A computer model of X-ray mammography has been developed which uses realistic high-resolution voxel phantoms to model the breast. These phantoms have 400-micron voxels and simulate in three dimensions adipose and fibroglandular tissues, Cooper’s ligaments, ducts and skin. The Monte Carlo computer model calculates the dose to each tissue and image properties such as energy imparted and noise per pixel and scatter-to-primary ratios (S/P). It allows dosimetry for individual breast tissues, and studies of the dependence of image properties on breast structure. The application of the program is illustrated with calculations using a Mo/Mo spectrum at 28kV for a phantom 50 mm thick. The value of S/P (no grid case) was 0.28 at the chest wall with a maximum of 0.71. Variations in S/P due to tissue inhomogeneity were 25%. Doses (normalised to 100 for adipose tissue) were: fibroglandular tissue 92; ducts 87; skin 284; Cooper’s ligaments 147. Calculations of the incident air kerma to average glandular dose conversion factor were made for 10 phantoms. Each had the same thickness, but different spatial distribution of tissues within a fixed central region of average glandularity 70%. The values showed variations of only 1%. It is concluded that the use of a computer model of mammography incorporating a realistic breast has significant advantages over models using homogeneous phantoms for dosimetry and the study of image properties.
Supported by Commission of the European Communities Grant CT2000-0036 and US Department of Defense Grant DAMD 17-
98-1-8169.