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Weekly UpdatesSAN DIEGO -- Various [T*.sub.2]-weighted MRI techniques are improving the ability to detect microbleeds, which are clear markers of hypertensive vasculopathy, cerebral amyloid angiopathy, and other underlying small vessel disease.
In fact, the more clinicians like Dr. Steven M. Greenberg look for them, the more they're finding.
"At some point we will not be able to get any more sensitive," Dr. Greenberg said at the International Stroke Conference. "The brain is not one big micro-bleed. There will be some limit to how many we pick up, but for now the harder we look the more we find."
Dr. Greenberg, a neurologist with the Hemorrhagic Stroke Research Program at Massachusetts General Hospital and Harvard Medical School, Boston, noted that advances in detection are coming at a time when the incidence of hypertensive vasculopathy and cerebral amyloid angiopathy are expected to rise as the Baby Boom generation continues to age. "These are not the only causes of microbleed, but these two processes are common and highly age associated," he said. "With the aging of the population these have become important entities."
Histology remains the accepted standard for detecting microbleeds, but Dr. Greenberg considers MRI superior to pathology from a practical standpoint because it has "tremendous sampling ability. Because of the sensitivity of MRI for iron deposits, we're able to get a full survey of the brain. While we'll never be able to replace the specificity of pathology for either identifying the bleeds themselves or the vascular diseases that go along with them, the sensitivity of MRI will supersede what is practical in an autopsy exam."
Compared with conventional [T.sub.2]-weighted MRI, [T*.sub.2]-weighted MRI does not use a refocusing pulse, "so the dephasing caused by the paramagnetic iron is accentuated by the [T*.sub.2]-weighted technique," Dr. Greenberg explained. "Lesions that are entirely invisible on [T.sub.2] sequence become highly visible on the [T*.sub.2] technique. So until you've looked with a [T*.sub.2] technique you really haven't done a proper test to look for microbleeds."
The most common micro-bleed mimics seen on MRI include mineral deposits, flow voids, bone/air artifacts, cavernous malformations, metastatic melanoma, and diffuse axonal injury.
According to detection criteria outlined by a group of experts including Dr. Greenberg, microbleeds should be black on [T*.sub.2] -weighted MRI; blooming on [T.sub.2] weighted MRI; devoid of signal hyperintensity on [T.sub.1]-or [T.sub.2]-weighted sequences; round or ovoid rather than linear; at least half surrounded by brain parenchyma; and be distinct from other potential mimics such as iron /calcium deposits, bone, or flow void; and should have a clinical history that excludes traumatic brain injury (Lancet Neurol. 2009;8:165-74).
The best clue to underlying disease may be location, Dr. Greenberg said. A deep hemispheric/brainstem pattern is suggestive of hypertensive vasculopathy while a strictly lobar pattern is suggestive of cerebral amyloid angiopathy.
Advances in [T*.sub.2]-weighted MRI techniques are leading to further improvements in detection rates. These include increasing the echo time, which allows greater time for the dephasing of the paramagnetic signal; thinner scanning sections, which increase the signal-to-noise ratio; increasing the magnetic field strength; and image postprocessing techniques such as susceptibility-weighted imaging, which "incorporates the phase shift as well as the magnitude effects of the iron deposits to increase their visibility."
Such advances in detection "will change our understanding of the prevalence of microbleed lesions," Dr. Greenberg said. "This is a story that has to play itself out before we'll know where we land.
"It's a growth area--one that we as researchers and clinicians need to better understand the implications of"
Dr. Greenberg disclosed that he has received grant and research support from the National Institutes of Health and the Alzheimer's Association.
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