Cerebrovascular Disease

Clinical Applications of Cerebral SPECT Imaging in Cerebrovascular Disease

Stroke occurs when an artery supplying blood to a portion of the brain becomes blocked (an ischaemic stroke) or ruptures (a haemorrhagic stroke). Stroke represents a major health problem with an average of over 800,000 patients experiencing strokes each year in the USA. The prevalence of stroke is approximately 1.2 percent worldwide with Australia having a higher prevalence of 1.7 percent and Canada being slightly lower at 1.1 percent. Nearly half of all stroke patients are left with reduced mobility. Haemorrhagic stroke seems to be more common in Japan. The risk of having a first stroke is twice as high for those of African descent compared to those of European descent.

Neuroimaging in Stroke

In the setting of acute cerebrovascular events, CT imaging is generally relied upon due to ease of access and rapid data acquisition, particularly in the context of active stroke management with clot retrieval techniques and thrombolytic therapies being more widely implemented.  MR is highly sensitive in the detection of ischaemic lesions, particularly in the subacute setting, and in delineating the extent of ischaemic infarction.Cerebral SPECT imaging is useful in delineating the extent of ischaemic infarction and correlates well with severity of neurologic deficits and clinical outcomes. It may be useful in demonstrating the ischaemic penumbra at the margins of an ischaemic infarct, which may be salvaged with neurointerventional procedures.

Technical Details of SPECT Imaging in Stroke

Cerebral SPECT (Single Photon Emission Computed Tomography) imaging using cerebral blood tracers can aid in the evaluation of cerebrovascular diseases. The commercially available radiotracers Tc-99m HMPAO (hexamethylpropyleneamine oxime) and Tc-99m ECD (ethyl cysteinate dimer) are used for evaluation of cerebral perfusion. These tracers demonstrate a generally linear relationship to blood flow at a wide range of flow rates, with cerebral uptake and retention of tracer occurring within the first 1-2 minutes following intravenous injection of tracer, allowing for a “snapshot” of cerebral blood flow within a narrow time window.

SPECT in Stroke Assessment

Cerebral SPECT imaging does not simply demonstrate the presence or absence of vascular occlusion. Rather, localized cerebral blood flow is regulated by the brain region itself. As the activity of a particular brain region increases, so does its need for oxygen and glucose. By a signalling mechanism involving neurons, glial cells, and the arterioles, the brain region calls for increase localized blood flow to meet its needs. Thus, cerebral SPECT imaging shows active brain tissue, inactive brain tissue, and compromised brain tissue.

Figure1: Coronal (top row) and horizontal (middle row) tomograms of a patient with left middle cerebral artery ischaemic stroke. (Colour scale shows normal levels of cerebral blood flow in yellow. Colour scale is on the left). Bottom image shows patient scan data compared to a normative database in a 3-D reconstruction image. Here, the colour scale indicates grey for areas that do not differ significantly from the normative database. In contrast, areas of green, light blue, and dark blue represent areas of more than 2, 3, and 4 SD below the mean perfusion of the normative database, respectively. Statistically significant increases in perfusion are illustrated in the red colour scale.
SPECT in Evaluation of Carotid Stent Benefits

Several studies have the demonstrated benefits of carotid endarterectomy and carotid stenting, in carefully selected patients with acute stroke or risk of stroke. Benefits are generally seen where there is a moderate to severe stenosis in a carotid artery, with recent ischaemia (transient ischaemic attack or infarction) in the territory of the stenosed vessel. The cerebral circulation is complex with multiple arterial inputs to the Circle of Willis, with altered haemodynamicsresulting from gradual occlusion of one of more vessels over time. In additional, there is highly responsive intracerebral autoregulation to maintain blood flow, particularly when cerebral perfusion pressure is reduced.  Cerebral SPECT imaging allows a global assessment of the integrated effects of these factors, as it measures cerebral cortical and subcortical grey matter blood flow. Furthermore, dynamic assessment using intravenous acetazolamide or inhaled CO2provides an accurate measure of cerebrovascular reserve. Both acetazolamide and inhaled CO2 cause vasodilation of cerebral microvasculature. By challenging the vascular system with additional flow demands, these techniques reveal if the smaller arteries fed by the carotid arteries can support the flow demand. This technique can be useful in decision making when patients are being considered for carotid revascularizationthat are at high surgical risk. Simply put, SPECT neuroimaging can give a reasonable assessment of the degree of benefit that can be expected from these surgical interventions. Furthermore, manypatients with carotid stenosesdo not correspond precisely with the enrolment criteria used in prior carotid surgery trials, and the trial results cannot be more widely applied. Clinical judgement is required and an objective measure of cerebrovascular reserve can help guide decision making.

Figure 2: Patient with high grade carotid stenosis and multiple co-morbidities. The left hemisphere (on the right of the image) is able to augment its blood flow with acetazolamide, but there is restriction of flow to the right side so there is relative deterioration in right perfusion, compared to the left.
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