External Vibration Multi-Directional Ultrasound Shearwave Elastography
H. Zhao, P. Song, D. Meixner, R. Kinnick, M. Callstrom, W. Sanchez, M. Urban, A. Manduca, J. Greenleaf, S. Chen
IEEE Trans Med Imaging vol. 33 issue 11: 2140-2148, November 2014
Due to limitations of biopsy and the increasing burden of liver disease, there is a great need for noninvasive staging of liver fibrosis. Elastography methods based on shear waves produced by ultrasound radiation force show great promise for liver fibrosis staging, but have limited penetration and high failure rate in obese patients. This is a serious limitation considering the obesity prevalence (35.7% in U.S.A.) and the increasing number of nonalcoholic steatohepatitis patients who are in need of liver fibrosis staging and are commonly obese.
Here we propose a novel technology, External Vibration Multi-Directional Ultrasound Shearwave Elastography (EVMUSE), which uses external mechanical vibration to produce shear waves with superior penetration for measurements in obese patients (Fig. A). EVMUSE uses directional filtering to decompose the complex wave field (with multiple waves traveling in different directions randomly over time) into simpler uni-directional wave fields (Fig. B). Elastography images calculated from each component wave field using a 2D time-of-flight method (accounting for arbitrary propagation angle) are compounded to produce a final shear wave speed image.
EVMUSE can reach 14 cm in a phantom with 0.7 dB/MHz/cm attenuation (Model 040GSE, CIRS Inc.), compared to 6.5 cm with the commercial Aixplorer® scanner (Fig. C). It can image to 12 cm in a healthy volunteer with Body Mass Index of 40 (Fig. D). Preliminary EVMUSE studies in 10 patients indicate a promising correlation (r = 0.86) with Magnetic Resonance Elastography. In summary, EVMUSE shows promise to meet the vital need of liver fibrosis staging in obese patients.