Book Description

Accurate control of the radio-frequency (RF) electromagnetic fields in Magnetic Resonance Imaging (MRI) is necessary to ensure patient safety and provide high-quality diagnostic capabilities. Precise control is however becoming increasingly difficult to achieve, given the recent trends toward high fields and transmitter array systems. At high fields, imaging is performed in a frequency regime where the wavelength is on the order of, or smaller than, the dimensions of the human body. This leads to prominent wave behavior, non-uniform field patterns, and increased power deposition. Multi-element transmitter array systems with independent phase and amplitude control of their elements support methods that can mitigate these problems. However, in turn, they demand high fidelity RF reproduction and may lead to undesired electromagnetic interactions between elements of the arrays and with interventional devices. Frequency-offset Cartesian feedback can be used to address all of these issues. In combination with the use of polyphase error amplifiers (to implement a low-IF control bandwidth), Cartesian feedback can be used with MRI power amplifiers and transmit coils to increase the fidelity of RF reproduction, without the in-bandwidth DC-offsets and quadrature mismatches that may lead to imaging artifacts such as bright spots and ghosting. In addition, the control system, which includes autotuning circuitry for stability and vector multipliers circuitry for feedback manipulation, can be used to tune the series output impedance of these amplifiers, thereby reducing the likelihood of interactions between elements of transmit array systems. Furthermore, a miniaturized variation of the control system (called Active Cable Trap) can be used on guidewires to suppress undesired currents elicited by coupling with the RF fields of the transmit coils. In an era of rapid progress in high field MRI for clinical applications, the frequency offset Cartesian feedback method and system thus promises to address many of the challenges faced by designers of multi-element transmitter array systems.