1. Field Map Constrained Nonrigid Registration Method for Correction of Distortions in Echo Planar Images
Yong Li1, Ning Xu1, J. Michael Fitzpatrick1,2, Victoria L. Morgan2, David R. Pickens2, Benoit M. Dawant1,2
1Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN, USA
2Vanderbilt University Institute for Imaging Science, Department of Radiology and Radiological Sciences, Vanderbilt University , Nashville, TN, USA

2. Introduction
Two common retrospective distortion correction methods for Echo Planar (EP) images
Field map based [1,2]
Field map measurement
Regularization & unwrapping
Nonrigid registration based [3-5]
Constraints &Optimization
[1] P. Jezzard,et al.,MRM,34:65-73,1995
[2] R. Cusack, et al., NeuroImage, , 2003
[3] C. Studholme, et al., TMI, , 2000
[4] J. Kybic, et al., TMI,
[5] Y. Li, et al., Med Im 2006: Im Proc, in press

3. Methods Unidirectional Deformation Jacobian factor
Rephasing factor (for GE images)
Physical Constraints
Initial Deformation Field
Final Deformation Field
Nonrigid Registration
Corrected EPI
Field map
Spatial Adaptation of Transformation Properties
EP Image

4. Results
Fig. 1 a. distortion free EP image; b. distorted image; c. difference between a and b; d, e: differences between distortion-free EP image and corrected images with field map method (d) and our approach (e); f: histogram of difference images.

5. Results Fig. 2 a: reference image
b: distorted image overlaid on reference image
c, d: images corrected with SPM FieldMap toolbox [6] (c) and our approach (d) overlaid on reference image
(b, c and d are magnified to show the differences)
[6]

6. Conclusions Acknowledgements
Combination of field map and nonrigid registration methods for distortion correction in EP images is proposed.
Field map is used both to initialize and to constrain the nonrigid registration algorithm, which improves on results obtained with either method alone.
Ongoing work includes evaluating this approach on real fMRI data.
Acknowledgements
NIH grant R01-NS