High-Speed RF Modulation System for 32 Parallel Transmission Channels at 7T

Introduction: Higher field strengths in MRI are well known for their potential to increase image resolution due to higher signal-to-noise ratio (SNR). However, due to the high dielectric permittivity of human tissue and higher resonance frequencies, and thus shorter wavelengths on the order of the human body size, RF interference artifacts are likely to occur. To overcome these constraints, parallel transmission concepts using multichannel transmit systems have gained significance throughout the past decade. An important part of such concepts is the modulation of the transmit signal using individual phases and amplitudes for each channel to adapt the resulting transmit field to restore the desired image homogeneity. Moreover, such systems facilitate applications in which only well-defined regions-of-interest in the body are excited for dedicated investigation. In earlier work [1], we gained experience with RF shimming using an 8-channel I/Q modulator system that allowed switching of shims at a rate of only about 1 kHz. In this work, we present a modular and extendable digitally controlled modulator system which is designed for high-speed and independent phase and magnitude control of 32 parallel transmission channels at 7T MRI for dynamic modulation during an MR sequence, thus allowing spoke trajectories and TX Sense in addition to static RF shimming. Concept: Each of the 32 channels of the parallel transmit system consists of an individual modulator and power amplifier. The exciter channels of a commercial 8-channel pTx system (Step 2, Siemens Healthcare) are split into sub channels by 1:8 signal splitters utilizing microstrip circuit technology with industrial surface-mount power divider components. Two I/Q modulators, two selectors, and two variable-gain amplifier ICs are placed on a single PCB to provide two of 32 channels in one shielded package (cassette) with the RF output ports feeding the power amplifiers that are remotely placed near the magnet [2]. Fig.1 presents the RF feed network subdivided into two parts, with each half of the system providing one RF power splitter board, the DC power supply, and the control interface for 8 I/Q modulator cassettes (16 channels). Each modulator channel can be individually assigned to one of four channels of the MRI exciter by controlling the selector. A parallel digital interface is used to control the selectors and modulators to enable the adaptive combination of selected coil elements for optimization of the transmit sensitivity and to allow dynamic RF modulation.