RF coil modeling and analysis in high field MRI: lessons learned

Until recently, RF coil design for magnetic resonance imaging (MRI) has primarily been based upon past experience and simplified circuit models for the coils. For MRI systems with static magnetic field strengths of 1.5 tesla and below, the traditional approaches have been very successful. Because the frequency of operation of the coil is proportional to the static magnetic field strength, the traditional approaches have not been as successful for fields over 2 tesla. The use of full wave analysis and design tools becomes especially important when the coil dimensions approach a significant fraction of a wavelength. For 4 tesla systems and higher, the wavelength is 180 cm or less. Even more important, inside the human head the wavelength is 20 cm or less. Numerical modeling of these coils at high fields has been done previously. We discuss the lessons we have learned using the finite difference time domain (FDTD) method to model the birdcage coil and TEM resonator coil. The results of our modeling show that many of the approximations used to model lower field systems are inaccurate for the higher field systems. The results demonstrate that improvements in the B/sub 1/ field homogeneity and the specific absorption rate (SAR) can be made with proper choices in exciting the coil.