On the Field Strength Dependence of Bi‐ and Triexponential Intravoxel Incoherent Motion (IVIM) Parameters in the Liver

Abstract

Background Studies on intravoxel incoherent motion (IVIM) imaging are carried out with different acquisition protocols. Purpose To investigate the dependence of IVIM parameters on the B0 field strength when using a bi‐ or triexponential model. Study Type Prospective. Study Population 20 healthy volunteers (age: 19–28 years). Field Strength/Sequence Volunteers were examined at two field strengths (1.5 and 3T). Diffusion‐weighted images of the abdomen were acquired at 24 b‐values ranging from 0.2 to 500 s/mm2. Assessment ROIs were manually drawn in the liver. Data were fitted with a bi‐ and a triexponential IVIM model. The resulting parameters were compared between both field strengths. Statistical Tests One‐way analysis of variance (ANOVA) and Kruskal–Wallis test were used to test the obtained IVIM parameters for a significant field strength dependency. Results At b‐values below 6 s/mm2, the triexponential model provided better agreement with the data than the biexponential model. The average tissue diffusivity was D = 1.22/1.00 μm2/msec at 1.5/3T. The average pseudodiffusion coefficients for the biexponential model were D* = 308/260 μm2/msec at 1.5/3T; and for the triexponential model D1* = 81.3/65.9 μm2/msec, D2* = 2453/2333 μm2/msec at 1.5/3T. The average perfusion fractions for the biexponential model were f = 0.286/0.303 at 1.5/3T; and for the triexponential model f1 = 0.161/0.174 and f2 = 0.152/0.159 at 1.5/3T. A significant B0 dependence was only found for the biexponential pseudodiffusion coefficient (ANOVA/KW P = 0.037/0.0453) and tissue diffusivity (ANOVA/KW: P < 0.001). Data Conclusion Our experimental results suggest that triexponential pseudodiffusion coefficients and perfusion fractions obtained at different field strengths could be compared across different studies using different B0. However, it is recommended to take the field strength into account when comparing tissue diffusivities or using the biexponential IVIM model. Considering published values for oxygenation‐dependent transversal relaxation times of blood, it is unlikely that the two blood compartments of the triexponential model represent venous and arterial blood. Level of Evidence: 1 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2019;50:1883–1892.

An optimized b‐value distribution for triexponential intravoxel incoherent motion (IVIM) in the liver

“…Table 1 provides an overview of currently published values. One thing to note is that D * 2 reported in Riexinger et al 15 is roughly by 1 order of magnitude larger than the D * 2 values reported in the other publications, 5,10,11,13,14 which most likely can be attributed to the lower minimal b-value used in Riexinger et al 15 The optimization performed here is based on the assumption that these large D * 2 values are a good representation of the real values and that consequently very small b-values, say below 1 s/mm 2 , are needed to sample such large D * 2 values. Triexponential IVIM parameters obtained with such very small b-values have so far been reported only for the liver to our knowledge.…”

“…24 A similar effect was observed for f 1 and f 2 , which both were reported to decrease by ~40% in the liver if the TE is reduced from 100 ms to 50 ms. 25,26 Therefore, we assume here that f 1 and f 2 scale identically with the TE and the following values were used in the optimization: f 1 , f 2 = {1.0 ⋅ (17.0%, 13.7%) , 0.8 ⋅ (17.0%, 13.7%) , 0.6 ⋅ (17.0%, 13.7%)} . The factors 1.0, 0.8, and 0.6 shall account for a TE range of ~45-100 ms. 24,26 The absolute values for f 1 and f 2 (i.e., 17.0% and 13.7%) were set to the mean of the values measured in the 10 first volunteers of Riexinger et al 15 Concerning the pseudo-diffusion coefficients, we assumed that their true values might be larger or smaller than reported in Riexinger et al 15 given their large fit uncertainty. Hence, D * 1 = 0.085mm 2 ∕s ⋅ {0.6, 1.0, 1.4} and D * 2 = 2.55mm 2 ∕s ⋅ {0.6, 1.0, 1.4} was used and all 9 combinations of these values were probed.…”

Age and gender dependence of liver diffusion parameters and the evidence of intravoxel incoherent motion modelling of perfusion component is constrained by diffusion component

“…Recently it was demonstrated that, with careful selection of b-values and improved image post-processing, PF and Dslow measurement can be quite reproducible, and Dfast can be moderately reproducible [3][4][5]. There are also strong evidences that the relationship between liver DWI signal and b-value does not follow bi-exponential decay; instead, it can be better fitted by an addition of very fast component with a tri-exponential decay model [5][6][7][8][9][10][11][12]. However, the fitting of tri-exponential decay model can be quite instable at individual study subject's level [6,8,13].…”

Value of intravoxel incoherent motion in detecting and staging liver fibrosis: A meta-analysis

“…Although still controversial, our findings together with most previous studies indicated that RT method offers no advantage in fitting IVIM parameters and could be substituted by FB method, which is usually more comfortable for the patients[ 45 - 47 ]. In addition, Riexinger et al[ 48 ] recently found that IVIM parameters of the liver showed a significant dependency on the applied field strength, hence we also conducted subgroup analysis in this regard. Commonly speaking, 3.0 T is much more sensitive to magnetic susceptibility induced artifacts and eddy current related distortion[ 37 ], however, our results indicated higher diagnostic performance of IVIM in 3.0 T scanners with AUC of 0.904, compared with 1.5 T scanners with AUC of 0.839.…”