An Improved Element Design for 64-Channel Planar Imaging

Abstract

Investigation of highly accelerated MRI has developed into a lively corner in the hardware and methodology arena in recent years. At the extreme of (one-dimensional) acceleration, our group introduced Single Echo Acquisition (SEA) imaging, in which the need to phase encode a 64×N(readout) image is eliminated and replaced with the well-localized spatial information obtained from an array of 64 very narrow, long, parallel coils. The narrow coil width (2mm) that facilitates this is accompanied by a concomitant constraint on the useful imaging depth. This note describes a 64-element planar array, constructed within the same 8×13cm total footprint as the original SEA array, still enabling full acceleration in one dimension, but with an element design modified to increase the imaging depth. This was accomplished by lowering the outer conducting legs of the planar pair with respect to the center conductor and adding a geometric decoupling configuration away from the imaging field of view. The element has been called a dual-plane pair in that the current carrying rungs in the imaging FOV function exactly as the planar pair, but are simply placed in two separate planes (sides of PCB in this case).

Figure 1

Illustrations of the planar pair element (a, left) and the dual-plane pair element (a, right)and simulated relative field sensitivity patterns of both elements (b). The dual-plane pair element has higher sensitivity than the planar pair due to the dropped traces for current return and thus decreased field cancellation, enabling greater imaging depth

Figure 2

(a) Front (imaging side) of the 64-channel dual-plane pair element array on a multi-layer board. The imaging side of the coil is free of all components and consists only of 64 signal (center conductor) traces. (b) Back (component side) of the 64-channel array. The geometric decoupling loops are fabricated at the end of the dual-plane pair element, opposite the feed point. A zoomed view of the decoupling loop is shown using circuit-board drawing software for clarity. The current path is denoted by yellow arrows. The center conductor of the dual-plane pair element on the top (imaging) layer is shown in red. It connects through a via to the bottom layer (shown in blue). At the end of the segment, the center conductor connects to the middle layer (shown in pink) through a via, and the coil is geometrically decoupled from its adjacent neighbors by overlap between the middle and bottom layers, spaced 0.012″ apart. On the far right inset, the zoomed match and tune region is shown with, from top to bottom, the fixed capacitor for full-wave effect compensation, the varactor diode for tuning, and the tunable match capacitor.

Figure 3

Decoupling matrix indicated by measured S21 values [dB] for the 64-channel dual-plane pair element array. The average nearest neighbor coupling was measured as S21 = 19.9dB, and average next-nearest neighbor coupling was measured at S21 = -25.7dB. The “bulges” visible in the matrix are most likely explained by coupling within the four ribbon cables used to interface to the

Figure 4

Fully encoded transmit-receive reference image taken with the volume coil (left). The white rectangle outlines the zoomed region selected to demonstrate the relative capabilities of the planar pair and dual-plane pair element arrays. The sum-of-squares comparison images on the right show the dual-plane pair element to have a five-fold increase in SNR at depth over the planar pair element – equivalent to that of the volume coil. In addition, the wider and deeper sensitivity pattern of the dual-plane pair element improves the “striping artifact” seen when using the planar pair

Figure 5

Single echo acquisition (SEA) images reconstructed retrospectively from the fully-encoded data sets from the planar pair and the dual-plane pair arrays. The dual-plane pair array enables SEA imaging at depths not previously possible. The array also shows significant improvements in SNR close to the coil due to an increase in the effective voxel size due to the wider and deeper sensitivity pattern – a fact which can adversely affect the achievable resolution, as described in the text. The 2.5cm long decoupling loops located outside the 8×13cm main imaging region do detect signal, visible as the region of decreased intensity that “completes” the circular phantom in the dual-plane