In Vivo Mechanical Anisotropy Assessment in Renal Cortex Using ARFI Peak Displacement

The kidney is an anisotropic organ, with higher elasticity along versus across nephrons. The degree of elastic anisotropy in kidney may be diagnostically relevant if properly exploited; however, if improperly controlled, anisotropy may confound stiffness measurements. The purpose of this study is to demonstrate a novel method for selectively exploiting or obviating elastic anisotropy in kidney using Acoustic Radiation Force Impulse (ARFI)-induced peak displacement (PD). The kidneys of three pigs were imaged in vivo at baseline, with venous ligation, and with arterial ligation, and then kidneys were extracted and imaged ex vivo. Imaging was performed with ARF excitation impulses having F/1.5 or F/5.0 focal configurations, and data were acquired with the transducer oriented along and across nephrons alignment in the renal cortex. In addition to ARFI PD, shear wave velocity was measured along and across nephrons to estimate longitudinal and transverse shear elastic moduli. Elastic anisotropy was then assessed as the ratio of PD across versus along nephrons, and as the ratio of shear moduli along versus across nephrons. PD ratio using the F/1.5 ARF linearly correlated with shear moduli ratio (R-2 = 0.95). However, PD ratio using the F/5.0 ARF was approximately 1.0 and had weak correlation to shear moduli ratio (R-2 = 0.56). Further, the average difference in PD measured along versus across nephrons was 2.91 mu m for the F/1.5 ARF but was 0.2 mu m for the F/5.0 ARF. These results suggest that the F/1.5 ARF excitation exploited elastic anisotropy in the renal cortex, while the F/5.0 ARF excitation obviated it.