Theory of Double Tuned TEM Resonators and Workbench Validation in a Frequency Range of 100-350 MHz

INTRODUCTION Imaging and spectroscopy at high static field (>3T) is proving to be very successful since it yields increased SNR, allowing improved spatial resolution, and also provides better spectral resolution. The single tuned TEM resonator design has been proposed [1-2] for its improved RF characteristics with respect to the standard birdcage coil. The theory of Multiconductor Transmission Lines (MTL) [3] has been used to calculate the N/2+1 modes of the single tuned TEM resonator, made of N identical coupled coaxial [4-6] or microstrip [7] elements. However, despite the important applications of high field multinuclei MRI, only one paper [2] reported the measured frequency response of a double tuned TEM resonator. In this study we present the MTL modelling of double tuned TEM resonators and workbench validation in a frequency range of 100350 MHz. MTL MODELLING The MTL model of a single tuned TEM resonator made by N identical coaxial elements was previously described [4-6]. It was shown that the frequency response of the TEM resonator can be calculated as S=-log(P). The function P is the determinant of the matrix [1 -GGexp(2gmLt)], where Lt is the length of the TEM resonator and G =TGT are the matrices of the modal reflection coefficients at the two resonator ends G. To calculate G it is necessary to know the modal admittance of the coaxial transmission line element Y0 =TY0T , where Y0 is a circulant matrix. This matrix is diagonalised by T, where Tmn=(1/√N)exp[-j2π(m-1)(n-1)/N]. For determination of G it is also important to know the modal decomposition of the impedance of the termination load line Zc=TZcT [3], where Zc is the matrix of the load impedances. Following the MTL model previously reported [5-6], the load impedance Zc is a diagonal matrix with elements given by Zcii=jωLpii+1/[jω(C lii +/-