Fifty-seventh annual meeting of the American association of physicists in medicine
SU-E-P-28: Mammography System Collimation Assessment Using Unfors/RaySafe DXR+: How to Increase the Measurement Accuracy
Unfors DXR rulers (Hopkinton, MA) are used to measure the X-ray field to light field coincidence for mammography systems with a small SID of approximately 66cm. However, due to its bulky size (14.5mm thick with 2.5mm sensors) thickness correction is needed.
Unfors DXR rulers were used to evaluate the collimation of three mammography systems: GE Essential, GE 2000D, and Hologic Selenia with various compression paddles. When measuring the X-ray field to light-field coincidence using DXR rulers, the light-field is aligned to the zero mark on top of the ruler which is 14.5mm from the base of the ruler, while the X-ray sensor inside the ruler is at 7mm from the base of the ruler. Error occurs because of this 7.5mm difference. To resolve this problem, the error is projected to different reference planes for GE and Hologic mammography systems and the correction factors (CFs) are generated. In order to generate the CFs, the image size at the image receptor is measured/calculated for GE and Hologic mammography systems with various compression paddles using the images stored in PACS. For the shifting compression paddle, the movement is also measured at the image receptor plane and CFs are generated based on the X-ray field locations.
The generated CFs were used to calculate the X-ray field to light field coincidence. The CFs ranges from 1.0mm for Hologic Selenia with 18×24cm compression paddle to 2.9mm for GE Essential with 24×30.7cm compression paddle.
CFs were generated for 3 mammography systems with various compression paddles when measuring the X-ray field to light field coincidence using Unfors DXR rulers. The application of these correction factors adjusts the X-ray field to light field misalignment calculation up to 0.55% of the SID which is over a quarter of the 2% SID pass/fail criteria for mammography systems.