Analysis of optical density and contrast in mammograms

S Meeson1, KC Young1, ML Ramsdale1, MG Wallis2 and J Cooke3

1National Co-ordinating Centre for the Physics of Mammography, St. Luke's Wing, Royal Surrey County Hospital, Guildford GU2 5XX. 2Warwickshire, Solihull & Coventry Breast Screening Centre, Coventry and Warwickshire Hospital, Stoney Stanton Road, Coventry CV1 4FH. 3Jarvis Breast Screening Centre, 60 Stoughton Road, Guildford GU1 1LJ, UK.

The objective of this project is the development of tools for the UK NHSBSP to quantitatively assess image quality in clinical films, for the purposes of optimising imaging procedures and audit. As an initial step 120 mammograms of 46 women on a single day of screening were digitised and analysed to produce indices of optical density (OD) and contrast. Analysis was performed on three regions of interest: pectoral muscle, main breast, and skin edge. Two radiologists independently graded the quality of information in the different parts of each mammogram, and categorised breast type as either dense, mixed density or fatty. Measurements of contrast and OD generally correlated well with the opinions of the radiologists. For the oblique mammograms the mean OD in the main breast ranged between films from 1.25 to 2.24 with a mean of 1.69 0.02. In the cranio-caudal mammograms the mean OD in the main breast ROI ranged from 1.14 to 1.94 with a mean of 1.61 0.05. The OD for a quality control film of a 40 mm block oPMMA exposed on the same day with this system was 1.53. A contrast index (CI) was calculated 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 breasts, 1.50 0.10 for mixed density breasts and 2.05 0.23 for dense breasts. A review of the radiologist assessments indicated that the main breast was satisfactorily displayed when glandular and fatty tissues were displayed within the OD range 0.8 to 2.9. An analysis of the dynamic range requirements for these mammograms indicated that 17 % exceeded that available between these suggested OD limits.

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