Susceptibility-induced distortion that varies due to motion: Correction in diffusion MR without acquiring additional data

Jesper L.R Andersson,Mark S Graham,Ivana Drobnjak,Hui Zhang,Jon Campbell

doi:10.1016/j.neuroimage.2017.12.040

Jesper L.R Andersson, Mark S Graham + Show 3 more

Open Access

https://doi.org/10.1016/j.neuroimage.2017.12.040

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Abstract

Because of their low bandwidth in the phase-encode (PE) direction, the susceptibility-induced off-resonance field causes distortions in echo planar imaging (EPI) images. It is therefore crucial to correct for susceptibility-induced distortions when performing diffusion studies using EPI. The susceptibility-induced field is caused by the object (head) disrupting the field and it is typically assumed that it remains constant within a framework defined by the object, (i.e. it follows the object as it moves in the scanner).However, this is only approximately true. When a non-spherical object rotates around an axis other than that parallel with the magnetic flux (the z-axis) it changes the way it disrupts the field, leading to different distortions. Hence, if using a single field to correct for distortions there will be residual distortions in the volumes where the object orientation is substantially different to that when the field was measured.In this paper we present a post-processing method for estimating the field as it changes with motion during the course of an experiment. It only requires a single measured field and knowledge of the orientation of the subject when that field was acquired. The volume-to-volume changes of the field as a consequence of subject movement are estimated directly from the diffusion data without the need for any additional or special acquisitions. It uses a generative model that predicts how each volume would look predicated on field change and inverts that model to yield an estimate of the field changes. It has been validated on both simulations and experimental data. The results show that we are able to track the field with high accuracy and that we are able to correct the data for the adverse effects of the changing field.

Highlights

When an object, such as the human head, is placed in a homogeneous magnetic field it will disrupt that field and it will no longer be homogeneous
Looking at movies of the simulated data (Fig. S1 in the supplementary material) after correcting for eddy currents, subject movement and a static susceptibility field it was clear that in the areas with a strong offresonance field from susceptibility it was not sufficient to correct with a single field
The susceptibility-by-movement effects were obvious as apparent shape changes between volumes

Summary

Introduction

When an object, such as the human head, is placed in a homogeneous magnetic field it will disrupt that field and it will no longer be homogeneous. Typical off-resonance values for the worst affected areas (near sinuses and ear canals) are ~150 Hz for a 3T scanner, which leads to signal displacements of up to 15 pixels. This must be resolved and typical solutions aim at finding the offresonance field (field map) either through direct measurement (Jezzard and Balaban, 1995; Robson et al, 1997) or by acquiring at least two EPI images that are affected differently by the off-resonance field (Andersson et al, 2003; Morgan et al, 2004; Holland et al, 2010; Irfanoglu et al, 2015). Once the field is known it is easy to correct for the first order effects of the distortions (Jezzard and Balaban, 1995; Munger et al, 2000)

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Results

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