Magnetization transfer

Magnetization transfer (MT) imaging is an MRI technique that can be used to exploit the contrast between tissues where H protons are present in three states:

• bound to macromolecules
• in free water
• as water in the hydration layer between the macromolecules and the free water

To assess MT, an off-resonance RF pulse is applied at a different frequency to the Larmor frequency of free-water protons. This pulse saturates protons bound to macromolecules but not those in free water. The saturated macromolecule-bound protons partially transfer their magnetization to protons in the hydration layer and free water. Some free water protons thus become saturated. When another radiofrequency pulse is applied, this time at the Larmor frequency of the free-water protons, the signal from the free water is reduced due to the pre-saturation of some free-water protons. The difference between the signals achieved with and without the off-resonance pulse can be compared and is referred to as the Magnetization Transfer Ratio (MTR).