capillary optics; total reflection; X-ray focusing; collimation; X-ray diffraction

Due to its short wavelength, high energy and ability to penetrate substances, X-rays are widely used in medical treatment, material analysis, non-destructive testing, astronomical research and other fields. However, X-rays are produced by high-speed electron flow hitting a metal target, and the emitted rays are divergent and irregular, which seriously weakens the energy of the ray beam and brings great difficulty to the application of X-rays. X-rays can cause certain damage to the human body. In addition, the energy of X-rays is large, and the refraction angle is small when passing through different media, and it is difficult for ordinary optical lenses to focus and collimate them.

Therefore, the development of an optical device for collimating and focusing X-rays is of great significance for the application of X-rays in the medical field. The proposal of X-ray capillary optics and the development of X-ray capillary optical lens are major breakthroughs in X-ray collimation and focusing. Capillary optics appeared in the 1980s. Based on this theory, an X-ray capillary optical lens was developed. The lens can focus and collimate X-rays and effectively control the propagation direction of rays. With the development of this technology, its application field has been extended to the field of neutron rays.

In 1984, Muradin A.Kumakhov proposed the emerging optics of capillary optics in Moscow. Capillary optics is based on the principle of total reflection: when X-rays are incident on the smooth inner wall of a 3-50 μm hollow glass capillary tube at a grazing incidence angle (the angle between the human radiation and the surface of the tube wall) not greater than the critical angle of total reflection θc, The ray will be transmitted between the tube walls by total reflection, and the transmission principle is shown in Figure 1. In the X-ray capillary, the hollow glass capillary acts as a waveguide, and the X-ray is transmitted through multiple reflections, instead of only one or two reflections in other grazing-incidence mirror systems, so that the direction of the X-ray can be effectively controlled.

The critical angle of total reflection θ_c=ωm/ω, the unit of θ_c is rad, ωm is the characteristic function of the capillary material, which mainly depends on the density ρ of the reflective material, and φ is the energy of the particle, which mainly depends on the X-ray wavelength λ. After multiple capillaries are arranged in a special way, the divergent X-ray beam can be focused or the divergent beam can be turned into a quasi-parallel beam. Transmission through the capillary can reduce the attenuation of rays. The radiant energy transmitted in the tube is proportional to L^-1, while in free space, the radiant energy is proportional to L^-2 (L is the ray transmission distance). This X-ray transmission principle is also applicable to neutron beams. Capillary optics can effectively control thermal neutron beams. In experiments, neutron capillary devices can be used to obtain high-energy neutron fluxes, thereby promoting the application of neutron optics in medical and other scientific research. development in the field. According to the theory of capillary optics, Kumarhof developed the first capillary optical lens in 1985. Since then, capillary optics have developed very rapidly in the past 20 years, and many X-ray analysis instruments based on capillary devices have appeared, such as X-ray spectrometers ( X-ray spectrometers), X-ray microscopes (X-ray microscopes,), X-ray diffract meters (X-ray diffract meters), etc., the following mainly introduces the most important optical device – capillary optical lens.

Fig. 1 Schematic diagram of total reflection inside the tube

X-ray capillary device classification X-ray capillary optics are mainly divided into two categories, namely: X-ray capillary optics (polymerized X-ray capillary optics) and single X-ray capillary optics. The former can focus a large divergent X-ray beam (~500μm) at a short working distance, the focal spot is 10~100mm from the exit end, and each hollow glass capillary can effectively refract the X-ray to the center of the capillary optical lens On the axis; the latter focal spot is relatively small (50mm~20μm), but the working distance is shorter. The X-ray capillary optical lens, also known as the Kumarhof lens, is a bundle of capillary arrays composed of thousands of hollow glass fiber tubes closely arranged in a hexagonal cross section. According to its function, the capillary optical lens can be subdivided into focusing lens, collimating lens and semi-focusing lens. Fig. 2 is a schematic diagram of the principle.

Fig. 2 Schematic diagram of the working mode of three types of lenses

The focusing lens can converge the X-rays emitted by a point ray source within a radian solid angle to form a high-energy X-ray source; the collimating lens can transform the divergent X-ray beam into a parallel X-ray beam with a large area and uniform intensity; semi-focusing Lenses are often used as relay coupling devices. The appearance of the component is shown in Figure 3, and the internal structure is shown in Figure 4.

Fig. 3 Outline structure of capillary optical lens

Fig. 4 Internal structure of capillary optical lens

The greatest advantage of using X-ray capillary optics is the ability to obtain high-intensity micron, submicron converging and collimated X-ray beams that are otherwise difficult to obtain. Such devices have important applications in medical treatment, materials research, X-ray lithography, and X-ray lithography for making high-density computer memory chips. The following focuses on the application of X-ray capillary optics in mammography, focused beam therapy, protein crystallography and other technologies.

Capillary optics have important applications in medical CT imaging and are widely used in soft tissue imaging, especially in mammography and angiography. Breast cancer is one of the deadliest cancers and the most common malignant tumor among women. It is most common in women between the ages of 35 and 50, and its incidence ranks first among female malignant tumors. About 1.2 million people worldwide suffer from this disease every year. .

Every year in the United States, 186,000 women are diagnosed with breast cancer, and 46,000 of them die from it. In my country, the incidence of breast cancer is increasing year by year. Therefore, it is particularly important for the diagnosis of breast cancer. Mammography is commonly employed to detect breast cancer by the observation of image contrast between normal tissue and slightly denser, cancerous lesions, or by the observation of small, microcalcified masses. The accuracy of diagnostic medical images is related to the contrast and resolution of the images. In traditional treatment methods, limited by the limited size of the light source, a small focus can improve the diagnostic effect, but cannot achieve full-field imaging.

The small incident angle brought about by the small focus makes the field of view smaller, and at the same time limits the output power, so that the patient receives X-rays for a longer time. The use of capillary optics will improve the quality of captured images, suppress scattering, effectively increase geometric magnification without affecting focus, and increase image resolution to 50μm or better, enabling earlier detection of breast cancer. Moreover, the required X-ray dose can be greatly reduced, and the damage to the patient’s body can be reduced.

The traditional X-ray treatment technology generally uses high-energy X-rays, and the parallel X-ray beams are irradiated on the tumor through a slit collimator. Gamma rays are generally used to reduce the absorbed dose of the surface skin, but for medium-voltage drugs, the ray energy is usually around 150 keV, which will still cause relatively large damage to the healthy part of the patient. The use of focusing capillary lens can reduce the ray energy to about 100 keV, while increasing the dose of irradiated tumor, reducing the absorbed dose of healthy parts around the tumor and the surface skin, so as to achieve a better therapeutic effect.

Such devices also have important applications in protein structure identification and analysis. The object studied by protein crystallography is the protein crystal size is very small, the crystal diffraction is very weak, and the stability is poor. Under the irradiation of synchrotron radiation, the typical protein crystal life is 2~3min. Conventional X-ray diffraction techniques require crystals with a minimum size of 100 μm, which is obviously not suitable for most applications; the use of X-ray capillary lenses can enhance the intensity of the X-ray light source and reduce the size of the ray beam to 10~15 μm.

Therefore, the protein studied The crystal size can be reduced to 20 μm. This technique can be used in pharmaceutical manufacturing, sample analysis. For example, the structure of viruses such as influenza and pneumonia can be analyzed. The capillary optical lens can also be applied to other medical fields, which will not be listed here. With the development of capillary optical technology and the improvement of manufacturing technology, the scope of application will be further broadened.

X-ray capillary optics is one of the fastest growing X-ray optics technologies. In addition, great progress has been made in the research of capillary optics in the field of neutrons in the past ten years. In capillary optics research, the research of fabrication process is a difficult point. With the development of capillary optical lens manufacturing technology, capillary optical elements with new structures will be continuously developed, the range of transmitted ray energy will be further expanded, and the accuracy will be greatly improved, and there will be broader applications in the medical field.

If some domestic enterprises engaged in the production of hard optical fiber devices introduce or transform existing equipment and master the relevant production processes, they can completely realize the localization of capillary optical devices, promote the vigorous development of capillary optics in my country, and promote its application in medical-related fields.