Mechanism of SWI of occult CMBs. Breaks in the exceedingly small blood vessels, like capillaries or postcapillary venules, are followed by extravasation of a thin amount of blood. The wall of these microvessels, made by interconnected endothelial cells, covered by a glia basement membrane with embedded pericytes (A), prevents blood from leaking out. If vessel damage occurs in healthy people, a multilayer of platelets is immediately formed to close the break, avoiding blood extravasation (B). However, if platelets are severely reduced, like in ITP, blood cells can escape and invade the cerebral parenchyma (C), inducing glia cells activation into macrophages that phagocytize escaped cells and degrade the hemoglobin into nontoxic hemosiderin deposits (D), preventing the toxicity of free iron. Hemosiderin-laden macrophages (E) may persist indefinitely. Because of their high iron content, SWI allows the detection of CMBs, that appear as small black dots, as shown in the encircled area of the axial brain representation.