Development of a High Resolution Photoelectronic Digital Radiographic Imaging System for Higher Energy Mammography

We have constructed a high resolution photoelectronic digital radiographic imaging system for mammography. The system consists of an Agfa MR Detail S mammographic screen (Gd202S:Tb) fiber optically coupled to a Tektronix TK2048M 2048 x 2048 CCD with an imaging area of 55.3 mm x 55.3 mm. Analysis of the application of this imaging system to mammography using a model of radiographic imaging which accurately reflect the sensitometric and signal-to-noise properties of this imaging system, has demonstrated the optimal radiographic techniques for imaging soft tissue nodules and microcalcifications. The SNRdetail for a 5mm soft tissue nodule and a 0.25 mm microcalcification have been calculated as a function of energy for constant exposure and constant dose. The SNRdetail for the 5mm nodule increases with energy and peaks just above the k-edge of the phosphor for constant exposure. The SNRdetai, for the 0.25 mm microcalcification peaks at approximately 23 keV. When the SNRdetail is calculated using exposures which result in constant dose as a function of energy, as measured by the average glandular dose (AGD), the SNRdetail for the soft tissue nodule reveals two equivalent peaks at 23 keV and at 50 keV. The constant dose SNRdetail for the 0.25 mm microcalcification peaks at approximately 20 keV. Experimental results using 3 mm lucite and a 0.5 mm aluminum test object on a 5 cm lucite block reveal very good correspondence between estimates and experimentally measured SNRmajdmura and SNRdetail for both the lucite and the aluminum. Measurement of the MTF reveals significant degradation due to the fiber optic coupling. We conclude that for photoelectronic digital mammography, the optimal dose efficient energy for imaging soft tissue lesions is 23 keV and 50 keV. For microcalcifications, the optimal dose efficient energy for imaging is approximately 20 keV.