Real-time shimming with FID navigators

Abstract

Purpose: To implement a method for real-time field control using rapid FID navigator (FIDnav) measurements and evaluate the efficacy of the proposed approach for mitigating dynamic field perturbations and improving $T_2^{*}$ -weighted image quality.

Methods: FIDnavs were embedded in a gradient echo sequence and a subject-specific linear calibration model was generated on the scanner to facilitate rapid shim updates in response to measured FIDnav signals. To confirm the accuracy of FID-navigated field updates, phantom and volunteer scans were performed with online updates of the scanner B₀ shim settings. To evaluate improvement in $T_2^{*}$ -weighted image quality with real-time shimming, 10 volunteers were scanned at 3T while performing deep-breathing and nose-touching tasks designed to modulate the B₀ field. Quantitative image quality metrics were compared with and without FID-navigated field control. An additional volunteer was scanned at 7T to evaluate performance at ultra-high field.

Results: Applying measured FIDnav shim updates successfully compensated for applied global and linear field offsets in phantoms and across all volunteers. FID-navigated real-time shimming led to a substantial reduction in field fluctuations and a consequent improvement in $T_2^{*}$ -weighted image quality in volunteers performing deep-breathing and nose-touching tasks, with 7.57% ± 6.01% and 8.21% ± 10.90% improvement in peak SNR and structural similarity, respectively.

Conclusion: FIDnavs facilitate rapid measurement and application of field coefficients for slice-wise B₀ shimming. The proposed approach can successfully counteract spatiotemporal field perturbations and substantially improves $T_2^{*}$ -weighted image quality, which is important for a variety of clinical and research applications, particularly at ultra-high field.

Keywords: $$$ {\mathrm{T}}_2^{\ast } $$$ -weighted imaging; B0 inhomogeneity; FID navigators; artifact correction; real-time shimming.