We design a crisscrossed double-layer birdcage (DLBC) coil by modifying the coil geometry of a standard
single-layer BC (SLBC) coil to enhance the homogeneity of transmitting magnetic flux density (B₁⁺) along the main magnetic field (B₀)-direction for small-animal magnetic resonance imaging (MRI) at 300 MHz. The performance assessment of the crisscrossed DLBC coil is conducted by computational analysis with the finitedifference time domain method (FDTD) and compared with SLBC coil in terms of the B₁ and the B₁⁺ distribution. As per the computational calculation studies, the mean value in the two-dimensional B₁⁺ map obtained at the mid-axial slice with the proposed DLBC coil is slightly lower than that obtained with the SLBC coil, but the B₁⁺ value of the DLBC coil in the outermost plane (40 mm away from the central plane) shows improvements of 19.3% and 24.8% over the SLBC coil B₁⁺ value when simulating a spherical phantom and realistic mouse body modeling. These simulation results indicate that, the B₁⁺ homogeneity along the z-direction was improved by using DLBC configuration. Our approach enables B₁⁺ homogeneity improvement along the zdirection, and it can also be applied to ultra-high field (UHF) MRI systems.