The difference between high-density shadow and low-density shadow is determined by the density of the lesion and the difference in density between the lesion and the surrounding normal tissue. If a brain hemorrhage is found in the skull, this is a high-density shadow, and the CT value of the lesion is high. Generally it is around 70-90HU, and the density of the lesion is high compared with the normal brain tissue, which is shown as white and bright on the CT film. If low-density shadow is found on the brain, it generally indicates the possibility of brain infarction, and the density is low compared with normal brain tissue, showing a darker color. Both high-density and low-density shadows need to be combined with clinical symptoms and medical history to determine what lesion is present.