Figure 1: A high resolution CE-MRA of carotid stenosis that shows vascular structure differences. A) Stenosis of internal carotid artery. B) Altered blood flow due to stenosis. C) Normal blood flow.
Functional magnetic resonance imaging (fMRI) is currently the method of choice for studying brain activation. It is non-invasive and readily available compared to position emission tomography (PET).
One drawback of fMRI is its indirect measure of activation through blood flow. As a result, subjects with cerebrovascular disease may not demonstrate a typical blood flow response. This is particularly important in the study of stroke, normal aging, and Alzheimer’s disease.
In this project, subjects with known carotid artery disease in one vessel underwent fMRI and transcranial Doppler (TCD) ultrasound for comparison. It is possible to measure the hemodynamic response due to activation using fMRI or stress challenge using TCD. These response curves are compared across hemispheres in the motor cortex with the visual cortex serving as a baseline. Furthermore, patients that go on for treatment are studied post-surgery to verify that the vascular response has normalized.
The contrast-enhanced magnetic resonance angiogram, as seen in Figure 1, demonstrates a carotid stenosis (A). The effect of this disease downstream (in the brain) is visible (B). Note that the diseased vessels are of much smaller caliber than the unaffected side©. It is not detectable in this image, but the autoregulation mechanism (an autonomic response that maintains proper local blood pressure) is compromised downstream of the stenosis due to the decreased flow. This was measured with TCD ultrasound.
Figure 2: An fMRI map of a visually cued bilateral motor task in a subject with an occlusion in the right internal carotid artery.
Figure 2 shows functional activation maps generated from a visually cued bilateral motor task. The color regions represent areas that show a statistically significant correlation with the bilateral finger movement task performed in the magnet. The left hemisphere response is representative of a normal response. Note that the right motor cortex is lacking in a response. This is due to the compromised autoregulation in this vascular bed as a result of the stenosis.
Figure 3: A BOLD time course of a bilateral motor task. Note the blunted, delayed response in the right motor cortex (red) compared to the more normal responses in the left motor cortex (blue) and visual cortex (yellow).