Dataset Information

A method for the dynamic correction of B0-related distortions in single-echo EPI at 7T.

ABSTRACT: We propose a method to calculate field maps from the phase of each EPI in an fMRI time series. These field maps can be used to correct the corresponding magnitude images for distortion caused by inhomogeneity in the static magnetic field. In contrast to conventional static distortion correction, in which one 'snapshot' field map is applied to all subsequent fMRI time points, our method also captures dynamic changes to B0 which arise due to motion and respiration. The approach is based on the assumption that the non-B0-related contribution to the phase measured by each radio-frequency coil, which is dominated by the coil sensitivity, is stable over time and can therefore be removed to yield a field map from EPI. Our solution addresses imaging with multi-channel coils at ultra-high field (7T), where phase offsets vary rapidly in space, phase processing is non-trivial and distortions are comparatively large. We propose using dual-echo gradient echo reference scan for the phase offset calculation, which yields estimates with high signal-to-noise ratio. An extrapolation method is proposed which yields reliable estimates for phase offsets even where motion is large and a tailored phase unwrapping procedure for EPI is suggested which gives robust results in regions with disconnected tissue or strong signal decay. Phase offsets are shown to be stable during long measurements (40min) and for large head motions. The dynamic distortion correction proposed here is found to work accurately in the presence of large motion (up to 8.1°), whereas a conventional method based on single field map fails to correct or even introduces distortions (up to 11.2mm). Finally, we show that dynamic unwarping increases the temporal stability of EPI in the presence of motion. Our approach can be applied to any EPI measurements without the need for sequence modification.

SUBMITTER: Dymerska B

PROVIDER: S-EPMC5832018 | biostudies-literature | 2018 Mar

REPOSITORIES: biostudies-literature

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A method for the dynamic correction of B<sub>0</sub>-related distortions in single-echo EPI at 7T.

Dymerska Barbara B Poser Benedikt A BA Barth Markus M Trattnig Siegfried S Robinson Simon D SD

NeuroImage 20160707

We propose a method to calculate field maps from the phase of each EPI in an fMRI time series. These field maps can be used to correct the corresponding magnitude images for distortion caused by inhomogeneity in the static magnetic field. In contrast to conventional static distortion correction, in which one 'snapshot' field map is applied to all subsequent fMRI time points, our method also captures dynamic changes to B<sub>0</sub> which arise due to motion and respiration. The approach is based ...[more]

PMID: 27397624

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Project description:BackgroundA reduction in scan time of 4D Flow MRI would facilitate clinical application. A recent study indicates that echo-planar imaging (EPI) 4D Flow MRI allows for a reduction in scan time and better data quality than the recommended k-space segmented spoiled gradient echo (SGRE) sequence. It was argued that the poor data quality of SGRE was related to the nonrecommended absence of respiratory motion compensation. However, data quality can also be affected by the background offset compensation.PurposeTo compare the data quality of respiratory motion-compensated SGRE and EPI 4D Flow MRI and their dependence on background correction (BC) order.Study typeRetrospective.SubjectsEighteen healthy subjects (eight female, mean age 32?±?5 years).Field strength and sequence1.5 T. [Correction added on July 26, 2019, after first online publication: The preceding field strength was corrected.] SGRE and EPI-based 4D Flow MRI.AssessmentData quality was investigated visually and by comparing flows through the cardiac valves and aorta. Measurements were obtained from transvalvular flow and pathline analysis.Statistical testsLinear regression and Bland-Altman analysis were used. Wilcoxon test was used for comparison of visual scoring. Student's t-test was used for comparison of flow volumes.ResultsNo significant difference was found by visual inspection (P = 0.08). Left ventricular (LV) flows were strongly and very strongly associated with SGRE and EPI, respectively (R2 = 0.86-0.94 SGRE; 0.71-0.79 EPI, BC0-4). LV and right ventricular (RV) outflows and LV pathline flows were very strongly associated (R2 = 0.93-0.95 SGRE; 0.88-0.91 EPI, R2 = 0.91-0.95 SGRE; 0.91-0.93 EPI, BC1-4). EPI LV outflow was lower than the short-axis-based stroke volume. EPI RV outflow and proximal descending aortic flow were lower than SGREs.Data conclusionBoth sequences yielded good internal data consistency when an adequate background correction was applied. Second and first BC order were considered sufficient for transvalvular flow analysis in SGRE and EPI, respectively. Higher BC orders were preferred for particle tracing. Level of Evidence 4 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2020;51:885-896.

Dataset Information

A method for the dynamic correction of B0-related distortions in single-echo EPI at 7T.

Publications

A method for the dynamic correction of B<sub>0</sub>-related distortions in single-echo EPI at 7T.

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