An orthogonal shim coil for 3T brain imaging

Abstract

We designed and implemented an orthogonal shim array consisting of shim coils with their planes perpendicular to the planes of neighboring RF coils. This shim coil improves the magnetic field homogeneity by minimizing the interference to RF coils. Methods: Using realistic off-resonance maps of the human brain, we first evaluated the performance of shim coils in different orientations. Based on simulations, we developed a 7-channel orthogonal shim array, whose coil plan was perpendicular to neighboring RF coils, at the forehead. A programmable open-source current driver supplied shim currents. Results: The 7-channel orthogonal shim array caused only marginal SNR loss to the integrated 32-channel RF-shim array. The 7-channel orthogonal shim array itself improved the magnetic field homogeneity by 30% in slice-optimized shimming, comparable to the baseline shimming offered by the scanner's 2nd order spherical harmonic shimming. Conclusion: Orthogonal shim coils can improve the field homogeneity while maintaining high image SNR.

A 31‐channel integrated “AC/DC” B0 shim and radiofrequency receive array coil for improved 7T MRI

“…The five most inferior and posterior neck loops were not set up for shimming based on (i) the difficulty of running wires all the way through the coil to these loops, and (ii) preliminary B 0 shimming simulations suggesting that they provide only minor gains for B 0 ‐shim performance. To bring the number of shim channels up to 31, six multi‐turn shim‐only loops were added over the face area to target prefrontal cortex, an idea previously explored experimentally by Juchem et al 33 and Zhou et al, 34 as well as in simulations 1 . These 4‐ to 6‐cm dia.…”

Physical limits to human brain B0 shimming with spherical harmonics, engineering implications thereof

“…10 shows that strong inhomogeneity regions still remain even after global shimming at very high degree. And although inhomogeneity values at and geometry in some studies [11,29,44].…”

Distortion‐free, high‐isotropic‐resolution diffusion MRI with gSlider BUDA‐EPI and multicoil dynamic B0 shimming

“…To mitigate this issue, dynamic B 0 shimming with a custom-built 32-channel integrated ΔB 0 /Rx array (AC/DC coil) 54 was used to reduce the B 0 inhomogeneity and hence the level of distortion differences between the blip-up/down data. Our previous study 45 showed that use of the dynamic B 0 shimming method [55][56][57] with the AC/DC coil 54 could significantly mitigate the local field inhomogeneity by more than 50%, which when combined with advanced parallel imaging can enable dMRI acquisition with single-shot EPI at about 8× distortion mitigation compared with the nonaccelerated single-shot EPI with standard second-order shim. This includes a factor of approximately 2× distortion reduction from improved B 0 homogeneity achieved by the AC/DC coil compared with baseline global shimming with the scanner's built-in first/second-order spherical harmonic shims.…”