Numerical Simulation of the Field and Frequency Limits for Volume coils in MRI

As signal to noise ratio in magnetic resonance imaging (MRI) improves with increasing static magnetic field strength there is a strong incentive to develop the technology to acquire images at higher fields. While magnet technology has made it possible to generate fields of 10 Tesla and more, limitations of the radio frequency (RF) hardware prevent the acquisition of high quality images over full regions of interest in clinical applications. Since its introduction, the birdcage coil [1] has become a very popular choice for volume imaging and this paper investigates the high frequency limits of several different sizes and designs of birdcage coils. Unloaded coils are simulated using commercial Method of Moments (MoM) software (FEKO [12]) and the capacitances and radii of the conductors are varied within practical limits to determine a maximum frequency of operation for each coil. Results show that practical birdcages are frequency limited to about 400 MHz for a typical head coil (27cm inside diameter, 30cm shield diameter and 25cm length); 128 MHz for a small whole body coil (60cm inside diameter, 64cm shield diameter and 70cm length) and 100 MHz for a large whole body coil (60cm inside diameter, 64cm shield diameter and 100 cm length). These limits are reduced when interactions with the load are taken into account and experience shows that a typical head coil is limited to about 200 MHz in practical experiments.