Assessment of renal artery stenosis using intravoxel incoherent motion diffusion-weighted magnetic resonance imaging analysis

Objectives: Diffusion-weighted magnetic resonance imaging is a powerful tool to assess renal morphology. However, its quantitative index, apparent diffusion coefficient (ADC), derived from a conventional monoexponential model can vary with both functional and structural alterations as well as the choice of b values. In contrast, the intravoxel incoherent motion (IVIM) biexponential model provides independent parameters that may represent broader aspects of renal pathophysiology. We hypothesized that IVIM analysis is capable of detecting early morphological and functional changes in the swine kidney distal to renal artery stenosis (RAS). Materials and Methods: Domestic pigs divided into 3 groups (n = 6-7 each) were studied for 16 weeks. Unilateral RAS was induced in 2 groups, of which 1 group was fed with a high-cholesterol diet to induce early atherosclerosis (ARAS), whereas the other (RAS) consumed regular diet. The third group included healthy pigs that served as control sham. Renal function, hemodynamics, tubular function, and morphology were assessed using multidetector computed tomography and histology. Diffusion-weighted magnetic resonance images were acquired at 3T and analyzed using monoexponential and biexponential models. Parameters of ADC and IVIM (diffusivity [D_t], flow-dependent pseudodiffusivity [D_p], and fluid fraction [f_p]) were calculated in the cortex and medulla of the stenotic (STK) and contralateral kidneys (CLKs). Results were analyzed using analysis of variance, Student t test, and regression analysis. Results: In both RAS and ARAS, the STK shrank and the CLK underwent hypertrophy. Glomerular filtration rate and renal blood flow declined in STKs, and CLKs manifested hyperfiltration. In addition, ARAS kidneys showed reduced mean transit time in distal tubular segments. Apparent diffusion coefficient and diffusivity both decreased in STK of RAS and ARAS. D_p and f_p were elevated in both the STK and CLK of RAS and more prominently in ARAS. The STK cortical ADC and D_t correlated inversely with the degree of fibrosis and directly with glomerular filtration rate. Furthermore, D_p correlated with tubular injury score in all kidneys. Conclusions: Apparent diffusion constant and Dt both correlated with cortical and medullary fibrosis; however, IVIM-derived parameters can detect subtle functional and structural changes in the post-STK and may also serve as markers for tubular injury.