Assessment of intravoxel incoherent motion MRI with an artificial capillary network: analysis of biexponential and phase-distribution models.

Purpose: To systematically analyze intravoxel incoherent motion (IVIM) MRI in a perfusable capillary phantom closely matching the geometry of capillary beds in vivo and to compare the validity of the biexponential pseudo-diffusion and the recently introduced phase-distribution IVIM model.

Methods: IVIM-MRI was performed at 12 different flow rates ( ) in a capillary phantom using 4 different DW-MRI sequences (2 with monopolar and 2 with flow-compensated diffusion-gradient schemes, with up to values between and ). Resulting parameters from the assessed IVIM models were compared to results from optical microscopy.

Results: The acquired data were best described by a static and a flowing compartment modeled by the phase-distribution approach. The estimated signal fraction of the flowing compartment stayed approximately constant over the applied flow rates, with an average of in excellent agreement with optical microscopy ( ). The estimated average particle flow speeds showed a highly significant linear correlation to the applied flow. The estimated capillary segment length of approximately agreed well with optical microscopy measurements. Using the biexponential model, the signal fraction was substantially underestimated and displayed a strong dependence on the applied flow rate.

Conclusion: The constructed phantom facilitated the detailed investigation of IVIM-MRI methods. The results demonstrate that the phase-distribution method is capable of accurately characterizing fluid flow inside a capillary network. Parameters estimated using the biexponential model, specifically the perfusion fraction , showed a substantial bias because the model assumptions were not met by the underlying flow pattern.