Adaptive integrated parallel reception, excitation, and shimming (iPRES-A) with microelectromechanical systems switches

Abstract

Purpose: Integrated parallel reception, excitation, and shimming coil arrays with N shim loops per radio-frequency (RF) coil element (iPRES(N)) allow an RF current and N direct currents (DC) to flow in each coil element, enabling simultaneous reception/excitation and shimming of highly localized B₀ inhomogeneities. The purpose of this work was to reduce the cost and complexity of this design by reducing the number of DC power supplies required by a factor N, while maintaining a high RF and shimming performance.

Methods: In the proposed design, termed adaptive iPRES(N) (iPRES(N)-A), each coil element only requires one DC power supply, but uses microelectromechanical systems switches to adaptively distribute the DC current into the appropriate shim loops to generate the desired magnetic field for B₀ shimming. Proof-of-concept phantom experiments with an iPRES(2)-A coil and simulations in the human abdomen with an 8-channel iPRES(4)-A body coil array were performed to demonstrate the advantages of this innovative design.

Results: The iPRES(2)-A coil showed no loss in signal-to-noise ratio and provided a much more effective correction of highly localized B₀ inhomogeneities and geometric distortions than an equivalent iPRES(1) coil (88.2% vs. 32.2% lower B₀ root-mean-square error). The iPRES(4)-A coil array showed a comparable shimming performance as that of an equivalent iPRES(4) coil array (52.6% vs. 54.2% lower B₀ root-mean-square error), while only requiring 8 instead of 32 power supplies.

Conclusion: The iPRES(N)-A design retains the ability of the iPRES(N) design to shim highly localized B₀ inhomogeneities, while drastically reducing its cost and complexity. Magn Reson Med 80:371-379, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: B0 shimming; RF coil; coil array; shim coil; switches.

Figure 1

a: An iPRES(1) coil element cannot shim B₀ inhomogeneities that are spatially smaller than the RF coil element (shown by the red trace). b: An iPRES(N) coil element with N smaller RF-isolated shim loops can shim localized B₀ inhomogeneities more effectively, but requires N DC power supplies. c: An iPRES(N)-A coil element uses a switch matrix to distribute the DC current from a single power supply into the appropriate shim loops, thereby maintaining a high shimming flexibility, while reducing the cost and complexity of the system.

Figure 2

RF coil before (a) and after (b) iPRES(2)-A integration with the addition of four RF-isolated MEMS switches (numbered 1 to 4) within the RF coil perimeter. SNR maps before (c) and after (d) iPRES(2)-A integration showing no appreciable change in SNR.

Figure 3

Circuit diagrams for each of the four unique iPRES(2)-A switch states shown in Table 1 (a: state 1, b: state 2, c: state 3, d: state 4, with the active DC current paths shown in gray, blue, green, and magenta, respectively), overlaid onto the corresponding basis B₀ maps. The MEMS switch control lines have been omitted for clarity.

Figure 4

Circuit diagrams and B₀ maps with the single loop or figure-eight perturbation before shimming (a,d), showing highly localized B₀ inhomogeneities, after shimming with the optimal switch state and DC current applied to the iPRES(2)-A coil (b: state 1, e: state 3), showing a drastic reduction in B₀ inhomogeneities, and after shimming with the iPRES(1)-equivalent switch state (c,f), showing only partial or minimal reduction in B₀ inhomogeneities.

Figure 5

a: 8-channel iPRES(1), iPRES(2), iPRES(3), iPRES(4), and iPRES(4)-A body coil arrays, with the RF coil elements shown in gray and the interior shim loop traces shown in red. b: Experimental B₀ map acquired in a representative axial slice in the abdomen of a healthy volunteer after linear shimming. c–g: Simulated B₀ maps after shimming with the coil arrays shown in (a) placed around the abdomen, showing that the shimming performance of the iPRES(4)-A coil array is comparable to that of the iPRES(4) coil array. Contour lines derived from the anatomical images are overlaid on the B₀ maps. The B₀ RMSE and reduction in RMSE relative to the baseline are shown on the bottom left and bottom right of each B₀ map, respectively.

Figure 6

Forty unique switch states of an iPRES(4)-A coil element, with the RF coil element shown in gray, the interior shim loop traces shown in green, and the active DC current paths shown in red.

Figure 7

iPRES(N)-A shim optimization method. An 8-channel iPRES(2)-A coil array is shown for simplicity, but extension to any N value is straightforward. A shim optimization is first performed for an equivalent iPRES(N) coil array (step 1), which is then used to determine the optimal switch states (step 2) and DC currents (step 3) to apply in the iPRES(N)-A coil array.

Figure 8

EPI images corresponding to the B₀ maps shown in Figure 4. Perturbation loops were used to introduce B₀ inhomogeneities and the iPRES(2)-A coil was used for both imaging and B₀ shimming. EPI images with no perturbation (a,e); with the single loop or figure-eight perturbation before shimming (b,f), showing severe geometric distortions (red arrows); after shimming with the optimal switch state and DC current applied to the iPRES(2)-A coil (c,g), showing a drastic reduction in distortions (green arrows); and after shimming with the iPRES(1)-equivalent switch state (d,h), showing only partial or minimal reduction in distortions (orange and red arrows).