Integrated RF/shim coil array for parallel reception and localized B0 shimming in the human brain

Abstract

The purpose of this work was to develop a novel integrated radiofrequency and shim (RF/shim) coil array that can perform parallel reception and localized B0 shimming in the human brain with the same coils, thereby maximizing both the signal-to-noise ratio and shimming efficiency. A 32-channel receive-only head coil array was modified to enable both RF currents (for signal reception) and direct currents (for B0 shimming) to flow in individual coil elements. Its in vivo performance was assessed in the frontal brain region, which is affected by large susceptibility-induced B0 inhomogeneities. The coil modifications did not reduce their quality factor or signal-to-noise ratio. Axial B0 maps and echo-planar images acquired in vivo with direct currents optimized to shim specific slices showed substantially reduced B0 inhomogeneities and image distortions in the frontal brain region. The B0 root-mean-square error in the anterior half of the brain was reduced by 60.3% as compared to that obtained with second-order spherical harmonic shimming. These results demonstrate that the integrated RF/shim coil array can perform parallel reception and localized B0 shimming in the human brain and provide a much more effective shimming than conventional spherical harmonic shimming alone, without taking up additional space in the magnet bore and without compromising the signal-to-noise ratio or shimming performance.

Keywords: B(0) shimming; Coil array; Human brain; RF coil; Shim coil.

Fig. 1

(A) Schematic diagram of the integrated RF/shim coil array, consisting of 16 conventional RF coils surrounding the posterior half of the brain (shown in gray) and 16 RF/shim coils surrounding the anterior half of the brain (shown in color), with the same DC current applied in coils symmetrically located on the right and left sides of the brain (shown with the same color). (B) Picture of the integrated RF/shim coil array. (C) Schematic diagram of one of the RF/shim coils, consisting of the original RF coil (shown in black) and the inductor L₁, chokes L₂, capacitor C, shielded twisted-pair cable, and DC power supply (shown in red) added to allow a DC current to flow in the closed loop (red arrows).

Fig. 2

(A) S₁₁ parameter of a representative loaded coil before (blue curve) and after (red curve) converting the original RF coil to an RF/shim coil. SNR maps in the same axial slice from two representative RF/shim coils (B) and two representative conventional RF coils (C) of the integrated RF/shim coil array and from four representative RF coils of the commercial 32-channel head coil array (D). The two coils in (B) are shown in orange in Fig. 1A, the two coils in (C) are those adjacent to them in the posterior direction, and the four coils in (D) were chosen to match the location of the four coils in (B) and (C). A: anterior, P: posterior, R: right, L: left.

Fig. 3

Basis B₀ maps acquired in a phantom with a DC current of 1 A separately applied in each pair of RF/shim coils. The color of the outline of each B₀ map matches the color of the RF/shim coils shown in Fig. 1A.

Fig. 4

(A) B₀ maps acquired in vivo without DC currents in four representative slices. The B₀ RMSE in the anterior half of the brain is shown at the top. (B) B₀ field generated by the RF/shim coils with optimal DC currents (i.e., sum of the basis B₀ maps weighted by the optimal DC currents). The average and maximum DC current amplitudes applied in the RF/shim coils are shown at the top. (C) B₀ maps with optimal DC currents predicted from the shim optimization (i.e., sum of (A) and (B)). (D) B₀ maps acquired in vivo with optimal DC currents. The B₀ RMSE in the anterior half of the brain and percent reduction with respect to (A) are shown at the top.

Fig. 5

EPI images acquired in the same slices as in Fig. 4 without DC currents (A) and with optimal DC currents (B), with overlaid contour lines derived from the undistorted fast spin-echo images. The arrows point to susceptibility-induced distortions in the frontal brain region.