C and W of lung CT do not represent nodule density. In clinical work, in order to make the CT image can achieve the best display of tissue structure and lesions, it is necessary to choose different window techniques based on the range of their CT values, which include window width and window position. 1.W for window width: Window width refers to the range of CT values included in the 16 gray levels on the image, and tissues within this CT value range are displayed in different simulated shades of gray. Increasing the window width increases the number of layers on the image and decreases the inter-tissue contrast; decreasing the window width decreases the number of layers on the image and increases the inter-tissue contrast. 2. C stands for window position: window position is the center position of the window, the same window width, due to the different window position, includes a different range of CT values. Raising the window position makes the image displayed on the fluorescent screen darker; lowering the window position makes the image whiter.