BLOOD-BRAIN-BARRIER PERMEABILITY IN RATS IS ALTERED BY EXPOSURE TO MAGNETIC-FIELDS ASSOCIATED WITH MAGNETIC-RESONANCE-IMAGING AT 1.5-T

We have previously reported that exposure of rats to low-field (0.15 T ) magnetic resonance imaging (MRI) increases blood-brain barrier (BBB) permeability. However, a number of investigators have failed to obser ve this effect when high-field MRI (1.5 T or higher) is used. Therefor e, we investigated whether or not we would observe changes using our t echnique at these higher fields. Adult male Sprague-Dawley rats were a naesthetised and then exposed to a 22.5 min imaging or sham procedure. Immediately following exposure, rats were injected with 1 MBq of Gd-1 53-DTPA intracardially and then immediately re-exposed for an addition al 22.5 min. The rats were killed lh following the second MRI exposure , at which time the brain was resected and 3 ml of venous blood collec ted. The ratio of radioactivity per gram of brain to radioactivity per milliliter of blood, known as the brain-blood partition coefficient, was determined and used as a measure of BBB permeability. Groups of an imals had different exposures. Group 1 (n = 9) was exposed to a clinic ally relevant MRI procedure. Group 2 (n = 20) was exposed to the same procedure except the rf specific absorption rate (SAR) was reduced to 25% and the animals were positioned 15 cm from imager centre to increa se the time-varying magnetic field from 0.4 to 2.8 T/s. For the sham e xposures (n = 21), the animals were placed in the imager with the stat ic field ramped down to zero and exposed to a sound recording simulati ng a MRI examination. In a third group (n = 11), exposure to only the static 1.5 T was investigated, while in a fourth group (n = 15), the e xposure field was only to a static 1.89 T. Sham controls for groups 3 (n = 12) and 4 (n = 19) were silent. Group 1 showed an increase in BBB permeability (5.7 +/- 0.5, mean +/- SEM, P < 0.04, n = 9); however, g roup 2 showed a decrease (3.9 +/- 0.2, P < 0.002, n = 20). Static fiel d exposure of 1.5 T increased permeability (5.7 +/- 0.5, P < 0.05, n = 11) as did 1.89 T (4.7 +/- 0.4, P < 0.02, n = 15). Although there was no difference between sound and silent shams, shams done at night (4. 8 +/- 0.2, n = 32) were significantly greater (P < 0.0004) than those done during the day (3.7 +/- 0.2, n = 19). Our results suggest changes in the rf and/or gradient field exposure can produce opposite effects on BBB permeability. Further, experiments involving only rf and only gradient field.-exposure are needed. The estimation of changes in BBB permeability by measuring the partition coefficient of compounds norma lly excluded by the BBB offers an objective quantitative technique whi ch can detect changes of the order of 25%. This sensitivity is needed to explore the important, albeit subtle, effects of magnetic fields. ( C) 1994 Wiley-Liss, Inc.