magnetisation transfer ratio
magnetisation transfer ratioAn MRI value that corresponds to the ratio of longitudinal magnetisation transferred from the hydrogen nuclei of water with restricted motion—because they are bound to macromolecules (proteins and lipids)—to hydrogen nuclei of water with unrestricted motion. The MTR may be of use in identifying epileptogenic foci in the brain.
MTR relies on the fact that macromolecules in the brain often contain water that is “bound” and unable to participate in magnetic decay with adjacent water molecules after stimulation; when stimulated by a 90º pulse, a compartment containing bound water will undergo a transverse magnetisation decay with its own T2. While this T2 is too long to be visible with conventional MRI, the compartment transfers magnetisation to the free water compartment; the fractional effect on the magnetisation of the free water corresponds to the MTR, which can be quantified and is proportional to the relative sizes of the two compartments. The MTR further depends on the proportion of bound water in the tissue compartment, which ranges from 40% in the white matter to 0% in the CSF. The MTR therefore provides information on macromolecular structure.