Stroke is the third leading cause of death in the United States. It is more common in males and its incidence increases with age. Risk factors include hypertension, smoking, diabetes, obesity, and heredity. The vast majority of strokes result from vascular causes, either ischemic (most common) or hemorrhagic. Ischemic CVAs, in turn, are subclassified as either thrombotic or embolic. Hemorrhagic CVAs most commonly result from primary intracerebral hemorrhage (e.g., hypertensive hemorrhage). Occasionally, vasospasm occuring as a result of a subarachnoid hemorrhage may result in an ischcemic CVA.
Stroke is a clinical diagnosis, and CT enables evaluation for hemorrhage to determine whether anticoagulation may be given. CT findings in stroke vary according to the age of the infarction. CT findings may be broken down as follows:
INFARCT AGE: CT findings
Less than 12 hours: usually normal.
12 to 24 hours: ill-defined focal areas of decreased density +/- evidence of mass effect.
Two to four days: focal areas of decreased density, often wedge-shaped and conforming to a
vascular territory, with evidence of increasing mass effect.
Four to seven days: gyral contrast enhancement.
Two to eight weeks: mass effect resolves, though enhancement may persist.
â€śOldâ€ť infarctions: focal areas of decreased density consistent with focal encephalomalacic changes
within the affected vascular distribution.
Tc-99m HMPAO is a cerebral blood flow (CBF) radiopharmaceutical and shares the following properties with other Tc-99m CBF agents. It is lipophilic; is of small molecular size; has a neutral charge; has a high first-pass extraction by the brain (80%); distributes across the blood brain barrier in proportion to CBF with a favorable gray/white matter uptake; and is fixed (i.e., is converted by glutathione into a more hydrophilic molecule) within brain cells with minimal subsequent redistribution. Brain uptake reaches peak activity within 10 minutes after IV injection, and approximately 3.5-7% of the injected dose is trapped within the brain. Note that functional brain scan imaging requires rigid adherence to a standard protocol to include careful regulation of the scanning environment, since a patientâ€™s response to stimuli to include lighting, noise, and positioning can effect changes in the scintigraphic perfusion pattern.
Diamox, also known as the carbonic anhydrase inhibitor acetazolamide, has supplanted CO2 in obtaining â€śstressâ€ť images of the brain. Like CO2, it causes vasodilation of the cerebral vessels, and its net effect on the cerebral circulation is similar to that of dipyridamole on the coronary circulation: a fourfold increase in flow in the normal cerebral vessels with concomitant perfusion defects in areas of cerebrovascular compromise.
In the future, the ability to assess a patientâ€™s cerebrovascular reserve as described above may prove especially useful in identifying patients with evidence of cerebrovascular reserve with defects that could potentially be amenable to intervention (e.g., carotid endarterectomy, cerebral bypass surgery).
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