End cap design for bird cage coils in nuclear magnetic resonance imaging

Two end cap designs for bird cage coils for application in nuclear magnetic resonance head imaging are presented. The objectives of the new design are to improve the B₁ field homogeneity within the coil and to improve the SNR. Design I extends the legs and one end ring of the bird cage coil onto the end cap. The radius of the end ring on the end cap is optimized to maximize the axial field of view with a limited B₁ field homogeneity. Design II uses a metal disk plate as a mirror placed at one end of the bird cage coil; it is analyzed by using a moment method. The size of the metal plate is optimized. An analysis of the two end cap designs is presented. The results show that in design I the optimal radius of the end ring on the end cap is about 2.5 times smaller than the coil radius; in design II, the inner radius of the disk mirror should be as small as possible and the mirror should be as close as possible to the coil for B₁ field homogeneity; making the outer radius of the disk larger than the coil radius does not much improve the B₁ field homogeneity