DW images are also T2-weighted due to the long probe time of the magnetic field gradients. Difficulties in the interpretation of DWI semeiology can arise due to this double weighting. For instance, a hyperintense focus can be both an area with restricted diffusion and one with increased T2, since a T2 increase is a common feature of many brain lesions that are not necessarily accompanied by diffusion abnormalities. To avoid losing much of the diagnostic ability of DWI, a map of ADC values (in s/ mm2) must thus be calculated to separate diffusion-in-ducedsignalchanges from thosecausedbyT2weighting.
Changes in ADC values seem to occur before morphological changes become apparent, thereby playing an important role in in vivo tissue characterization (as in tumours) and in monitoring response to treatment [21-23] (Fig. 7.3).
As mentioned above, a pair of DWI acquisitions with low and high diffusion weighting and the same TE make it possible to calculate ADC maps.
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