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Spatial Unwarping

Echo-planar imaging (EPI) enables rapid functional imaging but introduces spatial distortion. Unwarping methods estimate and correct this distortion.

Multi-echoMEDICROMEO

Multi-echo correction

Partial Fourier acquisition and in-plane acceleration can reduce EPI distortion, but may also reduce signal-to-noise ratio and do not eliminate distortion. Common correction methods use a separately acquired field map, as in FSL FUGUE, or images with reversed phase encoding, as in FSL TOPUP.

Van et al. (2026) introduced frame-wise Multi-Echo DIstortion Correction (MEDIC). MEDIC estimates field maps from phase data acquired as part of a multi-echo sequence. It requires no separate distortion-correction scan and estimates a field map for every volume, making the correction responsive to head motion.

MEDIC undistortion: native image (left), fieldmap (middle), undistorted (right)
MEDIC undistortion: native image (left), fieldmap (middle), undistorted (right)

The MEDIC authors provide the reference implementation in warpkit. niimath provides a compact, high-performance implementation through its -medic function.

Multi-echo correction pipelineEnd-to-end runtime (seconds) Multi-echo correction pipelineEnd-to-end runtime (seconds)MEDICMEDIC (OpenMP)niimathniimath (OpenMP)
Illustrative Spatial Unwarping benchmark figure

Benchmark source

Nonlinear spatial unwarping A deformed coordinate lattice and accent-colored axial EPI image are corrected to an undistorted anatomical reference. Spatial unwarping A nonlinear deformation field restores EPI geometry to anatomical space 01 Distorted EPI geometry The lattice and anatomy share one deformation field 02 After nonlinear correction Corrected anatomy returns to reference space UNWARP deformation lattice undeformed outline
Nonlinear correction of spatially distorted EPI anatomy

Links

  • Dymerska et al. describe Rapid Open-source Minimum spanning treE algOrithm (ROMEO) phase unwrapping.
  • Van et al. (2026) introduce Multi-Echo DIstortion Correction (MEDIC).
  • ROMEO.jl provides Julia source code under the permissive MIT license.
  • warpkit provides C and Python code for MEDIC under a Washington University license that permits noncommercial use.