Magnetization transfer consists in unmasking the tissues carrying protons bound to macromolecules by reducing the signal.
To do this, a preparation RF pulse is sent, with a shift of roughly 1500 Hz in relation to the resonance frequency of free protons (water and fat) to saturate the magnetization fo protons in the bound pool.
The permanent exchanges between the free protons in the water close to the macromolecules (and their bound protons) results in a magnetization transfer of the bound protons toward the free protons.

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Thus we observe a reduction in the MRI signal from the free protons in the tissues containing protons bound to macromolecules submitted to a high-power off-resonance RF pulse.