In normal tissues Ti values are related to macro-molecule concentration, water binding and water content. This is the reason for the basic Ti contrast in the brain: myelin causes white matter to have a shorter T1 than grey matter (Figure 5.2). The absolute T1 of each tissue will depend on field strength (Figure 5.3). T1 can be changed in pathology. For instance, oedema around tumours or in inflammatory acute MS lesions leads to an increase in T1. T1 is also increased in chronic MS lesions, probably as a result of the reduction in myelin and increase in water content. Conversely, T1 is reduced at the rim of active MS lesions by the presence of cellular debris which constitute extra-relaxation centres in the fluid. Bottomley's review (Bottomley etal., 1987) remains the most extensive review of T1 (and T2) values in normal and pathological tissues across a range of field strengths.
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