2013 Volume 22 Issue 1 Pages 133-138
Quality assurance studies of mammography X-ray equipment have shown that the concentration distribution is not even in the direction of the X-ray tube axis (i.e. from the nipple to the chest wall), a phenomenon known as the heel effect. In the present study, we investigated a similar effect in a direction perpendicular to the tube axis. We carried out two experiments. First, the film concentration was measured at 81 locations on an RMI156 phantom used for quality assurance studies of mammographic X-ray equipment. Second, we used one to five acrylic plates each 10mm thick and of sufficient size to cover the whole area of mammography. Photographs were taken at a total of 108 locations, including those 1 to 17cm along the tube axis direction and 1 to 23cm in a direction perpendicular to the tube axis. The differences in the measured concentration were represented as graphs. Results: In the tube axis direction, the concentration was lower at locations near the nipple than at those near to the chest wall, the decrease in concentration being gradual at locations within about 9cm from the chest wall, and then rapidly becoming more marked at further distances apart. The maximum difference in concentration was about 0.5. In the direction perpendicular to the tube axis, a slight left-to-right increase of concentration was noted up to about 7cm distant from the tube axis, followed by a gradual decrease. The maximum difference was about 0.25. Conclusion: The presence of an uneven concentration distribution may create problems with quality assurance of mammographic X-ray equipment and also the reading of clinical mammograms. Further study will be necessary to clarify the reason for this uneven concentration distribution.
