One of the earliest signs of acute ischemic stroke on noncontrasted head CT is the dense vessel sign, which will be hyperdense relative to the contralateral side and represents acute thrombus. This is typically present at the proximal portion of the vessel and can be seen within 90 minutes [2]. A later sign of acute ischemia is loss of the gray–white differentiation of the effective territory which can become apparent in the first 3 hours [4]. Using comparison to the contralateral, normal hemisphere is a powerful tool in both findings. These findings are very highly specific but not very sensitive [2–4].

If the noncontrasted head CT is negative or equivocal for acute stroke, a brain MRI can be utilized for higher sensitivity evaluation. Hyperintense DWI signal correlating with hypointense ADC signal is indicative of ischemic stroke in the acute phase. Restricted diffusion in the setting of stroke means simply that water cannot escape brain cells and cytotoxic edema ensues. Restricted diffusion can be seen as early as 30 minutes of the event [6] and sensitivity is >90% [6–8]. Hypointense ADC signal will normalize over the course of days to weeks and is useful to age the infarct [7, 8].