Noninvasive In Vivo Assessment of Renal Tissue Elasticity During Graded Renal Ischemia Using MR Elastography

Objectives: Magnetic resonance elastography (MRE) allows noninvasive assessment of tissue stiffness in vivo. Renal arterial stenosis (RAS), a narrowing of the renal artery, promotes irreversible tissue fibrosis that threatens kidney viability and may elevate tissue stiffness. However, kidney stiffness may also be affected by hemodynamic factors. This study tested the hypothesis that renal blood flow (RBF) is an important determinant of renal stiffness as measured by MRE. Material and Methods: In 6 anesthetized pigs MRE studies were performed to determine cortical and medullary elasticity during acute graded decreases in RBF (by 20%, 40%, 60%, 80%, and 100% of baseline) achieved by a vascular occluder. Three sham-operated swine served as time control. Additional pigs were studied with MRE 6 weeks after induction of chronic unilateral RAS (n = 6) or control (n = 3). Kidney fibrosis was subsequently evaluated histologically by trichrome staining. Results: During acute RAS the stenotic cortex stiffness decreased (from 7.4 +/- 0.3 to 4.8 +/- 0.6 kPa, P = 0.02 vs. baseline) as RBF decreased. Furthermore, in pigs with chronic RAS (80% +/- 5.4% stenosis) in which RBF was decreased by 60% +/- 14% compared with controls, cortical stiffness was not significantly different from normal (7.4 +/- 0.3 vs. 7.6 +/- 0.3 kPa, P = 0.3), despite histologic evidence of renal tissue fibrosis. Conclusion: Hemodynamic variables modulate kidney stiffness measured by MRE and may mask the presence of fibrosis. These results suggest that kidney turgor should be considered during interpretation of elasticity assessments.