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A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse.
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Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
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Generation and Coherent Control of Pulsed Quantum Frequency Combs
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Composite pulse combinations for chirp excitation.

Sreya Das1, Navin Khaneja1

  • 1Systems and Control Engineering Department, Indian Institute of Technology, Bombay, India.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|December 23, 2022
PubMed
Summary

Composite pulses, like CHORUS, efficiently manage high-field NMR limitations. These broadband excitation sequences correct signal distortion caused by resonance offset effects and RF power constraints.

Keywords:
Adiabatic pulseBroadband excitationCHORUSChirp pulseComposite pulsesDouble-refocussing

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Area of Science:

  • Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Physical Chemistry
  • Spectroscopic Techniques

Background:

  • High-field NMR faces limitations like resonance offset effects and RF power constraints, leading to signal intensity distortion.
  • Broadband excitation is crucial for overcoming these limitations in NMR spectroscopy.
  • Existing methods struggle to fully compensate for phase dispersion caused by offset frequencies.

Purpose of the Study:

  • To introduce and analyze the CHORUS (Composite pulses with forward, reverse, and their combinations) sequence for broadband excitation in high-field NMR.
  • To develop composite pulse sequences that effectively remove phase dispersion caused by offset resonance frequencies.
  • To provide analytical explanations and experimental validation for the proposed CHORUS sequences.

Main Methods:

  • Utilized phase-modulated chirp pulses in a composite pulse sequence framework.
  • Implemented forward and reverse sweep mechanisms within the excitation pulse.
  • Employed a high-field NMR spectrometer (BRUKER 750 MHz) for experimental verification.
  • Performed MATLAB simulations to analyze pulse performance.

Main Results:

  • Developed four novel combinations of composite pulses within the CHORUS sequence.
  • Demonstrated that the CHORUS sequence effectively removes non-uniform phase dispersion.
  • Validated simulation results with experimental data obtained on a 750 MHz NMR spectrometer.
  • Showcased the ability of composite pulses to combine adiabatic and non-adiabatic rotations for improved excitation.

Conclusions:

  • Composite self-refocusing CHORUS excitation pulses effectively compensate for phase dispersion in high-field NMR.
  • The CHORUS sequence offers an efficient solution for broadband excitation, overcoming limitations of standard methods.
  • The combination of analytical explanations, simulations, and experimental verification confirms the efficacy of the proposed pulse sequences.