26-07-2014, 03:49 PM
Magnetic Resonance Spectroscopy
[attachment=66284]
DEFINITION
Magnetic resonance spectroscopy (MRS) is a non-invasive tool to measure the chemical composition of tissues (in vivo) and characterize functional metabolic processes in different parts of the human organs.
It is a research technique that exploits the magnetic properties of certain atomic nuclei. It determines the physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.
Most frequently, NMR spectroscopy is used by chemists and biochemists to investigate the properties of organic molecules, although it is applicable to any kind of sample that contains nuclei possessing spin.
INTRODUCTION
The nuclei of many elemental isotopes have a characteristic spin (I). Some nuclei have integral spins (e.g. I = 1, 2, 3 ..), some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ..), and a few have no spin, I = 0 (e.g. 12C, 16O, 32S ). Isotopes of particular interest and use to organic chemists are1H, 13C, 19F and 31P, all of which have I = 1/2. Since the analysis of this spin state is fairly straight forward, our discussion of NMR will be limited to these and other I = 1/2 nuclei
The following features lead to the NMR phenomenon:
1. A spinning charge generates a magnetic field, as shown by the animation on the right. The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.
Fig 1.2a
2. In the presence of an external magnetic field (B0), two spin states exist, +1/2 and -1/2. The magnetic moment of the lower energy +1/2 state is aligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field. Note that the arrow representing the external field points north.
The difference in energy between the two spin states is dependent on the external magnetic field strength, and is always very small. The following diagram illustrates that the two spin states have the same energy when the external field is zero, but diverge as the field increases. At a field equal to Bx a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field).
BASIC PRINCIPLES USED IN MRS
The principle of NMR usually involves two sequential steps:
The alignment (polarization) of the magnetic nuclear spins in an applied, constant magnetic field H0.
The perturbation of this alignment of the nuclear spins by employing an electro-magnetic, usually radio frequency (RF) pulse. The required perturbing frequency is dependent upon the static magnetic field (H0) and the nuclei of observation.
MRS exploits the fact that when a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet
3 BASIC MRS TECHNIQUES
3.1 Chemical Shift:
In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule.
Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy.
The chemical shift provides information about the structure of the molecule. The conversion of the raw data to this information is called assigning the spectrum.
For example, for the 1H-NMR spectrum for ethanol (CH3CH2OH), one would expect signals at each of three specific chemical shifts: one for the CH3 group, one for the CH2 group and one for the OH group.
A typical CH3 group has a shift around 1 ppm, a CH2 attached to an OH has a shift of around 4 ppm and an OH has a shift around 2–3 ppm depending on the solvent used
3.2 J-coupling:
Scalar or J-couplings (also called indirect dipole dipole coupling) are mediated through chemical bonds connecting two spins.
It is an indirect interaction between two nuclear spins which arises from hyperfine interactions between the nuclei and local electrons. J-coupling contains information about bond distance and angles.
Most importantly, J-coupling provides information on the connectivity of molecules.
In NMR spectroscopy, it is responsible for the appearance of many signals in the NMR spectra of fairly simple molecules.
Coupling to n equivalent (spin ½) nuclei splits the signal into a n+1 multiplet with intensity ratios following Pascal's triangle as described on the right. Coupling to additional spins will lead to further splitting of each component of the multiplet
E.g. coupling to two different spin ½ nuclei with significantly different coupling constants will lead to a doublet of doublets (abbreviation: dd ). Note that coupling between nuclei that are chemically equivalent (that is, have the same chemical shift) has no effect on the NMR spectra and couplings between nuclei that are distant (usually more than 3 bonds apart for protons in flexible molecules) are usually too small to cause observable splitting.
4 FORMATION OF IMAGE:
A spin echo uses a 908 RF pulse along with a slice-selective gradient. This selective 908 excitation pulse flips the magnetization within the slice to the transverse plane and magnetization is dephased by the first gradient.
An 1808 pulse is applied at the middle of the sequence and the magnetization is rephased by the second gradient.
The amplitude of the spin echo is affected by T2 relaxation; the resulting images are T2 weighted.
Spin echo is the least artifact-prone sequence and generates a high signal-to noise ratio.
Long TR in SE sequence times is incompatible with 3D acquisitions.
[attachment=66284]
DEFINITION
Magnetic resonance spectroscopy (MRS) is a non-invasive tool to measure the chemical composition of tissues (in vivo) and characterize functional metabolic processes in different parts of the human organs.
It is a research technique that exploits the magnetic properties of certain atomic nuclei. It determines the physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.
Most frequently, NMR spectroscopy is used by chemists and biochemists to investigate the properties of organic molecules, although it is applicable to any kind of sample that contains nuclei possessing spin.
INTRODUCTION
The nuclei of many elemental isotopes have a characteristic spin (I). Some nuclei have integral spins (e.g. I = 1, 2, 3 ..), some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ..), and a few have no spin, I = 0 (e.g. 12C, 16O, 32S ). Isotopes of particular interest and use to organic chemists are1H, 13C, 19F and 31P, all of which have I = 1/2. Since the analysis of this spin state is fairly straight forward, our discussion of NMR will be limited to these and other I = 1/2 nuclei
The following features lead to the NMR phenomenon:
1. A spinning charge generates a magnetic field, as shown by the animation on the right. The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.
Fig 1.2a
2. In the presence of an external magnetic field (B0), two spin states exist, +1/2 and -1/2. The magnetic moment of the lower energy +1/2 state is aligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field. Note that the arrow representing the external field points north.
The difference in energy between the two spin states is dependent on the external magnetic field strength, and is always very small. The following diagram illustrates that the two spin states have the same energy when the external field is zero, but diverge as the field increases. At a field equal to Bx a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field).
BASIC PRINCIPLES USED IN MRS
The principle of NMR usually involves two sequential steps:
The alignment (polarization) of the magnetic nuclear spins in an applied, constant magnetic field H0.
The perturbation of this alignment of the nuclear spins by employing an electro-magnetic, usually radio frequency (RF) pulse. The required perturbing frequency is dependent upon the static magnetic field (H0) and the nuclei of observation.
MRS exploits the fact that when a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet
3 BASIC MRS TECHNIQUES
3.1 Chemical Shift:
In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule.
Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy.
The chemical shift provides information about the structure of the molecule. The conversion of the raw data to this information is called assigning the spectrum.
For example, for the 1H-NMR spectrum for ethanol (CH3CH2OH), one would expect signals at each of three specific chemical shifts: one for the CH3 group, one for the CH2 group and one for the OH group.
A typical CH3 group has a shift around 1 ppm, a CH2 attached to an OH has a shift of around 4 ppm and an OH has a shift around 2–3 ppm depending on the solvent used
3.2 J-coupling:
Scalar or J-couplings (also called indirect dipole dipole coupling) are mediated through chemical bonds connecting two spins.
It is an indirect interaction between two nuclear spins which arises from hyperfine interactions between the nuclei and local electrons. J-coupling contains information about bond distance and angles.
Most importantly, J-coupling provides information on the connectivity of molecules.
In NMR spectroscopy, it is responsible for the appearance of many signals in the NMR spectra of fairly simple molecules.
Coupling to n equivalent (spin ½) nuclei splits the signal into a n+1 multiplet with intensity ratios following Pascal's triangle as described on the right. Coupling to additional spins will lead to further splitting of each component of the multiplet
E.g. coupling to two different spin ½ nuclei with significantly different coupling constants will lead to a doublet of doublets (abbreviation: dd ). Note that coupling between nuclei that are chemically equivalent (that is, have the same chemical shift) has no effect on the NMR spectra and couplings between nuclei that are distant (usually more than 3 bonds apart for protons in flexible molecules) are usually too small to cause observable splitting.
4 FORMATION OF IMAGE:
A spin echo uses a 908 RF pulse along with a slice-selective gradient. This selective 908 excitation pulse flips the magnetization within the slice to the transverse plane and magnetization is dephased by the first gradient.
An 1808 pulse is applied at the middle of the sequence and the magnetization is rephased by the second gradient.
The amplitude of the spin echo is affected by T2 relaxation; the resulting images are T2 weighted.
Spin echo is the least artifact-prone sequence and generates a high signal-to noise ratio.
Long TR in SE sequence times is incompatible with 3D acquisitions.