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

Abstract

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 (0.2...2.4 mL/min) in a capillary phantom using 4 different DW-MRI sequences (2 with monopolar and 2 with flow-compensated diffusion-gradient schemes, with up to 16 b values between 0 and 800 s/mm2). 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 f of the flowing compartment stayed approximately constant over the applied flow rates, with an average of f = 0.451 ± 0.023 in excellent agreement with optical microscopy (f = 0.454 ± 0.002). The estimated average particle flow speeds v=0.25...2.7 mm/s showed a highly significant linear correlation to the applied flow. The estimated capillary segment length of approximately 189μm agreed well with optical microscopy measurements. Using the biexponential model, the signal fraction f 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 f, showed a substantial bias because the model assumptions were not met by the underlying flow pattern.