QUANTIFICATION OF INTRAVASCULAR AND EXTRAVASCULAR CONTRIBUTIONS TO BOLD EFFECTS INDUCED BY ALTERATION IN OXYGENATION OR INTRAVASCULAR CONTRAST AGENTS

A simple model is presented that allows quantitative separation of the contributions of signals from water in blood and extravascular parenc hyma due to changes in blood oxygenation, induced either by brain acti vation or by alteration of inspired oxygen. The separation is based on the progressive attenuation of the signals in the vasculature of diff erent levels when bipolar field gradient pulses are applied. Diffusion -weighted spin-echo echo-planar imaging sequences were used to measure signal changes under various conditions in both animals and human vol unteers. Normoxic-hyperoxic episodes were induced in rats before and a fter injection of a superparamagnetic iron oxide contrast agent. Signa l changes produced by visual stimulation were measured in human volunt eers, and in volunteers subject to alternating normoxic-hyperoxic epis odes, and with administration of Gd-DTPA, Analysis of the results with our simple model suggests that the apparent diffusion coefficient inc reases and R2 ( = 1/T-2) decreases upon brain activation, with a large component from extravascular water related to the decrease in the blo od deoxyhemoglobin concentration. Furthermore, this study suggests tha t apparent diffusion coefficient of the extravascular component alone may provide localization of neuronal activation.