CT values generally range from -1000HU to +1000HU. CT values are a response to the density of human tissues and are generally expressed in units of HU. The CT value of air is the lowest, close to -1000HU; the density of fat is between -70HU and -90HU; after that is the density of water, which is around 0HU; the density of soft tissue is generally around 30-50HU; the density of brain hemorrhage is generally around 50-90HU; the density of calcification and bone is increasingly high, and the highest can reach +1000HU; metal-like substances whose CT values are even higher, such as the steel plates implanted in the body.