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NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

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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.

Double Resonance Techniques: Overview

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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.
Spin decoupling is usually achieved by...

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Updated: May 22, 2026

Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease

Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease

Published on: December 18, 2016

Enhanced refocusing of fat signals using optimized multipulse echo sequences.

Ashley M Stokes¹, Yesu Feng, Tanya Mitropoulos

¹Department of Chemistry, Center for Molecular and Biomolecular Imaging, Duke University, Durham, NC 27708-0346, USA.

Magnetic Resonance in Medicine

|May 26, 2012

Summary

This summary is machine-generated.

Altering pulse timings in magnetic resonance imaging (MRI) with Uhrig

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Fat-Water Phantoms for Magnetic Resonance Imaging Validation: A Flexible and Scalable Protocol

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Published on: September 7, 2018

Area of Science:

Magnetic Resonance Imaging
Biophysics
Medical Physics

Background:

Endogenous magnetic resonance contrast provides functional information based on localized fat composition in vivo.
Conventional Carr-Purcell-Meiboom-Gill (CPMG) sequences use equal pulse timings.
Fatty tissues exhibit complex morphology, chemical shifts, and J-couplings, influencing MRI signals.

Purpose of the Study:

To investigate the impact of unequal pulse timings in Uhrig's dynamical decoupling (UDD) multipulse echo sequences on MRI signal intensity.
To compare UDD sequences with conventional CPMG sequences for fat signal characterization.
To explore the potential for designing tailored pulse sequences to optimize fat signal refocusing for enhanced MRI applications.

Main Methods:

Simulated UDD and CPMG pulse sequence timings to determine optimal time delays.
Experimental validation using excised and in vivo fatty tissues.
Analysis of signal intensity variations based on tissue type, sequence parameters, and interpulse spacings.

Main Results:

Unequal pulse timings in UDD sequences significantly alter signal intensity compared to CPMG sequences.
Fatty tissues showed particularly strong differences due to their complex chemical structure and couplings.
Chemical structure and stimulated echoes are key mechanisms for fat refocusing under multipulse sequences.
Optimized refocusing of fat chemical shifts and J-couplings can be achieved by tailoring pulse sequences.

Conclusions:

Specialized UDD pulse sequences can be designed to optimize fat signal refocusing, offering tailored contrast for specific fat types.
Improved echo refocusing directly translates to signal enhancements, particularly beneficial for applications like intermolecular multiple quantum coherence (IMQC) imaging.
This approach enhances functional information derived from endogenous magnetic resonance contrast in fatty tissues.