B0 Distortion Correction in EPI Douglas N Greve MGH Martinos Center.

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B0 Distortion Correction in EPI Douglas N Greve MGH Martinos Center

B0 Inhomogeneity Caused by Air-Tissue Boundary –Orbital Frontal –Inferior/Medial Temporal 3T-EPI Trio 128x128 2mm3 TE=40 ESP=0.94ms RO=120ms (cf 30ms) Thanks to Brad Dickerson Voxel Shift in PE (EPI) Blur (Spiral) Dropout Phase Encode Direction

B0 Inhomogeneity Map Magnitude Phase (may need to be unwrapped) Shift Map (mm) Phase Encode Direction Undistorted Space B0 Field Map Two conventional images (can be done with EPI, in theory) Different TEs (eg, diff=2.46ms for 3T – fixed in Siemens – grrr!) Need Magnitude and Phase PE Voxel Shift (mm)

B0 Distortion Correction Mag/ShiftMap (Undistorted) EPI (Distorted)EPI (Corrected) PE Voxel Shift (mm)

Tools (based on FSL) epidewarp.fsl --exf example_func.nii.gz --mag mag.nii.gz --dph dph.nii.gz --vsm vsm.nii.gz --exfdw example_func_dewarped.nii.gz --tediff esp 0.94 Performs phase unwrapping TE Diff is that of B0 Map (in DICOM) EPI Echo Spacing (esp) not available in Siemens DICOM

B0 Distortion Correction Summary Caused by Air-Tissue Boundary Dependent upon head position (interaction with motion), slice orientation and thickness Increases with Readout time –Resolution, Pixel Bandwidth/Noise, Parallel Imaging Increases with field strength EPI – Voxel Shift in Phase Encode Direction Spiral – Blur Dropout affects both EPI and Spiral Multi-site issues (field strength, echo spacing) Correction: Shift voxels back (EPI), could be combined with motion correction.