Effects of large vessels in functional magnetic resonance imaging at 1.5T

To further investigate the effects of large vessels on the activation maps generated with functional magnetic resonance imaging at 1.5T, we studied activation of the human visual and motor cortex using a multitude of dedicated FLASH and echo‐planar imaging (EPI) scanning techniques. Both slice and volume scans were performed to assess relative contributions of T₂* effects, in‐flow, and phase‐shift effects, specifically within and around the larger vessels (around 1 mm in diameter). The contrast mechanism in single‐slice FLASH studies appeared to be predominantly sensitive to in‐flow and phase effects of the blood water within these larger vessels, and their relative contributions were dependent on experimental parameters and vascular geometry. The contrast mechanism in gradient echo EPI studies was governed predominantly by T₂* effects in tissue water (and to a lesser extent cerebrospinal fluid) surrounding the larger vessels.