6F-1 Phase Aberration Effects in Static Compression Elasticity Imaging

There are currently several clinical studies underway to evaluate the use of elastography in breast imaging, mainly with compression elastographic techniques. Clinical ultrasound image quality is compromised by tissue velocity inhomogeneities, resulting in poor image resolution and contrast. This phase aberration can be especially evident in breast and abdominal imaging where fatty tissue components are present. The images used for elastographic estimations could be degraded by phase aberration and result in decreased performance of elastography. This paper presents an analysis of contrast and contrast-to-noise ratio (CNR) in strain images with different experimental phantom and ex-vivo porcine aberrators. Controlled axial compression strain elastography data were obtained to isolate the effect of aberration from other error contributions to elastography imaging. Several 2D thin rubber silicon aberrators, ranging from 25-100ns rms amplitude and 3-6mm FWHM, were placed between a clinical breast imaging 1D array and an elasticity phantom (E=32 kPa background with a 10mm diameter, E=77 kPa spherical lesion). Summed RF signals were collected from static axial multi-step compressions to induce 10% average strain at surface (about 1% frame-to-frame strain). Strain images were formed using a 2D phase sensitive, correlation-based speckle tracking technique with adaptive strain estimation. Speckle brightness lesion levels were assessed in all B-mode images; compared to the unaberrated case, the images with the 25ns and 100ns aberrators showed reductions of 57% and 88%, respectively. The strain image results show a decrease in strain CNR with increasing aberrator strength: CNR= 1.63, 1.39, 1.16, 0.84, and 0.63, for 3mm FWHM aberrators at 0, 25, 50, 75, and 100ns rms amplitudes, respectively. The average image-to-image correlation values in the lesion decreased from 0.97 to 0.88 for 0ns and 100ns rms aberrators. The results showed 1.57, 1.56, 1.49, 1.50, and 1.35 strain contra- st values for aberrator strengths from 0ns to 100ns rms. Similar decreases in strain contrast and CNR were seen with ex-vivo porcine aberrators (46ns and 124ns rms). An analysis of the sources of error (RF SNR and phase aberration changes) is presented. The initial results presented here indicate that phase aberration does decrease the strain CNR levels, but does not severely hinder the detectability of the lesion in this strain imaging scenario. Therefore, in clinical practice, phase aberration could contribute an additional source of error to compression strain imaging