MRI visualizes acute and chronic hematomas, but also old, clinically non-apparent cerebral microbleeds that are not detected on CT. Microbleeds have a hypointense appearance on MRI and are usually smaller than 5-10 mm. Pathological studies have shown that microbleeds seen with GRE MRI usually correspond to hemosiderin-laden macrophages adjacent to small vessels and are indicative of previous extravasation of blood . One review  included 53 case series studies involving 9073 participants, 4432 of whom were people with cerebrovascular diseases. Significant variations in MRI magnet strength, flip angle, slice gap and slice thickness were found as well as inconsistent definitions of microbleed size (44% chose a diameter of <5 mm). The authors found a 5% prevalence of microbleeds in healthy adults, rising to 34% (95% CI 31-36) in people with ischemic stroke, and to 60% (95% CI 57-64) in people with non-traumatic intracerebral hemorrhage (ICH). Microbleeds were seen in 83% (95% CI 71-90) of ICH cases with recurrent ICH .
Hypertension, cerebral amyloid angiopathy, getting older, and, less commonly, cerebral autosomal dominant arteriopathy with silent infarcts and leu-koaraiosis (CADASIL) have been identified as important risk factors for microbleeds [37-39]. Microbleeds have been suggested as markers of a bleeding-prone angiopathy [40, 41]. The results of several case reports and small series suggest that patients with microbleeds might be at increased risk of hemorrhage when on antithrombotic or thrombo-lytic therapy. By contrast, the results of two large studies did not show an increased risk of hemorrhage in patients with microbleeds who were treated with intravenous tissue plasminogen activator [42, 43].
Although there are still many studies ongoing, microbleeds are considered to bear prognostic significance for any future bleeding event and have been confirmed as a common finding in patients with cerebral amyloid angiopathy. There they are most commonly found in lobar brain regions . By contrast, in patients with intracerebral hemorrhage due to hypertensive disease, microbleeds are most commonly found in deep and infratentorial regions, although hypertension can also contribute to lobar microbleeds. A pattern of multiple hemorrhages without an underlying cause and restricted to lobar regions in an elderly patient is highly indicative of a diagnosis of cerebral amyloid angiopathy according to the Boston Criteria. A particularly noteworthy finding is that the total number of microbleeds predicts the risk of future symptomatic intracerebral hemorrhage in patients with lobar hemorrhage and probable cerebral amyloid angiopathy .
Old, clinically non-apparent cerebral microbleeds can be visualized on MRI, and have been suggested as markers of a bleeding-prone angiopathy.
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