CT low-density shadow refers to the black or near-black shadow in the imaging examination, and the common lesions of low-density shadow are cerebral infarction, cerebral hemorrhage, cerebral edema and so on. 1. Cerebral infarction: cerebrovascular infarction caused by ischemia, hypoxia, necrosis and softening of brain tissues, mainly manifested as impaired consciousness and limb paralysis, CT examination is preferred, CT shows low-density foci, the sulcus of the brain is mildly shallow, and the gray-white matter of the nucleus pulposus area is poorly demarcated. 2. Cerebral hemorrhage: primary non-traumatic hemorrhage within the brain parenchyma, commonly seen in hypertension combined with small arteriosclerosis, early manifestations of headache, dizziness. Later, numbness of facial limbs, visual impairment, paralysis, etc. appear. CT examination of early imaging, manifested as regular clumps, peripheral appearance of low-density rim shadow, brain parenchyma internal enlargement. 3. Cerebral edema: increased water in the brain tissue caused by trauma, tumor, hemorrhage, brain volume increases, often manifested as increased intracranial pressure headache, vomiting, epilepsy, for CT examination is a large low-density foci, shallow cerebral sulcus, gray matter and white matter junction blurred. If the CT shows a low density shadow, you should consult a doctor in time, under the formal guidance of the doctor, to identify the cause of the disease, symptomatic treatment.