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Quantitative ultrasound methods can provide valuable information about the microstructure of a material or tissue. This works well when the common assumptions of homogeneity, isotropy, and diffuse scattering conditions are valid. In biological tissues, however, these assumptions are often violated because the microstructure of biological tissues is often heterogeneous and anisotropic. The microstructure of biological tissues can change with disease, and therefore accurate identification and description of a tissue's microstructure can offer important clinical insight. To address the challenge of evaluating the microstructure of biological tissues, here we introduce a novel parameter called the correlation length ratio (CLR), a ratio of lateral to axial correlation lengths for backscattered echo signals. We developed it to determine the presence of fiber-like structures in soft tissues by comparing this value in tissue to a threshold determined from a reference material that is homogeneous, isotropic, and provides diffuse scattering. We tested this novel parameter in phantoms with spherical scattering sources, in an anisotropic phantom (containing elongated fibers), and in human biceps muscle. We found that the CLR accurately detected the presence of elongated structures in both the anisotropic phantom and muscle. These results encourage further exploration of this novel parameter in microstructurally complex tissues. [ABSTRACT FROM AUTHOR]

Accurate determination of the complex shear modulus of soft tissues and soft-tissue-like materials in the 10–300 Hz frequency range is very important to researchers in MR elastography and acoustic radiation force impulse (ARFI) imaging. A variety of instruments for making such measurements has been reported, but none of them is easily reproduced, and none have been tested to conform to causality via the Kramers–Kronig (K-K) relations. A promising linear oscillation instrument described in a previous brief report operates between 20 and 160 Hz, but results were not tested for conformity to the K-K relations. We have produced a similar instrument with our own version of the electronic components and have also accounted for instrumental effects on the data reduction, which is not addressed in the previous report. The improved instrument has been shown to conform to an accurate approximation of the K-K relations over the 10–300 Hz range. The K-K approximation is based on the Weichert mechanical circuit model. We also found that the sample thickness must be small enough to obtain agreement with a calibrated commercial rheometer. A complete description of the improved instrument is given, facilitating replication in other labs. [ABSTRACT FROM AUTHOR]