Analysis of optical density and contrast in mammograms

The objective of this project is the development of tools for the UK NHSBSP to assess image quality quantitatively in clinical films, for the purposes of optimizing imaging procedures and audit. As an initial step, 120 mammog rams of 46 women on a single day of screening were digitized and analysed t o produce indices of optical density (OD) and contrast. Analysis was perfor med on three regions of interest (ROI): pectoral muscle, main breast and sk in edge. Two radiologists independently graded the quality of information i n the different parts of each mammogram, and categorized breast type as eit her "dense", "mixed density" or "fatty" Measurements of contrast and OD gen erally correlated well with the opinions of the radiologists. For the obliq ue mammograms, the mean OD in the main breast ranged between films from 1.2 5 to 2.24 with a mean of 1.69 +/- 0.02. In the craniocaudal mammograms, the mean OD in the main breast ROI ranged from 1.14 to 1.94 with a mean of 1.6 1 +/- 0.05. The OD for a quality control film of a 40 mm block of PMMA expo sed on the same day with this system was 1.53. A contrast index (CI) was ca lculated for each mammogram as the difference between the points of maximum and minimum OD in the main breast. Mean CI was 1.02 +/- 0.09 for fatty bre asts, 1.50 +/- 0.10 for mixed density breasts and 2.05 +/- 0.23 for dense b reasts. A review of the radiologist assessments indicated that the main bre ast was satisfactorily displayed when glandular and fatty tissues were disp layed within the OD range 0.8-2.9. An analysis of the dynamic range require ments showed that 17% of films had a dynamic range that lay above that calc ulated using the suggested OD limits.