2D ARFI and Viscoelastic Response (VisR) anisotropy imaging in skeletal muscle

Tissue mechanical anisotropy has been shown to be diagnostically relevant in numerous clinical applications. Anisotropy can be assessed using acoustic radiation force-based techniques, including Acoustic Radiation Force Impulse (ARFI) and Viscoelastic Response (VisR) ultrasound. In this work, ARFI peak displacement (PD), as well as VisR relative elasticity (RE) and relative viscosity (RV), were implemented to image mechanical anisotropy in ex vivo porcine posas major muscle. Two psoas major samples were imaged in this work. The first sample was imaged whole. The second sample was sectioned into a 1 cm-diameter cylinder (with cylinder height oriented along the muscle fibers) and embedded into an isotropic gelatin phantom for imaging. ARFI and VisR were performed on both muscle samples using a linear array transducer mounted to a translation and rotation stage. By rotating the transducer 90
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as it stepped across a 2D imaging field of view, PD, RE, and RV were measured along and across muscle fibers. Then, the ratios of PD, RE, and RV measurements made along versus across the muscle fibers were calculated, and these ratios were parametrically rendered in to 2D images representing degree of anisotropy. In the whole psoas major, PD anisotropy ratio was 1.08 +− 0.11, the RE anisotropy ratio was 0.94 +− 0.09 and the RV ratio was 0.94 +− 0.08. These PD and RE anisotropy ratios suggest that the longitudinal shear elastic modulus was greater than the transverse shear elastic modulus, which agrees with previous studies on muscle anisotropy. The RV results suggest that the longitudinal viscous modulus was greater than the transverse viscous modulus. Similar PD, RE, and RV anisotropy ratios were measured in the embedded psoas major section, and contrast ratios were 0.21, 0.14, and 0.30 for PD, RE and RV 2D anisotropy images, respectively. CNR for PD, RE, and RV anisotropy images were 3.26, 2.09 and 2.41, respectively. These results demonstrate that ARFI and VisR anisotropy imaging are relevant for delineating the spatial distribution of anisotropic features in tissue.