Ｅｘｐｏｓｕｒｅ Ｉｎｄｅｘ導入時のキャリブレーションにおける誤差要因の検討

Abstract

Exposure indicators of digital radiography systems have been termed by manufacturers individually. The International Electrotechnical Commission (IEC) provided a concept of exposure index (EI) that unified the exposure indicators in order to recognize the deviations from the intended exposure. Although the IEC determined the calibration conditions between the exposure indicator and the air kerma at the detector surface, the tolerance was described was limited concerning the beam conditions. The purpose of this study is to evaluate the calibration error regarding the geometry and the added filtrations. The relative errors of calibrations based on the IEC condition were derived using the value of exposure indicators at EI value of 200, 500 and 1000 for field size of 10×10 cm, 20×20 cm, 30×30 cm and 43×43 cm, and source-image receptor distance (SID) of 100 cm, 120 cm, 150 cm and 200 cm. The beam qualities using added filtrations of 0.5 mm Cu+2 mm Al, 0.6 mm Cu and 21 mm Al were adjusted to RQA 5. The air kerma at the detector surface over the clinical use ranged from 0.18 to 26.3 µGy. Consequently, the relative errors for the calibrations were less than 6% at a field size of more than 10 × 10 cm and at SID of more than 100 cm with all added filtrations.