Optimal detection of blood-brain barrier defects with Gd-DTPA MRI-the influences of delayed imaging and optimised repetition time.

A computer simulation of Gd-DTPA enhancement in blood-brain barrier defects was used to find the tissue concentration as a function of time after bolus injection for a variety of lesion permeability and leakage space values. High permeability lesions start to decay less than 10 min after injection; while low permeability lesions may not reach their maximum concentration until at least 2 h after injection. The minimum detectable permeability was calculated for a range of leakage space values. For a leakage space of 0.1, 2 h after a standard 0.1 mmol/kg injection a permeability surface area product as low as 0.0005 min-1 still gives detectable enhancement, while 6 min after injection the permeability must be at least six times higher to give detectable enhancement. The simulation shows that the effect of triple dose compared to standard dose cannot be found using cumulative dose experiments where the triple dose is fractionated over a period of 10-30 min.