It is well known that R2′ is sensitive to both mesoscopic and macroscopic sources of magnetic field inhomogeneity. The former represents the effect of blood vessels, which underlies the BOLD response, and the latter is caused by disturbance of the magnetic field by the head. Here we compare perfect field homogeneity (Δω=0), plotted as a solid line, with a through slice gradient Δω=20 Hz, plotted as a dashed line. Shifting of the dashed line with respect to the solid line emphasises that macroscopic field inhomogeneity effects must be minimised.