Please talk about the advantages and disadvantages of digital spot film and film spot film of X-ray machine.

1 imaging principle of traditional x-ray film

The imaging process of traditional film X-ray machine is based on photochemical theory, while digital X-ray machine is based on optoelectronics theory. Silver halide is the main photosensitive material in X-ray film, and the particle size and granularity of silver halide in emulsion are one of the most important parameters. Because the image of the object is composed of silver halide reduced to granular silver. In the photosensitive process, silver halide particles act alone, and each particle forms a developing unit of the latent image. In the normal exposure range, the number of developable particles increases with the increase of exposure. The smallest diameter of silver halide particles in the photosensitive layer is only 50nm, and most of them are between 0.1-4 μ m. Silver halide particles are large and easy to be photosensitive, while silver halide particles are small and difficult to be photosensitive. The film with small silver halide particles has low sensitivity, and vice versa; The smaller the silver halide particles, the better the resolution and texture. In the preparation of X-ray film, impurities are intentionally added, and very small and uniform impurity particles and silver halide emulsion are evenly coated on the film. The more uniform the film quality, the more uniform the distribution of photosensitive centers, and the better the resolution and texture of the image.

The crystal structure of silver halide is regular hexagon. Such an ideal structure is stable and has no photosensitivity, that is, it will not be photosensitive. Only the defective lattice structure arrangement can cause the weak links of the crystal structure, and these weak links become the photosensitive centers, which make the silver halide crystal photosensitive. When the photosensitive layer of the film is exposed, light quantum acts on the silver halide crystal. Halogen ions first absorb light quantum, release a free electron, and then become halogen atoms. After halogen atoms form halogen molecules, they leave the lattice structure and are absorbed by gelatin, and free electrons quickly move to the photosensitive center and are fixed. In this way, the photosensitive center becomes a charged body with negative electric field and absorbs a lot of electrons. Under the action of electric field, the silver ions in the crystal lattice are attracted by the electric field, and then the electrons gathered in the photosensitive center are captured and reduced to silver atoms. The reduced metallic silver atoms are also fixed on the photosensitive center, which makes the photosensitive center expand further, and the expanded photosensitive center constantly captures photolytic electrons. Over and over again, the photosensitive center keeps growing, and exposure to a certain extent is appropriate. At this time, the development center formed by the photosensitive center constitutes the latent image core of the image, and the latent image is composed of countless development centers, and the image we need is formed through the later chemical development and fixing process.

Principle of digital x-ray imaging

Digital X-ray imaging equipment refers to X-ray equipment that digitizes and processes X-ray transmission images and then converts them into analog images for display. According to the different imaging principles, this kind of equipment can be divided into computer radiography (CR) system, digital subtraction angiography (DSA) system and digital radiography (DR) system.

CR records X-ray images with storage media, converts the stored signals into optical signals through laser scanning, and then converts them into yes signals with photomultiplier tubes. After A/D conversion, it is input into a computer for processing and becomes a high-quality digital image.

DR can be divided into direct digital photography (DDR) and indirect digital photography (IDR).

DDr refers to the method of directly converting X-rays into electrical signals and then forming digital signals by using one-dimensional or two-dimensional X-ray detectors. One-dimensional detector has multi-wire proportional chamber, gas ionization chamber and so on. Fan-shaped plane X-rays are used for scanning projection, and then magnified to synthesize a two-dimensional image. The two-dimensional detector includes amorphous selenium flat panel detector (FPD), which directly converts X-rays into digital signals. There is also an amorphous silicon flat panel detector, which is first converted into visible light by flashing light-emitting crystals, and then converted into digital electrical signals. The flat panel detector box contains analog-to-digital conversion. From the outside, X-rays directly output digital signals through the detector box. In addition, X-ray charge-coupled devices can directly convert X-rays into digital signals.

IDR refers to the method that X-ray images get analog images of X-ray information through X-ray film or image intensifier-TV imaging chain, and then convert them into digital signals.

At the end of 1970s, the research of digital X-ray photography (DR) began. On the basis of I. I-TV system, the analog video signal is digitized by A/D converter to realize computer processing. From the end of 1970s to 1980s, the long-range DDR used X-ray scanning projection imaging, and in the mid-1990s, DDR using FPD appeared.

1997 STERLING, TRIXELL and other companies introduced amorphous selenium and amorphous silicon x-ray detectors. The principle of amorphous selenium detector is to coat amorphous selenium on thin film transistor (TFT) array. The cell size of each TFT is 139* 139(um), and the number of cells in the range of 14* 17 inches is 2560*3027. The incident X-ray photons generate electron-hole pairs in the selenium layer. Under the action of external electric field, electrons and holes move in opposite directions to form a current, which is integrated into a storage charge in a thin film transistor. The storage capacity of each transistor corresponds to the energy and quantity of incident photons, and each thin film transistor becomes a pixel. A field effect transistor is also made in each pixel range as a switch, which is triggered by the control circuit to transmit the charges stored in the pixels to the external circuit one by one. Pixel signals are amplified and converted into 14-bit digital signals, and then these data are constructed into images.

The principle of amorphous silicon detector is different, and its pixels are all composed of photodiode and thin film transistor. Photodiode is made of amorphous silicon hydride, which generates current under visible light irradiation. The photodiode matrix is covered with a scintillation crystal layer, and X-ray photons are converted into visible light photons through the scintillation crystal layer, which excites the photodiode to generate current, and the current is integrated on the photodiode's own capacitance to form stored charge, and the stored charge amount of each pixel is proportional to the incident X-ray photon energy. Under the action of the control circuit, the charge stored in each pixel is read out according to a certain rule, and the digital signal of 14bit is output, and the image is established by the computer. Because the dynamic range of the detector can reach 1: 10000, and the signal readout is 14bit, while the dynamic range of traditional film is 1: 1000, the density resolution of DR is higher than that of traditional film.

At present, GE has developed an integral digital flat panel on the original basis. There is a blind area about 300um wide on the original spliced amorphous silicon detector, which will not show the structure of 250um*450um calcification furnace at all. The data reading time of the whole amorphous silicon digital flat panel detector is extremely short, only 0. 125 seconds, which is at least 10 times faster than other digital flat panels. The time interval between two exposures is less than 0.2 seconds, which, combined with the highest DQE and high-efficiency circulating refrigeration temperature control technology, lays a foundation for advanced clinical applications of DR, such as energy subtraction, three-dimensional imaging and image mosaic. All-amorphous silicon digital flat panel detector directly digitizes the data on each data line, which avoids the noise brought by multiplexer and improves the data reading speed.

3 Conclusion

With the development of standardization and standardization in hospitals, doctors need to understand the principles and correct use of traditional film imaging and digital imaging, so as to standardize the quality control of images. This is an important measure to reduce misdiagnosis and medical disputes in modern hospitals.

Ok!!