## 2 Materials and Methods
### Magnetic Resonance (MR) Experiment  
The prototype elliptical whole-body birdcage RF coil was tested in TX-RX mode for a narrow-bore whole-body 1.5 T MRI system (Chorus, Coretech Co. Ltd., Seongnam-si, Korea) equipped with a 20 kW RF power amplifier (AN8102, Analogic Corporation, Peabody, MA, USA). The ports of the RF coil were connected to a hybrid coupler integrated with a TX/RX switch for quadrature excitation. B1\({}^{+}\)-field mapping experiments were performed on a cylindrical phantom (inner diameter of 400 mm; length of 140 mm) comprising a copper sulfate solution (1 g CuSO4\(\cdot\)5H2O per 1000 mL of distilled H2O) to measure the B1\({}^{+}\)-field uniformity, and in vivo experiments were performed on a healthy male human volunteer (30 years old, 70 kg) using a composite RF pulse B1 mapping method (further details are provided in Supplementary Materials Section B) comprising two acquisitions with different RF pulses (#1-90\({}^{\circ}\)y-180\({}^{\circ}\)x-90\({}^{\circ}\)y and #2-90\({}^{\circ}\)y-0\({}^{\circ}\)x-90\({}^{\circ}\)y) in a single scan [40]. T1-weighted spin-echo MR images were acquired for SNR measurement in both the phantom and the in vivo human volunteer. The sequence parameters are shown in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Uniform Phantom**} & \multicolumn{2}{c}{**In Vivo Human**} \\ \hline \multirow{2}{*}{**Sequence**} & **Composite** & \multirow{2}{*}{**T\({}_{1}\)-Weighted Spin-Echo**} & \multirow{2}{*}{**Composite Spin-Echo**} & \multirow{2}{*}{**T\({}_{1}\)-Weighted Spin-Echo**} \\  & **Spin-Echo** & & & \\ \hline Nominal FA (\({}^{\circ}\)) & 180 & 90 & 180 & 90 \\ \hline TE/TR (ms) & 25/1600 & 15/500 & 15.4/600 & 13.2/600 \\ \hline FOV (mm\({}^{2}\)) & 500 \(\times\) 500 & 500 \(\times\) 500 & 420 \(\times\) 420 & 420 \(\times\) 420 \\ \hline Acquisition matrix & 256 \(\times\) 256 & 128 \(\times\) 128 & 128 \(\times\) 128 & 256 \(\times\) 256 \\ \hline Slice thickness (mm) & 5 & 5 & 8 & 8 \\ \hline Average & 2 & 2 & 1 & 1 \\ \hline Scan time (min) & 10 min 38 s & 4 min 16 s & 4 min 1 s & 2 min 41 s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: MR sequence parameters. Figure 2: (**a**) Schematic illustration of split-type elliptical birdcage showing positions of legs, capacitors, and ports; (**b**) unfolded view; detuning circuit is not shown for simplicity; (**c**) constructed split-type elliptical birdcage; (**d**) implemented setup for transmission and reception at 1.5 T MRI. For reference, all of the MR sequences were performed on a commercial two-port circular birdcage RF coil (20 legs; diameter, 58 cm; length, 60 cm). The study was approved by our institutional review board, and written informed consent was obtained from all volunteers before the experiments commenced. FA maps, SNR maps, and other post-processing visualizations were reconstructed from raw MR data using MATLAB. The FA maps were smoothed using a 3 \(\times\) 3 median filter for each image. The SNR maps were calculated in the acquired phantom and in vivo MR images. The mean SNR was calculated in the ROI using Equation (2), as follows [41]:  
\[SNR_{mean}=0.66\times\frac{\textit{Mean signal of ROI}_{B}}{\textit{Average standard deviation of ROI}_{s}} \tag{2}\]  
where the mean signal of ROI\({}_{B}\) represents the mean signal values in a two-dimensional (2D) ROI (diameter, 30 cm) within the phantom, and the average standard deviation of the _ROI\({}_{S}\)_ represents the average standard deviation of noise in the four small _ROI\({}_{S}\)_ (diameter, 6 cm) placed diagonally outside the phantom at the corner of the MR image. For the MR images acquired from quadrature volume RF coils, a factor of 0.66 (Rayleigh distribution correction factor) was necessarily multiplied to obtain the true SNR [42].