A digitally controlled class-e amplifier for MRI

Abstract

Radio-frequency (RF), or B1field is used for slice selection purposes in Magnetic Resonance Imaging (MRI). In an MRI scanner thisfield is handled by the RF chain which consists on a pulse generator unit, a high power amplifier and a transmit coil. Relatively low effciency linear power amplifiers are used. These amplifiers are placed in the system room, far from the transmit coil. In this work we propose to design the amplifier and the coil as a single unit, aiming to decrease cost and complexity while improving performance. Splitting the coil into multiple elements makes possible to drive these elements with individual amplifiers in Transmit Array (TxArray) mode. Also these elements are integrated as the load network of the amplifier, in this case a high effciency Class-E amplifier. The Class-E amplifier was modified for the intended application and it was digitally controlled. For the pulse generation two dierent methods were applied. One was by controlling separately the phase and amplitude of the pulse. The other method generates simultaneously phase and amplitude by controlling the switching pattern of the amplifier. An amplifier of output power 100W with effciency up to 88% was developed. As a step toward 36 channel complete system, 2 element prototype's operation was tested in a 3T Tim Trio Siemens scanner. In overall, Class-E amplifier is shown to be a promising candidate for on-coil RF excitation in TxArray in terms of size, cost, effciency and complexity.