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Generating accurate tip angles for NMR outside the rotating-wave approximation
Christopher P Bidinosti1, Geneviève Tastevin2, Pierre-Jean Nacher2
1Department of Physics, University of Winnipeg, Winnipeg, MB R3B 2E9, Canada.
Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|November 26, 2022
Summary
Accurate tip angles in low-field nuclear magnetic resonance (NMR) require careful pulse design. Strategies involving phase-dependent shifts and pulse timing can achieve precise control outside the rotating-wave approximation.
Area of Science:
- Nuclear Magnetic Resonance (NMR) Spectroscopy
- Magnetic Resonance Imaging (MRI) Physics
Background:
- Accurate tip angles are crucial for nuclear magnetic resonance (NMR) applications.
- The rotating-wave approximation (RWA) may fail in low static fields with linear rf polarization, causing deviations in Bloch sphere trajectories.
Purpose of the Study:
- To investigate deviations from expected Bloch sphere trajectories in low-field NMR under linear rf fields.
- To identify strategies for achieving accurate tip angles when the RWA is invalid.
Main Methods:
- Numerical simulations of NMR pulse sequences.
- Low-field NMR experiments using 1H and 3He nuclei.
- Analysis of phase-dependent Bloch-Siegert shifts and pulse timing effects.
Main Results:
- Rectangular rf pulses show phase-dependent deviations, exacerbated by pulse transients.
- Phase-dependent Bloch-Siegert shifts and optimized pulse timings can correct trajectories.
- Bloch-Siegert shift magnitude depends on pulse shape and rf coil characteristics.
Conclusions:
- Accurate tip angle generation outside the RWA in low-field NMR is achievable with specific strategies.
- Consideration of pulse phase, timing, and rf coil parameters is essential for precise control.
- Demonstrated experimental validation with 1H and 3He NMR.
Keywords:
Bloch–Siegert shiftCounter-rotating fieldLinear rf fieldLow-field NMR/MRIRotating-wave approximationTRASE (transmit array spatial encoding) MRIrf pulsesrf transientsMore Related Videos
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