Resistor-free and one-board-fits-all ratio adjustable power splitter for add-on RF shimming in high field MRI

Abstract

Ratio adjustable power splitter (RAPS) circuits were recently proposed for add-on RF shimming. Previous RAPSs split the input RF signal with a Wilkinson splitter or 50-Ω-terminated hybrid coupler into two branches, delay these two signals with cable/microstrip line phase shifters, and recombine them with another hybrid coupler. They require resistors to provide high output isolation and a cable/microstrip line library to realize desired splitting ratios. Here we propose a novel resistor-free RAPS circuit in which the Wilkinson splitter/50-Ω-terminated hybrid is replaced with a resistor-free T-junction splitter. A novel sliding mechanism was employed to further combine the T-junction's output arms with subsequent phase shifters and realize a one-board-fits-all design. The resistor-free RAPS was theoretically analyzed, simulated, and validated on workbench and MRI experiments. The resistor-free RAPS's splitting ratio has a tan/cot dependence on the phase/length difference between the T-junction output arms. The ratio can be continuously adjusted to any value by sliding the input arm without additional cable/microstrip libraries, largely saving time and effort when determining the best RF weights in practice. The fabricated resistor-free RAPS has a compact size, excellent input impedance matching, and a low insertion loss. Potential safety concerns caused by unwanted power dissipation on RF resistors are eliminated. The simulation and MRI experiments demonstrated that the resistor-free RAPS functions well on a widely-used Tx coil.

Keywords: One-board-fits-all; Power splitter; RF shimming; Resistor-free; Ultrahigh field.

Figure 1

Evolution of the resistor-free RAPS. a) Conventional RAPS consisting of a hybrid, a Wilkinson splitter, and a pair of phase shifters between them [23]. b) The Wilkinson splitter was replaced with a resistor-free T-junction power splitter. c) The phase shifters and output arms of the T-junction splitter were combined so that the ratio could be adjusted by simply sliding the input arm of the T-junction. With a total phase (i.e., φ1 +φ2) of π/2, this resistor-free RAPS can realize any output ratio. A π-shaped matching circuit was added to the input arm to ensure impedance matching of the input port. The gray dots in b) and c) represent the splitting points.

Figure 2

a) EM simulation set up for the resistor-free RAPS on a birdcage coil. The turquoise sphere represents the phantom, the orange represents the conductor, and the blue pieces separating the conductor are capacitors or ports. The circuit diagram shows the configuration and capacitor values of the birdcage coil. b) Simulated S-parameter plots versus frequency of the birdcage coil. Both ports (I and Q) were tuned to the desired resonant mode (k = 1), with S₁₁<−20 dB and S₁₂<−15 dB. c) Circuit simulation of the resistor-free RAPS with input and outputs terminated with 50-Ω ports, corresponding to the setup in practical bench tests. d) EM and RF circuit co-simulation setup on the birdcage coil. φ₁ and φ₂ are the phase delays that determine the splitting ratio.

Figure 3

a) CAD model of the resistor-free RAPS circuit. b) Fabricated device based on the CAD model.

Figure 4

Single-port (a,b) and two-port (c,d,e) experimental setups of the resistor-free RAPS on the Nova birdcage coil. a), b) B₁⁺ maps were obtained by connecting RAPS’s two outputs alternatively to port I of the birdcage coil. c), d) Measurement of individual B₁⁺ maps from ports I and Q of the birdcage coil. These individual maps were used to predict the combined B₁⁺ maps with resistor-free RAPS circuits inserted. e) Measurement of combined B₁⁺ maps with RAPS’s two outputs connected to birdcage coil’s two ports (I and Q).

Figure 5

Circuit simulation results of an ideal resistor-free RAPS circuit. a) Matching of the input port. b) Insertion loss of the whole circuit. c) Phase difference between two outputs. d) Output isolation. e) Relationship between the output power splitting ratio and (φ1-φ2). Port 1 is the input and ports 2,3 are the outputs.

Figure 6

Predicted and co-simulated B₁⁺ fields. The predicted B₁⁺ fields were calculated based on the circuit-simulated S-parameters and the EM-simulated individual B₁⁺ maps. The co-simulated B₁⁺ fields were generated by the EM and circuit co-simulation (Figure 2d).

Figure 7

a) Measured matching b) insertion loss c) phase shift between two output ports and d) isolation between two output ports.

Figure 8

B₁⁺ mapping results of the single-coil experiment using port I of the Nova birdcage coil, with the ratio adjusted to nominal power splitting ratios of 1, 2, 4 and 8. In single-coil experiments, the birdcage coil’s I port was alternately connected to one output of the resistor-free RAPS circuit, with the other output terminated with 50 Ω.

Figure 9

B₁⁺ mapping results of the two-port/coil experiment using two quadrature ports of the Nova birdcage coil. In two-port/coil experiments, ports I and Q of the birdcage coil were connected to the two outputs of the resistor-free RAPS circuit with different splitting ratios.