895 Eur Radiol (2012) 22:891–899 Table 3 Mono- and bi-exponential fitting results for five regions in the liver Mono-exponential fit Bi-exponential fit Radj2≥0.479 Segment ADC (×10−3 mm2/s) Region 1 2 3 4 5 P-value Discussion 5/8 6/7 2/3 7 5 1.107±0.101 1.204±0.055 1.423±0.118 1.239±0.090 1.107±0.102 <0.001* Radj2≥0.969 ffast (%) 29±5 24±3 47±9 24±5 29±4 <0.001† Dfast (×10−3 mm2/s) 33.5±12.1 37.8±11.0 17.8±7.1 46.8±19.8 43.6±9.7 <0.001† fslow (%) 71±5 76±3 53±9 76±5 71±4 <0.001† Dslow (×10−3 mm2/s) 0.923±0.148 1.038±0.052 0.900±0.228 1.062±0.118 0.954±0.120 0.140* The aim of this study was to demonstrate that mono- exponential diffusion in the liver is contaminated by microperfusion. We suspected that variation of ADCs in different regions of the liver is due to microperfusion contamination and that therefore the bi-exponential model is required. We found that the ADC of normal liver tissue is substantially dependent on the location; however, the bi- exponentially fitted Dslow is not dependent on the location of the measurement. We used terminology according to the suggestions of Guiu and Cercueil who convincingly advocated for the use of Dslow to describe molecular diffusion properties and Dfast to describe microperfusion [31]. Our results agree with earlier studies reporting signifi- cant differences among ADCs obtained in different regions in the liver [19, 21, 25, 26, 30]. ADCs in our study (1.107 to 1.423 × 10−3 mm2/s) were consistent with those of previous reports, which showed a large range (0.69 to 2.69 × 10−3 mm2/s) of ADCs for normal liver tissue [15, 25, 32]. The results on Dslow were supported by Luciani et al.; they also found no significant differences between Dslow of normal liver tissue [13]. They found that Dslow varied between 1.02 and 1.16 × 10−3 mm2/s; this is comparable to our range (0.98 to 1.18 × 10−3 mm2/s) when we simulated their results using our data and similar b-values. The demonstrated regional dependence of the ADC contrary to the non-regional dependence of Dslow can be explained from the differences between the mono- and the bi-exponential models. The mono-exponentially fitted ADC is to a high degree sensitive to microperfusion, which was already shown by Le Bihan et al. in the brain [2]. When the DWI sequence contains b-values in the microperfusion range, and the microperfusion is relatively high compared with the diffusion component, then the model will result in a high ADC compared with Dslow. When there is no microperfusion, which has been reported for fibroglandular breast tissue, the ADC will be comparable to Dslow [33]. Hence the ADC of the liver is to a large extent dependent Data are medians ± median absolute deviations. Both mono- and bi-exponential fits were determined with seven b-values (0, 50, 100, 250, 500, 750 and 1000). Radj2 is the goodness-of-fit correlation coefficient. P values indicate the significance of differences between different regions in the liver. *One-way ANOVA. †Non-parametric Kruskal–Wallis Fig. 2 An example of ADC and Dslow maps of the medial liver. a ADC map (left) and Dslow map (right) of the medial liver which were scaled equally from 0.1×10−3 to 2×10−3 mm2/s. The Dslow map shows a more homogeneous distribution of the bi-exponential fitted slow diffusion component throughout the liver. The ADC map shows significantly different values between different regions of the liver, especially near blood vessels and in the left lobe. b The anatomical position of the ADC and Dslow maps