The physical basis of magnetization transfer imaging

Magnetization transfer imaging (MTI) refers to an application of magnetic resonance imaging (MRI) designed as a means of exploring characteristics of non-water components in tissue. Compared with conventional MR imaging, where differences in brightness on images reflect differences in observed relaxation times, MTI theory incorporates the influence of additional parameters, specifically those reflecting the exchange of protons between molecules of water and molecules of more-solid, structural components. Thus, MTI offers the potential of a window on tissue structure, and structural components that are normally not resolvable with MRI. The theory arises from the idea that the spin magnetization of macromolecular components of tissue can be indirectly observed via observation of the conventionally visible spins of tissue water. The extent of influence of the magnetization transfer process can be reflected in a normalized index derived from two magnetic resonance images, which is known as the magnetization transfer ratio. Exploitation of this effect is of current interest in multiple sclerosis because it offers the potential for gains in specificity as well as sensitivity of the magnetic resonance examination.