Renal tubular dilation and fibrosis after unilateral ureter obstruction revealed by relaxometry and spin-lock exchange MRI

We evaluated the use of quantitative MRI relaxometry, including the dispersion of spin-lock relaxation with different locking fields, for detecting and assessing tubular dilation and fibrosis in a mouse model of unilateral ureter obstruction (UUO). C57BL/6 J and BALB/c mice that exhibit different levels of tubular dilation and renal fibrosis after UUO were subjected to MR imaging at 7 T. Mice were imaged before UUO surgery, and at 5, 10 and 15 days after surgery. We acquired maps of relaxation rates and fit the dispersion of spin-lock relaxation rates R-1 rho at different locking fields (frequencies) to a model of exchanging water pools, and assessed the sensitivity of the derived quantities for detecting tubular dilation and fibrosis in kidney. Histological scores for tubular dilation and fibrosis, based on luminal space and positive fibrotic areas in sections, were obtained for comparison. Histology detected extensive tubular dilation and mild to moderate fibrosis in the UUO kidneys, in which enlargement of luminal space, deposition of collagen, and reductions in capillary density were observed in the cortex and outer stripe of the outer medulla. Relaxation rates R-1, R-2 and R-1 rho clearly decreased in these regions of UUO kidneys longitudinally. While R-1 showed the highest detectability to tubular dilation and overall changes in UUO kidneys, S-rho, a parameter derived from R-1 rho dispersion data, showed the highest correlation with renal fibrosis in UUO. While relaxation parameters are sensitive to tubular dilation in UUO kidneys, S-rho depends primarily on the average exchange rate between water and other chemically shifted resonances such as hydroxyls and amides, and provides additional specific information for evaluating fibrosis in kidney disease.

Automated summary

Utilizing quantitative MRI relaxometry, particularly spin-lock relaxation dispersion analysis, shows promise in detecting and evaluating tubular dilation and fibrosis in a mouse model of UUO. Changes in relaxation rates provide valuable information on tubular dilation and fibrosis progression, with specific parameters like S-rho showing high correlations with renal fibrosis. This technique offers additional specific insights into fibrosis assessment in kidney disease beyond traditional histological methods.