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

Abstract

Purpose To test an integrated “AC/DC” array approach at 7T, where B0 inhomogeneity poses an obstacle for functional imaging, diffusion‐weighted MRI, MR spectroscopy, and other applications. Methods A close‐fitting 7T 31‐channel (31‐ch) brain array was constructed and tested using combined Rx and ΔB0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape‐matched 31‐ch reference receive‐only array for RF safety, signal‐to‐noise ratio (SNR), and inter‐element noise correlation. We characterize the coil array’s ability to provide global and dynamic (slice‐optimized) shimming using ΔB0 field maps and echo planar imaging (EPI) acquisitions. Results The SNR and average noise correlation were similar to the 31‐ch reference array. Global and slice‐optimized shimming provide 11% and 40% improvements respectively compared to baseline second‐order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups. Conclusion Adding ΔB0 shim capability to a 31‐ch 7T receive array can significantly boost 7T brain B0 homogeneity without sacrificing the array’s rdiofrequency performance, potentially improving ultra‐high field neuroimaging applications that are vulnerable to off‐resonance effects.

In Vivo Magnetic Resonance Spectroscopy Methods for Investigating Cardiac Metabolism

“…Also, slice-by-slice (slab-selective compared to the volume shim) shimming approaches have greatly improved the utility of parallel transmission with improved homogenous B 1 field across the slabs in the heart while decreasing the SAR level [65,66]. These studies aim to eventually include dynamic shim updating on the selected volume in the heart, as has been experimented with within the brain studies at ultra-high-field ≥7 Tesla [67,68]. Overall considerations on the heart shimming include: (i) using slab-selective approaches shimming to minimize the field distribution variations (shimming standard deviation (SD)), (ii) optimizing high-order shims, (iii) temporal B 0 -field variations with regards to cardiac cycle and respirationinduced, (iv) shimming of oblique orientations, (v) develop dynamic shimming interface by rapid field mapping using slab-specific or cardiac phase-specific mapping sets, and (vi) the emerging use of artificial intelligence in compensating the field variations [45,63,66,[69][70][71].…”

Real‐time shimming with FID navigators

“…While previous work has shown that FIDnavs can retrospectively compensate for higher-order spatiotemporal field changes, the second-order shim coils on the scanner are not designed for rapid switching. Higher-order field stabilization requires use of custom shim array coils 42,43 and will be investigated in future work.…”

Caterpillar traps: A highly flexible, distributed system of toroidal cable traps

“…Toroids have been previously used as inline chokes; in spatially distributed, rigid, inductively coupled traps; and as transceive coils in interventional devices. 9,[17][18][19] We propose to use them as subcomponents in our distributed design because they are mostly self-shielded, especially with a large number of turns. Caterpillar traps are illustrated and compared to conventional traps in Figure 1A.…”