Magnetization transfer

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Magnetization transfer (MT), in

MRI, refers to the transfer of nuclear spin polarization and/or spin coherence from one population of nuclei to another population of nuclei, and to techniques that make use of these phenomena.[1] 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, Magnetic dipole–dipole interaction, J-coupling, Residual dipolar coupling, Nuclear Overhauser effect, Spin–spin relaxation, 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

NMR
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 T1,T2, 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 to highlight abnormalities in brain structures. (The MTR is (Mo-Mt)/Mo.)

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

References

  1. S2CID 233710016
    .

External links
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