Magnetization transfer contrast: Applications in brain imaging

Growing interest in the mechanism based on magnetization transfer (MT) between the immobile protons of macromolecular structures in tissues and the free protons of the water surrounding them led us to analyse the technique applied to MR sequences to generate this Mew type of contrast. We studied the physical and chemical mechanisms underlying this exchange of energy: finding that the dipole-dipole interaction occurring during relaxation process is primarily responsible for the energy exchange by diffusion, whereas the chemical processes are of little importance. We describe the technical features of selective saturation of bound protons, comparing the types of impulses coupled to the sequences and their characteristics to establish the advantages and disadvantages. We demonstrate how magnetization transfer occurs in many different tissues, including those of the brain, rich in macromolecular components. The first clinical use of magnetization transfer was with MR angiography in which saturation impulses suppress signals from the parenchyma, enhancing the contrast with vessels. We then focused on the application of magnetization transfer to characterise different brain diseases caused by infection, tumours, haemorrhage and demyelination. In many cases MT offers more specific information than that yielded by T1, T2 and PD weighted scans alone because it is sensitive to various tissue factors like the paramagnetic components of changes in acute aid subacute haemorrhage. The full range of potential applications of this techniques have yet to be defined.