Hybrid-pair ratio adjustable power splitters for add-on RF shimming and array-compressed parallel transmission

Abstract

Purpose: A ratio adjustable power splitter (RAPS) circuit was recently proposed for add-on RF shimming and array-compressed parallel transmission. Here we propose a new RAPS circuit design based on off-the-shelf components for improved performance and manufacturability.

Theory and methods: The original RAPS used a pair of home-built Wilkinson splitter and hybrid coupler connected by a pair of connectorized coaxial cables. Here we propose a new hybrid-pair RAPS (or HP-RAPS) circuit that replaces the home-built circuits with two commercially available hybrid couplers and replaces connectorized cables with interchangeable microstrip lines. We derive the relation between the desired splitting ratio and the required phase shifts for HP-RAPS and investigate how to generate arbitrary splitting ratios using paired meandering and straight lines. Several HP-RAPSs with different splitting ratios were fabricated and tested on the workbench and MRI experiments.

Results: The splitting ratio of an HP-RAPS circuit has a tan or cot dependence on the meandering line's additional length compared to the straight line. The fabricated HP-RAPSs exhibit accurate splitting ratios as expected (<4% deviations) and generate transmit fields that well agree with predicted fields. They also demonstrated a low insertion loss of 0.33 dB, high output isolation of -26 dB, and acceptable impedance matching of -16 dB.

Conclusion: A novel HP-RAPS circuit was developed and implemented. It is easy-to-fabricate/reproduce with minimal expertise. It also preserves the features of the original RAPS circuit (ratio-adjustable, small footprint, etc.) with lower insertion loss.

Keywords: RF shimming; parallel transmission; power splitter; ratio adjustable; ultrahigh fields.

Figure 1.

(a): Circuit diagram of the Hybrid-Pair Ratio Adjustable Power Splitter (HP-RAPS). (b-c): Top view and side view of the HP-RAPS model. The straight and meandering lines were 2.54 cm apart to match the size of hybrid coupler. The length of the straight and meandered lines were both 2.68 cm. The meander’s width (a and b) was swept to investigate its relation with the phase difference and thus the splitting ratio, while the trace width of meandering conductor (w) was varied for impedance optimization. The gap between adjacent meanders (g) in the meandering line was kept to a fixed value of 0.9 mm. The width of the straight line was kept to a fixed value of 1.18 mm based on the standard equations. (d): Photograph of a fabricated HP-RAPS.

Figure 2.

Impedance matching (a) and insertion loss (b) of the microstrip lines with different trace widths. The solid line represents the meandering line, and the dotted line represents the straight line.

Figure 3.

Bench tests of the boards with nominal amplitude splitting ratios of 1:1 (a, e, and i), 1:2.5 (b, f, and j), 1:5 (c, g, and k), and 1:7.5 (d, h, and l). The corresponding power splitting ratios expressed in dB are: 0 dB, 7.96 dB, 13.98 dB and 17.50 dB. The power splitting (dB) is shown in a)-d), port matching conditions in e)-h), and isolation between the output ports in i)-l).

Figure 4.

$B_1^{+}$ mapping results with different HP-RAPS circuits (nominal splitting ratios of 1, 2.5, and 5). (a): $B_1^{+}$ mapping results of the single-coil experiment using one port of the Nova birdcage coil. (b): $B_1^{+}$ mapping results of the single-coil experiment using one coil of the 8-channel ICE-decoupled loop array [26]. (c): $B_1^{+}$ mapping results of the two-coil experiment using two quadrature ports of the Nova birdcage coil. (d): $B_1^{+}$ mapping results of the two-coil experiment using two coils of the 8-channel ICE-decoupled loop array. In single-coil experiments, one coil was alternately connected to one output of the HP-RAPS circuit, with the other output terminated with 50 Ω. In the two-coil experiments, two coils were connected to the two outputs of HP-RAPS circuits.