Ultrasonic mechanical relaxation imaging

Ultrasonic mechanical relaxation (UMR) imaging is proposed as a research tool for visualizing viscoelastic properties of tumors. Soft tissues behave mechanically as water-based polymers, similar to gelatin, with time-varying viscoelastic properties that depend on the chemical environment; e.g., acidic conditions alter the polymer-fiber surface charge density that determines cross-linking. We hypothesize that changes in the tumor microenvironment can be imaged using UMR methods. UMR techniques include elastic strain and viscous creep relaxation images produced by applying and holding a step-stress stimulus while recording RF echo frames at a high rate. Local displacements estimated from motion occurring between frames in the echo sequence yield strain images whose contrast is weighted by the viscous relaxation parameters. The relaxation parameters are estimated and mapped for each pixel using the strain time series to produce parametric UMR images. UMR images in gelatin suggest it is possible to explore the microenvironments of living tumors, where the viscoelastic properties are influenced by the changes in pO₂, pH, and collagen density that predict metastatic potential and resistance to treatment.