Abstract
OBJECTIVE: Clinical hepatic diffusion weighted imaging (DWI) generally relies on mono-exponential diffusion. The aim was to demonstrate that mono-exponential diffusion in the liver is contaminated by microperfusion and that the bi-exponential model is required. METHODS: Nineteen fasting healthy volunteers were examined with DWI (seven b-values) using fat suppression and respiratory triggering (1.5 T). Five different regions in the liver were analysed regarding the mono-exponentially fitted apparent diffusion coefficient (ADC), and the bi-exponential model: molecular diffusion (D (slow)), microperfusion (D (fast)) and the respective fractions (f (slow/fast)). Data were compared using ANOVA and Kruskal-Wallis tests. Simulations were performed by repeating our data analyses, using just the DWI series acquired with b-values approximating those of previous studies. RESULTS: Median mono-exponentially fitted ADCs varied significantly (P < 0.001) between 1.107 and 1.423 × 10(-3) mm(2)/s for the five regions. Bi-exponential fitted D(slow) varied between 0.923 and 1.062 × 10(-3) mm(2)/s without significant differences (P = 0.140). D (fast) varied significantly, between 17.8 and 46.8 × 10(-3) mm(2)/s (P < 0.001). F-tests showed that the diffusion data fitted the bi-exponential model significantly better than the mono-exponential model (F > 21.4, P < 0.010). These results were confirmed by the simulations. CONCLUSION: ADCs of normal liver tissue are significantly dependent on the measurement location because of substantial microperfusion contamination; therefore the bi-exponential model should be used. KEY POINTS: Diffusion weighted MR imaging helps clinicians to differentiate tumours by diffusion properties. Fast moving water molecules experience microperfusion, slow molecules diffusion. Hepatic diffusion should be measured by bi-exponential models to avoid microperfusion contamination. Mono-exponential models are contaminated with microperfusion, resulting in apparent regional diffusion differences. Bi-exponential models are necessary to measure diffusion and microperfusion in the liver.