The artifacts arising from large vessels pose serious problems to the interpretation of oxygenation sensitive fMRI data. It is generally believed that microvascular changes are specific to the underlying region of neuronal activation. However, MRI gradient echo (GE) is sensitive to vessels of all dimensions, and there is concern that macrovascular changes distal to the site of neuronal activity can be induced [20]. This has been known as the brain–vein problem. For laboratories not equipped with EPI, gradient echo (GE) sensitive to variations in T2 and magnetic susceptibility are the only realistic sequences available for fMRI acquisition, so the problem is particularly acute.
In addition, there is a non-deoxyhemoglobin-related problem, especially acute in conventional MRI. This is the inflow problem of fresh blood that can be time-locked to stimulation. Such nonparenchymal and macrovascular responses can introduce error in the estimate of activated volumes.
