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NMR Spectroscopy: Spin–Spin Coupling01:08

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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
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An experiment is a planned activity carried out under controlled conditions. The purpose of an experiment is to investigate the relationship between two variables. When one variable causes change in another, we call the first variable the explanatory or independent variable. The affected variable is called the response or dependent variable. In a randomized experiment, the researcher manipulates values of the explanatory variable and measures the resulting changes in the response variable. The...
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Magnetic Resonance Imaging Quantification of Pulmonary Perfusion using Calibrated Arterial Spin Labeling
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A general framework for optimizing arterial spin labeling MRI experiments.

Joseph G Woods1, Michael A Chappell1,2, Thomas W Okell1

  • 1Wellcome Centre for Integrative Neuroimaging, FMRIB, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom.

Magnetic Resonance in Medicine
|December 28, 2018
PubMed
Summary
This summary is machine-generated.

This study presents a framework to optimize Arterial Spin Labeling (ASL) MRI protocols for accurate perfusion measurements. Optimizing scan parameters significantly reduced estimation errors in cerebral blood flow (CBF) and arterial transit time (ATT).

Keywords:
accuracyarterial spin labelingcerebral blood flowmulti-delayoptimal experimental designperfusion

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Area of Science:

  • Medical Imaging
  • Biophysics
  • Neuroimaging

Background:

  • Arterial Spin Labeling (ASL) MRI is a non-invasive technique for measuring tissue perfusion.
  • ASL MRI is limited by signal-to-noise ratio (SNR), necessitating rigorous optimization of scan protocols for accurate hemodynamic parameter estimation.
  • Optimizing ASL protocols is crucial for reliable quantitative perfusion measurements.

Purpose of the Study:

  • To present a general framework for optimizing ASL MRI experiments to achieve maximum accuracy in perfusion and hemodynamic parameter estimates within a fixed scan time.
  • To demonstrate the effectiveness of this framework by optimizing post-labeling delays (PLDs) in a multi-PLD pseudo-continuous ASL (pcASL) sequence.
  • To validate the optimized protocol's performance using computational simulations and in vivo human data.

Main Methods:

  • A framework utilizing the Cramér-Rao lower bound (CRLB) was developed to minimize predicted parameter estimation errors.
  • ASL protocols were optimized for accuracy in cerebral blood flow (CBF) alone, or for both CBF and arterial transit time (ATT).
  • Optimized protocols were compared against conventional evenly spaced multi-PLD and single-PLD protocols using simulations and in vivo data from healthy volunteers.

Main Results:

  • Simulations and in vivo experimental results showed excellent agreement with the predicted performance of all tested ASL protocols.
  • Optimizing solely for CBF accuracy led to a 48% reduction in CBF errors compared to the reference multi-PLD protocol and a 15% reduction compared to the single-PLD protocol.
  • Optimizing for both CBF and ATT accuracy reduced CBF errors by 37% relative to the multi-PLD protocol, while maintaining ATT accuracy.

Conclusions:

  • The proposed framework effectively designs ASL MRI experiments to minimize measurement errors.
  • Protocol optimization is dependent on specific experimental requirements and desired parameter accuracy.
  • This approach enhances the reliability and accuracy of quantitative perfusion imaging with ASL MRI.