Preparation method of customized mitral valve forming ring

Abstract

The invention relates to the technical field of medical instruments, and discloses a preparation method of a customized mitral valve forming ring. The technical problems that in the prior art, the specification and model selection of a mitral valve forming ring is limited, and the valve ring cannot be completely matched with the physiological structure of a patient are solved. The method comprises the following steps: obtaining shape parameters of a mitral valve forming ring at a lesion of a patient through multi-modal imageology imaging; a three-dimensional model of the diseased region is constructed, a lesion model is reconstructed, and a healthy mitral valve forming ring structure model is obtained; fusion printing forming is conducted through a micro-scale laser powder bed, a laser beam is scanned according to a certain strip width, and in the fusion process of each layer, the laser scanning path rotates by a certain angle relative to the scanning path of a base plate or the scanning path of the previous layer. It is guaranteed that the artificial mitral valve ring can be perfectly matched with the physiological structure of a patient, and individual differences are reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of medical devices, and in particular to a method for preparing a customized mitral valve forming ring. Background Art

[0002] Mitral valve disease ranks first in the prevalence of heart valve diseases. Mitral valve disease mainly includes mitral valve regurgitation, mitral valve stenosis and mitral valve prolapse, of which mitral valve regurgitation accounts for 65%, which is mainly due to mitral valve lesions caused by mitral valve leaflets, valve ring, papillary muscles, chordae tendineae, etc., which makes the mitral valve of the heart not close tightly when the heart contracts, and blood flows from the left ventricle to the left atrium.

[0003] Mitral valvuloplasty and mitral valve replacement are the two main surgical methods used by cardiac surgery to treat mitral valve disease. In the past, artificial valve replacement was the main surgical method for the treatment of mitral valve disease. However, according to relevant studies, mitral valvuloplasty after retaining the valve and subvalvular structure can effectively maintain the patient's left ventricular morphology and function, and effectively reduce the mortality rate of the operation, and prevent the risk of long-term anticoagulation after surgery. Literature reports that the mortality rate of mitral valve replacement is 1.8% to 18.1%, which is higher than the mortality rate of mitral valvuloplasty (0% to 6.1%). Mitral valvuloplasty now accounts for 41% of mitral valve surgeries. Therefore, it has gradually been widely used by cardiac surgery. At present, the number of mitral valvuloplasty surgeries is increasing year by year.

[0004] At present, the mitral annuloplasty rings sold in my country are all of fixed sizes, and the specifications of mitral annuloplasty rings of different brands are very limited. Doctors can only choose the "suitable" size of annuloplasty ring according to the physiological structure of the patient's valve ring, which makes it impossible to fully match the patient's physiological structure. After treatment, some patients still have reflux due to loose valve leaflets. At the same time, the mitral annuloplasty ring of recurrent patients changes in shape again, and they need to undergo a second operation to replace the mitral annuloplasty ring model, forming a vicious circle. Therefore, there is an urgent need to develop a personalized mitral annuloplasty ring.

[0005] For the development of such a mitral valvuloplasty ring, the choice of materials is extremely important. Since the mitral valve ring is a saddle-shaped non-planar spatial conformation, and its three-dimensional shape and size change with the cardiac cycle, the use of commercial fixed-specification mitral valvuloplasty rings may cause left ventricular outflow tract obstruction in some patients. Through a lot of research and testing, we found that the NiTi memory alloy widely used in the clinical treatment of cardiac implants not only has excellent corrosion resistance and biocompatibility, but also has a unique shape memory effect (SME) and superelasticity (SE). The use of NiTi memory alloy to prepare mitral valvuloplasty rings can greatly make up for the shortcomings of existing mitral valvuloplasty rings, and is an ideal material for manufacturing new mitral valvuloplasty rings. However, due to the poor machining performance of NiTi memory alloy, it is difficult to form complex structures by machining after smelting and casting in traditional processes, which greatly limits the promotion and application of NiTi memory alloys.

[0006] The Chinese patent document 201610754840.6 in the prior art discloses a method for preparing a valvuloplasty ring, comprising: using a clamp to sleeve a suture layer on a metal piece covered with a silicone layer on the circumference; the clamp comprises a guide section and a main section, the guide section is conical, the main section is hollow and tubular, the main section is arranged at the end of the guide section with a larger diameter, and the inner dimension of the main section is slightly larger than the outer circumferential dimension of the silicone layer, so that the silicone layer can be inserted into the inner cavity of the main section.

[0007] However, in the implementation of the above scheme, there are at least the following technical problems: the mitral annuloplasty ring has limited specifications and models, and the annuloplasty ring cannot completely match the patient's physiological structure. Therefore, it is urgent to propose a method for preparing a customized mitral annuloplasty ring. Summary of the invention

[0008] In view of the above technical problems, the present disclosure provides a method for preparing a customized mitral valvuloplasty ring, which solves the technical problems in the prior art that the mitral valvuloplasty ring has limited specifications and models and the valve ring cannot fully match the patient's physiological structure.

[0009] According to one aspect of the present disclosure, a method for preparing a customized mitral annuloplasty ring is provided, characterized in that it comprises the following steps: (1) Data acquisition: The shape parameters of the mitral annuloplasty ring at the patient's lesion site are obtained through multimodal imaging; (2) Model processing: construct a three-dimensional model of the lesion site based on the shape parameters of the mitral annuloplasty ring at the lesion site collected in step (1), and reconstruct the lesion model to obtain a healthy mitral annuloplasty ring structure model; reconstruction of the lesion model includes the following steps: a. determine the influence of the shape and size of the mitral annuloplasty ring structure on the opening and closing of the valve leaflets; b. based on the mitral annuloplasty ring structure model, use a macro-scale numerical simulation method to analyze the mechanical characteristics of the mitral annuloplasty ring in the human body environment, reveal the evolution law of the stress field and the deformation field, and establish a three-dimensional model; (3) Model forming: The healthy mitral valve shaping ring structure model reconstructed in step (2) is formed by microscale laser powder bed fusion printing. The fusion printing includes the following steps: (3.1) Strip scanning: The laser beam is scanned according to a certain strip width. During the fusion printing process, an ultra-narrow single-pass fusion width is achieved through an ultra-thin powder layer thickness and an ultra-small spot diameter. The calculation formula of the laser spot diameter after the focusing lens is as follows: ; In the formula, is the focal spot diameter, is the focused spot size, is the laser wavelength, is the focal length of the focusing lens, is the laser spot size at the time of incidence, is the beam quality factor of the laser; From the above formula, we can conclude that: by a. reducing the focal length ; b. Select high-quality lasers to make the beam quality factor Close to 1; c. The laser spot size is enlarged by a large-aperture scanning galvanometer ; These three methods work together to reduce the focus spot diameter to achieve an ultra-small spot; The focusing lens is a telecentric field lens to avoid the distortion of the light spot shape at the edge of the forming chamber; (3.2) Layer-by-layer rotation: During the melting process of each layer, the laser scanning path is rotated by a certain angle relative to the substrate or the scanning path of the previous layer; the above steps (3.1) to (3.2) are repeated until a customized mitral annuloplasty ring is obtained.

[0010] In some embodiments of the present disclosure, the data acquisition method in step (1) is CT imaging.

[0011] In some embodiments of the present disclosure, in step (3), the strip width is 0.1-10 mm, and the rotation angle is 10-90°.

[0012] In some embodiments of the present disclosure, the process conditions adopted for microscale laser powder bed melting in step (3) are: laser power 10-100 W, laser spot diameter 10-30 μm, laser scanning speed 100-2000 mm / s, laser spot spacing 10-100 μm, and powder layer thickness 5-20 μm.

[0013] In some embodiments of the present disclosure, the process conditions adopted for microscale laser powder bed melting in step (3) are: laser power of 30-40 W, spot diameter of 20-25 μm, laser scanning speed of 600-1000 mm / s; laser spot spacing of 30-50 μm; and powder layer thickness of 5-15 μm.

[0014] In some embodiments of the present disclosure, the powder raw material used in the microscale laser powder bed melting in step (3) is a pre-alloyed NiTi powder prepared by a vacuum induction atomization method, and the particle size of the pre-alloyed NiTi powder is 0-45 μm, wherein more than 50% of the powder particle size is less than 15 μm.

[0015] In some embodiments of the present disclosure, before the melt printing, a powder drying step is also included: the pre-alloyed NiTi powder is subjected to a vacuum drying and impurity removal treatment at 80-120° C. for 5-10 h.

[0016] In some embodiments of the present disclosure, the process conditions of the microscale laser powder bed melting also include: reducing oxygen in the forming chamber and filling with a protective gas, the oxygen content in the forming chamber is not more than 1000 ppm, the protective gas is argon or nitrogen, and the protective gas pressure is 10-20 mbar.

[0017] In some embodiments of the present disclosure, a NiTi substrate with the same composition is selected in the microscale laser powder bed melting process.

[0018] In some embodiments of the present disclosure, the large-aperture scanning galvanometer in step c comprises a lens with a light-through hole, and the diameter of the light-through hole is 10-30 mm to allow a large-diameter light beam to pass through.

[0019] The beneficial effects of the present invention are: 1. Develop personalized mitral valvuloplasty rings, and achieve precision manufacturing by combining 3D printing technology with CT ultrasound imaging technology. This not only greatly reduces the production cost and cycle of mitral valvuloplasty rings, but also significantly improves the long-term efficacy of mitral valvuloplasty, providing strong support for overcoming the major difficulties of heart valve diseases.

[0020] 2. Improve the adaptability of the mitral valvuloplasty ring. Utilize the superelastic properties of NiTi memory alloy to achieve adaptive adjustment of the mitral valvuloplasty ring, thereby significantly improving its application effect.

[0021] 3. Microscale LPBF technology is used to achieve precise control of laser spot energy, ensuring printing accuracy and efficiency, thereby successfully completing the manufacturing process of the mitral annuloplasty ring.

[0022] 4. Ensure that the artificial mitral valve ring can perfectly match the patient's physiological structure and reduce individual differences.

[0023] 5. The superelasticity of NiTi alloy is achieved through process, and the superelasticity of NiTi alloy is used to achieve the application effect of adaptive adjustment of mitral valvuloplasty ring, which has better therapeutic effect than conventional rigid and semi-rigid mitral valvuloplasty rings.

[0024] 6. Use micro-scale laser powder bed melting technology to form a mitral valve forming ring, so as to achieve precise control of phase change temperature and high-quality and high-precision forming effect.

[0025] 7. The density of NiTi prepared by the preparation process reaches more than 99% and it has super elasticity. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of CT reconstruction imaging of the patient's lesion; Figure 2 This is a schematic diagram of the shape extraction of the mitral annuloplasty ring at the patient's lesion; Figure 3 Create a schematic for a custom-made mitral annuloplasty ring model; Figure 4 Schematic diagram of the deformation field of the forming ring simulated at the macro scale; Figure 5 Schematic diagram of the stress field of the forming ring simulated at macro scale; Figure 6 Schematic diagram of deformation field during macro-scale simulation of printing process; Figure 7 Schematic diagram of the density of NiTi alloy under a microscope; Figure 8 When a 6% cyclic compression test is performed at human body temperature, the loading rate is 10 -4 s -1 Strain and stress changes of the customized mitral annuloplasty ring in the state; Fig. 9 This is a schematic diagram of the printed mitral annuloplasty ring; Fig.10 A schematic diagram of a printed mitral annuloplasty ring from another perspective; Fig.11A schematic diagram of a printed mitral annuloplasty ring after it is removed from a forming substrate and the support is removed; DETAILED DESCRIPTION

[0027] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention. Example 1

[0028] This example discloses a method for preparing a customized mitral annuloplasty ring, see Figures 1 to 11 ; The steps include: (1) Data acquisition: The shape parameters of the mitral annuloplasty ring at the patient's lesion site are obtained through multimodal imaging; (2) Model processing: construct a three-dimensional model of the lesion site based on the shape parameters of the mitral annuloplasty ring at the lesion site collected in step (1), and reconstruct the lesion model to obtain a healthy mitral annuloplasty ring structure model; reconstruction of the lesion model includes the following steps: a. determine the influence of the shape and size of the mitral annuloplasty ring structure on the opening and closing of the valve leaflets; b. based on the mitral annuloplasty ring structure model, use a macro-scale numerical simulation method to analyze the mechanical characteristics of the mitral annuloplasty ring in the human body environment, reveal the evolution law of the stress field and the deformation field, and establish a three-dimensional model; (3) Model forming: The healthy mitral valve shaping ring structure model reconstructed in step (2) is formed by microscale laser powder bed fusion printing. The fusion printing includes the following steps: (3.1) Strip scanning: The laser beam is scanned according to a certain strip width. During the fusion printing process, an ultra-narrow single-pass fusion width is achieved through an ultra-thin powder layer thickness and an ultra-small spot diameter. The calculation formula of the laser spot diameter after the focusing lens is as follows: ; In the formula, is the focal spot diameter, is the focused spot size, is the laser wavelength, is the focal length of the focusing lens, is the laser spot size at the time of incidence, is the beam quality factor of the laser; From the above formula, we can conclude that: by a. reducing the focal length ; b. Select high-quality lasers to make the beam quality factor Close to 1; c. The laser spot size is enlarged by a large-aperture scanning galvanometer ; These three methods work together to reduce the focus spot diameter to achieve an ultra-small spot; The focusing lens is a telecentric field lens to avoid the distortion of the light spot shape at the edge of the forming chamber; The large-aperture scanning galvanometer in step c includes a lens with a light-through hole, and the diameter of the light-through hole is 10-30 mm to allow a large-diameter light beam to pass through.

[0029] (3.2) Layer-by-layer rotation: During the melting process of each layer, the laser scanning path is rotated by a certain angle relative to the substrate or the scanning path of the previous layer; the above steps (3.1) to (3.2) are repeated until a customized mitral annuloplasty ring is obtained.

[0030] The data acquisition method in step (1) is CT imaging.

[0031] In the microscale laser powder bed melting preparation process in step (3), strip scanning and layer-by-layer rotation steps are adopted, the strip width is 0.1-10 mm, and the rotation angle is 10-90°.

[0032] The process conditions for microscale laser powder bed melting in step (3) are as follows: laser power of 10-100 W, laser spot diameter of 10-30 μm, laser scanning speed of 100-2000 mm / s, laser spot spacing of 10-100 μm, and powder layer thickness of 5-20 μm.

[0033] The process conditions for microscale laser powder bed melting in step (3) are as follows: laser power of 30-40 W, spot diameter of 20-25 μm, laser scanning speed of 600-1000 mm / s, laser spot spacing of 30-50 μm, and powder layer thickness of 5-15 μm.

[0034] The powder raw material used in the micro-scale laser powder bed melting in step (3) is a pre-alloyed NiTi powder prepared by a vacuum induction atomization method, wherein the particle size of the pre-alloyed NiTi powder is 0-45 μm, wherein more than 50% of the powder particle size is less than 15 μm. Before the melt printing, a powder drying step is also included: the pre-alloyed NiTi powder is vacuum dried at 80-120° C. to remove impurities for 5-10 h.

[0035] The process conditions of the microscale laser powder bed melting also include: reducing oxygen in the forming chamber and filling with protective gas, the oxygen content in the forming chamber is not more than 1000 ppm, the protective gas is argon or nitrogen, and the protective gas pressure is 10-20 mbar.

[0036] A NiTi substrate with the same composition is selected during the microscale laser powder bed melting process.

[0037] The strip width is 0.1-10 mm. The strip scanning method is to control the vector length of the laser scanning during the laser powder bed melting process to the set strip width. The conventional laser powder bed melting technology for processing NiTi memory alloy generally uses a strip width greater than 5 mm, while the microscale laser powder bed melting technology used in the present invention has a smaller laser spot, finer powder and thinner layer thickness, so the laser molten pool is finer and the strip can be narrower. Narrower strips help to reduce residual stress in the printed components and reduce the cracking tendency of NiTi memory alloy. At the same time, narrow strips can suppress the powder raising phenomenon generated by ultrafine metal powder during microscale laser powder bed melting and forming, which can improve printing quality. In summary, the strip width selected here helps to achieve defect-free high-quality additive manufacturing of NiTi memory alloy.

[0038] The angle of each layer is rotated 10-90° so that the melt paths of each layer do not overlap, thus avoiding the accumulation of residual stress. At the same time, it can also make the NiTi structure more refined and reduce the anisotropy of its performance.

[0039] The laser power is 10-100 W (preferably 30-40 W). The selection of laser power is closely related to microscale laser powder bed melting. Due to the smaller laser spot, finer powder and thinner layer thickness, the laser power required for microscale laser powder bed melting is generally much smaller than the laser power for conventional laser powder bed melting. For example, the laser power used in other conventional laser powder bed melting when printing NiTi memory alloy is generally greater than 100 W. In the present invention, the use of a laser power of 30-40 W can control the heat input and prevent defects such as warping, deformation, cracking, and pores caused by excessive laser energy input.

[0040] The laser spot diameter is 10-30 μm (preferably 20-25 μm). The laser spot diameter used in other conventional laser powder bed melting is generally above 80 μm, while the laser spot diameter used in the present invention is about 1 / 3 of that, which can effectively improve the accuracy of the NiTi memory alloy customized mitral valve shaping ring and achieve the printing of a shaping ring with a smaller limit shaping size. λ 1064nm, f 170mm, D 10-12mm, M 2 is 1.05.

[0041] The laser scanning speed is 100-2000 mm / s (preferably 600-1000 mm / s). The laser scanning speed used in other conventional laser powder bed melting is generally 1000-1500 mm / s or below 600 mm / s, while the laser scanning speed of the present invention is not within this range. This feature is mainly determined by the material and process constraints of microscale laser powder bed melting, namely, "fine powder, small spot, thin layer thickness, narrow melting channel, and fast cooling rate". The laser scanning speed within this range can ensure the forming quality of the NiTi memory alloy customized mitral valve forming ring.

[0042] The laser spot spacing is 10-100 μm, and the powder layer thickness is 5-20 μm (preferably the laser spot spacing is 30-50 μm, and the powder layer thickness is 5-15 μm). The laser spot spacing used in other conventional laser powder bed melting is relatively large, generally above 100 μm, and the powder layer thickness is 40 μm and above. This results in the poor forming accuracy of the NiTi memory alloy that can be achieved by conventional laser powder bed melting, high surface roughness, and a "step effect" caused by the thick powder layer. The spot spacing and powder layer thickness of the micro-scale laser powder bed melting technology used in the present invention are smaller than those of conventional laser powder bed melting, especially the powder layer thickness is reduced by more than 50%, thereby greatly improving the "step effect" and improving the printing accuracy and surface finish of the NiTi memory alloy.

[0043] The powder particle size distribution is 0-45 μm, and most of the powder particle size is less than 15 μm. The powder particle size used in conventional laser powder bed melting is 15-53 μm, while the particle size range used in the present invention is smaller. In particular, the particle size of most powders melted by conventional laser powder bed is concentrated at around 30 μm, while the particle size of most powders melted by the microscale laser powder bed of the present invention is only half of that. This feature helps to improve the surface quality of the NiTi memory alloy customized mitral valve shaping ring, so that it has better performance under the cyclic load conditions of the cardiac cycle.

[0044] 80-120℃ vacuum drying and impurity removal treatment for 5-10h. Other conventional laser powder bed melting processes generally do not dry the powder, but this vacuum drying treatment before printing is more important for the microscale laser powder bed melting process used in the present invention, because the finer powder used has a larger specific surface area and is more likely to absorb water and agglomerate. Drying and impurity removal under the conditions set in the present invention can remove moisture attached to the surface of the ultrafine powder, reduce powder agglomeration, and improve powder fluidity and spreadability, thereby ensuring the printing quality of the NiTi memory alloy customized mitral valve forming ring.

[0045] Although some preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0046] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of this application and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for preparing a customized mitral annuloplasty ring, characterized in that: The steps include: (1) Data acquisition: The shape parameters of the mitral annuloplasty ring at the patient's lesion site are obtained through multimodal imaging; (2) Model processing: construct a three-dimensional model of the lesion site based on the shape parameters of the mitral annuloplasty ring at the lesion site collected in step (1), and reconstruct the lesion model to obtain a healthy mitral annuloplasty ring structure model; reconstruction of the lesion model includes the following steps: a. determine the influence of the shape and size of the mitral annuloplasty ring structure on the opening and closing of the valve leaflets; b. based on the mitral annuloplasty ring structure model, use a macro-scale numerical simulation method to analyze the mechanical characteristics of the mitral annuloplasty ring in the human body environment, reveal the evolution law of the stress field and the deformation field, and establish a three-dimensional model; (3) Model forming: The healthy mitral valve shaping ring structure model reconstructed in step (2) is formed by microscale laser powder bed fusion printing. The fusion printing includes the following steps: (3.1) Strip scanning: The laser beam is scanned according to a certain strip width. During the fusion printing process, an ultra-narrow single-pass fusion width is achieved through an ultra-thin powder layer thickness and an ultra-small spot diameter. The calculation formula of the laser spot diameter after the focusing lens is as follows: ； In the formula, is the focal spot diameter and the focused spot size, is the laser wavelength, is the focal length of the focusing lens, is the laser spot size at the time of incidence, is the beam quality factor of the laser; From the above formula, we can conclude that: by a. reducing the focal length ; b. Select high-quality lasers to make the beam quality factor Close to 1; c. The laser spot size is enlarged by a large-aperture scanning galvanometer ; These three methods work together to reduce the focus spot diameter to achieve an ultra-small spot; The focusing lens is a telecentric field lens to avoid the distortion of the light spot shape at the edge of the forming chamber; (3.2) Layer-by-layer rotation: During the melting process of each layer, the laser scanning path is rotated by a certain angle relative to the substrate or the scanning path of the previous layer; the above steps (3.1) to (3.2) are repeated until a customized mitral annuloplasty ring is obtained.

2. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: The data acquisition method in step (1) is CT imaging.

3. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: The strip scanning width of step (3) is 0.1-10 mm, and the rotation angle is 10-90°.

4. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: The process conditions for microscale laser powder bed melting in step (3) are as follows: laser power of 10-100 W, laser spot diameter of 10-30 μm, laser scanning speed of 100-2000 mm / s, laser spot spacing of 10-100 μm, and powder layer thickness of 5-20 μm.

5. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: The process conditions for microscale laser powder bed melting in step (3) are as follows: laser power of 30-40 W, spot diameter of 20-25 μm, laser scanning speed of 600-1000 mm / s; laser spot spacing of 30-50 μm; and powder layer thickness of 5-15 μm.

6. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: The powder raw material used in the microscale laser powder bed melting in step (3) is a pre-alloyed NiTi powder prepared by a vacuum induction atomization method, wherein the particle size of the pre-alloyed NiTi powder is 0-45 μm, wherein more than 50% of the powder particle size is less than 15 μm.

7. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: Before the melt printing, a powder drying step is also included: the pre-alloyed NiTi powder is vacuum dried at 80-120° C. to remove impurities for 5-10 h.

8. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: The process conditions of the microscale laser powder bed melting also include: reducing oxygen in the forming chamber and filling with protective gas, the oxygen content in the forming chamber is not more than 1000 ppm, the protective gas is argon or nitrogen, and the protective gas pressure is 10-20 mbar.

9. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: A NiTi substrate with the same composition is selected during the microscale laser powder bed melting process.

10. The method for preparing a customized mitral annuloplasty ring according to claim 1, characterized in that: The large-aperture scanning galvanometer in step c comprises a lens with a light-through hole, wherein the diameter of the light-through hole is 10-30 mm, so that a large-diameter light beam can pass through.

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