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Related Concept Videos

¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)

When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
NMR Spectroscopy Of Amines01:19

NMR Spectroscopy Of Amines

In proton NMR spectroscopy, primary amines and secondary amines showcase their N–H protons as a broad signal in the chemical shift range between δ 0.5 and 5 ppm. The exact position in this range depends on several factors, including sample concentration, hydrogen bonding, and the type of solvent used. Since amine protons undergo fast proton exchange in solution, the protons are labile and therefore do not participate in any splitting with adjacent protons. Thus, the observed peak is broad and...
Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei in a...
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the others.
¹H NMR Signal Integration: Overview00:58

¹H NMR Signal Integration: Overview

The intensity of a signal, which can be represented by the area under the peak, depends on the number of protons contributing to that signal. The area under each peak is shown as a vertical line called an integral, with the integral value listed under it, as seen in the proton NMR spectrum of benzyl acetate. Each integral value is divided by the smallest integral value to obtain the ratio of the number of protons producing each signal. The ratio reveals the relative number of protons and not...

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Quantitative Bayesian model-based analysis of amide proton transfer MRI.

Michael A Chappell1, Manus J Donahue, Yee Kai Tee

  • 1Institute of Biomedical Engineering, Department of Engineering Science, University of Oxford, Oxford, UK. michael.chappell@eng.ox.ac.uk

Magnetic Resonance in Medicine
|September 26, 2012
PubMed
Summary
This summary is machine-generated.

A new Bayesian method accurately quantifies Amide Proton Transfer (APT) effects using MRI. This approach corrects for confounding factors, offering a more precise way to measure APT concentration and exchange rates in biological tissues.

Keywords:
amide proton transferchemical exchange saturation transfermagnetization transfer

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Area of Science:

  • Biomedical Imaging
  • Magnetic Resonance Imaging
  • Quantitative MRI

Background:

  • Amide Proton Transfer (APT) MRI contrast arises from proton exchange between amide groups and water.
  • Traditional APT quantification relies on asymmetry analysis, which can be confounded by factors like spillover and field inhomogeneity.
  • Independent quantification of amide concentration and exchange rate is challenging with existing methods.

Purpose of the Study:

  • To develop and validate a Bayesian method for quantifying APT effects.
  • To enable independent quantification of amide proton concentration and exchange rate.
  • To correct for confounding effects in APT MRI measurements.

Main Methods:

  • A Bayesian approach was applied using a three-pool model (water, amide protons, magnetization transfer).
  • The method was tested using simulations, creatine phantoms with varying pH, and in vivo MRI data (n=7).
  • Off-resonant spectra were sampled and fitted to an exchange model.

Main Results:

  • The Bayesian method accurately quantified APT effects with a root-mean-square error < 2%, outperforming traditional asymmetry analysis.
  • It effectively corrected for confounding effects of field variation and magnetization transfer.
  • In vivo results yielded approximate APT concentration (relative to water) of 3 × 10(-3) and an exchange rate of 15 s(-1).

Conclusions:

  • Bayesian modeling provides an accurate and robust method for quantifying APT effects in MRI.
  • This approach offers improved correction for confounding factors compared to standard techniques.
  • While promising, further optimization of sampling strategies may be needed for absolute accuracy of exchange rate quantification.