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IntroductionDigital X-ray imaging systems are widely used in medical imaging and industrial non-destructive testing because of their advantages in image acquisition display storage and transmission. Such traditional medical imaging and industrial non-destructive testing X-ray imaging systems generally have a large working area, but the image resolution is not high, generally does not exceed 31 pixels / mm; and in practical applications, some observation and analysis of small components Internal structure work, such as inspection of integrated circuits, non-destructive testing of materials and components, and flaw detection of tiny parts, these practical applications do not require a large working area, but require high image clarity, and Athletes, troops, and field workers are diagnosed in time for injury. This work requires timely diagnosis of injured patients, and requires a high image clarity. A portable digital x-ray cone-coupled CCD imaging system is also required.
In recent years, flat-panel X-ray image intensifiers have been developed at home and abroad. The working principle is to use the microchannel plate as a conversion element from X-ray to electron, and the subsequent electron multiplying element, and the microchannel plate is closely focused and imaged with the fiber optic panel phosphor screen behind it. This is the core component of the portable X-ray imager currently on the market. This X-ray imager is directly visual and cannot output video signals. It cannot be directly connected to a computer for quantitative measurement and analysis. Although the X-ray image intensifier screen image can also be coupled to the CCD through an optical lens, the imaging volume is relatively large, and since the light energy loss is large, finally, the signal-to-noise ratio of the image is significantly reduced.
The portable digital X-ray cone-coupled CCD imaging system described in this paper uses a light cone as an optical relay element to couple an enhanced image of the X-ray II image intensifier fiber optic panel screen output to the photosensitive surface of the CCD, thereby forming a high The resolution, digital imaging system, and the spatial resolution of its system imaging were tested and analyzed.
1 Imaging system structure and working principle
The portable digital X-ray cone-coupled CCD imaging system described in this paper uses an X-ray image intensifier, which is an II image intensifier with an effective working diameter of 50 mm, an operating voltage of 35 to 70 kV, and an operating current of 220 to 500 μA. The fluorescent output screen is a fiber optic panel. The CCD is a SONY CCD with an effective working area of ​​6.4 mm × 4.8 mm and a pixel size of 8 μm x 8 μm. The cone ratio of the light cone is 6:1, and the structural diagram of the system is shown in Fig. 1.
　　
During operation, X-rays are projected onto the input screen by the object being detected, inducing a visible fluorescent image that is enhanced by image brightness enhancement by the image intensifier. Photoelectron emission is generated at the image intensifier photocathode to form a photoelectron image. Under the action of the high-voltage electrode, the photoelectron is accelerated and focused onto the output panel of the fiber-optic panel of the image intensifier, converted into a visible light image, projected onto the photosensitive surface of the CCD, and the optical image is converted into a video signal by the CCD chip.
1.1 Influence of coupling efficiency of light cone and CCD on signal-to-noise ratio of imaging system
The coupling of the light cone and the CCD is actually the use of an optical adhesive to assemble two discrete components (light cone and CCD) in series. Thus, the image transmission from the phosphor screen of the image intensifier to the surface of the CCD chip does not require an optical lens, but can be directly performed by the light cone. This has the advantages of improving the utilization efficiency of light energy and greatly reducing the imaging volume. The coupling efficiency of the light cone and the CCD reflects the coupling of the light cone and the CCD.
Light cone and CCD coupling efficiency (ηFOT) refers to the light emitted from the X-ray image intensifier fiber optic panel screen, the loss through the cone and coupling medium (Eabsorb) and the beam energy (ECCD) propagating to the CCD photosurface The ratio of the total light energy (Eall) emitted by the X-ray image intensifier to the panel of the fiber optic panel is expressed in mathematical formula as
　　
In the formula, ATaper is the effective numerical aperture of the light cone; TR is the transmittance of the light after the reflection loss of the front and back ends of the light cone; TA is the transmission of the light cone through the cone of light, due to absorption and internal reflection loss of the light cone core material. Overshoot; KC is the effective fill rate of the light cone; Tepoxy is the transmittance of light through the coupling medium.
It can be seen from equation (1) that the efficiency of the coupling of the light cone and the CCD is related to the effective numerical aperture of the light cone, the effective filling rate of the light cone, and the absorption loss of the light cone and the coupling medium. An effective way to improve the coupling efficiency between the light cone and the CCD is to use a large numerical aperture light cone, reduce the reflection loss of the light cone end face and the absorption loss of the light transmission medium, and improve the transmittance of the light cone and the coupling medium.
In an imaging system, the size of the signal n that can be detected by the X-ray image intensifier, the light cone, and the entire CCD device can be expressed as
　　
Where ηCCD is the quantum conversion efficiency of CCD; ηFOT is the coupling efficiency of light cone; nintensifier is the photon quantity of the image intensifier output.
The signal noise detected by the entire coupling device is mainly generated by the X-ray image intensifier and CCD, and the signal noise can be expressed as
　　
The signal noise generated by the n2intensifier-noise X-ray image intensifier; n2CCD-noise is the signal noise generated by the CCD. Therefore, the signal-to-noise ratio of the entire coupled device is
　　
It can be seen from equation (4) that when the gain of the X-ray image intensifier is particularly high, the signal noise n2CCD-noise generated by the CCD is negligible, and the signal noise can be considered as independent of the characteristics of the light cone and the CCD coupling device. When the gain of the X-ray image intensifier is relatively low, when the noise of the CCD is greater than the noise of the X-ray image intensifier, the signal-to-noise ratio of the entire coupled device is
　　
This shows that improving the coupling efficiency of the light cone and the CCD can improve the signal-to-noise ratio of the entire coupled device.
1.2 Coupling of light cone and CCD
The small end face of the light cone should be cut into a fiber block corresponding to the size of the effective working area of ​​the CCD. Then, the two end faces of the light cone are polished according to the regulations, and at the same time, the end face of the light cone is guaranteed. Good parallelism and number of apertures. In order to protect the ruthenium surface from being oxidized or contaminated by dust, the target surface together with the circuit board frame is sealed with a quartz glass protection window. Since the light cone is an end face image, it must be in close contact with the CCD target surface. Therefore, the quartz glass window of the CCD must first be removed. The removal of the CCD quartz glass window is usually removed in a clean room by mechanical means while keeping the CCD surface clean.
There is no direct contact between the light cone and the surface of the CCD chip. There is a coupling medium in the middle. The coupling medium is a low-viscosity photosensitive adhesive with a refractive index of 1.56 after fixing. In this way, not only the direct contact between the small end face of the light cone and the CCD is avoided, but also the damage of the CCD surface is more important. More importantly, due to the existence of the coupling medium, the loss and scattering of light at the interface can be reduced, and the coupling efficiency of the light cone and the CCD is improved. To maintain image resolution, at the same time, it also plays a role in fixing the CCD and the light cone.
In the process of coupling, firstly, the photosensitive adhesive is applied to the target surface of the CCD, and then, while adjusting, while observing at the big end of the light cone, the light cone is lightly squeezed to uniformly apply the photosensitive adhesive on the CCD target surface. When there is no bubble generation, the thickness of the photosensitive paste is made as small as possible, because the smaller the thickness of the photosensitive paste, the better the quality of the image formation. The coupling process is mainly to avoid damage to the CCD filament pins, unevenness of the glue, bubble generation and moiré.
1.3 Coupling of light cone and X-ray II image intensifier
For the optical coupling between the big end of the cone and the output screen of the X-ray II image intensifier, the ordinary optical glass output screen can no longer be used, and can be replaced with a fiber optic panel or a thin transparent mica. The experiment selected a fiber optic panel to replace the ordinary optical glass as the output screen of the image intensifier. The fiber optic panel has a fiber diameter of about 5 μm.
The coupling between the light cone and the X-ray II image intensifier is equivalent to the coupling between the two flat glass. During the coupling process, the image in the coupling process can be transmitted to the monitor through the CCD camera for real-time observation. In the process, problems such as unevenness of glue application, generation of bubbles, and moiré are mainly avoided.
2 Spatial resolution analysis and detection of imaging systems
2.1 Analysis and testing of spatial resolution
The spatial resolution of the imaging system was tested using Figure 1. The spatial resolution of the measured system is shown in Fig. 2. In general, the image can be improved to some extent by the process of removing noise, adjusting the contrast of the image, and correcting the CCD pixel inconsistency. From the results of the test, the spatial resolution of the system reached 41 pixels/mm. Compared with the resolution test results of the X-ray image intensifier, it is shown that the low-viscosity photosensitive adhesive has little effect on the image quality, and still maintains good resolution after coupling.
　　
2.2 Experimental results of human imaging
The device of Fig. 1 is used to observe various parts of the human body, and Fig. 3 is an X-ray photograph of a human finger bone, a palm, and a wrist. From the perspective of the fluoroscopy, the images of each part are relatively clear, and can accurately diagnose the fractures and bone hyperplasia of these parts.
　　
3 Conclusion
The portable digital X-ray cone-coupled CCD imaging system studied in this paper has the characteristics of small size and high resolution. The spatial resolution of the system shows that the spatial resolution of the system can reach 41pixel/mm, and the imaging effect of the whole system. Better, able to meet the requirements of non-destructive testing and medical imaging of small parts. At the same time, this imaging system is easy to carry, low in cost and wide in application range, and is easy to realize production in China.
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