A New Approach for Estimating Liver IVIM Fast Diffusion Coefficient in Two-Compartment Model and Fast Diffusion Fraction in Three-Compartment Model: Theory and In Vivo Validation of Two Centers' Data.

The intravoxel incoherent motion (IVIM) three-compartment model (3CM) potentially offers better classification for perfusion-rich pathologies. However, 3CM is often associated with less fitting stability than the two-compartment model (2CM). For liver intravoxel IVIM data, when the fit-starting b-value is adjusted from 2 s/mm2 to bi s/mm2, PFi, which indicates the perfusion fraction at bi, gradually decreases as the initial fitting b-value bi increases and eventually approaches zero. We define Yi = ln [(1 - PFi)/PFi]. With a fitted line between Yi and bi, the intercept with the y-axis is the predicted value of Y0 that follows the bi-exponential model. The PF0 calculated from it eliminates the signals of other very rapid perfusion components and retains a more stable perfusion fraction measurement. The slope of the fitted line is (Dfast-Dslow). Dslow can be calculated by the standard IVIM model. Because the Yi fitting assumes that the signal attenuation is bi-exponential, and the portion of the signal strength that exceeds the bi-exponential model at very low b-values is not used, the perfusion fraction detected is the fast decay perfusion component (Ffast) without the very fast decay perfusion component (Fvfast). In our study on healthy livers, Dataset-1 with 3.0 T data had 17 subjects scanned twice with a 10-12 days' interval, and paired scan-rescan had 13 subjects; Dataset-2 with 1.5 T data had 20 subjects scanned twice during the same scan session, and paired scan-rescan had 17 subjects. For the 2CM, IVIM parameters Dslow, Dfast, and PFi were obtained with a nonlinear least square (NLLSQ) fit. Parameters obtained with segmented fitting were used for Yi analysis. Results show that conventional fitting and Yi fitting derived very comparable IVIM parameters in mean value. Dfast obtained with Yi fitting showed better scan-rescan stability than conventional segmented fitting. Perfusion fraction parameters showed more favorable scan-rescan stability with Yi fitting.