To characterize the anisotropic elasticity of tissue, the authors apply diffusing wave spectroscopy (DWS) to detect the particle dynamics induced by ultrasound forcing. The elastic properties of tissue are modified by microscopic changes in ultrastructure that can occur during the growth and spread of cancer. DWS is a technique that measures the motion of scattering particles (e.g., blood cells in microvasculature or scattering particles in colloidal dispersions) by modeling the temporal decay of the wave autocorrelation function, using a dynamic particle model. One such model assumes Brownian motion allowing the recovery of the diffusion coefficient, D
. When ultrasound forcing is applied to the region-of-interrogation (ROI), translational and rotational modes are superimposed on the Brownian motion, dynamics that are influenced by the elastic properties of the tissue and can be detected by DWS. The challenge is to identify and separate the ultrasound-induced dynamics from the thermal fluctuations owing to Brownian motion. Mazumder and colleagues, from the Indian Institute of Science in Bangalore, India, describe a means of separating these signals by examining the difference between the mean-squared displacement of Brownian particles in the presence and absence of ultrasound-induced dynamics. By analyzing the decay of this modulation, they were able to extract the elasticity tensor from the ROI. In the paper, they present explicit expressions for characterizing orthotropic materials, such as pork fat, and isotropic, such as agar; they confirm their predictions by experimentally recovering resonant modes from agar (0.5–1.5%) and pork slab and estimating Young’s and shear moduli for these materials. Because structural changes affecting tissue elasticity often occur during the progression of cancer, the proposed technique of using resonant ultrasound spectroscopy to characterize anisotropy of soft tissue represents an exciting potential tool detecting cancer progression in soft-tissue organs where visual changes are often difficult to observe.
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