B₀ concomitant field compensation for MRI systems employing asymmetric transverse gradient coils

Abstract

Purpose Imaging gradients result in the generation of concomitant fields, or Maxwell fields, which are of increasing importance at higher gradient amplitudes. These time-varying fields cause additional phase accumulation, which must be compensated for to avoid image artifacts. In the case of gradient systems employing symmetric design, the concomitant fields are well described with second-order spatial variation. Gradient systems employing asymmetric design additionally generate concomitant fields with global (zeroth-order or B0) and linear (first-order) spatial dependence. Methods This work demonstrates a general solution to eliminate the zeroth-order concomitant field by applying the correct B0 frequency shift in real time to counteract the concomitant fields. Results are demonstrated for phase contrast, spiral, echo-planar imaging (EPI), and fast spin-echo imaging. Results A global phase offset is reduced in the phase-contrast exam, and blurring is virtually eliminated in spiral images. The bulk image shift in the phase-encode direction is compensated for in EPI, whereas signal loss, ghosting, and blurring are corrected in the fast-spin echo images. Conclusion A user-transparent method to compensate the zeroth-order concomitant field term by center frequency shifting is proposed and implemented. This solution allows all the existing pulse sequences—both product and research—to be retained without any modifications.

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Improving apparent diffusion coefficient accuracy on a compact 3T MRI scanner using gradient nonlinearity correction

“…3D “gradwarp” correction for only spatial encoding distortion in the source images was enabled for the C3T and MR750w, while the software version on the Signa HDxt was limited to 2D “gradwarp” correction. The acquisitions on the C3T employed both real‐time gradient preemphasis and frequency shifting to compensate for additional concomitant field terms due to the asymmetric gradient design …”

Reduced acoustic noise in diffusion tensor imaging on a compact MRI system

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“…For DTI processing, an independently-validated tenth- and even-ordered spherical harmonics-derived gradient field map [45] was used to correct for gradient nonlinearity of both spatial distortion and diffusivity-encoding [46–47]. …”