SUSCEPTIBILITY CHANGES FOLLOWING BOLUS INJECTIONS

The general mechanism of bulk magnetic susceptibility (BMS) induced MR I contrast following a bolus injection is elaborated. Combining radiol abeled tracer data for the first pass of a bolus injection through the human brain with the application of Wiedemann's law allows us to calc ulate the lower limit for the time course of the vascular BMS followin g the injection of any contrast agent. Superparamagnetic iron oxide pa rticles produce a much larger effect than any mononuclear Ln(III) chel ate. We also calculate the BMS changes occurring after a dilution bolu s injection (of isosmolal physiological saline) subsequent to a prior slow infusion of an intravascular contrast agent. This technique bears some resemblance to the increasingly important approach that exploits changes in only the level of blood oxygenation. The calculation indic ates that contrast changes after the dilution bolus injection are smal ler than those following Ln(III) agent injections but larger than thos e due to changes in blood oxygenation and suggests a way to possibly e nhance the latter. We present an in vivo study demonstrating the dilut ion bolus injection technique in the mouse brain, and that features it s rapid repeatability. Extrapolation of these results to the human, ho wever, indicates that the saline volumes required for venous injection s, except possibly for cardiac studies, would be prohibitively large. Smaller, catheter-delivered arterial bolus injections are feasible. We also suggest a method for using an agent bolus injection to measure t he parenchymal BMS, and thus the iron content of pathologically iron-l oaded tissue.