Rebecca C. Booi

Combination of digital mammography with semi-automated 3D breast ultrasound

This paper describes work aimed at combining 3D ultrasound with full-field digital mammography via a semi-automatic prototype ultrasound scanning mechanism attached to the digital mammography system gantry. Initial efforts to obtain high x-ray and ultrasound image quality through a compression paddle are proving successful. Registration between the x-ray mammogram and ultrasound image volumes is quite promising when the breast is stably compressed. This prototype system takes advantage of many synergies between the co-registered digital mammography and pulse-echo ultrasound image data used for breast cancer detection and diagnosis. In addition, innovative combinations of advanced US and X-ray applications are being implemented and tested along with the basic modes. The basic and advanced applications are those that should provide relatively independent information about the breast tissues. Advanced applications include x-ray tomosynthesis, for 3D delineation of mammographic structures, and non-linear elasticity and 3D color flow imaging by ultrasound, for mechanical and physiological information unavailable from conventional, non-contrast x-ray and ultrasound imaging.

Automated Ultrasound Scanning on a Dual-Modality Breast Imaging System Coverage and Motion Issues and Solutions

We are developing an automated ultrasound imaging‐mammography system wherein a digital mammography unit has been augmented with a motorized ultrasound transducer carriage above a special compression paddle. Challenges of this system are acquiring complete coverage of the breast and minimizing motion. We assessed these problems and investigated methods to increase coverage and stabilize the compressed breast.

Diagnosing Cysts With Correlation Coefficient Images From 2-Dimensional Freehand Elastography

We compared the diagnostic potential of using correlation coefficient images versus elastograms from 2‐dimensional (2D) freehand elastography to characterize breast cysts.

P3D-4 3D Breast Elastography with a Combined Ultrasound/Tomography System

We have produced high quality strain images in a breast phantom and in 7 human subjects with 3D ultrasound (US) breast elastography using a combined US/tomography system. All radiofrequency (RF) images in this study were acquired using a GE Logiq 9 scanner and a linear 1D array operating at 7.5 MHz on a stand-alone, mammography-mimicking unit. To determine 3D elastography efficacy, a breast phantom (ATS BB-1) was imaged using static compression at 0.5% axial steps up to 2.5% strain over 7-17 elevational steps with slice thicknesses 15-60% of the elevational beamwidth. RF images were correlated using 3D, phase-sensitive speckle tracking algorithms and accumulated, estimated displacements were converted to strain images. Image quality was assessed via correlation coefficient (R) and strain contrast-to-noise ratio (CNR). Results indicated that R remained high and nearly constant (0.96-0.98) for a 0.5% strain step under all conditions. Elevational slice thicknesses of les 30% of the elevational beamwidth sizes produced the highest CNR because thicker slices did not sufficiently meet Nyquist requirements. At slice thicknesses of 35% elevational beamwidth, at least 7 elevational slices were required to meet 3D speckle tracking algorithm spatial requirements in the elevational direction (filter ges kernel = 1 elevational speckle spot). Moving beyond these minimum requirements produced the greatest improvement in CNR with 3D tracking: acquiring elevational planes over 3 speckle spots produced a 90% CNR improvement over 2D analogous. Acquiring up to 4.5 speckle spots (17 elevational planes) increased the CNR by a total of 130%. Additionally, elevational slices off the center axis confirmed on-axis results. Human subject motion was addressed before applying these results in vivo. Volume data acquisition must occur within a patient breath hold (les 10 sec). Thus, all 7 human subjects (1 cancer, 6 fibroadenomas) were imaged using quasistatic elastography as they held their breath. Five axial compression steps were acquired at 0.3-0.7% strain for 7-11 elevational planes with slice thicknesses 30% of the elevational beamwidth (spatially equivalent to 2.0-4.6 speckle spots). When minimal out-of-plane motion was present, there was no significant difference in correlation coefficient values created from 3D and fast (<0.5 sec) 2D acquisition. Thus, potential motion artifacts introduced by 3D data acquisition have been minimized. Of the 7 lesions imaged, 3 were visible on both 3D and 2D and elastograms, with the 3D elastograms depicting CNRepsiv of 7-11% better than 2D. This suggests that 3D elastography on the combined system holds great potential for improving an already clinically valuable imaging technique

Evaluation of 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 /spl mu/m 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.

Breast ultrasound image improvement by pixel compounding of compression sequence

Pixel compounding is a technique that synthesizes the information of an image sequence involving slow decorrelation of the speckle to form a detail-recovered and speckle reduced image. To avoid extra data acquisition time and patient exposure, reuse of the existing data is desirable. In the procedure of elasticity imaging, a set of B-mode images with slight changes due to deformation is produced, which provides an ideal input for the pixel compounding. The improvement in image quality is evaluated quantitatively using a figure-of-merit (FOM) that indicates the quality of boundary information recovery and the contrast-to-noise ratio (CNR) over the phantom images. The increase in average CNR is from 0.4 in the original images to 0.8 in the pixel compounded images. The improvement in average FOM is from 0.15 to more than 0.5 on a scale of 0 to 1. In vivo results with a breast cyst, a fibroadenoma, and a breast cancer are also presented and the image quality improvement is subjectively evaluated. The results suggest that B-mode breast images from compression procedures are suitable data for pixel compounding, and that a speckle-reduced and detail-recovered or detail-maintained image can be produced. The improved imaging may provide alternative or better information for detection and diagnosis. A similar approach could be extended to elasticity imaging with other modalities.

Applying in vitro elasticity imaging results to optimize in vivo breast lesion characterization using a combined 3D USs/digital X-ray system

Ultrasound-based reconstructive elasticity imaging has great potential for diagnosis and characterization of breast lesions. Applying external strain with a mammographic paddle as part of a combined 3D US/Digital X-ray system provides more uniform deformation and breast stability, offering opportunities to improve image fidelity. In this study, we examined phantom and in vivo strain image quality with three GE transducers (M12L, 10L, 7L) each operating at several frequencies between 5- 10 MHz and 3 TPX paddle thicknesses to predict optimal in vivo results with the combined system. Out-of-plane motion was measured by translating an ultrasonic transducer across a breast phantom (ATS, model BB-1) in 50um steps over 400um. Each image was correlated to the first in the sequence to determine rate of elevational decorrelation. Next, in-plane, strain-limited decorrelation was evaluated by correlating images at 0.25-1.0% steps up to 5% strain using two-pass 2D speckle tracking algorithms and accumulation. Adaptive strain estimation (ASE) was applied to maximize CNR throughout the final strain image. Overall the 10L transducer caused the least decorrelation due to out-of-plane motion (R = 0.97 at 7.5MHz and 400um elevational translation). In-plane decorrelation was also primarily strain- limited with the 10L transducer at 7.5MHz, with R = 0.9 for a 1.6% strain step. Accumulated strain images after ASE demonstrated a CNR = 13.6 with the 10L transducer at 7.5 MHz. Of the 3 paddles (0.25, 1.0, 2.5 mm) used in the phantom study, the 2.5mm paddle created strain images with less artifacts than the thinner paddles by providing more uniform deformation during compression. Next, we evaluated sources of in vivo chest wall motion in 7 subjects to minimize patient motion during the scan. Based on these experiments, patients were asked to breathe shallowly during exams as it caused less decorrelation due to chest wall motion (Ravg = 0.96 over 91 frames). In vivo results were acquired with the 10L at 7.5 MHz using continuous compression over 2.1 seconds. Strain images clearly distinguished between tissue types when accumulated up to 5% strain. In vivo results are limited by out-of-plane motion but are expected to improve with 3D tracking. These early successes indicate that using elasticity imaging with the combined system can potentially characterize breast lesions and monitor therapies.

Sub-pixel compounding from elasticity imaging data

Sub-pixel compounding is a technique that synthesizes the information of an image sequence to form a betterresolved and speckle reduced image. To avoid extra data acquisition time and patient exposure, reuse of the existing data is highly desired. In elasticity imaging, a set of images with slight changes due to deformation is produced, which provides an ideal input for the sub-pixel compounding process. In this paper, a brief review of the resolution enhancement techniques in ultrasound imaging will be provided, and then, a diffusion-regularized, least square approach is presented for sub-pixel compounding image reconstruction. Based on the results, we suggest that (1) B-mode images from elastic imaging are suitable data for sub-pixel compounding and a speckle noise reduced higher-resolution image is a co-product of elasticity imaging; (2) for breast diagnosis, resolution improvement is of strong interest since better depiction of the interior and exterior structures of a tumor provides important detection and diagnostic information; (3) a similar approach could be extended to elasticity imaging with other modalities.