Magnetic resonance imaging (MRI) is a medical imaging technique in widespread use for viewing the structure and function of the human body. MRI systems provide soft-tissue contrast, such as for diagnosing many soft-tissue disorders. MRI systems generally implement a two-phase method. The first phase is the excitation phase, in which a magnetic resonance signal is created in the subject with a main, polarizing magnetic field, B0, and a radio frequency (RF) excitation field, B1+. The second phase is the acquisition phase, in which the system receives an electromagnetic signal emitted as the precessing nuclei induce a voltage in a receive coil via the Faraday effect. After the excitation and precession phase, the nuclear magnetic moments relax back into alignment with the main magnetic field with the characteristic time T1 (e.g., about 1 second in the brain). These two phases are repeated pair-wise to acquire enough data to construct an image.
Higher magnetic field strength scanners have been recently used to improve image signal-to-noise ratio and contrast. However, a spatial variation in the magnitude of the RF excitation magnetic field, B1+, occurs with main magnetic field strengths of, for example, 7 Tesla. This undesirable non-uniformity in the excitation across the region of interest is commonly referred to as “center brightening,” “B1+ inhomogeneity” or “flip angle inhomogeneity.”
Newer-generation MRI systems have the capability of generating RF pulses with a spatially tailored excitation pattern to mitigate the B1+ inhomogeneity inherent to high magnetic fields by exciting a spatial inverse of the inhomogeneity. In these systems, multiple radio-frequency pulse trains are transmitted in parallel over independent radio-frequency transmit channels, e.g., the individual rods of a whole-body antenna. This method, referred to as “parallel transmission” or “parallel excitation,” exploits variations among the different spatial profiles of a multi-element RF coil array. Parallel excitation has enabled several important applications beyond the mitigation of B1+ inhomogeneity, including flexibly shaped excitation volumes, and minimization and management of power deposition in tissue as measured by specific absorption rate (SAR).
Unfortunately, in parallel transmission systems, power from one channel may be coupled, i.e., partially delivered, to other channels. Such coupling interferes with the incident waves of the pulses of the other channels. Coupling also reduces the power efficiency of the MRI system, insofar as power coupled from one channel to another is redirected to resistive loads for dissipation in order to protect the power amplifiers. This power is therefore lost and cannot be used to excite the MRI signal.