Computed tomography in acute stroke

The goal of this chapter is to familiarize the reader with the use of computed tomography (CT) and advanced CT in the diagnosis, treatment, and management of suspected acute stroke and to develop an understanding of the role, strengths, and weaknesses of each application of CT. Key points necessary to understanding image interpretation and patient triage will be highlighted. CT: considerations CT offers multiple applications to aid in the detection and characterization of acute stroke. Rapid ongoing improvements in scanner technology continue to expand its role. Unenhanced CT, the traditional initial screening modality in the acute setting of suspected stroke, can reveal the presence of an infarct or intracranial hemorrhage, and is sufficient for screening patients for standard IV thrombolytic therapy. Focal edematous hypodensity on CT, in the setting of abrupt onset of stroke symptoms, is specific for tissue with a high probability of irreversible infarction, although sensitivity - especially at early times postictus (<3-6 hours) - is likely less than that of magnetic resonance (MR) diffusion-weighted imaging (DWI). CT angiography (CTA) in the acute setting serves to detect proximal large vessel occlusion. It is fast, accurate, and can be obtained without delaying the administration of IV thrombolysis. CT perfusion (CTP) extends traditional anatomic imaging to evaluate stroke physiology (including capillary-level hemodynamics), improves the accuracy of differential diagnosis by excluding stroke mimics, and aids in outcome prediction. When DWI is not available, appropriately thresholded CTP-cerebral bloodflow (CBF) maps can help to identify critically hypoperfused tissue with a high probability of infarction in the absence of early, robust reperfusion (Table 7.1). CTA source images (CTA-SI), the parenchymal axial images obtained as part of the CTA vascular acquisition, offer additional views of perfusion hemodynamics in the detection of infarct, as well as the assessment of collateral circulation - especially aiding in the detection of a "malignant" collateral pattern - the near complete absence of collateral flow on CTA - which is highly specific for poor tissue and clinical outcome.