Magnetization transfer

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Magnetization transfer (MT) refers to the transfer of longitudinal magnetization from free water protons to hydration water protons in NMR and MRI.

In magnetic resonance imaging of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and 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 frequency lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 tesla). This also results in slower transverse magnetization dephasing and hence longer T2. Conversely, hydration water molecules are slowed down by interaction with solute molecules and hence create field inhomogeneities that lead to wider resonance frequency spectrum. This corresponds to faster dephasing and very short T2 (~1 ms).

Using a ninety degree off-resonance pulse excitation, the hydration proton longitudinal magnetization (M_z) is eliminated and transverse magnetization (M_xy) is created. However, due to rapid dephasing of M<sub>xy, only M_z loss will remain for hydration proton soon after excitation. Since hydration water may freely exchange with free water, the loss of M_z will also be introduced into the pool of free water. This causes T2 signal decay in free water. 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 and T2 differences.