Improved dynamic distortion correction for fMRI using single‐echo EPI and a readout‐reversed first image (REFILL)

Abstract

The boundaries between tissues with different magnetic susceptibilities generate inhomogeneities in the main magnetic field which change over time due to motion, respiration and system instabilities. The dynamically changing field can be measured from the phase of the fMRI data and corrected. However, methods for doing so need multi‐echo data, time‐consuming reference scans and/or involve error‐prone processing steps, such as phase unwrapping, which are difficult to implement robustly on the MRI host. The improved dynamic distortion correction method we propose is based on the phase of the single‐echo EPI data acquired for fMRI, phase offsets calculated from a triple‐echo, bipolar reference scan of circa 3–10 s duration using a method which avoids the need for phase unwrapping and an additional correction derived from one EPI volume in which the readout direction is reversed. This Reverse‐Encoded First Image and Low resoLution reference scan (REFILL) approach is shown to accurately measure B0 as it changes due to shim, motion and respiration, even with large dynamic changes to the field at 7 T, where it led to a > 20% increase in time‐series signal to noise ratio compared to data corrected with the classic static approach. fMRI results from REFILL‐corrected data were free of stimulus‐correlated distortion artefacts seen when data were corrected with static field mapping. The method is insensitive to shim changes and eddy current differences between the reference scan and the fMRI time series, and employs calculation steps that are simple and robust, allowing most data processing to be performed in real time on the scanner image reconstruction computer. These improvements make it feasible to routinely perform dynamic distortion correction in fMRI.

Predicting dynamic, motion‐related changes in B0 field in the brain at a 7T MRI using a subject‐specific fine‐trained U‐net

“…Some of those methods require modification to the sequence which may not be desirable, such as jittering the TE, 45 and it has been shown that even for an approach which uses unmodified, single-echo EPI, 46 careful consideration of eddy currents and appropriate corrections are necessary. 47 Our NN approach has similar accuracy to those methods but is more flexible, in that it is expected to be applicable to fast imaging with non-Cartesian readouts (such as spiral and wave). It is also capable of predicting the change of the B 0 with high temporal resolution without measuring extra data during the subsequent sequences in the particular volunteer's MRI acquisitions.…”

“…In some specific cases (e.g., fMRI), the B 0 maps can be calculated from the phase of single‐echo EPI sequences. Some of those methods require modification to the sequence which may not be desirable, such as jittering the TE, 45 and it has been shown that even for an approach which uses unmodified, single‐echo EPI, 46 careful consideration of eddy currents and appropriate corrections are necessary 47 . Our NN approach has similar accuracy to those methods but is more flexible, in that it is expected to be applicable to fast imaging with non‐Cartesian readouts (such as spiral and wave).…”

Dynamic field mapping and distortion correction using single‐shot blip‐rewound EPI and joint multi‐echo reconstruction

“…However, acquiring an even number of echoes with rEPI (e.g., two‐echo rEPI) are prone to residual phase errors between odd and even EPI readout lines, as each image is acquired with the same readout gradient polarity. Recent work based on readout‐reversed first echo 13 demonstrated the capability to correct for EPI odd‐even phase errors for fMRI time series, which can be potentially integrated with rEPI to enable robust $$$ \Delta {B}_0 $$$ estimation with an even number of echoes. Note that conventional me‐EPI with under‐sampling can be considered as a special case of rEPI with a large rewinder blip that traverses from − k y_max to k y_max in a single step.…”

“…One type of navigator‐free method uses the conventional EPI readout and acquires one EPI image after each excitation. A dynamic $$$ \Delta {B}_0 $$$ map is calculated from the phase of each image volume by subtracting a phase offset term (due to non‐ B 0 related contribution, e.g., B 1 field inhomogeneity) from the image phase and then dividing the residual phase by the TE 9–13 . These methods typically require acquisition of a reference $$$ \Delta {B}_0 $$$ map using conventional multi‐echo gradient echo sequence and use phase extrapolation to obtain reliable phase offset estimation when subject motion is large 12 …”

“…A dynamic ΔB 0 map is calculated from the phase of each image volume by subtracting a phase offset term (due to non-B 0 related contribution, e.g., B 1 field inhomogeneity) from the image phase and then dividing the residual phase by the TE. [9][10][11][12][13] These methods typically require acquisition of a reference ΔB 0 map using conventional multi-echo gradient echo sequence and use phase extrapolation to obtain reliable phase offset estimation when subject motion is large. 12 Another type of navigator-free dynamic ΔB 0 mapping method calculates a ΔB 0 map using multiple EPI images acquired with different TEs.…”