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MRI with zero echo time: hard versus sweep pulse excitation.

Markus Weiger1, Klaas P Pruessmann, Franciszek Hennel

  • 1Bruker BioSpin AG, Faellanden, Switzerland. markus.weiger@bruker-biospin.ch

Magnetic Resonance in Medicine
|March 8, 2011
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Comparing radiofrequency (RF) excitation methods for zero echo time MRI, this study found that while sweep excitation offers higher signal-to-noise ratio (SNR) in ideal conditions, hard pulses are superior when acquisition gaps are necessary.

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Pulse Sequence Design
  • Quantitative MRI

Background:

  • Zero echo time (TE) MRI techniques enable imaging of tissues with very short T2 relaxation times.
  • Achieving zero TE requires simultaneous radiofrequency (RF) excitation and data acquisition during a readout gradient.
  • Two main strategies for spatially nonselective excitation exist: short, hard RF pulses and frequency-swept pulses (e.g., SWIFT).

Purpose of the Study:

  • To compare the performance of hard pulse versus frequency-swept RF excitation for zero TE MRI.
  • To evaluate T2 sensitivity, signal-to-noise ratio (SNR), and SNR efficiency of both excitation methods.
  • To investigate the impact of acquisition gaps, necessary for transmit-receive switching, on SNR.

Main Methods:

  • Simulations and experimental MRI data acquisition were used for comparison.
  • Evaluation metrics included T2 sensitivity, SNR, and SNR efficiency.
  • The study specifically analyzed the effects of RF pulse type and acquisition gapping.

Main Results:

  • Both hard pulse and sweep excitation methods demonstrated comparable T2 sensitivity.
  • In ideal scenarios without acquisition gaps, sweep excitation yielded higher SNR due to larger feasible flip angles.
  • Acquisition gapping significantly reduced SNR, particularly impacting the sweep method due to a reduced acquisition duty cycle.

Conclusions:

  • Hard pulse excitation is advantageous for zero TE MRI when acquisition gaps are unavoidable, especially with sufficient RF amplitude or in the presence of short T2 tissues.
  • Sweep excitation offers potential SNR benefits in idealized, continuous acquisition scenarios.
  • The choice between hard pulse and sweep excitation depends on specific sequence constraints, particularly the need for transmit-receive switching.