Film based scatter measurement in mammography

Film based measurements of scatter to primary ratio (s/p) have been made using the "beam stop" technique. Phantom material, radiographically equivalent to 100% glandular tissue, 100% adipose tissue and 50% glandular: 50% adipose tissue was used. These measurements have been made at 25 and 28 kVp for non grid and 28 and 30 kVp with grid for phantom thicknesses varying from 3 cm to 7 cm and 'D shaped' field sizes varying from 80 to 290 cm2. Results indicate that s/p is predominantly uniform across a mammographic image with a reduction towards the edge of the phantom. The values for s/p agree with previously published values determined from Monte Carlo calculation. Scatter primary ratio was found to be directly proportional to phantom thickness with little effect from field size. The effect of beam energy was significant in the non-grid case but reduced in significance with the use of a grid. The average measurement error was estimated at 4.5%. No variation in scatter primary ratio was detected between glandular and adipose phantom materials in this study.