The higher positive rate of cerebral infarction in CT examination is after 48 hours of the onset of cerebral infarction, this is because after cerebral infarction occurs after the blockage of blood vessels, resulting in ischemia, hypoxia and necrosis of brain tissues in the area of blood supply, but the early brain tissue is only a loss of function, and has not been completely liquefied, which may not be able to be shown on the cranial CT, so within 48 hours of the cranial CT is often suggestive of no obvious abnormal density. After 48 hours, the brain tissue is completely necrotic and liquefied, and will show a low density shadow, which is clearer on CT. In the early stage, the edges of this low-density shadow are blurred, and the density in the middle is not particularly different from that in the surrounding area, so it can be judged as a fresh infarction. With the passage of time, the density of the lesion will gradually become lower, and the contrast with the surrounding area is clear, and the edges will become sharp, and the interior is full of proliferating glial cells, so this kind of CT film can identify the infarction as an old cerebral infarction.