Characterization of fast and slow diffusion from diffusion-weighted MRI of pediatric Crohn's disease

Abstract

Purpose: To characterize fast and slow diffusion components in diffusion-weighted magnetic resonance imaging (DW-MRI) of pediatric Crohn's disease (CD). Overall diffusivity reduction as measured by the apparent diffusion coefficient (ADC) in patients with CD has been previously demonstrated. However, the ADC reduction may be due to changes in either fast or slow diffusion components. In this study we distinguished between the fast and slow diffusion components in the DW-MRI signal decay of pediatric CD.

Materials and methods: We acquired MRI from 24 patients, including MR enterography (MRE) and DW-MRI with 8 b-values (0-800 s/mm(2)). We characterized fast and slow diffusivity by intravoxel incoherent motion (IVIM) model parameters (f, D*, D), and overall diffusivity by ADC values. We determined which model best described the DW-MRI signal decay. We assessed the influence of the IVIM model parameters on the ADC. We evaluated differences in model parameter values between the enhancing and nonenhancing groups.

Results: The IVIM model described the observed data significantly better than the ADC model (P = 0.0088). The ADC was correlated with f (r = 0.67, P = 0.0003), but not with D (r = 0.39, P = 0.062) and D* (r = -0.39, P = 0.057). f values were significantly lower (P < 0.003) and D* values were significantly higher (P = 0.03) in the enhancing segments, while D values were not significantly different between the groups (P = 0.14).

Conclusion: For this study population the IVIM model provides a better description of the DW-MRI signal decay than the ADC model. The reduced ADC is related to changes in the fast diffusion rather than to changes in the slow diffusion.

Fig. 1

Scatter plots of the ADC_all values versus: (A) the IVIM fast diffusion fraction parameter (f); (b) the IVIM fast diffusion parameter (D*), and; (C) the IVIM slow diffusion parameter (D). The black line represents the trend as modeled using linear regression. The ADC_all values are strongly correlated with the f values while having only non-significant correlation with the D* and D values.

Fig. 2

Representative example of an enhanced patient. (A) the DW-MRI image (b-value=600 s/mm²) with bright region in the abnormal ileum (yellow arrow); (B) the ADC_0-600 map presents restricted diffusion (dark region) in the abnormal ileum; (C) the IVIM-D map shows similar value to the ADC_0-600; (D)–(E) the IVIM-D* and IVIM-f maps, and; (F) the signal decay plot shows almost no fast-decay compartment.

Fig. 3

Representative example of a nonenhanced patient. (A) the DW-MRI image (b-value=600 s/mm²) with normal region in the ileum (yellow arrow); (B) the ADC_0-600 map presents normal diffusion (bright region) in the ileum; (C) the IVIM-D map shows, however, much lower value as compared to the ADC_0-600; (D) – (E) the IVIM-D* and IVIM-f maps show high values of the fast diffusion components, and; (F) the signal decay plot shows that most of the signal decay is associated with the fast diffusion component rather than with the slow diffusion component.

Fig. 4

Box-plot representation of the DW-MRI parameters values distributions for non-enhancing and enhancing ileal segments. The overall diffusion variations between the enhancing and non-enhancing ileal segments as measured by the ADC (A–C) are eliminated when calculating slow diffusion measurements using the IVIM model (D). The variations in the fast diffusion components (E–F) are the actual contributors to the overall diffusion variation as observed in the ADC model.