Maximising BOLD sensitivity through automated EPI protocol optimisation

Steffen Volz,Martina F Callaghan,Oliver Josephs,Nikolaus Weiskopf

doi:10.1016/j.neuroimage.2018.12.052

Abstract

Gradient echo echo-planar imaging (GE EPI) is used for most fMRI studies but can suffer substantially from image distortions and BOLD sensitivity (BS) loss due to susceptibility-induced magnetic field inhomogeneities. While there are various post-processing methods for correcting image distortions, signal dropouts cannot be recovered and therefore need to be addressed at the data acquisition stage. Common approaches for reducing susceptibility-related BS loss in selected brain areas are: z-shimming, inverting the phase encoding (PE) gradient polarity, optimizing the slice tilt and increasing spatial resolution. The optimization of these parameters can be based on atlases derived from multiple echo-planar imaging (EPI) acquisitions. However, this requires resource and time, which imposes a practical limitation on the range over which parameters can be optimised meaning that the chosen settings may still be sub-optimal. To address this issue, we have developed an automated method that can be used to optimize across a large parameter space. It is based on numerical signal simulations of the BS loss predicted by physical models informed by a large database of magnetic field (B0) maps acquired on a broad cohort of participants. The advantage of our simulation-based approach compared to previous methods is that it saves time and expensive measurements and allows for optimizing EPI protocols by incorporating a broad range of factors, including different resolutions, echo times or slice orientations. To verify the numerical optimisation, results are compared to those from an earlier study and to experimental BS measurements carried out in six healthy volunteers.

Highlights

Gradient echo echo-planar imaging (GE EPI) (Mansfield, 1977) is used for most functional magnetic resonance imaging studies due to its high acquisition speed and its sensitivity to the blood oxygenation level-dependent (BOLD) effect (Ogawa et al, 1990).EPI quality can suffer substantially from image distortions and BOLD sensitivity loss (BS) caused mainly by magnetic field inhomogeneities
From top to bottom, maps of the optimal z-shim gradient moment (a), the optimal slice angulation (b) and the maximum BS gain with the optimal z-shim gradient and slice angulation compared to the standard EPI protocol with no shim gradient or slice angulation (c)
For the optimal z-shim gradient, a negative gradient moment was found to yield the highest BS in the orbitofrontal cortex and a positive gradient moment in the temporal lobes in case of the transverse acquisition (Fig. 3a) as previously reported for the measurements with a negative phase encoding (PE) gradient

Summary

Introduction

EPI quality can suffer substantially from image distortions and BOLD sensitivity loss (BS) caused mainly by magnetic field inhomogeneities These inhomogeneities originate from differences in the magnetic susceptibility between tissue and air and are especially prominent in the orbitofrontal cortex (OFC), the medial temporal and the inferior temporal lobes (Ojemann et al, 1997; Devlin et al, 2000; Lipschutz et al, 2001). The differences in susceptibility can be reduced directly by placing diamagnetic materials with susceptibilities similar to tissue around the participant (Fritzsche et al., 1995) or using oral shim coils (Hsu and Glover, 2005; Wilson and Jezzard, 2003) Such shimming is limited to a relatively small area and situations with strong susceptibility gradients. The design of these pulses is computationally expensive and leads to prolonged RF pulse durations, echo time TE and repetition time TR, and often reduce signal-to-noise-ratio (SNR) in well-shimmed areas (Cho and Ro, 1992)

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