Evaluating Thin Compression Paddles for Mammographically Compatible Ultrasound
We are developing a combined digital mammography/3D ultrasound system for breast cancer imaging to better detect and/or characterize breast lesions. Scanning a GE Logiq 9 M12L transducer array over a mammographic compression paddle/plate introduces an attenuating layer with sound speed and impedance different from that of tissue. This reduces signal level and affects beam focusing, Making the choice of a suitable paddle is essential for accurate sonographic detection of lesions. Similar work has been reported, but we present a more complete characterization of image quality through mammographic paddles of varying materials, (e.g., Lexan, Polyurethane, TPX, Mylar) and thicknesses. Quantitative measures such as spatial and contrast resolution, signal strength, and range lobe levels were compared to images without a paddle. In vivo patient studies compared images with standard handheld scans to images with 0.25, 1.0, and 2.5 mm thick paddles to examine restricted access problems, coupling issues, and overall lesion clarity. For mammography, filters were added to account for differences in X-ray transmission properties between the tested paddle and the standard mammography paddle. When lateral beamforming corrections were implemented to partially account for the speed of sound through the paddles, experiments conducted on 25 μm line targets with several plastic paddles between 0.25-2.5 mm thick demonstrated image quality measures close to those with no paddle present. In some paddles <1.0 mm thick, a worst-case 5% reduction in linear spatial resolution and a maximum 4 dB signal loss averaged over 4 cm occurred. In those better paddles up to 2.5 mm thick, range lobe levels were consistently 35-40 dB lower than the signal maximum. Areas of restricted access (such as near the chest wall) were minimized by imaging in trapezoidal (virtual convex) format. TPX paddles <2.5 mm were the most ideal for ultrasound and mammogram imaging requirements and, after accounting for signal loss through the paddle, appearance of cysts was comparable to images obtained from handheld, direct contact sweeps.
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