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Susceptibility-weighted imaging (SWI) has proven to be a very sensitive technique for the identification of cerebral microbleeds (CMBs). Increased sensitivity to CMBs may allow assessment of the rate of microhemorrhage development or regression, allowing more precise analysis of the natural history of disease, or better assessment of response to therapy. Early recognition may be advantageous to patients treated with anticoagulant or aspirin therapy in that they are at increased risk for subsequent and possibly fatal hemorrhage. There are in the order of 1.4 million people a year affected by traumatic brain injury (TBI). For these patients, computed tomography (CT) is the mainstay of imaging in the emergency setting and does a good job detecting major bleeds. Radiation damage to blood vessels and radiation necrosis can result from treating tumors and disease with potentially damaging doses of radiation. Future semiautomated methods hold promise for evaluating large numbers of patients using SWI.
This chapter describes the vascular pathologies that underlie cerebral microbleeds (CMBs), concentrating on the two commonest disorders: hypertensive small vessel disease (SVD) and cerebral amyloid angiopathy (CAA). It also describes the process of blood degradation, and the correlation of imaging with histological findings. The chapter concentrates on hypertensive arteriopathy and CAA, which is usually diagnosed following symptomatic lobar intracerebral hemorrhage (ICH). Hypertensive arteriopathy is thought to be caused by a forced dilation of resistance vessels: that is, those vessels that regulate the blood supply volume to the distal capillary bed. The first event in a hemorrhage is the extravasation of all constituents of blood along with plasma. Extravasation may occur by rhexis (rupture of a vessel wall) or by diapedesis affecting arterioles, veins or capillaries. From the perspective of neuroimaging, CMBs are focal deposits of hemosiderin and can be visualized with MRI.