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 $D_1^{*}$ = 81.3/65.9 μm² /msec, $D_2^{*}$ = 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.

Keywords: IVIM; intravoxel incoherent motion; liver.