Receiver coil arrays are widely used in MRI for obtaining high local SNR and improved imaging speed [1]. The most commonly used decoupling methods are overlap (for neighboring coils) and low-input-impedance preamplifiers (for non-neighboring coils) [2]. As the channel number in coil arrays increases, element coils become smaller. At some point, the overlapping method becomes impractical because of the limits on the available space for matching networks and preamplifiers. Furthermore, overlapped neighboring coils increase the mutual inductance between next-neighbor coils and degrade the preamplifier decoupling method for very small coils [3]. Therefore, new decoupling methods that can decouple very small coils in receiver arrays should be developed. Here, we present a new decoupling method together with a corresponding coil array.

The layout of the corresponding coil array is shown first in Figure 1. The coil array has overlapped receiver coils in the direction aligned along $$$\overrightarrow{B_{0}}$$$ and gapped receiver coils in the direction transverse to $$$\overrightarrow{B_{0}}$$$. The layout is designed considering the uniformity of array sensitivity. All the receiver coils have the same shape and orientation. Only 16 channels are shown in Figure 1, but it is easy to extend the number of channels in two dimensions.

All decoupling setups have the same structure. Each consists of four small decoupling loops connected to the corners of four neighbor receiver coils separately. These four decoupling loops are strongly overlapped with each other, thus, coupling among receiver coils is counteracted by the strong negative coupling between these decoupling loops. The layout of a representative decoupling setup is shown in Figure 2a (top-view) and Figure 2b (substrate). The four decoupling loops stay in four different layers, with distances between layers in agreement with current microPCB technology (Dyconex AG, Switzerland). The topview layout (Figure 2a) of the decoupling setup is generated by us with the purpose of counteracting the coupling among receiver coils. By using this decoupling setup and the proposed coil array layout, the mutual inductance between next neighbor coils can be kept small enough and the preamplifier method can work effectively.

The performance of the newly designed coil array is simulated and results are compared with the traditional overlapped array (abbreviated as Overlapped Array). Limited by computer memory, the 2D array is represented by two 1D 10-channel arrays: one aligned along $$$\overrightarrow{B_{0}}$$$ (abbreviated as A_Array) and the other transverse to $$$\overrightarrow{B_{0}}$$$ (abbreviated as T_Array). Since the A_Array has overlapped coils similar to the traditional Overlapped Array, very similar performance is found (not shown). Therefore, only the comparison between the T_Array and the Overlapped Array are shown.

First, noise correlation is compared with a Gapped Array as a reference. Seen from Figure 3, the largest noise correlations, coming from neighboring coils, are 44.9%, 48.2% and 56.4% in the three arrays, respectively. Obtained intrinsic SNR maps, without acceleration (R=1) and with strong acceleration (R=9), are compared in Figure 4 and Figure 5, respectively. SNR was represented by $$$\frac{B_{1-xy}^-(\triangle V)}{\sqrt{R_{sample}}}$$$ for simplicity. With strong acceleration, the T_Array has better performance than the Overlapped Array, especially at the sample periphery.

Both the new decoupling method and the overlapping method use overlap to decouple neighboring coil, but overlap in two methods contributes differently to the noise correlation of receiver coils. In the overlapping method, the overlapped sensitivities of receiver coils contribute a lot to noise correlation, while in the new decoupling method, the overlapped sensitivities of small decoupling loops only have a minor contribution to the noise correlation because of the gaps between coils and sample. In highly accelerated parallel imaging, lower noise correlation means lower g-factor, and higher SNR, as seen in Figure 5. It could be predicted that with more receiver channels, higher acceleration factor can be successfully applied.

Considering the low noise correlation, the new decoupling method is similar to the transformer decoupling method [4]. However, every two coupled coil loops need one pair of transformers in the transformer decoupling method, so the new decoupling method needs fewer decoupling setups. Similar to the transformer decoupling method, a disadvantage of the new decoupling method is the large resistance induced by the decoupling loops.

A new decoupling method has been developed which is specifically useful for very small receiver coils in coil arrays. Compared to the traditional overlap method, the new decoupling method contributes less to noise correlation between receiver coils, and has better performance in highly accelerated imaging especially at the sample periphery.