Correcting B0 inhomogeneity-induced distortions in whole-body diffusion MRI of bone

Leonardino A Digma,Aaron B Simon,Christopher C Conlin,Ana E Rodríguez-Soto,Kanha Batra,Rebecca Rakow-Penner,Tyler M Seibert,Troy S Hussain,Christine H Feng,Asona J Lui,Joshua Kuperman,Anders M Dale,Allison Y Zhong,Roshan Karunamuni,Michael E Hahn

doi:10.1038/s41598-021-04467-2

Abstract

Diffusion-weighted magnetic resonance imaging (DWI) of the musculoskeletal system has various applications, including visualization of bone tumors. However, DWI acquired with echo-planar imaging is susceptible to distortions due to static magnetic field inhomogeneities. This study aimed to estimate spatial displacements of bone and to examine whether distortion corrected DWI images more accurately reflect underlying anatomy. Whole-body MRI data from 127 prostate cancer patients were analyzed. The reverse polarity gradient (RPG) technique was applied to DWI data to estimate voxel-level distortions and to produce a distortion corrected DWI dataset. First, an anatomic landmark analysis was conducted, in which corresponding vertebral landmarks on DWI and anatomic T2-weighted images were annotated. Changes in distance between DWI- and T2-defined landmarks (i.e., changes in error) after distortion correction were calculated. In secondary analyses, distortion estimates from RPG were used to assess spatial displacements of bone metastases. Lastly, changes in mutual information between DWI and T2-weighted images of bone metastases after distortion correction were calculated. Distortion correction reduced anatomic error of vertebral DWI up to 29 mm. Error reductions were consistent across subjects (Wilcoxon signed-rank p < 10–20). On average (± SD), participants’ largest error reduction was 11.8 mm (± 3.6). Mean (95% CI) displacement of bone lesions was 6.0 mm (95% CI 5.0–7.2); maximum displacement was 17.1 mm. Corrected diffusion images were more similar to structural MRI, as evidenced by consistent increases in mutual information (Wilcoxon signed-rank p < 10–12). These findings support the use of distortion correction techniques to improve localization of bone on DWI.

Highlights

Diffusion-weighted magnetic resonance imaging (DWI) of the musculoskeletal system has wide-ranging applications, such as visualization of vertebral fractures, ligament tears, and assessment of bone quality with aging and osteoporosis[1,2]
This acquisition reduces the echo-planar imaging (EPI) readout time, which in turn limits the time allowed for the spin dephasing that leads to B0 inhomogeneity distortions
We used reverse polarity gradient (RPG) to estimate the magnitude of these distortions in images of bone and to determine whether correcting for these distortions would improve anatomic correlation of skeletal DWI with T2-weighted images

Summary

Introduction

Diffusion-weighted magnetic resonance imaging (DWI) of the musculoskeletal system has wide-ranging applications, such as visualization of vertebral fractures, ligament tears, and assessment of bone quality with aging and osteoporosis[1,2]. Assessing response to medical or radiation treatment demands highly accurate localization and measurement of lesion size, both of which may be affected by distortions induced by B0 inhomogeneity. The resulting field is used to correct the diffusion data set Both FSL’s topup and the reverse polarity gradient (RPG) technique are specific implementations of this a pproach[11,12,15]. The latter, R PG12, has been applied to DWI of prostate[16], breast tissue[17,18], and brain[12,19], and has been shown to improve anatomic localization. We performed a secondary analysis to explore how these distortions affect localization of bone metastases

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Conclusion