In magnetic resonance imaging or NMR of macromolecular samples, such as protein solutions, at least two types of water molecules, free (bulk) and bound (hydration), are present.
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.
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.
An extension of MT, the magnetization transfer ratio (MTR) has been used in neuroradiology to highlight abnormalities in brain structures.
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".