The effect of geometric and recording system unsharpness in mammography

Modulation transfer functions (MTFs) and test object radiographs were used to study the effect of geometric and recording system unsharpness in mammography with the CGR Senographe x-ray unit. Results show that geometric unsharpness can be a significant factor in the detection of microcalcifications within the breast, depending on the size and shape of the focal spot, the focal spot-to-recording system distance, and the object-to-recording system distance (o.r.d.). A new recording system for mammography, the DuPont Lo-dose system, requires approximately 1/15th the exposure of a direct x-ray film, such as Kodak RP/M, to provide mammograms with comparable photographic density. With the Lo-dose system, geometric unsharpness can be reduced by use of a specially designed long cone with an increased focal spot-to-recording system distance. This cannot be accomplished with direct x-ray films because the x-ray unit is operating at near-maximum output conditions even when short cones are used. Although direct x-ray films have a higher resolution than the Lo-dose system, at certain o.r.d.s total resolution is found to be affected more significantly by geometric unsharpness than by the Lo-dose recording system. In several cases, clinical results show improved detection of microcalcifications at larger o.r.d.s by the Lo-dose system with a long one, combined with a reduction by a factor of 15 in patient exposure.