title: Direct radiofrequency phase control in MRI by digital waveform playback at the Larmor frequency.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Biophysics, Medical College of Wisconsin, Milwaukee, Wisconsin, USA.
authors.forename: Andrzej
authors.initials: A
authors.lastname: Jesmanowicz
authors.forename: Andrew
authors.initials: A
authors.lastname: Nencka
authors.forename: James S
authors.initials: JS
authors.lastname: Hyde
mesh_terms: D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005246:Feedback; D006801:Humans; D007089:Image Enhancement / Q000295:instrumentation* / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*