Novel single x-ray absorptiometry method to solve for volumetric breast density in mammograms with paddle tilt

摘要

We report on the design and validation of a breast tissue equivalent phantom for automated measurement of breast composition in film/screen and digital mammography systems. This phantom is a multi-step phantom made of a single material and containing nine lead positioning markers around its periphery. The markers allow for the phantom position to be solved relative to the x-ray gantry. The phantom was adhered to the top of the mammographic compression paddle such that it projected an attenuation image onto the unused corner of the image without overlapping with CC- nor MLO-view breast edges. The markers and their centroids were identified using automatic morphological image processing operations. The phantom, and thus the paddle, orientation is then obtained by minimizing a simple least-square error function of the difference between a pseudo projection image of the phantom markers at known coordinates and the actual marker image. Fibroglandular-equivalent breast attenuation values were found directly from step phantom projections. Fat attenuation values were derived from the attenuation coefficient ratios of fat to fibroglandular tissue. Finally, breast density was calculated by comparing image pixel values to the fat/fibroglandular references at the same thickness. Multiple scans of a test object (a density step phantom with 7 densities) at nine different compression thicknesses and six paddle-tilt angles were acquired. We found the precision for determining the breast thickness to be 0.015 cm (standard deviation) and for determining individual paddle angles to be 0.05 degrees. Multiple clinical studies using the technique on film/screen and digital mammography machines are also currently under way.