Spatial Distortion Correction in EPI Sequences: Field Mapping Examples
Spatial distortion artifacts in EPI sequences (BOLD or DWI) due to slow sampling rates in the phase encoding direction can be corrected using B0/spatial field mapping techniques. This correction requires obtaining field maps under the same B0 shimming conditions and with identical FoV and adjustment volumes as the BOLD/DWI acquisitions. Grouping BOLD/DWI sequences with field map acquisitions is essential for accurate distortion correction. Three examples illustrate the process: correcting BOLD fMRI data, correcting DWI acquisitions, and successful/unsuccessful BOLD fMRI field mapping.
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Spatial Distortion in EPI Sequences (BOLD or DWI) Sampling rate of EPI sequence (e.g., BOLD or DWI) in phase encoding direction (PED) is slow, which makes spatial distortion artifacts in EPI image in PED Spatial distortion could be corrected by using B0/spatial field mapping, e.g., gre field mapping or spin-echo field mapping Field mapping MUST be acquired under same B0 shimming condition FoV and Adjustment volume MUST be identical those as BOLD/DWI acquisition Field maps acquired with different B0 shim condition from BOLD/DWI CANNOT be applied for distortion correction Typically, BOLD/DWI and Field map sequences are grouped, for example, fMRI, SpinEchoFieldMap_AP, and SpinEchoFieldMap_PA fMRI, and GREFieldMap fMRI, SpinEchoFieldMap_AP, and SpinEchoFieldMap_PA, and GREFieldMap DWI_AP, and DWI_b0_PA Note DWI_AP includes SpinEchoFieldMap_AP, and DWI_b0_PA is SpinEchoFieldMap_PA
Example 1: BOLD fMRI BOLD fMRI data; rsfMRI, fMRI1, fMRI2 Same FoV and Adjustment volume assigned for all three BOLD sequences Spin Echo Field Map acquired for distortion correction FoV and Adjustment Volume MUST be same as those of BOLD B0 shim values MUST be same as those of BOLD Pair of forward and backward phase encoding direction (PED) , SpinEchoFieldMap (forward & backward) Same shim values applied -> Correct scan!
Example 2: Diffusion Weighted Imaging (DWI) DWI acquisition Main DWI data acquired w/ multiple b value and diffusion gradient table PED P > A DWI1_PA, DWI2_PA b0 reference Same FoV and Adjustment volume as DWI data Same shim values as in DWI acquisition PED A > P DWI1_b0_AP, DWI2_b0_AP Same shim values applied -> Correct scan!
Example 3: BOLD fMRI fMRI data BOLD acquisition given FoV and Adjustment volume Field mapping Spin echo field mapping with forward-backward PED GRE field mapping Same FoV and Adjustment volume as in BOLD fMRI scan Success! Failure!
