Time course of BBB closing. Scanning was performed 4 hours and 1,3,7, and 17 days post-PDT treatment to determine changes in the BBB with time. Contrast flow rate, based on increasing volume of T1 signal intensity, on each of these days was calculated by rescanning the rats at several intervals following contrast injection. The rate of flow fell rapidly from 6.7 to 1.3 μl/minutes (80% decrease) from day 1 to day 3. PDT treatment was performed 4 hours following i.p. injection of 125 mg/kg ALA. All animals were subjected to light fluence and fluence rates of 17 J and 10 mW, respectively.