Controlling gray level variation in Contrast Enhanced Digital Mammography: design of a calibration procedure

Ideally, the gray level changes in a Contrast-Enhanced Digital Mammography (CEDM) sequence reflect the uptake and wash-out of contrast medium in the breast. While insignificant in standard mammography, gray level variations with time caused both by patient and system related factors, have been observed in clinical CEDM sequences. We have acquired phantom image series on digital mammography systems using a Mo/Cu anode-filter combination and a tube voltage between 45 and 49 kVp, in order to derive a model for gray level change with time as a function of system parameters. The gray level variation exhibits a fair degree of inter-series repeatability, and-strongly depends on the dose received by the detector and. timing of the image acquisition series. Moreover, for tissue-equivalent compositions, the relative gray level change with respect to the first image does not depend on the composition. We designed a calibration procedure that can be used-to compensate for the tiny system-dependent signal variation that has been observed. A global reduction of 80-93% of the variation has been demonstrated in sequences acquired on a breast shaped phantom. Local improvement is effective across the whole field of view. When imaging iodine inserts (0.5-2 mg/cm(2) concentration), the calibration increases the constancy with time of iodine signal on subtracted sequences by a factor of 4 (median value).