B0 Distortion Correction in EPI

Douglas N GreveMGH Martinos Center

B0 Inhomogeneity

Caused by Air-Tissue Boundary– Orbital Frontal– Inferior/Medial Temporal

3T-EPITrio128x1282mm3TE=40ESP=0.94msRO=120ms(cf 30ms)

Thanks to Brad Dickerson

• Voxel Shift in PE (EPI)• Blur (Spiral)• Dropout

Phase EncodeDirection

B0 Inhomogeneity Map

Magnitude

Phase (may need to be unwrapped)

Shift Map (mm)Phase Encode DirectionUndistorted 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 S

hift (mm

)

B0 Distortion Correction

Mag/ShiftMap (Undistorted)

EPI (Distorted) EPI (Corrected)

PE

Voxel S

hift (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 2.46

--esp 0.94

Performs phase unwrappingTE 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.