High-Power DC Controlled Variable Capacitors for MR Engineering - Project Summary/Abstract The objective of this proposal is to develop a new device for the engineering of radiofrequency (RF) transmit coil systems for MRI and MRS. RF coils have been an active and important area of research in MRI for decades. Research in receiver coils is accessible to most sites as multiple channel receivers are now nearly ubiquitous. Additionally, once the signals are digitized, processing (phase and amplitude adjustments) on the received signals are done during reconstruction. Finally, for tuning and adjusting the coils themselves, varactor diodes provide a voltage variable capacitance that can be used to manipulate low-power RF signals. The case is quite different with transmit arrays. Varactor diodes generally do not work due to the high voltages required in the RF transmit chain. Thus, phase shifting elements are often simply transmission lines, possibly switched with PIN diode switches. Tuning is fixed or adjusted manually or with switches. Switched elements can be bulky and result in discretization that may not be sufficient for some B1 shimming applications or applications such as Transmit SENSE. This is of particular importance in the transmit chain because all signal adjustments are done one time, prior to each transmission- not during reconstruction. This has limited transmit array design to a much smaller group of sites, primarily those with multiple channel transmitters, which can be quite costly. While eight channel transmitters are available on 7T systems, they are rarely available with more than two channels at 3T and below. This proposal aims to develop DC controlled variable capacitors for high-power applications. The project investigates two approaches: (1) using the drain-source voltage dependent output capacitance of MOSFET transistors that are biased off as variable capacitors and (2) using the DC bias voltage dependence of ferroelectric capacitors. We have defined three specific aims. Aim 1 will be to implement stand-alone high- powered DC controlled variable capacitors using both methods and to characterize them for thermal and RF voltage dependence. Aim 2 will use these devices to construct DC controlled phase shifters and variable power splitters. Both will be tested for thermal and RF level dependence and any waveform distortion impacts, on the bench and in MRI test. Finally, aim 3 will construct a four channel transmit array with independent amplitude and phase control on each channel without requiring multiple channel transmitters. This aim will again characterize B1 pattern control and stability. Successful implementation of the first aim will provide the MR community with a new device for tuning RF circuits, and lead to new methods to MR Engineering of RF transmit systems without resorting to discrete and bulky switching circuits. Overall success will both demonstrate the effectiveness of the proposed technology and provide immediate translation to users in the MR community.
