Jh. Zhong et al., QUANTIFICATION OF INTRAVASCULAR AND EXTRAVASCULAR CONTRIBUTIONS TO BOLD EFFECTS INDUCED BY ALTERATION IN OXYGENATION OR INTRAVASCULAR CONTRAST AGENTS, Magnetic resonance in medicine, 40(4), 1998, pp. 526-536
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.