The great brain versus vein debate.

From the earliest fMRI experiments, it was quickly appreciated by those working with BOLD at high field that the signal change originated from visible veins whose spatial localization was relatively coarse ("the macrovasculature"), and smaller vessels ("the microvasculature") that were not individually visible in BOLD images. It was expected that a functional brain imaging technique that was predominantly sensitive to the macrovasculature would not have the same effective resolution as one sensitive to the microvasculature. Elimination of the venous signal and enhancement of the microvascular one offered the tantalizing ability to image columnar and lamellar structures in the brain and distinguished fMRI from its predecessor techniques. This article reviews a brief history of how these signal sources were first identified and separated and some of the controversy associated with the "brain versus vein" debate.