Acquisition-to-acquisition variance maps. Acquisition-to-acquisition variance, axial slices, overlaid on a template brain. Very large variance (in red) is present at the edges of the brain and in correspondence of the course of large vessels (MCA: middle cerebral artery; PCA: posterior cerebral artery; ACA: anterior cerebral artery and frontal arteries; IA: insular arteries; CA: middle occipital and choroidal arteries; RS: sinus rectus). Most of the brain parenchyma, in contrast, displays lower variance (in yellow, green, and blue), especially in white matter. On the right, the histogram of the variance values shows a long right tail; isolated values reach σ2 = 30 000.