René van der Bel

| Chapter 6 | 100 Abstract In the kidneys, there is both blood flow through the capillaries and flow of pre-urine through the tubuli and collecting ducts. We hypothesized that diffusion-weighted (DW) MRI mea- sures both blood and pre-urine flow when using a tri-exponential intravoxel incoherent motion (IVIM) model. Our aim was to systematically investigate and optimize tri-exponential IVIM-analysis for the kidney and test its sensitivity to renal perfusion changes in humans. The tri-exponential fit probes the diffusion coefficient ( D ), the intermediate ( D * i ) and fast ( D * f ) pseudo-diffusion coefficients, and their signal fractions, f D , f i and f f . First, we studied the effects of fixing the D *-coefficients of the tri-exponential fit using in silico simulations. Then, using a 3T MRI scanner, DW images were acquired in healthy subjects (18–24 years) and we assessed the within-subject coefficient of variation (wsCV, n=6). Then, renal perfusion was modulated by Angiotensin II infusion during which DW imaging of the kidneys and phase contrast MRI of the renal artery was performed (n=8). Radioisotope clearing tests were used to assess the glomerular filtration rate. Simulations showed that fixing the D *-coefficients could potentially increase the fit stability, in fact decreased the precision of the model. Changes in D *-coefficients were translated into the f -parameters instead. Fixing D *-coefficients resulted in a stronger response of the fit pa- rameters to the intervention. Using this model, the wsCVs for D , f D , f i and f f were 2.4%, 0.8%, 3.5%, 19.4% respectively. f i decreased by 14% (p=0.059) and f f increased by 32% (p=0.004) between baseline and maximal Angiotensin II dose. f f inversely correlated to renal plasma flow (R=-0.70, p<0.01) and f i correlated to glomerular filtration rate (R=0.39, p=0.026). We validated kidney-specific method for IVIM analysis using a tri-exponential model. The model is able to track renal perfusion changes induced by Angiotensin II.