MRI measurement of change in vascular parameters in the 9L rat cerebral tumor after dexamethasone administration

Purpose: To demonstrate in the rat 9L cerebral tumor model that repeated MRI measurements can quantitate acute changes in the blood-brain distribution of Gadomer after dexamethasone administration. Materials and Methods: A total of 16 Fischer 344 rats were studied at 7T, 15 days after cerebral implantation of a 9L tumor. MRI procedures employed a T-One by Multiple Read Out Pulses (TOMROP) sequence to estimate R-1 (R-1 = 1/T-1) at 145-second intervals before and after administration of Gadomer (Bayer), a macromolecular contrast agent (CA). Two baseline studies preceded Gadomer administration and 10 subsequent R-1 maps tracked CA concentration in blood and brain for 25 minutes. Thereafter, either dexamethasone (N = 10) or normal saline (N = 6) was administered intravenously. A total of 90 minutes later a second series of 12 TOMROP measurements of Gadomer distribution was performed. The influx constant, K-1, plasma distribution volume, v(D), backflux constant, k(b), and interstitial space, v(e), were determined, and the test-retest differences of each of four vascular parameters were calculated. Results: Dexamethasone decreased K-1 approximately 60% (P = 0.02), lowered k(b) and v(D) (P = 0.03 and P < 0.01, respectively), and marginally but insignificantly decreased v(e). Conclusion: This noninvasive MRI technique can detect drug effects on blood-brain transfer constants of CAs within two hours of administration.