A METHOD FOR ACCURATE CALCULATION OF B-1 FIELDS IN 3 DIMENSIONS - EFFECTS OF SHIELD GEOMETRY ON FIELD-STRENGTH AND HOMOGENEITY IN THE BIRDCAGE COIL

A method for calculating B-1 field strength and homogeneity as functio ns of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theor y, and antenna-array theory. Calculations were performed for a 12-elem ent birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. Calculate d B-1 field strengths and homogeneities for the coil in 25 different s hields and in no shield are given. For configurations where the shield is longer than the coil, both B-1 field strength and homogeneity decr ease as shield diameter decreases or as shield length increases. In co nfigurations where the shield is shorter than the coil and has a diame ter of 25.6 cm, B-1 homogeneity is greater than in an unshielded coil. B-1 field strength was measured experimentally at 125 MHz in a birdca ge coil of the same geometry as the model within shields of four diffe rent diameters. Calculated results very closely matched experimental m easurement. (C) 1997 Academic Press.