  Purpose: To assess the utility of Laplacian fitting to describe the
differences in hyperpolarized 13C urea T2 relaxation in ischemic and healthy
rodent kidneys. Theory and Methods: Six rats with unilateral renal ischemia
were investigated. 13C urea T2 mapping was undertaken with a radial fast spin
echo method, with subsequent post-processing performed with regularised
Laplacian fitting. Results: Simulations showed that Laplacian fitting was
stable down to a signal to noise ratio of 20. In vivo results showed a
significant increase in the mono- and decrease in bi-exponential pools in IRI
kidneys, in comparison to healthy (14+-10% vs 4+-2%, 85+-10% vs 95+-3%,
p<0.05). Conclusion: We demonstrate, for the first time, the differences in
multi-exponential behaviour of 13C,15N2-urea between the healthy and ischemic
rodent kidney. The distribution of relaxation pools were found to be both
visually and numerically significantly different. The ability to improve the
information level in hyperpolarized MR, by utilizing the relaxation contrast
mechanisms is an appealing option, that can easily be adopted in large animals
and even in clinical studies in the near future.
