ACR Codes: 1.9
The term "stroke" encompasses a wide array of pathological causes of cerebral ischemia. The clinical manifestations which lead to the diagnosis of "stroke" may be a result of cerebral infarction, primary intracranial hemorrhage, subarachnoid hemorrhage, or venous occlusion. Of these, cerebral infarction is found to account for patient symptoms 80% of the time. More specifically large vessel infarcts (ICA, MCA, and PCA) account for up to 50% of all clinical strokes.
NECT of the Head is frequently used as an initial study in patients presenting with stroke symptoms because of its relative speed and efficiency in stratifying patients into "bleed" and "infarct" groups for treatment purposes. While NECT may be fairly effective at ruling out hemorrhage, ruling in infarction in the acute setting is more problematic. Up to 60% of NECT scans will be completely negative within the first few hours of a cerebral infarction. Even when NECT findings are found in the first 24 hrs, they can be quite subtle.
Somewhere between 6 to 24 hours after an acute infarct NECT may begin to reveal the "dense MCA sign," the "insular ribbon sign," and early cerebral hypodensities with associated sulcal effacement indicating evolving edema. The dense MCA sign is seen as hyperattenuation of the MCA in usually at the base of the brain on the affected side (the MCA is responsible for up to 40-50% of acute cerebral stokes). The insular ribbon sign is seen as loss of the normally distinct gray-white matter differentiation in the insular cortex along the sylvian fissure.
In cases where the NECT is non-specific or negative in the setting of clinical stroke, MRI has proven to be particularly helpful. Specific MRI sequences reveal acute infraction immediately following an event. The sensitivity is therefore far greater than NECT in the first few minutes to hours. Early MR findings include the loss of normal flow voids in occluded vessels, signal abnormalities consistent with edema in the affected regions, and most notably diffusion abnormalities seen on diffusion weighted images (DWI). In an acute stroke, hyperintensity on the DWI will correspond to hypointensity on the ADC maps. Further explanation of the physics behind these imaging sequences is beyond the scope of this discussion but can be easily found in any comprehensive Neuroradiology or MRI Physics text.
In this case, you should notice that the initial NECT of the Head demonstrates some early findings concerning for left sided ACA and MCA infarction. The follow up MRI with DWI, ADC, and MRA sequences reveal much more dramatically the extent and severity of this patient's condition.
Reference(s): 1. Osborn AG: Diagnostic Neuroradiology, pp341-355, Mosby, St. Louis 1994.
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Disease Topic 4965 
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