Magnetization transfer (MT), in
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
and
MRI
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...
, refers to the transfer of nuclear
spin polarization
Spin polarization is the degree to which the spin, i.e., the intrinsic angular momentum of elementary particles, is aligned with a given direction. This property may pertain to the spin, hence to the magnetic moment, of conduction electrons in fe ...
and/or spin coherence from one population of nuclei to another population of nuclei, and to techniques that make use of these phenomena.
There is some ambiguity regarding the precise definition of magnetization transfer, however the general definition given above encompasses all more specific notions. NMR active nuclei, those with non-zero spin, can be energetically coupled to one another under certain conditions. The mechanisms of nuclear-spin energy-coupling have been extensively characterized and are described in the following articles:
Angular momentum coupling
In quantum mechanics, the procedure of constructing eigenstates of total angular momentum out of eigenstates of separate angular momenta is called angular momentum coupling. For instance, the orbit and spin of a single particle can interact t ...
,
Magnetic dipole–dipole interaction
Magnetic dipole–dipole interaction, also called dipolar coupling, refers to the direct interaction between two magnetic dipoles.
Suppose and are two magnetic dipole moments that are far enough apart that they can be treated as point dipoles i ...
,
J-coupling
In nuclear chemistry and nuclear physics, ''J''-couplings (also called spin-spin coupling or indirect dipole–dipole coupling) are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins that ...
,
Residual dipolar coupling The residual dipolar coupling between two spins in a molecule occurs if the molecules in solution exhibit a partial alignment leading to an incomplete averaging of spatially anisotropic dipolar couplings.
Partial molecular alignment leads to an in ...
,
Nuclear Overhauser effect The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via cross-relaxation. A phenomenological definition of the NOE in nuclear magnetic res ...
,
Spin–spin relaxation
In physics, the spin–spin relaxation is the mechanism by which , the transverse component of the magnetization vector, exponentially decays towards its equilibrium value in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) ...
, and
Spin saturation transfer. Alternatively, some nuclei in a chemical system are labile and exchange between non-equivalent environments. A more specific example of this case is presented in the section ''Chemical Exchange Magnetization transfer''.
In either case, magnetization transfer techniques probe the dynamic relationship between two or more distinguishable nuclei populations, in so far as energy exchange between the populations can be induced and measured in an idealized NMR experiment.
Chemical Exchange Magnetization transfer
In
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...
or
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
of macromolecular samples, such as protein solutions, at least two types of water molecules, free (bulk) and bound (hydration), are present. Bulk water molecules have many mechanical degrees of freedom, and motion of such molecules thus exhibits statistically averaged behavior. Because of this uniformity, most free water protons have resonance frequencies very near the average Larmor frequency of all such protons. On a properly acquired NMR spectrum this is seen as a narrow Lorentzian line (at 4.8 ppm, 20 C). Bulk water molecules are also relatively far from magnetic field perturbing macromolecules, such that free water protons experience a more homogeneous magnetic field, which results in slower transverse magnetization dephasing and a longer
''T''2*. Conversely, hydration water molecules are mechanically constrained by extensive interactions with the local macromolecules and hence magnetic field inhomogeneities are not averaged out, which leads to broader resonance lines. This results in faster dephasing of the magnetization that produces the
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
signal and much shorter ''T''
2 values(<200 μs). Because the ''T''
2 values are so short, the
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
signal from the protons of bound water is not typically observed in MRI.
However, using an off-resonance saturation pulse to irradiate protons in the bound (hydration) population can have a detectable effect on the NMR signal of the mobile (free) proton pool. When a population of spins is saturated, such that the magnitude of the macroscopic magnetization vector approaches zero, there is no remaining spin polarization with which to produce an NMR signal. Longitudinal relaxation refers to the return of longitudinal spin polarization, which occurs at a rate described by T1. While the number of hydration water molecules may be insufficient to produce an observable signal, exchange of water molecules between the hydration and bulk population allows characterization of the hydration population, and measurement of the rate at which molecules are exchanging between bulk and bound sites. Such experiments are often termed saturation transfer or chemical exchange saturation transfer (CEST), because the signal of the bulk water is observed to decrease when the hydration population is saturated. Considering these techniques from the opposite perspective, that magnetization (i.e. spin polarization) is being transferred from the bulk water to the spin-saturated hydration population, allows one to conceptually unify chemical exchange methods with other techniques that transfer magnetization between nuclei populations. Since the extent of signal decay depends on the exchange rate between free and hydration water, MT can be used to provide an alternative contrast method in addition to ''T''
1,''T''
2, and proton density differences.
MT is believed to be a nonspecific indicator of the structural integrity of the tissue being imaged.
An extension of MT, the magnetization transfer ratio (MTR) has been used in
neuroradiology
Neuroradiology is a subspecialty of radiology focusing on the diagnosis and characterization of abnormalities of the central and peripheral nervous system, spine, and head and neck using neuroimaging techniques. Medical issues utilizing neuroradio ...
to highlight abnormalities in brain structures. (The MTR is (''M''
''o''-''M''
''t'')/''M''
''o''.)
A systematic modulation of the precise frequency offset for the saturation pulse can be plotted against the free-water signal to form a "Z-spectrum". This technique is often referred to as "Z-spectroscopy".
See also
*
Magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...
*
Magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
References
External links
The Role of Nonconventional MRI Techniques in Demyelinating DisordersMagnetic Resonance Findings in Amyotropic Lateral Sclerosis Using a Spin Echo Magnetization Transfer Sequence*Wolff SD & Balaban RS. Magnetization transfer contrast (MTC) and tissue water proton relaxation in vivo. Magnetic Resonance in Medicine. 1989;10(1):135-144.
*Mehta RC, Pike GB, Enzmann DR. Magnetization transfer magnetic resonance imaging: a clinical review. Topics in Magnetic Resonance Imaging. 1996;8(4):214-30.
*Tanabe JL, Ezekiel F, Jagust WJ, et al. Magnetization Transfer Ratio of White Matter Hyperintensities in Subcortical Ischemic Vascular Dementia. AJNR Am J Neuroradiol. 1999;20(5):839–844.
*Symms M, Jäger HR, Schmierer K, Yousry TA. A review of structural magnetic resonance neuroimaging. J Neurol Neurosurg Psychiatry. 2004 Sep;75(9):1235-44. Review. {{PMID, 15314108
* Lepage M, McMahon K, Galloway GJ, De Deene Y, Back SÅJ, Baldock C, 2002. Magnetization transfer imaging for polymer gel dosimetry. Phys. Med. Biol. 47 1881–1890.
Magnetic resonance imaging
Nuclear magnetic resonance