Basic principles of clinical medical imaging

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.

Therefore, the formation of X-ray images should meet the following three basic conditions: first, X-rays must have a certain penetrating power to penetrate the irradiated tissue structure; Secondly, there must be differences in the density and thickness of the penetrated tissue structure, so that the amount of X-rays remaining after being absorbed during the penetration process will be different; Thirdly, this differential residual X-ray is still invisible, and it must go through the imaging process, such as X-ray film, screen or TV screen display, in order to obtain X-ray images with black-and-white contrast and hierarchical differences.

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 structures with equal thickness and different densities, they will appear because of different absorption degrees. X-ray images with black and white (or light and dark) contrast and gradation differences are displayed on X-ray films or screens.

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.

Relationship between different density tissues (same thickness) and X-ray imaging

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. Therefore, X-ray projection can have different properties. 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.

A when x-rays pass through a trapezoid, the thick part absorbs more x-rays, but transmits less x-rays. The photo shows a white shadow, while the thin part shows a black shadow. There is a clear line between white shadow and black shadow. On the fluorescent screen, on the other hand, when B.X rays pass through a triangle, their absorption and shadow formation are similar to those of a trapezoid, but the black and white shadows are gradually transitional and there is no clear boundary. On the contrary, when C.X-rays pass through the tubular body seen on the fluorescent screen, the outer part of the tubular body absorbs more X-rays and transmits less X-rays, showing a white shadow, and the middle part shows a black shadow. The boundary between white shadow and black shadow is clear. Contrary to what you see on the screen.

It can be seen that the difference of density and thickness is the basis of image contrast and the basic condition of X-ray imaging. It should be pointed out that the role of density and thickness in imaging depends on which one is dominant. For example, in the chest, the ribs have high density but small thickness, while the great blood vessels of the heart have low density but large thickness, so the image of the great blood vessels of the heart is whiter than that of the ribs. Similarly, a large number of pleural effusion has a medium density, but its image is whiter than that of ribs because of its large thickness. It should be pointed out that the density of human tissue structure and the image density on X-ray film are two different concepts. The former refers to the mass of matter in the unit volume of human tissues, while the latter refers to the black and white images displayed on X-ray films. However, the density of a substance is directly proportional to its own specific gravity. The density of matter is high, the ratio is heavy, the amount of X-rays absorbed is large, and the image in the photo is white. On the contrary, the substance has low density, small specific gravity, less X-rays absorbed, and the image in the photo is dark. Therefore, the white shadow and black shadow in the photo, although also related to the thickness of the object, can reflect the density of the material. In terminology, high density and low density are usually used to represent white and black of an image. For example, white shadow, gray shadow and black shadow are represented by high density, medium density and low density respectively, and the density of matter is represented. When the density of human tissue changes, the white shadow and black shadow of the image are represented by increasing or decreasing the density.