Calibrated breast density methods for full field digital mammography: A system for serial quality control and inter‐system generalization

Abstract

Purpose: The authors are developing a system for calibrated breast density measurements using full field digital mammography (FFDM). Breast tissue equivalent (BTE) phantom images are used to establish baseline (BL) calibration curves at time zero. For a given FFDM unit, the full BL dataset is comprised of approximately 160 phantom images, acquired prior to calibrating prospective patient mammograms. BL curves are monitored serially to ensure they produce accurate calibration and require updating when calibration accuracy degrades beyond an acceptable tolerance, rather than acquiring full BL datasets repeatedly. BL updating is a special case of generalizing calibration datasets across FFDM units, referred to as cross-calibration. Serial monitoring, BL updating, and cross-calibration techniques were developed and evaluated. Methods: BL curves were established for three Hologic Selenia FFDM units at time zero. In addition, one set of serial phantom images, comprised of equal proportions of adipose and fibroglandular BTE materials (50/50 compositions) of a fixed height, was acquired biweekly and monitored with the cumulative sum (Cusum) technique. These 50/50 composition images were used to update the BL curves when the calibration accuracy degraded beyond a preset tolerance of ±4 standardized units. A second set of serial images, comprised of a wide-range of BTE compositions, was acquired biweekly to evaluate serial monitoring, BL updating, and cross-calibration techniques.

Technical challenges in generalizing calibration techniques for breast density measurements

“…The calibration methodology for mammograms was described previously . Briefly, calibration maps the image data into standardized units (SUs), which is a normalized effective x‐ray attenuation representation, ranging from (0,100).…”

“…A serial updating scheme is used to ensure that the baseline BTE reference data collected in the past are current. This approach permits updating the baseline reference data serially when calibration accuracy slips beyond tolerance . This monitoring involves acquiring a minimal number of BTE phantom images biweekly for each unit.…”

“…The accuracy of the characterized compression paddle for each unit was monitored serially with the Cusum technique described in our previous work adapted to DP images. Briefly, the Cusum argument is expressed asfor n > 0 and U 0 = 0, where m 0 and m n are the LREs for the DP taken at time = 0 and at serial time points referenced as n measured in days.…”

“…Upward drift is monitored with this DI formand downward by this formwhere k is the chart constant with . We used an empirically defined tolerance described previously of ±4SU for two consecutive serial points, which applies to reference phantom images. We note, k was adjusted for given acquisition (i.e., kV, target/filter combination, thickness) to detect the same tolerance ±4SU .…”

“…We used an empirically defined tolerance described previously of ±4SU for two consecutive serial points, which applies to reference phantom images. We note, k was adjusted for given acquisition (i.e., kV, target/filter combination, thickness) to detect the same tolerance ±4SU . When the deviation at any time point is less than the tolerance, the DI terms return zero, indicating in control behavior.…”