When blood flow to a portion of the brain is reduced, the survival of the tissue at risk depends on a number of modifying factors:

1. The duration of ischemia
2. The availability of collateral circulation
3. The magnitude and rapidity of the reduction of flow

******These factors also determine the speed of MRI signal evolution*******

These factors will determine the precise anatomic site and size of the lesion and, consequently, the extent of clinical deficit.