Optimized radiofrequency resonators for high field NMR clinical imaging

In high field (>4 Tesla) magnetic resonance imaging (MRI) systems, a major challenge is the design of radiofrequency (RF) coils that exhibit a good signal to noise ratio, tangential magnetic field (B/sub 1/) uniformity, and low specific absorption rate (SAR) in the biological tissues. As the static magnetic field increases, the frequency of operation rises and consequently the electrical dimensions of the head/body and the RF coil become comparable to the operational wavelength. Thus, for these high field systems, the need for an accurate full wave analysis is very apparent to accurately study the electromagnetic interactions between the head/body and the coil. In this work, in accordance with the suggestion of Chen et al. (1998) both the RF coil and the human head are modeled as a single system using the FDTD method. A complete analysis of the TEM resonator (Vaughan et al. 1994) is presented. The FDTD representation accurately models the geometrical structure of the coil including the excitation source, coaxial rods, and the shield. The simulations were performed using a newly developed anatomically detailed human head model with 18 tissue types. Furthermore, new excitation techniques that utilize phased array concepts are proposed and evaluated in 8, 16, and 24 struts TEM resonators.