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Updated: Aug 17, 2025

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Published on: November 11, 2013
Correcting image distortions from a nonlinear -gradient field in frequency-modulated Rabi-encoded echoes
Paul Wang1, Taylor Froelich2, Efraín Torres1,2
1Department of Biomedical Engineering, University of Minnesota, Minneapolis, Minnesota, USA.
Purpose:
To correct image distortions that result from nonlinear spatial variation in the transmit RF field amplitude ( ) when performing spatial encoding with the method called frequency-modulated Rabi encoded echoes (FREE).
Theory And Methods:
An algorithm developed to correct image distortion resulting from the use of nonlinear static field (B0 ) gradients in standard MRI is adapted herein to correct image distortion arising from a nonlinear -gradient field in FREE. From a -map, the algorithm performs linear interpolation and intensity scaling to correct the image. The quality of the distortion correction is evaluated in 1.5T images of a grid phantom and human occipital lobe.
Results:
An expanded theoretical description of FREE revealed the symmetry between this -gradient field spatial-encoding and standard B0 -gradient field spatial-encoding. The adapted distortion-correction algorithm substantially reduced image distortions arising in the spatial dimension that was encoded by the nonlinear gradient of a circular surface coil.
Conclusion:
Image processing based on straightforward linear interpolation and intensity scaling, as previously applied in conventional MRI, can effectively reduce distortions in FREE images acquired with nonlinear -gradient fields.
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