Direct radiofrequency phase control in MRI by digital waveform playback at the Larmor frequency.

PURPOSE: A scalable multiband and multichannel digital magnetic resonance imaging system has been developed with the goal of reducing the time needed for acquisition of a single volume of gradient-recalled echo-planar images of the brain.
METHODS: Transmit pulses are created by an offline computer equipped with a Pentek excitation card (PCIe model 78621) that was built around the Texas Instruments D/A converter (DAC5688).
RESULTS: The spectral purity of pulses made in this way surpasses the quality of pulses made by the standard modulators of the scanner, even when using the same pulse-creation algorithm. There is no need to mix reference waveforms with the magnetic resonance imaging signal to obtain inter-k-space coherency for different repetitions. The key was the use of a system clock to create the Larmor frequency used for pulse formation. The 3- and 4-fold slice accelerations were tested using phantoms as well as functional and resting-state magnetic resonance imaging of the human brain.
CONCLUSION: Synthesizers with limited modulation-time steps should be replaced not only because of the improved spectral quality of radiofrequency pulses but also for the exceptional coherence of pulses at different slice-selection frequencies.