MAGNETIZATION-TRANSFER - THEORY AND CLINICAL-APPLICATIONS IN NEURORADIOLOGY

Magnetization transfer, a new technique for improving image contrast i n magnetic resonance (MR) imaging, is based on application of off-reso nance radio-frequency pulses and observing their effects on MR images, as well as measuring the signal intensity with and without applicatio n of the pulses (ie, magnetization transfer ratio [MTR]). MTRs can be used to detect changes in the structural status of brain parenchyma th at may or may not be visible with standard MR techniques. Use of MTRs may allow subcategorization of multiple sclerosis lesions into those w ith very low MTR (demyelinated lesions) and slightly decreased MTR (ed ematous lesions). In cases of wallerian degeneration, use of MTRs appe ars to allow reliable detection of changes undetectable with MR imagin g or even light microscopy. In cases of infection with human immunodef iciency virus, MTRs seem to indicate that the macromolecular structure of white matter remains intact until relatively late in the course of disease. In cases of metastatic disease, MTRs of brain lesions indica te structural changes beyond the extent of the lesions seen on standar d MR images. These findings may be due to chronic edema, myelin loss, and perhaps previous undetected tumor. In addition to being a new meth od of providing contrast, the magnetization transfer technique enables semiquantitative, reproducible characterization of tissue and patholo gic entities, which could substantially improve the specificity of MR imaging.