EPFL | Biomedical Imaging Group

Unwarping of Unidirectionally Distorted EPI Images

J. Kybic, P. Thévenaz, A. Nirkko, M. Unser

IEEE Transactions on Medical Imaging, vol. 19, no. 2, pp. 80-93, February 2000.

Echo-planar imaging (EPI) is a fast nuclear magnetic resonance imaging (MRI) method. Unfortunately, local magnetic field inhomogeneities induced mainly by the subject's presence cause significant geometrical distortion, predominantly along the phase-encoding direction, which must be undone to allow for meaningful further processing. So far, this aspect has been too often neglected.

In this paper, we suggest a new approach using an algorithm specifically developed for the automatic registration of distorted EPI images with corresponding anatomically correct MRI images. We model the deformation field with splines, which gives us a great deal of flexibility, while comprising the affine transform as a special case. The registration criterion is least squares. Interestingly, the complexity of its evaluation does not depend on the resolution of the control grid. The spline model gives us good accuracy thanks to its high approximation order. The short support of splines leads to a fast algorithm. A multiresolution approach yields robustness and additional speed-up.

The algorithm was tested on real as well as synthetic data, and the results were compared with a manual method. A wavelet-based Sobolev-type random deformation generator was developed for testing purposes. A blind test indicates that the proposed automatic method is faster, more reliable, and more precise than the manual one.

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AUTHOR="Kybic, J. and Th{\'{e}}venaz, P. and Nirkko, A. and Unser,
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TITLE="Unwarping of Unidirectionally Distorted {EPI} Images",
JOURNAL="{IEEE} Transactions on Medical Imaging",
YEAR="2000",
volume="19",
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pages="80--93",
month="February",
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