Multi-coil B₀ shimming with a set of local shim coils^1,2 can provide a more effective correction of local B₀ inhomogeneities than spherical harmonic shimming. However, it requires an additional shim coil array inside or outside the RF coil, which decreases either the SNR or shimming performance due to RF shielding between the coils or an increased distance between the subject and shim coil array, respectively. To eliminate this trade-off, an integrated parallel reception, excitation, and shimming (iPRES) concept was proposed that integrates a DC shim loop onto each coil element of an RF coil array, so that both DC and RF currents can flow on the same coils simultaneously while remaining close to the subject, thereby maximizing both the SNR and shimming performance^3,4. However, since the DC shim loops are constrained to the RF coils, iPRES cannot effectively shim B₀ inhomogeneities that change within a coil. To address this limitation, an improved design, termed iPRES(N), was proposed with N independent DC shim loops within each RF coil, thus increasing the number of magnetic fields available for local shimming⁵. Despite these advantages, the additional DC loops require additional power supplies, which increases cost and complexity.

A shim coil array that can contour a magnetic field to the shape of local B₀ inhomogeneities by adapting the DC current path with a solid-state switch matrix and a single power supply has been proposed⁶. While this design can improve the B₀ homogeneity, only a single current is available to produce the magnetic fields required for shimming, which limits its effectiveness when multiple amplitudes and polarities are required. Here, we propose to integrate a shim switch matrix onto an iPRES(N) coil array to reduce the number of power supplies per RF coil while maintaining the increased shimming flexibility of iPRES(N) over iPRES. As a proof-of-concept, a switch matrix is integrated onto an iPRES(2) coil driven by a single power supply to produce four unique DC paths that are used to shim local B₀ inhomogeneities.

A switched iPRES(2) coil was created by using 4 solid-state bipolar junction transistors (Fig. 1). By activating different sets of transistors, four independent DC paths can be generated within the RF coil using a single DC power supply (Fig. 2). Coronal basis B₀ maps were acquired in a phantom on a 3T scanner using a multi-echo gradient-echo sequence with a 1A DC current applied to each of the four DC paths.

A 3 A DC current was then applied to an RF-isolated, figure-eight perturbation loop placed near the phantom to introduce localized B₀ inhomogeneities across the coil (Fig. 3A). A B₀ map was acquired with the perturbation applied, but no DC current in the coil. The optimal state to use and optimal DC current to apply in the corresponding DC path to shim this perturbation were then determined by using this B₀ map and the four basis B₀ maps. The experiment was repeated with a 2.5 A current applied to a single RF-isolated perturbation loop (Fig. 3D) that produced different B₀ inhomogeneities.

Fig. 2 shows the four independent basis B₀ maps available for shimming. Figs. 3B and 3E show the B₀ maps corresponding to the two perturbation loops. Figs. 3C and 3F show the B₀ maps after shimming with the switched iPRES(2) coil, demonstrating a significant improvement in B₀ homogeneity with up to an 86% reduction in the B₀ root-mean-square-error.These proof-of-concept experiments demonstrate that the switched iPRES(2) coil can adaptively shim B₀ inhomogeneities of different shapes within the RF coil using a single power supply. Such results cannot be achieved with an iPRES(1) coil, which lacks the spatial resolution to shim changing B₀ inhomogeneities within the RF coil, or an iPRES(2) coil, which requires an additional power supply. In general, a switched iPRES(N) coil array retains the advantage of having multiple magnetic fields available for B₀ shimming within each RF coil while reducing the number of power supplies required, thereby reducing cost and complexity.