A feasible study on using multiplexed sensitivity-encoding to reduce geometric distortion in diffusion-weighted echo planar imaging.

PURPOSE

Single-shot (SS) echo planar imaging (EPI) is commonly used for diffusion-weighted imaging (DWI) in radiation treatment planning. The geometric distortion of single-shot diffusion-weighted EPI (SS-DWEPI) limits its utility in precision radiotherapy. We aimed to investigate the use of multiplexed sensitivity-encoding DWEPI (MUSE-DWEPI) to reduce the geometric distortions to allow its use in brain radiotherapy.

METHODS

In a phantom study, phantom diameters measured using MUSE-DWEPI and SS-DWEPI were compared and the percentages of geometric distortion (%GD) were calculated. The shifting vectors of control points were also plotted and calculated. In a patient study, ten patients (six with post-surgery glioma, four with brain metastases) requiring MRI were enrolled, and the image distortion levels in SS-DWEPI and MUSE-DWEPI were compared using T2 Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (T2 PROPELLER) images as the reference. The tumor targets and four brain regions were delineated based on T2 PROPELLER, SS-DWEPI, and MUSE-DWEPI, using the Dice similarity coefficient (DSC) and the Hausdorff distance (HD) to quantify the level of geometric distortion.

RESULTS

In the phantom study, the most prominent image distortion was along the phase encoding direction in terms of %GD fluctuated from 7%-8% for SS-DWEPI (b = 0 s/mm and 1000 s/mm) and fluctuated from 2%-3% for MUSE-DWEPI (b = 0 s/mm and 1000 s/mm) with different positions and b values. The mean relative displacement of all control points was 4.45 ± 3.44 mm for SS-DWEPI b and 2.17 ± 1.9 mm for MUSE-DWEPI b close to the isocenter. Increasing the distance away from the isocenter in the z direction, the distortion increased more in SS-DWEPI. For all brain regions and targets involved, higher DSC values and lower HDs were obtained using MUSE-DWEPI than with SS-DWEPI (p < .01). The mean improvement in HD when switching from SS-DWEPI to MUSE-DWEPI was 3.65 ± 1.31 mm.

CONCLUSIONS

MUSE-DWEPI is an improvement upon SS-DWEPI in geometric distortion reduction, which might be a promising application strategy for DWI in radiotherapy.