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Related Concept Videos

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences01:17

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

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.
Atomic Nuclei: Magnetic Resonance01:05

Atomic Nuclei: Magnetic Resonance

The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
Insensitive Nuclei Enhanced by Polarization Transfer (INEPT)01:15

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Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) is an advanced Nuclear Magnetic Resonance (NMR) technique specifically designed to detect and enhance the signals of low-abundance nuclei, such as carbon-13 and nitrogen-15, in small molecules. The fundamental principle behind INEPT is the transfer of polarization from a more abundant and highly polarizable nucleus, typically hydrogen-1, to the low-abundance nucleus of interest. This process effectively boosts the NMR signal of the...
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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis. This...
Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

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...
NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...

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Tailored RF pulse for magnetization inversion at ultrahigh field.

Aaron C Hurley1, Ali Al-Radaideh, Li Bai

  • 1Sir Peter Mansfield Magnetic Resonance Centre, Department of Computer Science, University of Nottingham, UK.

Magnetic Resonance in Medicine
|October 28, 2009
PubMed
Summary

Researchers developed novel radiofrequency inversion pulses for 7 Tesla MRI. These pulses correct for magnetic field inhomogeneity, improving image quality for critical sequences like arterial spin labeling.

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Biophysics

Background:

  • Radiofrequency (RF) field inhomogeneity at ultrahigh fields (7 Tesla) degrades MRI quality.
  • This is especially problematic for inversion recovery sequences, impacting applications like quantitative arterial spin labeling (ASL).

Purpose of the Study:

  • To design and validate novel slice-selective inversion pulses for 7T MRI.
  • To address RF transmit field heterogeneity and improve image uniformity.

Main Methods:

  • Utilized a genetic algorithm search to generate tailored inversion pulses.
  • Incorporated frequency offset correction and time dilation techniques.
  • Optimized RF amplitude, frequency sweep, and gradient functions considering field inhomogeneity.

Main Results:

  • Developed a slice-selective inversion pulse with a good slice profile and uniform inversion.
  • The pulse demonstrated efficacy across typical RF amplitude ranges in the human head at 7T.
  • Achieved experimentally feasible pulse length and amplitude for brain imaging.

Conclusions:

  • Novel, algorithm-generated inversion pulses effectively compensate for 7T RF field inhomogeneity.
  • This advancement enhances the utility of inversion recovery-based MRI sequences at ultrahigh fields.
  • Improved pulse design enables more reliable quantitative ASL and other advanced MRI techniques.