Optimal radiographic techniques for digital mammograms obtained with an amorphous selenium detector

Abstract

Traditional film/screen mammograms are obtained using Molybdenum or Rhodium target x-ray tubes. The energy spectrum from these sources matches the limited latitude of film/screen systems. For digital imaging systems, the latitude is linear over a wide range of exposures and arbitrary H&D curves can be obtained with image processing. This allows the recorded contrast to noise ratio (CNR) to be optimized by considering a wide range of radiographic techniques. For this work, we modeled the radiographic process for a digital (amorphous selenium) mammography system. The optimal CNR relative to dose was determined for several target/filter combinations, for a wide range of kVp values, and for varying breast thickness. The target/filter combinations included: Mo/Mo, Mo/Rh, Rh/Rh, W/Al, W/Mo, W/Ag, and W/Sn. As breast thickness increased, the use of a tungsten target with a tin filter resulted in a 34% improvement in CNR for the same dose to the breast when compared to the use of a Molybdenum target with a Molybdenum filter. Notably, the W/Sn target/filter combination resulted in a significantly lower mA-s for the same breast dose (2/3 to 1/5 lower for a breast thickness from 4 to 8cm). In mammography applications, use of a Tungsten tube rather than the traditional Molybdenum tube should lead to significant reductions in exposure time and tube heat while maintaining similar image quality and dose.