2D Pulses using spatially dependent frequency sweeping

Magn Reson Med. 2016 Nov;76(5):1364-1374. doi: 10.1002/mrm.25973. Epub 2015 Nov 27.


Purpose: To introduce a method of designing two-dimensional (2D) frequency-modulated pulses that produce phase coherence in a spatiotemporal manner. Uniquely, this class of pulses provides the ability to compensate for field inhomogeneity using a spatiotemporally dependent trajectory of maximum coherence in a single-shot.

Theory and methods: A pulse design method based on a k-space description is developed. Bloch simulations and phantom experiments are used to demonstrate sequential spatiotemporal phase coherence and compensation for B1+ and B0 inhomogeneity.

Results: In the presence of modulated gradients, the 2D frequency-modulated pulses were shown to excite a cylinder in a selective manner. With a surface coil transmitter, compensation of the effect of B1+ inhomogeneity was experimentally verified, in agreement with simulation results. In addition, simulations were used to demonstrate partial compensation for B0 inhomogeneity.

Conclusion: The 2D frequency-modulated pulses are a new class of pulses that generate phase coherence sequentially along a spatial trajectory determined by gradient- and frequency-modulated functions. By exploiting their spatiotemporal nature, 2D frequency-modulated pulses can compensate for spatial variation of the radiofrequency field in a single-shot excitation. Preliminary results shown suggest extensions might also be used to compensate for static field inhomogeneity. Magn Reson Med 76:1364-1374, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Keywords: MRI; chirp; frequency-modulated pulse; hyperbolic secant; magnetic field inhomogeneity; spatiotemporal encoding.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Algorithms*
  • Image Enhancement / methods*
  • Image Interpretation, Computer-Assisted / methods*
  • Magnetic Resonance Imaging / methods*
  • Phantoms, Imaging
  • Reproducibility of Results
  • Sensitivity and Specificity
  • Signal Processing, Computer-Assisted*
  • Spatio-Temporal Analysis