The development of the hottest digital X-ray imagi

The development of digital X-ray imaging technology

in 1895, German physicist William roentgen discovered X-ray, which is considered to be a major discovery in the 19th century. After several months of technical breakthroughs, this "new light" was used to check fractures and determine the location of bullets in gunshot wounds. Although X-ray was first used for medical purposes, the theory of this new technology has also been applied to the field of nondestructive testing. For example, the early X-ray of zinc plate implied the possibility of welding quality control. In the early 20th century, X-ray was applied to boiler inspection

in the second half of the century, X-ray technology - although unchanged for a long time - has not changed significantly. X-rays emitted by ray sources pass through objects and are then received through films or fluorescent screens. The contrast and spatial resolution of the film are controlled by the speed of the film and the X-ray source. Using the fluorescent intensifying screen with film, a better image effect is obtained at low energy

in the 1950s, with the emergence of image intensifiers, great changes took place, and real-time clear images were obtained for the first time. Through the image amplifier, X-rays are collected from the fluorescent screen and focused on another screen, which can be observed directly or through high-quality TV or CCD cameras. For real-time imaging, although the image intensifier has strong performance, until recently, film was still selected to save large images, high-quality spatial resolution and contrast

however, each technology has its own shortcomings. Chemical processing x-ray film usually takes 20 minutes of lag time from image acquisition to technician's detection. If the film exposure is insufficient or the transmission angle is wrong, all procedures must be repeated, and it still takes 20 minutes. If you irradiate a lot of film, it will take several hours. In addition, the company must provide storage locations and trained employees to ensure the safe operation, storage and handling of film processing solutions. Although the spatial resolution of the film is good, due to the poor linearity and narrow contrast range of the film, coupled with the limitations of human eyes, the discrimination ability can not exceed 100 gray levels, so it is impossible to detect and obtain more accurate data from a wide range of film density

for image intensifier, its application range is limited by its huge protective volume and field of view, and the edge of the image is distorted. Only the image at the center is useful for some applications. In addition, the contrast and spatial resolution of image intensifier cannot be compared with other technologies. No matter how much film or image intensifier is archived and distributed, it is inconvenient. For image archiving of image intensifier, it needs to be converted into video format; For X-ray negatives, scan

digital field: computed radiography

since the introduction of computerized X-ray Technology (CR) in the 1980s, X-ray imaging has undergone great changes. Until this time, the real automatic inspection, defect identification, storage and relying on human interpretation of images or films were realized. CR provides useful computer assistance and image identification, storage and digital transmission, eliminating the film processing process and saving the resulting costs

cr is similar to film, but it replaces film and stores images inside it through the image storage board. In many cases, this technology can be easily renovated into a film based system, but it does not need film, chemicals, darkroom, related equipment and film storage

to reduce costs in these areas means to quickly recover investment. For example, Fred Morro of Dr NDT product department of Fuji company said, "we make a cost analysis for each customer, and the cost includes film, liquid medicine and liquid medicine treatment. Of course, these depend on the dosage, but the ROI is less than before"

envision of the United States successfully developed CMOS digital tablets, which saved the costs of film, processing and chemical recovery. It is estimated that about $6000 will be saved per 1000 X-ray films, excluding the cost of storing films and darkrooms

about performance, Morro said: the contrast of Cr is 12 bits or 4096 gray scale, which is similar to film. Morro also added that although its spatial resolution has not exceeded that of film, it is sufficient for most NDT applications. The accuracy of Cr is 5 pairs/mm (i.e. 100 m). Because of its large contrast range, Cr can be applied to all digital X-ray technologies. The full thickness range of most subjects can be obtained by one exposure, which is sometimes impossible for film. Through the computer, you can browse any interested location in the whole thickness range

like film, CR image plates can also be divided and bent, although the cost of storage plates is higher than that of film (14 × 17in), the price of the board is about 700 dollars, but it can be used thousands of times. Its life depends on the degree of mechanical wear, but it is actually cheaper than film. In addition, like film, the use conditions are very demanding, and it cannot be used in humid environment and extreme temperature conditions

cr has the advantage over other digital technologies: in most cases, only one image board reader is needed in the whole laboratory. This reader is independent of the image board, and users can purchase it separately, which is different from other digital technologies integrating acquisition and reading

cr is that it is similar to film and cannot be real-time. Although it is faster than film, the image board must be removed from the X-ray exposure chamber and then put into the reader. CR makes the film free X-ray technology a big step forward, but it can not provide all the advantages of X-ray digital technology

digital tablet

in the late 1990s, digital tablet came into being. This technology is different from the processing process of film or cr. it adopts X-ray image digital readout technology to truly realize the automation of X-ray NDT detection. Except that it cannot be divided and bent. The digital flat panel can be used in the same application range as film and Cr. it can be placed in the position of machinery or conveyor belt, and the parts that pass the inspection can also be inspected with multiple configurations for multi view inspection. During the two exposures, it is not necessary to change the film and store the fluorescent plate. It only takes a few seconds of data acquisition to observe the image, which is greatly improved compared with the production capacity of film and Cr

at present, two digital flat panel technologies are competing face-to-face in the market: amorphous silicon (a-Si) and amorphous selenium (a-Se). On the surface, these two kinds of flat plates operate in the same way: the extracted X-rays are converted into digital images through the panel. The panel does not need to be processed like film. It can collect data at the speed of one image in a few seconds, or collect live data at the speed of 30 images per second. In addition, because of their high accuracy and wide field of view, it is ideal to replace the image intensifier by displaying images at the speed of 30 per second. However, an amplitude frequency of 30 per second will reduce the accuracy of the image

for the flat panel technology of amorphous selenium, X-rays will impact the selenium layer, which directly converts X-rays into charges, and then converts charges into digital values of each pixel. This method is called direct image. Supporters say amorphous selenium provides better spatial resolution than amorphous silicon

it is generally called amorphous silicon plate (the name is not accurate, that is, amorphous silicon is used). X-rays first hit the scintillation layer on the plate, and the scintillation layer emits photoelectrons in proportion to the energy of the struck rays. These photoelectrons are collected by the silicon photoelectric diode array below, and they are converted into charges, and then these charges are converted into the digital value of each pixel. Because the intermediate medium that converts X-rays into light is the scintillation layer, it is called the indirect image method. The scintillation layer is generally composed of cesium iodide or oxysulfide, and cesium iodide is an ideal material. Its supporters said that the amplitude frequency of amorphous silicon plate is faster than that of amorphous selenium plate, which can reach 30 images per second

the spatial resolution of both technologies is close to that of film, but the contrast range far exceeds the performance of film

the dispute between the two is mainly theoretical. More about the mode conversion function, detection quantum efficiency and a large number of Einstein theories, it seems that they are similar, that is, who has a higher spatial resolution under the condition of ensuring the best contrast and minimum noise. Bedford, Massachusetts based hologic, which mainly develops selenium imaging plates, believes that the light generated by the scintillation layer of the indirect system will be slightly scattered before reaching the photodetector, so the effect is not good. For selenium plate imaging system, electrons are directly impacted by X-rays on the plate, resulting in very small scattering, so the image accuracy is high

"what I argue about is not just theoretical," said Ken Swartz of hologic Inc. "now, there are a large number of public studies comparing the image quality and production efficiency advantages produced by direct and indirect detectors". He pointed out that in April 2003, Ehsan samei and Michael Flynn published in med Phys' article "test and compare the detector performance of direct and indirect digital ray (DR) systems", through the comparison between hologic's direct imaging board and other indirect imaging boards by GE and Philips, it is concluded that the performance of amorphous selenium board is good when the accuracy requirement is less than 200 m; For accuracy greater than 200 m, amorphous silicon has good performance. Ken Swartz also pointed out that Kodak, Siemens, Philips, Agfa and instrumentarium have selected amorphous selenium detectors because of their excellent image quality to meet complex medical applications

here are the technical parameters of planar imaging boards from several manufacturers:

agfa company: the company provides 11 × 16 inch silicon plate, 12 bits (gray 409, and adopting digital controller to replace the original analog controller has become a necessary choice for the new generation of hydraulic fatigue testing machine 6), with an accuracy of 127 M. Agfa also assembled 14 of hologic × 17 inch selenium plate. These two kinds of plates can meet the temperature requirements on site

ge company: at present, GE company provides four kinds of digital silicon plates. The plate size ranges from 63 to 256 square inches. It can be operated in static mode or at the speed of 30 frames per second. All four plates have 14 bit (16000 gray scale) contrast, and the spatial resolution reaches 9 pairs/mm (55 m), without geometric amplification. " Ralf usinger, innovation and industrialization manager of ruag spac e (CH), said that

hologic Inc: 14% of the company × 17 inch selenium plate with an accuracy of 3.6 pairs/mm (139 m) and 14 bits (16000 gray). Hologic also produces 7.2 pairs/mm (70 m) flat plates

PerkinElmer company: the company's highest precision flat plate is 16 × 16 inches, with an accuracy of 200 m. Its 8 × The accuracy of the 8-inch plate is 400 m, the acquisition speed is 7 amplitude frequency/s, and its maximum receiving energy can reach 25 MeV. The contrast of all products is 16 bits or 65000 grayscale

varian: 12 of the company × 16 inch silicon plate with an accuracy of 3.97 pairs/mm (126 m). In the high-speed mode, the acquisition speed is 30 amplitude frequency/s, and the accuracy is 1.29 line pairs/mm (388 m). Varian (5) start the experimental program company also sells high-energy flat plates, which can receive energy up to 9 MeV, so it is possible to detect aluminum castings with a thickness of less than 27 inches. The contrast of the product is 12 bits or 4096 gray, and there are also 65000 gray versions