IN-VIVO VALIDATION OF THE BOLD MECHANISM - A REVIEW OF SIGNAL CHANGESIN GRADIENT-ECHO FUNCTIONAL MRI IN THE PRESENCE OF FLOW

Brain functional magnetic resonance imaging (FMRI) is possible because of local changes in blood flow and oxygenation levels. Understanding the role of each mechanism is important for interpreting FMRI results as well as for the design of the experiment itself. In this review, we address the role of flow and blood oxygen level dependence (BOLD) and how they can be used in conjunction with each other to enhance the BO LD effect. We also discuss the role of intra/extravascular signal chan ges in the presence of a vessel for the imaging situation and show how the ratio of these two contributions is likely to change as the blood volume fraction changes. Using the BOLD model itself, along with 3-D phase imaging, we show that blood oxygenation level in veins in vivo i s Y=0.544+/-0.029 as measured in 14 vessels in five subjects. Finally, we comment on the use of phase images themselves as a means to discri minate tissues with different levels of blood such as gray matter, whi te matter, and cerebrospinal fluid. (C) 1995 John Wiley & Sons, Inc.