## 2 Materials and Methods  
Previous studies have demonstrated that large BC coils produce more uniform magnetic fields under the loading condition, but the field intensity is lower. In contrast, smaller BC coils produce stronger field intensity at the expense of lower field uniformity and produce the central field effect [25]. To optimize the BC coil geometry and produce a uniform field with a large BC coil that has the intensity of a small BC coil, we propose the design of a BC coil with different ring diameters in which the rings are connected to each other. We compared our design with a head BC that is frequently used as conventional coil. The head BC had a length of 280 mm and diameter of 280 mm, as shown in Figure 1a. To gain a more complete understanding of how the magnetic field inside the brain and other human tissues can be improved, we used a dedicated simulation software [26] (Sim4 Life, Zurich, Switzerland) to solve the corresponding equations and obtain the magnetic field. The proposed coil design has a shape that resembles a yurt tent and is therefore referred to as the \"yurt BC coil\". This coil consists of a comparatively small ring with a diameter of 100 mm located at the top and a larger ring with a diameter of 600 mm located at the base. This coil consists of 16 legs; however, here, the legs between the rings are not  
Figure 1: Birdcage (BC) coil geometries in sagittal (**a**) head BC coil, (**b**) yurt BC coil, and (**c**) capacitor placement in the yurt small ring. connected directly but are instead connected in three sections. The first section, with a leg length of 372 mm and width of 5 mm, is connected to an imaginary diameter of 400 mm that is 340 mm away from the top ring. The second section has a length of 168 mm and width of 5 mm, which is connected from the previous section to a 600 mm diameter imaginary circle located at a length of 135 mm along the \(z\)-axis. Finally, the last section is connected to the 600 mm diameter ring at the base in a 324.5 mm long straight line with 5 mm width. The yurt BC coil design is illustrated in Figure 1b. The capacitor placement in the smaller ring of the yurt coil is shown in Figure 1c. The main concept in this design is that the focus of the magnetic field on the periphery of the small ring increases as the coil legs approach each other. The legs of the yurt BC coil are closer to the human head, which helps increase the field intensity compared with the head BC coil. The concept underlying this coil design is that the magnetic field focus can be changed from being off-center, and to maintain a strong field even when the distance between the legs and the subject is increased. The coil length is extended such that the field is focused on the brain area. However, extending the coil length also requires that the ring radius increases such that it is at least shoulder-wide, as shown in Figure 1.  
We conducted RF simulations using the finite-difference time-domain (FDTD) software Sim4 Life, thereby acquiring \(\mid B_{1}^{+}\mid\) field maps. The entire structure was considered to be a perfect electric conductor. A human model (DUKE) served as the basis for our simulations [27], with the basic position altered such that the arms were close to the body, consistent with standard MRI protocol. This human model consisted of over 100 tissues provided by the simulation software, and the electrical properties of each tissues were set to corresponding values at 200 MHz. The coils were excited by a Gaussian voltage current signal with a central frequency of 200 MHz and a bandwidth of 400 MHz. Notably, the coils were excited by two voltage sources and tuned with ring capacitors. A phase shift with a difference of 90\({}^{\circ}\) in total was applied between the voltage sources. For each coil type, the input power was normalized to 1 W. Figure 2 shows the S-parameters for each of the coils while loaded with the human model phantom, and the tuning of the head BC coils (Figure 2a) was done with ring capacitors of 4.1 pF.