A dynamic vector model of microstrip RF resonators for high-field MR imaging

This paper describes a dynamic vector model for modelling the electromagnetic characteristics of microstrip radio-frequency (RF) resonators for high field magnetic resonance imaging (MRI). A biological tissue-equivalent load having a circular cross section is assumed in the analysis. The dynamic model uses the well-known Green's function for cylindrically stratified media to characterize all six components of the electromagnetic field excited by the microstrip lines. The accuracy of the method as a function of its parameters is assessed and the results compared with those obtained from the quasi-static method often used at low frequencies. The limits of the quasi-static assumption are investigated by comparing values for the modal propagation constant and the terminating capacitances required to tune the cavity resonance over a frequency range of 100 MHz-1 GHz. The dynamic method is further used to analyse the modal content of a microstrip head resonator. Finally, a variational approach is used to assess the impact of the intermodal coupling for the case of small perturbations in the shape and the position of the cylindrical phantom.