High-Bandwidth, High-Efficiency RF Amplitude Modulation for On-Board Transmit Amplifiers

Introduction Switching RF amplifiers appear to be a promising alternative for on-board transmit amplifiers due to its high efficiency and high isolation to neighboring elements [1,2]. We have shown that the current mode class-D amplifier (CMCD) allows a direct control of the current through the coil, resulting in a B1 field, which is potentially less sensitive to load changes than some other alternatives [3]. In this work we present a novel system to modulate the amplitude of the RF output signal of a CMCD amplifier that keeps the intrinsic high efficiency and improves load insensitivity. In the proposed system (Fig. 1), an amplitude Sigma-Delta Modulated (SDM) signal will be sent to an on-board amplifier configuration. The signal will be demodulated and used as a control signal of a Pulse Width Modulated (PWM) controller that controls the gate of high frequency MOSFETs in a buck converter configuration [4]. This converter constitutes the power supply of the output stage of a CMCD amplifier that connects to an RF coil. The current on the coil is sensed and sent as a feedback signal to the PWM controller to directly control the current in the coil. Material and Methods Since the PWM switching frequency should be at least 10 times the desired AM bandwidth (BW), a switching frequency in the order of MHz is needed. For this, we used a 2MHz switching frequency for a theoretical 200kHz maximum BW. At this high switching frequency the resulting output inductor and capacitor are relatively small with values of 400nH and 4.7 μF respectively. We have used RF MOSFETs (MRF136) that have very low input capacitance. In a first experiment the converter was adjusted and tested with a PWM signal generated from a data-timing generator (Tektronix DTG7058 750Mb/s) that controls the MOSFETs . In a second step a PWM controller (TL1454, TI) was implemented that allows a switching frequency up to 2Mhz and has an integrated error amplifier with high BW and high slew rate (SR). Preliminary results were obtained in combination with a 7T transmit RF CMCD amplifier [3]. At this stage we used the output of an analog function generator as reference signal, the output current was sensed through a small coil physically coupled to a portion of the trace of the transmit coil. The sensed AM envelope was demodulated through a half wave rectifier. To analyze the load sensitivity of the system we loaded the coil with a bottle filled with saline and measured the peak of the RF signal generated in the coil while changing the relative distance between this and the transmit coil without retuning or rematching, the experiment was repeated using a voltage-mode, 50 Ω coil for comparison. Results In Fig 2A a clean converter output is shown. In this case there is no feedback and a PWM ramp wave controls the converter MOSFET gates. The buck converter output is used to power the output stage of the 7T CMCD amplifier and the feedback is closed through the coil sensor. Fig. 2B shows a 2Mhz PWM 10kHz triangle wave that controls the switching of the upper MOSFET (the control signal of the lower MOSFET is complementary). The output of the buck converter and the resulting AM RF signal in the coil measured with pick-up loop is shown in Fig. 2C and D. Finally Fig. 3 shows the load sensitivity curves of the closed loop current-mode configuration (blue) and an open loop voltage-mode configuration (red). For the current-mode configuration the overall drop in peak B1 field was 39% with respect to the initial value when the relative distance between the phantom and coil was changed by over a factor of 3, while for the tuned voltage-mode device the change in output was significantly higher. Discussion In the present work we showed the feasibility of designing a novel system for RF amplitude modulation to be implemented in onboard high efficiency amplifier. We successfully showed the system can handle up to 20kHz AM signal and higher; however further improvement needs to be done to the feedback network to approximate the practical BW value to theoretical maximum of 200kHz.We believe a system like this will operate with the high efficiency characteristic of switching mode converters (higher than 80%) and intrinsic load insensitivity due to the direct current feedback. References [1] Kobayashi H., et al., IEEE Trans. Mic. Th. Tech. 49(12) 2480-85 (2001) [2] Kurpad K.N., et al. Concepts in Magnetic Resonance, Vol. 29B(2) 75– 83 (2006) [3] J.A. Heilman, et al. Proc. ISMRM 2007:00171 [4] Xuefang Lin-Shi et al. IEEE Industrial Electronics, IECON 2006 32nd Annual Conference on Acknowledgement This abstract was partially supported by Siemens Medical Solutions Fig. 1 AM System implemented to a CMCD amplifier DS signal IRE