Can the low and high b-value distribution influence the pseudodiffusion parameter derived from IVIM DWI in normal brain?

Abstract

Background: Our study aims to reveal whether the low b-values distribution, high b-values upper limit, and the number of excitation (NEX) influence the accuracy of the intravoxel incoherent motion (IVIM) parameter derived from multi-b-value diffusion-weighted imaging (DWI) in the brain.

Methods: This prospective study was approved by the local Ethics Committee and informed consent was obtained from each participant. The five consecutive multi-b DWI with different b-value protocols (0-3500 s/mm²) were performed in 22 male healthy volunteers on a 3.0-T MRI system. The IVIM parameters from normal white matter (WM) and gray matter (GM) including slow diffusion coefficient (D), fast perfusion coefficient (D*) and perfusion fraction (f) were compared for differences among defined groups with different IVIM protocols by one-way ANOVA.

Results: The D* and f value of WM or GM in groups with less low b-values distribution (less than or equal to 5 b-values) were significantly lower than ones in any other group with more low b-values distribution (all P < 0.05), but no significant differences among groups with more low b-values distribution (P > 0.05). In addition, no significant differences in the D, D* and f value of WM or GM were found between group with one and more NEX of low b-values distribution (all P > 0.05). IVIM parameters in normal WM and GM strongly depended on the choice of the high b-value upper limit.

Conclusions: Metrics of IVIM parameters can be affected by low and high b value distribution. Eight low b-values distribution with high b-value upper limit of 800-1000 s/mm² may be the relatively proper set when performing brain IVIM studies.

Keywords: B-value; Brain; Diffusion weighted imaging; Intravoxel incoherent motion; Number of excitation; Pseudodiffusion.

Fig. 1

Flow diagram shows subjects selection process and multi-b-value DWI protocol. Note: Each set of low b-values was in addition to a high b-value distribution of 300, 500, 800, 1000, 1500, 2000, 3000 and 3500 s/mm², with NEX of 2,2,2,2,2,4,4 and 6, respectively

Fig. 2

Comparison of the bi- and mono-exponential fitting of the diffusion signal decay over a wide-range of b-values (up to 1000 s/mm²) in normal brain, with more obvious pseudodiffusion effect in GM than in WM. Note: Axial diffusion-weighted trace image (b = 1000 s/mm²) shows ROIs placed in right frontal white matter (WM) and gray matter (GM)

Fig. 3

Brain pseudodiffusion parametric maps from one representative subject. The top row depicts D* maps (a1-d1) calculated by 4 sets of multiple b-value DWI with different high b-value upper limit (500, 800, 1000, 1500 s/mm², respectively) and the same low b-value distribution (0, 30, 60, 90, 120, 150, 180, 200 s/mm²), meanwhile the bottom row shows corresponding f maps (a2-d2) derived from IVIM DWI model in normal brain. As the increase of high b-value upper limit, the red color area representing fast diffusion effect in brain tissue was reduced on pseudodiffusion images, reflecting increased fast diffusion effect. At the same time, much red color area was displayed at 500 s/mm² high b-value upper limit (a1 or a2), and relative absent red area on pseudodiffusion images when high b-value upper limit reached 1500 s/mm² (d1 or d2)

Fig. 4

Box plots for pseudodiffusion parameter of D* (a) and f (b) derived from IVIM DWI among groups with different distribution of low b-value and NEX in brain white matter (WM) and gray matter (GM)

Fig. 5

Box plots for pseudodiffusion parameter of D* (a) and f (b) derived from IVIM DWI among groups with different high b-value upper limit in brain white matter (WM) and gray matter (GM)