Magnetization Transfer

Magnetization transfer (MT), as commonly used in biomedical MRI, refers to the transfer of longitudinal magnetization from the hydrogen nuclei of water that has restricted motion to the hydrogen nuclei of water that moves with many degrees of freedom. The water with restricted motion is generally conceived as being bound to macromolecules,such as proteins and lipids through a series of hydrogen bonds. The free water pool is the bulk water of the cytosol. In this context, the hydrogen nuclei are typically referred to simply as protons.

In magnetic resonance imaging of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and bound (hydration), are found. Free water protons have faster average rotational frequency and hence less fixed water molecules that may cause local field inhomogeneity. Because of this uniformity, most free water protons have resonance frequencies lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). The high rotational frequency also results in fewer interactions with the environment so that the transverse magnetization dephasing is slower and the T₂ is long. Conversely, hydrated water molecules are slowed down by extensive interactions with the protons in the local macromolecules and hence magnetic field inhomogeneities are created that lead to wider resonance frequency spectrum. This results in faster dephasing of the magnetization that produces the NMR signal and much shorter T₂ values(<200 μs). Because the T₂ values are so short, the NMR signal from the protons of bound water is not typically observed in MRI.

However, using an off-resonance pulse excitation to saturate protons in the restricted pool can have a detectable effect on NMR signal from the mobile (free) proton pool. The transverse magnetization created is rapidly dephased and the longitudinal magnetization requires some time (approximately 5 times T₁) to return to equilibrium. Since the bound water may exchange magnetization with the free water, the loss of longitudinal magnetization will also be introduced into the pool of free water. This causes an increase in the T₁ of the free water and reduced signal from the free water in tissues in which the magnetization transfer mechanism is prevalent. 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 (M_o-M_t)/M_o.)