A unipolar head gradient for high-field MRI without encoding ambiguity.

Abstract

Purpose: MRI gradients with a conventional, bipolar design generally face a trade-off among performance, encoding ambiguity, and radiofrequency selectivity used to circumvent said ambiguity. This problem is particularly limiting in cutting-edge brain imaging performed at field strengths ≥ 7 T and using high-performance head gradients.

Methods: To address this issue, the present work proposes to fundamentally eliminate the encoding ambiguity in head gradients by using a unipolar z-gradient design that takes advantage of the signal-free range on one side of the imaging volume. This concept is demonstrated by implementation of a unipolar head gradient for operation at 7 T.

Results: Imaging in phantoms and in vivo demonstrates elimination of backfolding due to encoding ambiguity. At the same time, the unipolar design achieves efficiency on par with conventional bipolar design, resulting in high amplitude and slew-rate performance.

Conclusion: The prospect of gradient systems based on a unipolar design holds promise for all advanced neuroimaging that demands high gradient performance. It will make the greatest difference at 7 T and beyond, where the absence of ambiguity removes a key concern and constraint in terms of radiofrequency behavior and instrumentation.