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Axis-matching excitation pulses for CPMG-like sequences in inhomogeneous fields
Soumyajit Mandal1, Van D M Koroleva2, Troy W Borneman3
1Schlumberger-Doll Research, One Hampshire Street, Cambridge, MA 02139, United States.
Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|October 16, 2013
Summary
New axis-matching excitation pulses improve Carr-Purcell-Meiboom-Gill (CPMG) sequence performance in inhomogeneous fields. These pulses enhance signal bandwidth and echo amplitudes, significantly boosting signal-to-noise ratio for better relaxation time measurements.
Area of Science:
- Magnetic Resonance Imaging
- Pulse Sequence Optimization
- Nuclear Magnetic Resonance Spectroscopy
Background:
- Standard CPMG sequences suffer performance degradation in inhomogeneous magnetic fields.
- Broadband excitation and refocusing pulses can mitigate these issues.
- Existing methods may not fully optimize signal response for specific refocusing pulses.
Purpose of the Study:
- Introduce novel axis-matching excitation pulses for improved CPMG performance.
- Optimize signal generation by aligning excitation with refocusing pulse effective rotation.
- Enhance signal bandwidth, echo amplitudes, and signal-to-noise ratio (SNR).
Main Methods:
- Developed axis-matching excitation pulses tailored to specific refocusing pulses, accounting for imperfections.
- Designed pulses to generate magnetization along the effective rotation axis of the refocusing cycle.
- Utilized optimal control methods for generating phase-modulated axis-matching pulses.
- Implemented a new phase inversion scheme instead of standard 180° phase shifts.
Main Results:
- Axis-matching excitation pulses increased signal bandwidth and echo amplitudes by 30% compared to standard CPMG.
- Achieved over a factor of 2 increase in SNR with axis-matching pulses.
- Demonstrated reduction in transient amplitude decay for early echoes.
- Experimental results showed excellent agreement with theoretical predictions.
Conclusions:
- Axis-matching excitation pulses offer significant improvements in CPMG sequence performance, especially in inhomogeneous fields.
- These pulses enhance SNR and reduce signal transients, enabling more accurate measurements of short relaxation times.
- The developed methodology provides a powerful tool for optimizing NMR pulse sequences.
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