The principle of X-ray imaging technology

On the one hand, the reason why X-ray can make human body form images on screen or film is based on the characteristics of X-ray, that is, its penetrability, fluorescence effect and photographic effect; On the other hand, it is based on the difference of human tissue density and thickness. Because of this difference, when X-rays pass through various human tissues and structures, they are absorbed to different degrees, so the amount of X-rays reaching the screen or film is also different. In this way, an image with different black and white contrast is formed on the screen or X-ray. The tissue structure of human body is composed of different elements, which have different densities according to the total amount of various elements in the unit volume of various tissues. The density of human tissue structure can be summarized into three categories: high-density bone tissue and calcified lesions; Medium density includes cartilage, muscle, nerve, parenchymal organs, connective tissue and body fluids; Low density adipose tissue and gas existing in respiratory tract, gastrointestinal tract, sinus and mastoid. When X-rays with uniform intensity penetrate through different density tissue structures with the same thickness, X-ray images with black-and-white (or light-dark) contrast and different levels will appear on X-ray films or screens due to different absorption degrees.

In the human body structure, the rib density in the chest is high, which absorbs more X-rays and the photos are white. The lung has low gas density, less X-ray absorption and dark photos.

When X-rays penetrate low-density tissues, they are absorbed less, and there are more X-rays left, which makes the X-ray film more sensitive. Metal silver is reduced by photochemical reaction, so the X-ray film is black. Make the fluorescent screen produce more fluorescence, so the fluorescent screen is bright. High density tissue is just the opposite.

Lesions can also change the density of human tissues. For example, tuberculosis can produce moderate fiber changes and high calcification in low-density lung tissue. On the chest radiograph, a white shadow representing the lesion appears on the background of lung shadow. Therefore, pathological changes of different tissue densities can produce corresponding pathological X-ray images. The human tissue structure and organ morphology are different, and the thickness is also inconsistent. Its thick part and thin part are either clearly defined or gradually migrated. The thick part absorbs more X-rays and transmits less X-rays, while the thin part is the opposite. The contrast between black and white on the X-ray film and the screen, the difference between light and dark, and the relatively clear or gradually moving boundaries from black to white and from bright to dark are all related to their thickness differences.