Magnetisation transfer ratio of normal brain white matter: a normative database spanning four decades of life.

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Abstract
OBJECTIVES: To establish a normative database for magnetisation transfer ratio (MTR) measurements in the white matter of healthy adult brains. Such MTR values were evaluated for regional variation and evidence of differences associated with aging, sex, and handedness. METHODS: Forty one healthy volunteers, ranging in age from 16 to 55 years, underwent axial brain magnetisation transfer (MT) imaging on a 1.5 Tesla magnetic resonance scanner. Calculated MT images allowed evaluation of MTR from specific regions within the corpus callosum, cerebral hemispheres, and pons. RESULTS: Highest values were noted in the corpus callosum. No significant sex differences were seen for any region studied. Small but significant age related reductions in MTR were noted in the corpus callosum and other cerebral white matter regions studied. Comparing MTR values between young (16-35 years) and older (36-55 years) age groups, this was most apparent in the corpus callosum (40.82% units in the young group v 40.28% units in the older group, P < 0.05) and frontal white matter (39.65% units in the young group v 39.18% units in the older group, P < 0.005). In addition, values for MTR were analysed for evidence of hemispheric asymmetry. MTR values were higher in the left hemisphere for all regions studied, reaching significance in the centrum semiovale (37.75% units v 37.57% units, P < 0.05) and parieto-occipital white matter (37.67% units v 37.43% units, P < 0.05). No relation between such interhemispheric MTR differences and handedness was noted. CONCLUSIONS: Magnetisation transfer imaging shows significant age related changes in normal brain white matter. In addition to regional variations in MTR in the normal brain, there seem to be small but significant variations in MTR between the cerebral hemispheres. It is important to consider such normal variations when evaluating MTR in pathological states.