Application of finite difference time domain method for the design of birdcage RF head coils using multi-port excitations.

A three-dimensional finite difference time domain model was developed where the high pass birdcage coil and the imaged object are analysed as a single unit. A study was performed comparing linear, conventional quadrature, and four-port excitation at 64 MHz and 200 MHz for different coil loadings, namely muscle phantoms and an anatomically detailed human head model. A phase array concept was utilized to excite the birdcage coil in four ports. Two phase conditions were analyzed, the simple fixed phase and the variable phase. At 200 MHz, compared to the conventional quadrature drive, the four-port drive reduces the effects of the tissue-coil interactions leading to more uniform currents on the coil legs and consequently to a better B(1) field homogeneity. Also at 200 MHz, driving the coil in four ports provides an SAR distribution with peak values that are significantly less than those with linear or quadrature excitations.