As soon as blood supply ceases, ischemic cells are unable to withstand the constant osmotic pressure they are subjected to, and extracellular water floods into the cells causing them to swell, a process we call cytotoxic edema.
It is important to recognize that this is merely a redistribution of water, within the amount of increased intracellular water precisely offset by depletion of extracellular water. This therefore does not change the total amount of water in the affected part of the brain and so there is no change in density and no change in the volume of the tissue. This is very different to the changes we see later as the brain swells and becomes hypoattenuating.
When water from collateral vessels is recruited to replenish the depleted extracellular space, mass effect and reduction in density on CT occurs. This is more accurately called ionic edema, although no one uses the term clinically. In the hyperacute setting you will only have pure cytotoxic edema and this is not visible on CT. Then at this time you should look immediately for thromboembolism - hyperdense artery sign.
One area that is particularly useful is to look in the Sylvian fissure for thromboembolic hyperdensity, appearing as a dot in a vessel that is traveling from inferior to superior. Identifying a basilar tip thromboembolism is extremely important and you should have a low threshold to go on CT angiography in patients who present with top of the basilar syndrome. When you look to brain CT images concentrate on searching hyperdense artery sign.