Shear wave absolute vibro-elastography of the liver: method optimization and characterization

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Liver disease management can benefit from the measurement of liver stiffness and liver fat content. Liver stiffness can be assessed without biopsy by measuring the propagation speed of shear waves in tissue using magnetic resonance elastography (MRE), a technique in which a steady vibration is applied to tissue and the shear wave pattern throughout the tissue volume is measured. Fat content can also be measured with MR imaging. What is needed is MRI quality measurements at the cost and accessibility offered by ultrasound. In recent work with our partner, we have demonstrated that an electronically scanned 3D ultrasound transducer can be used to accurately measure steady-state wave motion induced by multi-frequency steady-state vibrations in tissue-mimicking phantoms and in healthy volunteers. The technique developed, called S-WAVE, currently does not provide real-time feedback to the operator on the quality of the image acquisition and does not provide the stiffness map in real time. Furthermore, imaging of patients is different than imaging healthy volunteers because their livers are much stiffer and have much higher fat content, and the patients involved are often large which makes scanning them difficult, whether by MRI or ultrasound. With our partner in this project, we will: (1) develop methods for real-time display of 2D shear-wave images during S-WAVE data acquisition and of the computed stiffness maps, and we will evaluate the usability and repeatability of the system in healthy volunteers; (2) compare shear and loss moduli acquired with S-WAVE and MRE, by using new image registration techniques, and also develop methods for imaging both elasticity and viscosity and their dependence on frequency; and (3) develop a model of fat content from ultrasound attenuation and shear wave attenuation, with MRI as the gold standard. To carry out the modeling required under (2) and (3), we will acquire healthy volunteer and patient data with the system developed in (1). Our technique has the advantage of being able to image the liver volume at depth, through a much simpler examination than by MRI. Its successful implementation has the potential to make a significant impact.