Cardiac Phase and Flow Compensation Effects on REnal Flow and Microstructure AnisotroPy MRI in Healthy Human Kidney

Background: Renal diffusion-weighted imaging (DWI) involves microstructure and microcirculation, quantified with diffusion tensor imaging (DTI), intravoxel incoherent motion (IVIM), and hybrid models. A better understanding of their contrast may increase specificity. Purpose: To measure modulation of DWI with cardiac phase and flow-compensated (FC) diffusion gradient waveforms. Study Type: Prospective. Population: Six healthy volunteers (ages: 22-48 years, five females), water phantom. Field Strength/Sequence: 3-T, prototype DWI sequence with 2D echo-planar imaging, and bipolar (BP) or FC gradients. 2D Half-Fourier Single-shot Turbo-spin-Echo (HASTE). Multiple-phase 2D spoiled gradient-echo phase contrast (PC) MRI. Assessment: BP and FC water signal decays were qualitatively compared. Renal arteries and velocities were visualized on PC-MRI. Systolic (peak velocity), diastolic (end stable velocity), and pre-systolic (before peak velocity) phases were identified. Following mutual information-based retrospective self-registration of DWI within each kidney, and Marchenko-Pastur Principal Component Analysis (MPPCA) denoising, combined IVIM-DTI analysis estimated mean diffusivity (MD), fractional anisotropy (FA), and eigenvalues (lambda i) from tissue diffusivity (D-t), perfusion fraction (f(p)), and pseudodiffusivity (D-p, D-p,D-axial, D-p,D-radial), for each tissue (cortex/medulla, segmented on b0/FA respectively), phase, and waveform (BP, FC). Monte Carlo water diffusion simulations aided data interpretation. Statistical Tests: Mixed model regression probed differences between tissue types and pulse sequences. Univariate general linear model analysis probed variations among cardiac phases. Spearman correlations were measured between diffusion metrics and renal artery velocities. Statistical significance level was set at P < 0.05. Results: Water BP and FC signal decays showed no differences. Significant pulse sequence dependence occurred for lambda(1), lambda(3), FA, D-p, f(p), D-p,D-axial, D-p,D-radial in cortex and medulla, and medullary lambda(2). Significant cortex/medulla differences occurred with BP for all metrics except MD (systole [P = 0.224]; diastole [P = 0.556]). Significant phase dependence occurred for D-p, D-p,D-axial, D-p,D-radial for BP and medullary lambda(1), lambda(2), lambda(3), MD for FC. FA correlated significantly with velocity. Monte Carlo simulations indicated medullary measurements were consistent with a 34 mu m tubule diameter. Data Conclusion: Cardiac gating and flow compensation modulate of measurements of renal diffusion.