality was evaluated by three radiologists and the following three-point rating method was employed: grade 1 (good image quality without diagnostic limitations), grade 2 (image was deteriorated, but there were no diagnostic limitations), and grade 3 (image was deteriorated with diagnostic limitations). RESULTS: Assuming that the equivalent dose was y (mSv), and E-mAs was x, a simple regression line, y=0.506x-0.494 (decision coefficient, R2=0.999), was obtained. A standard deviation (S.D.) less than 120 (E-mAs, 220-120) was judged as grade 1, an S.D. between 120 and 150 was judged as grade 2, and an S.D. higher than 150 was judged as grade 3, indicating that deterioration of the quality of images with reduced E-mAs affected the diagnosis by imaging at S.D. higher than 150. CONCLUSION: Radiation dose at the eye lens in HRCT could be reduced up to an equivalent dose corresponding to 70 mAs without compromising diagnostic quality in the phantom experiment.