Susceptibility Weighted Image showing a CAA patient
with numerous microbleeds. Collected at 1.5T.
A hot topic currently is Cerebral Microbleeds (CMBs, aka cerebral microhemorrhages) which manifest in diseases and conditions such as vascular dementia, stroke, cerebral amyloid angiopathy (CAA), traumatic brain injury (TBI), and even normal aging. CMBs reflect small areas of hemorrhage. The small "microhemorrhages" consist pathologically of collections of hemosiderin-laden macrophages, often associated with diseased blood vessels. It has been suggested that cerebral microbleeds may be a risk factor for intracranial hemorrhage. A patient with a minimal number of bleeds may have a high risk of intracerebral hemorrhage if treated with anti-platelet and anti-coagulant drugs, thus it is critical to detect their presence before any treatment. T2*-weighted gradient-echo magnetic resonance (MR) imaging, specifically Susceptibility Weighted Imaging, is the method of choice for detecting cerebral microbleeds because of its sensitivity to the field inhomogeneity caused by hemosiderin deposits.
Cerebral amyloid angiopathy: emerging concepts.Yamada M. Journal of Stroke. 2015;17(1):17-30.
Cerebral amyloid angiopathy is a disease most prevalent amongst the elderly, affecting approximately half of them. It is associated with Alzheimer's disease with a prevalence of 80-90%. CAA is classified into several types depending on which amyloid protein is involved but this review primarily focuses on amyloid-beta protein. CAA can mostly be found in the leptomeningeal and cortical vessels of the cerebral lobes and cerebellum. The nature of the disease is vascular in which small and medium arteries accumulate amyloid deposits into the media and adventitia of the vessel wall. Derived from the brain, amyloid moves through periarterial interstitial fluid drainage pathways to blood vessels for clearance, but aggregates on the vascular basement membrane. CAA is significantly related to lobar cerebral macrohemorrhage, as well as cerebral microbleeds, and cortical superficial siderosis. CMBs are noted in 16.7% - 32 % of AD patients, which is much higher than the general population (5 - 6%)-when examined with T2* GRE MRI; it is also noted in 78% of AD patients or mild cognitive impairment on ultra-high field strength 7T MRI. Cerebral hypoperfusion (occlusive small vessel disease may cause white matter lesions in the occipital lobe, which can be seen with MRI. Dementia is also noted in 74% of several CAA.
One can image CAA with multiple modalities, for example, inflammatory CAA is characterized by T2-hyperintense white matter lesions on MRI. For macrohemorrages cranial CT is an option, however,
SWI (susceptibility weighted imaging) is one of the most sensitive MR imaging techniques for detecting CMBs.
Cognitive impairment can be assessed with Doppler ultrasound and fMRI, consistent with the damage in the occipital lobe. Amyloid imaging with the PET ligand, PiB, reveals increased PiB binding which often shows greater occipital uptake of CAA-related intracerebral hemorrhage. Amyloid imaging, however, cannot discriminate between vascular from parenchymal deposition or Amyloid beta from other amyloid proteins. Biochemically, levels of the amyloid tau protein are higher in patients with probable CAA compared to controls, but lower than in AD. Amyloid beta -40 and -42 levels are significantly low in the CSF of CAA, which may suggest it being trapped in the cerebral vasculature. The Boston criteria are to be used when diagnosing CAA, but no disease-modifying therapies are available yet. The risk factors for CAA include old age, AD, blood-thinning and anti-thrombotic medications, hypertension, head trauma, CAA-related gene mutations, and apolipoprotein E gene in sporadic cases. Pathogenic mechanisms underlying the damage and rupture of CAA-affected vessels remain to be determined.
MR Innovations is on the cutting edge of detection, location, and quantification of cerebral microbleeds. For more information on SWI and MRI post-processing and analysis, please visit www.mrinnovations.com.
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