A method to measure renal inner medullary perfusion using MR renography

Objective: In the kidney, the medulla is most susceptible to damage in case of hampered perfusion or oxygenation. Due to separate regulation of cortical and medullary perfusion, measurement of both is crucial to improve the understanding of renal pathophysiology. We aim to develop and evaluate a physiologically accurate model to measure renal inner medullary (F_med) and cortical perfusion (F_cor) separately. Materials and methods: We developed a 7-compartment model of renal perfusion and used an iterated approach to fit 10 free parameters. Model stability and accuracy were tested on both patient data and simulations. Cortical perfusion and F_T (tubular flow or glomerular filtration rate per unit of tissue volume) were compared to a conventional 2-compartment filtration model. Results: Average (standard deviation) F_med was 37(23)mL/100 mL/min. Fitting stability as expressed by the median (interquartile range) coefficient of variation between fits was 0.0(0.0–5.8)%, with outliers up to 81%. In simulations, F_med was underestimated by around 8%. Intra-class correlation coefficients for F_cor and F_T as measured with the 2- and 7- compartment model were 0.87 and 0.63, respectively. Discussion: We developed a pharmacokinetic model closely following renal physiology. Although the results were vulnerable for overfitting, relatively stable results could be obtained even for F_med.