Tomoelastography of the native kidney: Regional variation and physiological effects on in vivo renal stiffness.

PURPOSE: To measure normal renal stiffness in adults, taking into account regional variation, hydration, and urinary status.
METHODS: Thirty-six healthy volunteers were examined by tomoelastography based on MR elastography at four frequencies, from 40 to 70 Hz and multifrequency shear wave speed recovery. Regional wave speeds were derived for the medulla, cortex (inner cortex and outer cortex), and renal pelvis, and examined for age-related effects. Subgroups were repeatedly examined for reproducibility, amount of prior water drinking, and urinary status. Variations in renal perfusion were simulated ex vivo using a porcine kidney subjected to venous water inflow at different pressures.
RESULTS: Shear wave speed (stiffness) of renal parenchyma was 2.46 ± 0.12 m/s (inner cortex: 2.91 ± 0.17 m/s; outer cortex: 2.52 ± 0.11 m/s; medulla: 2.15 ± 0.08 m/s) without side differences and a tendency toward softening with age (P = 0.028). Corresponding intraclass correlation for reproducibility coefficients were 0.78 (inner cortex: 0.80; outer cortex: 0.81; medulla: 0.80). Water drinking resulted in slightly higher values in inner cortex and lower values in medulla (both P = 0.039), which was consistent with the results in perfused specimens. A full bladder led to higher renal pelvis stiffness (P = 0.004), whereas renal parenchyma remained uninfluenced. Stiffness of the porcine renal cortex increased with venous inflow pressure, whereas medulla stiffness decreased.
CONCLUSIONS: Tomoelastography provides full field of view maps of renal stiffness with highly detailed resolution and sensitivity to physiological effects related to age and fluid-solid tissue interactions. These basic data could be used to compare pathological conditions in the future. Magn Reson Med 79:2126-2134, 2018.