Parallel Excitation in Ultrahigh Field Human MR Imaging and Multi-Channel Transmit System.

Abstract

Parallel excitation [1,2] with a multi-element Radio Frequency (RF) transceiver array [3-9] as a contemporary methodology has been advocated for human MR imaging at ultrahigh magnetic fields (7 Tesla and above). In ultrahigh field MRI, the required high operating frequency and thus shortened wavelength of radio frequency waves creates a complex wave behavior and increased phase variation of RF magnetic fields (i.e. B1 fields) in conductive and high dielectric biological samples, such as human body, resulting in inhomogeneous image distribution. The inhomogeneous image distribution consequently leads to difficulties in quantifying the MR signal intensity. With independent phase and amplitude control of each channel of a transceiver array, parallel excitation can be applied to perform B1 shimming to obtain uniform B1 distribution. In MR safety aspect, RF power required to excite the spins increases dramatically at ultrahigh fields compared with that at lower fields, e.g. 1.5T. The high RF excitation power results in high Specific Absorption Rate (SAR) in human body, ultimately increases tissue heating during MRI. It is demonstrated that by using the parallel excitation method, the RF excitation profile can be optimized, providing in a significantly reduced SAR and therefore safer MRI at ultrahigh fields. In fact, the emerging method of parallel excitation has become essential for ultrahigh field MRI in addressing B1 in homogeneity, increased SAR and tissue heating.