Shear wave based quantitative ultrasound imaging methods

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BACKGROUND: Clinical ultrasound (US) scanners typically provide brightness mode (B-mode) images where the acoustic impedance contrast between tissue structures is used to identify boundaries for diagnosis. Modern US systems also include Doppler modes (with or without contrast agents) for flow and tissue motion analysis, and elastography modes (strain and shear wave (SW) based) for assessing mechanical properties of tissues. Quantitative US (QUS) imaging is another field that aims at quantifying acoustic compression wave interactions with biological tissues. QUS techniques extract fundamental physical properties of tissues to provide information on sub-resolution properties that are not visible on B-mode or other imaging modes. STATE-OF-THE-ART POSITIONING: Since biological soft tissues are hydrated, viscoelasticity is suitable to represent their solid-like behavior using elasticity, and fluid-like behavior using viscosity. However, the assessment of elasticity through US imaging has been more often exploited than viscosity. In US elastography, SWs are usually generated through focused push beams followed immediately by the monitoring of the SW motion. Under certain assumptions, elasticity relies on the measurement of the SW speed, and viscosity on its frequency dispersion or SW attenuation. I will pursue the development of robust viscosity methods for SW elastography imaging. Also, based on intriguing preliminary data on changes in QUS backscatter properties during SW propagation, I am also proposing the development of a new field that I label: "Dynamic QUS backscatter imaging". OBJECTIVES: 1) Improve robustness of tissue viscosity assessment using SW propagation properties; 2) develop a new QUS concept based on SW propagation; 3) validate SW-based viscosity and QUS imaging methods using simulations, in vitro phantom data, and available in vivo human scans; and 4) develop an open-access simulation package combining SW tissue excitation and compression wave backscatter analysis for dynamic QUS backscatter imaging. HYPOTHESES: 1) SW attenuation imaging can be improved by allowing the spectral content of SWs to vary spatially, by no longer assuming a linear power law dependency for the attenuation, and by using a randomized sample consensus algorithm to seek attenuation versus distance in the framework of an optimization process. 2) The dynamic QUS backscatter method should reflect the correlation length of scatterers, which determines constructive and destructive compression wave interferences modulated by the SW propagation. The new dynamic QUS backscatter method may image local tissue oscillating scatterer size and position changes, and provide a new image contrast reflecting pathological states (different from standard backscatter QUS, SW speed, or SW attenuation). CONCLUSION: The development of SW-based technologies (viscosity and backscatter) should provide new imaging modalities for better biological tissue characterization.