A Method for Accurate Calculation ofB1Fields in Three Dimensions. Effects of Shield Geometry on Field Strength and Homogeneity in the Birdcage Coil |
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| Authors: | Christopher M. Collins Shizhe Li Qing X. Yang Michael B. Smith |
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| Affiliation: | aDepartment of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, 19104;bDepartment of Radiology, Center for NMR Research, Pennsylvania State University College of Medicine, Milton S. Hershey Medical Center, Hershey, Pennsylvania, 17033;cCenter for NMR Research, Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Milton S. Hershey Medical Center, Hershey, Pennsylvania, 17033 |
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| Abstract: | A method for calculatingB1field 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. CalculatedB1field 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, bothB1field 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,B1homogeneity is greater than in an unshielded coil.B1field 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. |
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