Yubin Cai^{1}, John Stager^{1}, Hsin-Jung Yang^{1}, Debiao Li^{1}, and Hui Han^{1}

^{1}Biomedical Imaging Research Institute, Cedars-Sinai Medical Center, Los Angeles, CA, United States

One appealing feature in iPRES coil implementation is that a traditional array can be simply modified by adding RF chokes

$$ Q[ S_{11} method] = \frac{2\omega_0}{\Delta\omega_{+3dB}} \textrm{ <or> } Q[ S_{12} method] = \frac{\omega_0}{\Delta\omega_{-3dB}} $$

Where $$$ \omega_0 $$$ is the resonant frequency. There are two methods to extract Q due to reciprocity between the transmission and the reception of antennas: 1) S11 method, where $$$ \Delta\omega_{+3dB} $$$ is the bandwidth between the +3dB points on the S11 plot and 2) S12 method, where $$$ \Delta\omega_{-3dB} $$$ is the bandwidth between the -3dB points on the S12 plot. Q of an iPRES coil in and out of the presence of a phantom, $$$ Q_{loaded} $$$ and $$$ Q_{unloaded} $$$ respectively, can lead to a numerical prediction of the coil’s signal sensitivity,

$$ SNR = SNR_0 \cdot \sqrt{1 - \frac{Q_{loaded}}{Q_{unloaded}}} $$

Where $$$ SNR_0 $$$ is the total available intrinsic SNR. Thus, by comparing Q ratio ($$$ =\frac{Q_{unloaded}}{Q_{loaded}} $$$) of two coils, the relativity of the SNRs can be determined.

The simulation setups in CST are shown in Fig. 1. The phantom simulated (20-cm-diameter, 17-cm-depth) has a conductivity of 1.109 S/m and relative permittivity of 72.84 at 123MHz. Further, the Q performance is investigated through a circuit-level S11 analysis, based on the circuit diagram shown in Fig. 2. The inductance and resistance of the coil are derived through classic analytic solution of loop antenna’s impedance

Fig. 4 provides coil engineers with a clue to consider the choke effect on iPRES coils. It can be observed that if ESR is equal to zero meaning chokes only store energy rather than dissipate, then the less $$$ L_{choke} $$$ is, the higher overall Q is; if ESR is nonzero, then the reverse is true. Intuitively, the overall coil Q is affected by how much current flows through the ESR in chokes. Maneuvering through the plot in Fig. 4 is not as freely as it might seem. Inductance of a choke is determined by many factors including the length of the conductor, coupling between turns, and geometries. Thus, although a 1uH choke seems to have an optimal Q, a poorly designed 1uH choke with ultra-high ESR might have a Q lower than that of a choke with less inductance. It is important to appreciate the trade-offs during the choke design phase. iPRES alike coils has been a rising topic and analytical work is helpful to understand such novel coils. Future work may include the effects of DC wiring, comprehensive Q and SNR evaluation, bench and scanner measurements.

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Fig. 1. CST simulation setups with tabulated
parameter values of conventional coil and two iPRES with same main coil
dimensions but different RF chokes, 1uH and 300nH. Both toroidal chokes share the same 18-mm outer diameter
and 0.644-mm wire diameter. AWG 16 and 18 copper wires are used for the
main loop and the chokes, respectively. A double S12
probe is positioned 5 cm away from the coil plane to obtain S12.

Fig. 2. Equivalent circuit model of iPRES coil
with RF chokes. L and R in the diagram comprise the coil impedance.

Fig. 3. CST simulated S12 of iPRES coils with a 1uH
or 300nH RF choke in or out the presence of the phantom. The extracted unloaded and loaded quality
factors are shown in the tables.

Fig. 4. Unloaded quality factor of iPRES coil with
respect to the ESR in the RF choke with different levels of choke inductance
values shown in the legend. Results are generated through circuit-level analysis. The red data points correspond to the two
illustrated iPRES coils that are studied through CST simulation and circuit
analysis.