Investigation of diffusion time dependence of apparent diffusion coefficient and intravoxel incoherent motion parameters in the human kidney.

Purpose: To characterize the diffusion time (Δ_eff) dependence of apparent diffusion coefficient (ADC) and intravoxel incoherent motion-related parameters in the human kidney at 3 T.

Methods: Sixteen healthy volunteers underwent an MRI examination at 3 T including diffusion-weighted imaging at different Δ_eff ranging from 24.1 to 104.1 ms. The extended mono-exponential ADC and intravoxel incoherent motion models were fitted to the data for each Δ_eff and the medullary and cortical ADC, (pseudo-)diffusion coefficients (D* and D) and flow-related signal fraction (f) were calculated.

Results: When all the data were used for fitting, a significant trend toward higher ADC with increasing Δ_eff was observed between 24.1 and 104.1 ms (median and interquartile range: 2.38 [2.19, 2.47] to 2.84 [2.36, 2.90] × 10^-3 mm²/s for cortex, and 2.28 [2.18, 2.37] to 2.82 [2.58, 3.11] × 10^-3 mm²/s for medulla). In contrast, no significant differences in ADC were found when only the data acquired at b-values higher than 200 s/mm² were used for fitting. When the intravoxel incoherent motion model was applied, cortical and medullary f increased significantly (cortex: 0.21 [0.15 0.27] to 0.37 [0.32, 0.49] × 10^-3 mm²/s; medulla: 0.15 [0.13 0.29] to 0.41 [0.36 0.51] × 10^-3 mm²/s). No significant changes in cortical and medullary D and D* were observed as diffusion time increased.

Conclusion: Renal perfusion and tubular flow substantially contribute to the observed increase in ADC over a wide range of Δ_eff between 24 and 104 ms.