Imaging x-ray imaging

(1) Generation of X-rays 1895, a German scientist Roentgen discovered high-energy rays, which are invisible to the naked eye, but can penetrate different substances and make fluorescent substances glow. Because the nature of this ray was not known at that time, it was called X-ray. In memory of the discoverer, it was later called Roentgen ray, and now it is simply called X-ray.

Generally speaking, high-speed electron flow can be blocked by matter to produce X-rays. Specifically, when an electron beam traveling at high speed in a vacuum tube hits a tungsten (or molybdenum) target, X-rays will be generated. Therefore, the X-ray generator mainly includes X-ray tube, transformer and console.

X-ray tube is a high vacuum diode with a cup-shaped cathode filled with filament. The anode consists of an inclined tungsten target and an attached radiator.

The transformer is set to provide X-ray tube filament power supply and high voltage. The former generally only needs less than 12V and is a step-down transformer; The latter needs 40 ~ 150 kV (generally 45 ~ 90 kV) as a step-up transformer.

The console is mainly used to adjust voltage, current and exposure time, including voltmeter, ammeter, timer, adjusting knob and switch.

The X-ray tube, transformer and console are connected by cables. See figure 1- 1- 1 for the main components and circuits of the X-ray machine.

The procedure of X-ray generation is to turn on the power supply, heat the filament of X-ray tube through the step-down transformer, and generate free electrons, which gather near the cathode. When the step-up transformer provides high-voltage electricity to the two poles of the X-ray tube, the potential difference between the cathode and the anode increases sharply, and the free electrons in the activated state are strongly attracted, so that the bundled electrons travel from the cathode to the anode at high speed and hit the anode tungsten target atomic structure. At this time, energy conversion takes place, in which about 1% of energy forms X-rays, and the remaining 99% is converted into heat energy. The former is mainly emitted by the window of X-ray tube, while the latter is emitted by heat dissipation facilities.

(II) Characteristics of X-ray X-ray is an electromagnetic wave with a very short wavelength. The wavelength range is 0.0006~50nm. The commonly used X-ray wavelength range for X-ray diagnosis is 0.008 ~ 0.03 1 nm (equivalent to 40 ~ 150 kV). In the electromagnetic radiation spectrum, it is between gamma rays and ultraviolet rays, which is much shorter than the wavelength of visible light and invisible to the naked eye.

In addition to the above general physical characteristics, X-rays also have the following characteristics related to X-ray imaging:

Penetration: X-rays have short wave length and strong penetrating power, and can penetrate various substances with different densities that ordinary visible light cannot penetrate, and are absorbed or attenuated to a certain extent during the penetration process. The penetration of X-ray is closely related to the voltage of X-ray tube. The higher the voltage, the shorter the wavelength of X-ray and the stronger the penetrating power. On the contrary, the lower the voltage, the longer the wavelength of X-rays generated, and the weaker the penetration. On the other hand, the penetrating power of X-rays is also related to the density and thickness of objects. X-ray penetrability is the basis of X-ray imaging.

Fluorescence effect: X-rays can excite fluorescent substances (such as zinc cadmium sulfide and calcium tungstate) to produce visible fluorescence. That is, X-rays act on the fluorescent substance, so that X-rays having a short wavelength are converted into fluorescence having a long wavelength. This conversion is called fluorescence effect. This feature is the basis of fluoroscopy.

Photographic effect: After X-ray irradiation, the film coated with silver bromide can be exposed to light, resulting in latent image. After development and fixing, silver ions (Ag) in the exposed silver bromide are reduced to metallic silver (Ag) and deposited in the film. This metallic silver particle is black on the film. The unexposed silver bromide is washed off from the X-ray film during fixing and developing, thus showing the transparency of the film base. According to the amount of metallic silver precipitation, a black-and-white image will be produced. Therefore, the photographic effect is the basis of X-ray imaging.

Ionization effect: X-rays passing through any substance can produce ionization effect. The ionization degree of air is directly proportional to the amount of X-rays absorbed by air, so the amount of X-rays can be calculated by measuring the ionization degree of air. X-rays entering the human body will also produce ionization, which will cause biological changes in the human body, that is, biological effects. It is the basis of radiation protection and radiotherapy. 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 different density structures with the same thickness, the situation as shown in Figure 1- 1-2 will appear due to 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.

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, on the contrary, pathological changes will also change the density of human tissue. 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 performances, as shown in figure 1- 1-3. 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 per unit volume in human tissues, while the latter refers to the black-and-white images displayed on X-rays. 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. X-ray image is the sum of the projections of X-ray beams penetrating through tissues with different densities and thicknesses in a certain part, and it is an image in which the projections of all layers are superimposed on each other on the penetration path. In orthographic X-ray projection, it includes not only the front, but also the middle and back. Due to overlapping, the projections of some tissue structures in the body can be well displayed due to cumulative gain, while the projections of other tissue structures in the body may be difficult or impossible to display due to weakening and cancellation.

Because the X-ray beam is projected conically from the X-ray tube onto the human body, the X-ray image will be enlarged to a certain extent and an accompanying image will be generated (Figure 1- 1-4). The clarity of the X-ray image is reduced.

Cone projection may also affect the X-ray image, as shown in figure 1- 1-5. The X-ray image of the central ray position, although enlarged, still maintains the original shape of the object, and there is no image distortion or distortion; On the other hand, the X-ray image of the edge ray part, due to oblique projection, not only magnifies but also distorts the illuminated object.