MAGNETIZATION-TRANSFER MAGNETIC-RESONANCE-IMAGING - A CLINICAL REVIEW

Magnetic resonance imaging has traditionally used the T1 and T2 relaxa tion times and proton density (PD) of tissue water (hydrogen protons) to manipulate contrast. Magnetization transfer (MT) is a new form of t issue contrast based on the physical concept that tissues contain two or more separate populations of hydrogen protons: a highly mobile (fre e) hydrogen (water) pool, H-f and an immobile (restricted) hydrogen po ol, H-r, the latter being those protons bound to large macromolecular proteins and lipids, such as those found in such cellular membranes as myelin. Direct observation of the PI, magnetization pool is normally not possible because of its extremely short T2 time (<200 mu s). But S aturation of the restricted pool will have a detectable effect on the mobile (free) proton pool. Saturation of the restricted pool decreases the signal of the free pool by transferring the restricted pool's sat uration. Exchange of magnetization between the free and restricted hyd rogen protons is a substantial mechanism for spin-lattice (T1) relaxat ion in tissues and the physical basis of MT. Through an appropriately designed pulse sequence, magnetization transfer contrast (MTC) can be produced. MT contrast is different from T1, T2, and PD, and it likely reflects the structural integrity of the tissue being imaged. A variet y of clinically important uses of MT have emerged. In this clinical re view of the neuroradiological applications of MT, we briefly review th e physics of MT, the appearance of normal brain with MT, and the use o f MT as a method of contrast enhancement/background suppression and in tissue characterization, such as evaluation of multiple sclerosis and other white-matter lesions and tumors. The role of MT in small-vessel visualization on three-dimensional time-of-flight magnetic resonance angiography and in head and neck disease and newer applications of MT are also elaborated.