## 4 Discussion and Conclusions  
The aim of this study was to present an RF coil circuit co-simulation approach to calculate the optimal capacitor for four-port split-type asymmetric elliptical birdcage RF coil and its construction procedure, which can generate a homogenous B\({}_{1}\)\({}^{+}\)-field and an acceptable transmission efficiency for the narrow bore 1.5 T MRI system. The RF coil circuit co-simulation approach previously reported was implemented for a multi-channel surface loop RF coil [29], which did not have characteristic B\({}_{1}\) resonance modes, as observed in the birdcage RF coil. In this work, the RF coil circuit co-simulation approach was extended to calculate the optimal capacitor of the proposed birdcage RF coil in the presence of loading as well as coupling between the excitation ports, which required more careful consideration of the optimal capacitor value to generate the desired resonance mode. The decoupling between the excitation ports was very crucial, as they had significant effects on the input power and the SAR deposition (effect of decoupling is shown in Supplementary Materials Section A). Additionally, the circular birdcage RF coil resonance modes occurred at the same frequency when using identical-value end-ring capacitors, however, the elliptical birdcage RF coil had horizontal and vertical resonance modes which needed to be maintained at the  
Figure 5: Experimental MRI results of in vivo human volunteer obtained in axial and coronal orientations: (**a**) SNR map acquired using T1w spin echo sequence; (**b**) FA map (nominal FA—180\({}^{\circ}\)) acquired using composite spin echo sequence in commercial circular birdcage (top row) and constructed split-type elliptical birdcage (bottom row). same resonant frequency using different-value end-ring capacitors. During the evaluation of the elliptical birdcage, it was discovered that the homogeneity of the B1+-field could be increased further if the leg positions became asymmetric around the circumference and the position of the legs shifted away from the shield in the horizontal axis. However, asymmetric legs had less B1+-field distribution near the empty region of the birdcage coil. To improve this, the legs were split, and the gaps between the splits were maintained identical to the gap between adjacent non-split legs. This method improved the B1+-field distribution in the aforementioned regions with a moderately higher peak 10 g average SAR compared with the circular birdcage. To further improve the B1+-field distribution, elliptical driving was implemented because the horizontal and the vertical modes had different B1+-magnitudes at the center of the RF coil. During EM simulation post-process analysis, it was discovered that the 60\({}^{\circ}\) phase difference between adjacent ports and a 180\({}^{\circ}\) phase difference between opposite ports generated the maximum uniform B1+-field distribution. Similarly, the different amplitudes of the input power at the horizontal (#2 = #4) and the vertical (#1 = #3) ports were investigated. It was also discovered that the B1+-field uniformity increased with the amplitude difference between the horizontal (#2 and #4) and the vertical (#1 and #4) ports up to 12 dB in the uniform phantom and up to 6 dB in the human \"Duke\" model. It is noteworthy that the input power of the vertical ports was lower than that of the horizontal ports. Furthermore, the four-port implementation in the closely shielded elliptical birdcage could be implemented if the ports were located diagonally, as in the case of the two-port circular birdcage. However, the maximum B1+-field uniformity achieved in this state was 5% lower than that achieved by horizontal-vertical port feeding. Based on the EM simulation, an experimental prototype of a split-type asymmetric elliptical birdcage was constructed and verified through MRI experiments. The calculated and measured end-ring capacitor values varied from 10% to 15% owing to the change in the ideal condition in the EM simulation to real complex conditions in the MRI system.