SU‐E‐I‐71: KVp Dependence of Transmitted Exposure for a Radiography Unit

Abstract

Purpose:To investigate the kVp dependence of the transmitted exposure for a radiography x‐ray unit.Methods:The study used a GE DiscoveryTM XR656 DR unit, a 30 (L) × 30 (W) × 25 cm thick Lucite phantom, two anthropomorphic phantoms (an Alderson RS‐310 chest phantom and a 3M skull phantom), an Unfors detector, and a Radcal 10×9‐6 ion chamber. We measured the entrance exposure and transmitted exposure of each phantom at 60, 70, 80, 90, 100, 110, 120 kVp for mAs range from 2.5 to 200 mAs, without any additional filter. The FOV is 30×30 cm for the Lucite and chest phantom (AP view), and 20×20 cm for skull phantom (Lateral view). The transmitted exposure was measured at the phantom center of the x‐ray exit side. For chest phantom, the transmitted exposures at 3 inch upper right and upper left from the center were also measured. We also checked the reproducibility and accuracy of the DR unit.Results:For each phantom, at every kVp and mAs setting, the transmitted exposure per mAs was calculated and normalized by the relative entrance exposure; the averaged transmitted exposure per mAs at each specific kVp was then determined. For chest phantom, the mean transmitted exposure per mAs was the average of three exit locations. The averaged transmitted exposure per mAs was fit as a power function of kVp. The result showed the transmitted exposure per mAs was approximately proportional to third power of the kVp for two anthropomorphic phantoms and forth power of the kVp for the Lucite phantom.Conclusion:The traditional assumption of fifth power kVp dependence to the transmitted exposure is inaccurate. At the normal radiography kVp range, the transmitted exposure is approximately proportional to third power of the kVp for a typical patient and up to forth power of the kVp for a large patient.