Correcting image distortions from a nonlinear -gradient field in frequency-modulated Rabi-encoded echoes.

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 (B ) 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 B -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.