Correcting unwanted diffusion weighting in diffusion-weighted-STEAM sequence for time-dependent diffusion kurtosis imaging

Purpose

This study proposes an optimized acquisition protocol to minimize the unwanted diffusion weighting in DW-STEAM and facilitate time-dependent diffusion kurtosis imaging (tDKI).

Methods

We first corrected the diffusion-direction-dependent shift by optimizing the diffusion gradient amplitude. We then proposed to use low-b-value in the reference image, instead of the conventional non-diffusion-weighted (b0) acquisition, and removed the crusher gradients. The tDKI measurements from the proposed strategy were compared with conventional DWIs that included crushers in the b0 image, in a water phantom and in healthy adults (n = 8) on 3 T, and the water exchange time (τ_ex) was calculated from the tDKI measurements. The optimal strategy was tested in five ex vivo human brains, and the results were compared with in vivo data.

Results

Neglecting the diffusion weighting in b0 images introduced an artificial time dependence in the apparent diffusivity of the water phantom, and resulted in elevated estimates of water exchange time (τ_ex) in in the in vivo data. Additionally, the AIC indicated that, even when diffusion weighting in the b0 images was accounted for, the kurtosis estimation remained less stable in the in vivo than with the crusher-free approach. In contrast, using low-b-value images as reference measurements yielded reasonable tDKI estimates, with τ_ex ranging from 15 to 40 ms for gray matter in vivo and from 20 to 60 ms for gray matter ex vivo.

Conclusions

The optimized DW-STEAM acquisition eliminated artificial diffusion-time dependence, enabling the acquisition of accurate tDKI data for mapping structural morphology and transmembrane permeability.