1.5T On-Coil Current-Mode Class-D (CMCD) Amplifier with Amplitude Modulation Feedback and Voltage-Mode Class-D (VMCD) Preamplifier

Material and Methods: The amplifier diagram is shown in Fig. 1. A digital encoded RF pulse is amplified and split into two out-of-phase digital signals through a high speed Emitter Coupled Logic (ECL) comparator. This is demodulated through a band-pass filter and further amplified through a cascade of high-speed differential amplifiers. Two stages of Voltage-Mode Class-D (VMCD) amplifiers boost the voltage amplitude of the out-of-phase signal to a high enough level to efficiently switch the MOSFET of the CMCD output stage. The amplitude of the RF signal is modulated by connecting the power stage to the output of a customized Buck regulator controlled by PWM [6]. The digitally encoded target RF envelope is sent to the system, demodulated and compared with the sensed RF envelope [7]. The actual transmitted RF amplitude is obtained by a wire loop coupled to the coil [8] followed by a low-pass filter with a cutoff frequency below the coil resonant frequency (63.6 MHz). A trigger signal is sent to the saw oscillator of the PWM control to avoid any false switching when there is no RF pulse. In order to measure output power with a 50Ω standard technique a 1: 25 impedance transformer was connected to the output stage. Decoupling between two oncoil transmitters was evaluated in a 1.5T clinical MRI scanner (Espree, Siemens) using the body coil as the receiver. A data timing generator (Tektronix DTG7058 750Mb/s) synchronized with the scanner was used to synthesize the digital encoded RF pulse and RF envelope for each of the channels. A FLASH sequence with 1ms Gauss RF pulse excitation (16ms TR, 7.8ms TE, 10mm slice thickness and 30x30cm FOV) was used to image a cylindrical phantom.