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

TL;DR

This paper introduces a unipolar head gradient design for high-field MRI that eliminates encoding ambiguity, enabling improved neuroimaging at 7T and beyond without sacrificing gradient performance.

Contribution

It proposes and demonstrates a unipolar gradient design that removes encoding ambiguity in high-field MRI, enhancing imaging quality and efficiency.

Findings

Elimination of backfolding due to encoding ambiguity in phantom and in vivo imaging.

Unipolar design achieves high gradient amplitude and slew-rate performance comparable to bipolar systems.

Potential to improve neuroimaging at 7T and higher fields by removing RF and instrumentation constraints.

Abstract

Purpose: MRI gradients with a conventional, bipolar design generally face a trade-off between performance, encoding ambiguity, and circumventing the latter by means of RF selectivity. This problem is particularly limiting in cutting-edge brain imaging performed at field strengths >= 7T 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 7T. 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 7T and beyond where the absence of ambiguity removes a key concern and constraint in terms of RF behaviour and instrumentation.