Simplified intravoxel incoherent motion diffusion-weighted MRI of liver lesions: feasibility of combined two-colour index maps

Background: To evaluate the feasibility of two-colour index maps containing combined diffusion and perfusion information from simplified intravoxel incoherent motion (IVIM) for liver lesion malignancy assessment.

Methods: Diffusion-weighted data from a respiratory-gated 1.5-T magnetic resonance sequence were analysed in 109 patients with liver lesions. With three b values (0, 50, 800 s/mm²) estimated diffusion coefficient D', perfusion fraction f', and apparent diffusion coefficient (ADC) maps were calculated and analysed for regions of interest (ROIs). D' and f' cutoff values were determined by differentiating haemangiomas from other lesions and focal nodular hyperplasias from other lesions, respectively. Combined I_Df index maps were generated with a voxel value set to 100, if both D' and f' voxel values were lower than their cutoff values (1,529.4 × 10^-6 mm²/s and 114.4 × 10^-3, respectively), otherwise to 0. Moreover, I_ADC index maps were generated from ADC cutoff value (1,338.5 × 10^-6 mm²/s) obtained by differentiating benign from malignant lesions. Discriminatory power was assessed for both I_Df and I_ADC. Index maps were displayed as two-colour overlays to b-800 images and visually assessed within the translucent hyperintense areas.

Results: For I_Df, the same diagnostic accuracy was achieved as for the combined use of parameters D' and f' (93.6%). Compared to I_ADC, I_Df showed a higher diagnostic accuracy. Visual judgment of I_Df yielded an accuracy (95.4%) similar to that of quantitative analysis (93.6%).

Conclusion: Voxel-wise combined two-colour index maps I_Df provide similar diagnostic accuracy as ROI-based combination of estimated IVIM parameters D' and f' and are suitable for visual assessment of liver lesion malignancy.