Comparison of image quality indicators among mammography facilities in Ontario

To compare the technical aspects of image quality of mammography machines in facilities across Ontario.

Eight mammographic physics consultants took measurements and calculated accreditation phantom scores, mean glandular dose, entrance exposure, average optical density, half value layer and the limiting resolution of the imaging chain for 100 mammography machines across Ontario.

Of the 100 machines, 39 were affiliated with the Ontario Breast Screening Program (OBSP), and the remaining 61 were applying to become OBSP affiliates. All of the OBSP facilities and 32 of the applicants were accredited with the Mammography Accreditation Program (MAP) of the Canadian Association of Radiologists (CAR). All OBSP facilities had passing phantom scores, and 84% of the applicants with CAR accreditation and 79% of those without had passing phantom scores. The mean glandular dose was 1.5 (range 1.0-1.9, standard deviation [SD] 0.22) mGy for OBSP facilities, 1.3 (0.8-1.8, SD 0.29) mGy for CAR accredited and 1.4 (0.9-1.9, SD 0.24) mGy for nonaccredited applicants. Mean entrance exposures were 798 (540-1280, SD 135) mR, 717 (430-980, SD 153) mR and 770 (520-930, SD 116) mR for the OBSP, accredited and nonaccredited facilities respectively. There were no appreciable differences in the mean optical densities (approx. 1.62 OD), mean half-value layers (approx. 0.34 mm Al) or mean resolutions measured with the large focal spot (nominal size of 0.3 mm) either parallel (approx. 16 line pairs/mm) or perpendicular (approx. 14 line pairs/mm) to the cathode-anode axis of the x-ray tube, among the 3 types of facilities.

Image quality is increased and variability is decreased in facilities participating in a province-wide screening program.