Cerebral microhemorrhages on susceptibility weight magnetic resonance images are an important biomarker of cerebral arteriopathies. The iron in small foci of bleeding is released from hemoglobin and its magnetic properties induce a focal inhomogeneity in the magnetic field in MR images. This results in a small magnetic dipole artifact in the image which on a typical clinical scanner looks like a black dot. This artifact is somewhat larger than the hemorrhage that causes it because it represents a dipole around the focus of iron; this has been termed the “blooming effect” and can range from overestimating the size of the lesion by 10% to several hundred percent. Generally, the smaller the lesion, the larger the blooming effect.
Microhemorrhages can be mimicked by vessels viewed end on, so it is important to review adjacent imaging slices to make sure lesions are spheroid and not linear. Calcium deposits can also induce susceptibility artifacts, this is most common in the basal ganglia, choroid plexus and in atherosclerotic vessels. When the interpretation of the MRI is difficult, correlated with computed tomography can confirm whether a small lesion is calcium or iron. Other rare mimickers of cerebral microhemorrhages are air embolism and mettallic emboli.
Cerebral amyloid angiopathy and hypertension are widely recognized as common causes of cerebral microhemorrhages, but numerous other causes have been reported. One of our ongoing projects is thoroughly document the causes of cerebral microhemorrhages and systematically determine the size and spacial pattern of these lesions. To this end, we have assembled cohorts of patients with infective endocarditis and with aortic dissection and found that both of these are associated with cerebral microhemorrhages at a high frequency.