Correcting image distortions from a nonlinear �1±gradient field in frequency-modulated Rabi-encoded echoes
Paul Wang, Taylor Froelich, Efraín Torres, Lance DelaBarre, Parker Jenkins, Jerahmie Radder, Michael Mullen, Michael Garwood
Abstract
Purpose
To correct image distortions that result from nonlinear spatial variation in the transmit RF field amplitude (B1+) 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 0) gradients in standard MRI is adapted herein to correct image distortion arising from a nonlinear B1±gradient field in FREE. From a B1±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 B1±gradient field spatial-encoding and standard B 0-gradient field spatial-encoding. The adapted distortion-correction algorithm substantially reduced image distortions arising in the spatial dimension that was encoded by the nonlinear B1+ 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 B1+ -gradient fields.