A Method for Accurate Calculation of B₁ Fields in Three Dimensions. Effects of Shield Geometry on Field Strength and Homogeneity in the Birdcage Coil

A method for calculating B₁ field strength and homogeneity as functions of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theory, and antenna-array theory. Calculations were performed for a 12-element birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. Calculated B₁ field strengths and homogeneities for the coil in 25 different shields and in no shield are given. For configurations where the shield is longer than the coil, both B₁ field strength and homogeneity decrease as shield diameter decreases or as shield length increases. In configurations where the shield is shorter than the coil and has a diameter of 25.6 cm, B₁ homogeneity is greater than in an unshielded coil. B₁ field strength was measured experimentally at 125 MHz in a birdcage coil of the same geometry as the model within shields of four different diameters. Calculated results very closely matched experimental measurement.