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.

Figure 1.

Schematic showing on-scanner calibration of FIDnav B₀ shim measurements, which is performed for each subject at the beginning of the acquisition. A pair of low-resolution images with reversed gradient encoding are acquired to create a forward model of FIDnav signal changes. Changes in B₀ shim offsets are simulated using the phase-corrected multi-channel reference data to calibrate a linear model for each subject. B₀ shim coefficients are estimated from the measured FIDnav signals in each slice using the pre-calculated subject-specific calibration and used to update the center frequency and gradient offsets in real time.

Figure 2.

First-order shim coefficients and frequency offsets measured from axial GRE field maps during applied X and Y gradient offsets and central frequency shifts across acquisitions with FID-navigated shimming switched off and on at 3T (A) and 7T (B). Applying B₀ shim measurements from FIDnavs using the linear calibration model accurately compensates for controlled field offsets.

Figure 3.

Measured ΔB₀ field maps in a volunteer scanned with applied X and Y gradient offsets of 10 μT/m and a shift in the center frequency of 10 Hz, with FID-navigated shimming switched off and on (A). B₀ shim offsets measured across nine volunteers with FID-navigated field control off and on (B). Applying ΔB₀ shim measurements computed from FIDnavs accurately compensates for controlled linear field offsets in volunteers.

Figure 4.

Zeroth- and first-order shim coefficients measured using FIDnavs for a single slice in a representative volunteer performing deep breathing (A) and nose touching (B) at 3T with real-time shimming switched off and on. Performing real-time field control based on FIDnav measurements reduces spatiotemporal field fluctuations.

Figure 5.

Boxplots showing standard deviation of zeroth- (A) and first-order (B) B₀ field fluctuations measured across all volunteers scanned at 3T during deep breathing and nose touching with FID-navigated shimming switched off and on. Real-time field control substantially reduces spatiotemporal field fluctuations across all volunteers.

Figure 6.

Axial T₂*-weighted GRE images acquired at 3T in two volunteers during continuous deep breathing with FID-navigated shimming switched off and on. Reference T₂*-weighted image acquired during normal breathing (FID-navigated shimming OFF) and difference images relative to this reference are shown for comparison. FID-navigated shimming visibly reduces ghosting and signal modulation artifacts induced by variable susceptibility changes during deep breathing.

Figure 7.

Axial T₂*-weighted GRE images acquired at 3T in two volunteers repeatedly performing a nose touching action inside the scanner with FID-navigated shimming switched off and on. Reference T₂*-weighted image acquired with no motion (FID-navigated shimming OFF) and difference images relative to this reference are shown for comparison. FID-navigated shimming successfully compensates for artifacts arising from the spatiotemporal field changes induced by arm motion.

Figure 8.

Boxplots showing peak signal-to-noise ratio (PSNR; A) and structural similarity (SSIM; B) relative to the reference T₂*-weighted image for deep breathing and nose touching scans across all ten volunteers. FID-navigated real-time shimming significantly improved image quality across all volunteers.

Figure 9.

Sagittal T₂*-weighted images acquired at 3T in a volunteer performing continuous deep breathing during the scan. A reference scan acquired during normal breathing is shown for comparison. Strong ghosting artifacts are induced by respiration in the inferior brain region, which are considerably improved with first-order FID-navigated shimming.

Figure 10.

Measured shim coefficients in a volunteer performing the nose touching task at 7T (A) and with FID-navigated shimming (B). Axial T₂*-weighted image acquired during nose touching without any phase stabilization (PS) correction applied (C); with PS; with FID-navigated shimming (E). A reference scan acquired without any intentional motion (with PS correction) is shown for comparison (F). Zoomed in sections (G-J) show that FID-navigated shimming successfully compensates for zeroth- and first-order field perturbations induced by nose touching, outperforming retrospective PS correction.