X-RAY IMAGE INTENSIFIER

Abstract: An X-ray image intensifier includes a vacuum enve­ lope having an input window for receiving X-rays, an input fluorescent screen (8) for converting X-rays received through the input window into light-rays, a photoelectric layer for converting the light-rays con­ verted by the input fluorescent screen (8) into elec­ trons, an anode and a converging electrode constituting an electron lens for accelerating and converging the electrons converted by the photoelectric layer, and an output fluorescent screen for converting the electrons accelerated and converged by the electron lens into a visible image. The light-rays transmission coefficient of a peripheral portion (8c) of the input fluorescent screen (8) is larger than that of a central portion (8b) thereof.

Description: The present invention relates to an X-ray image intensifier for converting an X-ray image into a visible image. Generally, X-ray image intensifiers are widely used in medical X-ray image pickup devices or X-ray industrial TV sets for industrial nondestructive tests. An X-ray image intensifier of this type has a vacuum envelope. The vacuum envelope has an input window for receiving X-rays. An arcuated substrate is arranged in the vacuum envelope so as to oppose the input window. An input fluorescent screen and a photoelectric layer are stacked in the above mentioned order on a surface of the substrate, which is opposite to the input widow side. An anode and an output fluorescent screen are arranged on the output side of the vacuum envelope. In addition, a converging electrode is arranged along an inner side wall of the vacuum envelope. X-rays radiated from an X-ray tube pass through an object to be imaged, the input window, and the substrate, and then converted into light-rays by the input fluorescent screen. The light is converted into electrons by the photoelectric layer. The electrons are accelerated and focused by an electron lens constituted by the focusing electrode and the anode. The electrons are converted into a visible image by the output fluorescent screen. The visible image is picked up by a TV camera, a cinecamera, or a spot camera, and the resultant image is used for a medical diagnosis. Of the fluorescent screens used in the X-ray image intensifiers, a fluorescent screen has been recently used, whose film thickness is greatly increased compared with conventional fluorescent screens. X-rays to be absorbed by an input fluorescent screen having a thickness of T can be given as: 1 - exp(-ψT) where ψ is an X-ray absorption coefficient. Fig. 1 shows a relation between the thickness of the input fluorescent screen and the absorption rate. Referring to Fig. 1, a material of the input fluorescent screen is cesium iodide (CsI), and an energy of X-rays is 60 KeV. The absorption index of X-rays is increased with an increase in film thickness, and hence X-rays can be efficiently used. As a result, an X-ray dose and can be reduced and image quality can be improved. When output images are observed after X-rays are uniformly radiated onto the X-ray image intensifier, it is sometimes found in an output image that a central portion is bright, whereas luminance is decreased toward a peripheral portion of the image (see Fig. 2). This is because compared with the central portion of the image, the peripheral portion of the image is expanded by a so-called electron lens in the X-ray image intensifier. With such an output luminance distribution, the dynamic range upon imaging cannot be effectively utilized for the entire screen surface. That is, a possible application range of the output image cannot be widened. FR-A-2 186 723 discloses an X-ray image intensifier screen with crystals formed in elongated columnar shape having a height-width ratio of 2:1 to 10:1. However, the columnar shaped crystals are arranged in an irregular distance from each other. Further their shape is varying deliberately throughout the entire screen. A known method of maximally flattening an output luminance range is disclosed in, e.g., Japanese Patent Disclosure (Kokai) No. 53-102663, wherein the film thickness of an input fluorescent screen is gradually increased from its central portion toward its peripheral portion. According to this method, the input fluorescent screen emits light by absorbing a larger number of X-rays at the peripheral portion than at the central portion. Therefore, in the output side, the luminance of the peripheral portion is increased, and the output luminance distribution can become close to a flat one. However, this method cannot be applied to the X-ray image intensifier using the above-described input fluorescent screen having a large film thickness. The reasons will be described below. First, for the purpose of understanding of the reasons, it is checked using a model how much light-rays emitted from the input fluorescent screen will reach the photoelectric layer when X-rays are uniformly incident onto the input fluorescent screen. Fig. 3 shows the model. The conversion amount of X-rays converted into light-rays at small portion dt located at depth t in the input fluorescent screen having film thickness T is proportional to the light-rays amount at position t . Since the distance from small portion dt to the photoelectric layer is T - t, if the absorption coefficient of light in the input fluorescent screen is set to be β, an amount of light-rays component of the light-rays converted by small portion dt and reaching the photoelectric layer, can be given as: αe -αt ·e -β(T-t) dt Therefore, the amount of light-rays component of the light-rays converted by the entire input fluorescent screen can be obtained by integrating the above formula as follows: α∫ T 0 e -αt ·e -β(T-t) dt where α is an X-ray absorption coefficient. This definite integral is calculated as follows: α/(β - α) × exp(-βT) × {exp [(β - α)T] - 1} Accordingly, the value of this definite integral reaches its peak value at a given value of T. After input fluorescent screens having various film thicknesses were actually manufactured and tested, a peak value of the light-rays amount was obtained at the photoelectric layer. Fig. 4 shows the test result. This data is obtained by measuring the luminance of an input fluorescent screen composed of CsI as a single-element film. In this case, an energy of X-rays is 60 KeV. If the film thickness of the central portion of the input fluorescent screen is set to be the one exhibiting this peak value so as to effectively use the X-rays, the above method of correcting the output luminance distribution cannot be applied. More specifically, even if the film thickness of the peripheral portion of the input fluorescent screen is increased with respect to the central portion, luminance is decreased. As a result, the plotted output luminance distribution shows a steep convex shape. In addition, if the film thickness is further increased, the resolution is degraded because of diffusion of light. That is, the film thickness corresponding to the peak value of emitted light-rays is regarded as the maximum film thickness to be practically used. Therefore, it is necessary to solve the problem, i.e., that when an input fluorescent screen having such film thickness is realized, the output luminance distribution cannot be effectively corrected. In addition, another problem will be described. If the film thickness is not uniform in one fluorescent screen, the X-ray absorption coefficient is changed depending on the quality of X-ray. For this reason, even if the output luminance distribution is made flat for a give X-ray quality, the output luminance distribution is not flat for other X-ray qualities. It is an object of the present invention to provide an X-ray image intensifier which can make an output luminance distribution flat even if an input fluorescent screen of a large film thickness type is used, and can minimize changes in output luminance distribution due to changes in quality of X-rays. According to one aspect of the present invention, there is provided an X-ray image intensifier comprising the features of claim 1. Further there is provided an input fluorescent screen comprising the features as claimed in claim 5. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a graph showing a relation between the film thickness of an input fluorescent screen and the X-ray absorption coefficient; Fig. 2 is a graph of an output luminance distribution; Fig. 3 is a view illustrating a state wherein light-rays emitted in the input fluorescent screen is attenuated; Fig. 4 is a graph showing a relation between the film thickness of the input fluorescent screen and the relative amount or emitted light-rays; Fig. 5 is a view of an X-ray image intensifier according to the present invention; Fig. 6 is a sectional view illustrating the input fluorescent screen used in the X-ray image intensifier in Fig. 5; Fig. 7 is a schematic view of an apparatus for forming a film of the input fluorescent screen in Fig. 6; Fig. 8 is a view for explaining the transmission coefficient of the input fluorescent screen in Fig. 6; Fig. 9 is a graph showing a relation between the light-rays absorption coefficient of the input fluorescent screen in Fig. 6 and the relative luminance; Fig. 10 is a graph showing the light-rays absorption coefficient distribution of the input fluorescent screen in Fig. 6; Fig. 11 is a graph showing the relative luminance distribution of the input fluorescent screen in Fig. 6; Fig. 12 is a graph showing a relation between the light-rays absorption coefficient of the input fluorescent screen in Fig. 6 and the relative luminance; Fig. 13 is a schematic sectional view showing a modification of the input fluorescent screen; and Fig. 14 is a view of a measuring device of transmittance. Referring to Fig. 5, reference numeral 2 denotes a vacuum envelope of an X-ray image intensifier. Vacuum envelope 2 has input window 4 for receiving X-rays. Arcuated board 6 is arranged in vacuum envelope 2 so as to oppose input window 4. Input phosphor screen 8 and photoelectric layer 10 are stacked in the above mentioned order on a surface of substrate 6, which is opposite to the input window 4 side. Input fluorescent screen 8 converts X-rays received through input window 4 into light. Photoelectric layer 10 converts the light-rays converted by input fluorescent screen 8, into electrons. In addition, anode 12 and output fluorescent screen 14 are arranged on an output side of vacuum envelope 2. Converging electrode 16 is arranged along an inner side wall of vacuum envelope 2. Anode 12 and converging electrode 16 form an electron lens for accelerating and converging the electrons converted by photoelectric layer 10. Output fluorescent screen 14 converts the electrons, which have been accelerated and converged by the electron lens constituted by anode 12 and converging electrode 16, into a visible image. X-rays radiated from X-ray tube 18 pass through object 20 to be imaged, input window 4, and board 6, and is then converted into light-rays by input fluorescent screen 8. The light is converted into electrons by photoelectric layer 10. The electrons are accelerated and focused by the electron lens constituted by anode 12 and converging electrode 16. The electrodes are converted into a visible image by output fluorescent screen 14. The visible image is picked up by a TV camera, a cinecamera, a spot camera, or the like, thereby performing a medical diagnosis or the like. As shown in Figs. 6 and 8, input fluorescent screen 8 is constituted by elongated columnar crystals arranged along a direction perpendicular to input fluorescent screen 8. Each columnar crystal 8a is composed of cesium iodide (CsI), which is activated by using an activator such as sodium. The diameters of pillar-like crystals 8a gradually increase as they extend from central portion 8b toward peripheral portion 8c of input fluorescent screen 8. The thickness of input fluorescent screen 8 is made to be substantially uniform throughout central and peripheral portions 8b, 8c. Fig. 7 schematically shows a film forming apparatus for forming input fluorescent screen 8 on board 6. Referring to Fig. 7, reference numeral 22 denotes a vapor source. Board 6 is supported by board support/rotating unit 24 above vapor source 22. Circular central heater 26, annular intermediate heaters 28, and annular peripheral heaters 30 are arranged above board 6. Central heater 26 heats a central portion of board 6. Intermediate heaters 28 heat an intermediate portion of substrate 6. Peripheral heaters 30 heat a peripheral portion of substrate 6. Heaters 26, 28, and 30 are driven by heater driver 32. In addition, temperature sensors (not shown) for monitoring the temperatures of substrate 6 are arranged near heaters 26, 28, and 30. Heaters 26, 28, and 30 are driven by heater driver 32 during deposition of CsI such that the temperatures of the central portion and peripheral portions are respectively kept at 150 to 200°C and 200 to 250°C, and the temperature of the intermediate portion of substrate 6 is kept within a temperature which falls between the temperatures of the central and peripheral portions. Heaters 26, 28, and 30 are driven by heater driver 32. The temperature gradient from the central portion to the peripheral portion may be linearly changed, or may be moderately changed near the central portion and more abruptly changed near the peripheral portion. In such an apparatus, when crystal seeds are attached to substrate 6 while the temperatures of the central and peripheral portions thereof are respectively kept at 150°C and 250°C, crystals 8a are grown such that the diameter of a central crystal is about 2 µm and the diameter of a peripheral crystal is about 6 µm. Each pillar-like crystal 8a is grown from a corresponding seed in the form of a pillar in a direction perpendicular to substrate 6. Thus, columnar crystals 8a, whose diameters increase as they extend toward the peripheral portions, can be obtained by setting the temperatures of substrate 6 to gradually increase as they extend from central portion 8b toward peripheral portion 8c of input fluorescent screen 8. As shown in Fig. 8, light-rays converted from the X-rays in columnar crystal 8a has been reflected fewer times on an inner wall of columnar crystal 8a with a resulting increase in the diameter of crystal 8a, and the loss of light-rays at an interface between columnar crystals 8a is thereby reduced. Since the transmission coefficient of central portion 8b is larger than that of peripheral portion 8c of input fluorescent screen 8, the amount of light-rays radiated from peripheral portion 8b of input fluorescent screen 8 toward fluorescent screen 10 is larger than that from central portion 8c. As described above, assuming that the X-ray absorption coefficient and light-rays absorption coefficient of input fluorescent screen 8 having thickness T are respectively set to be α and β, the amount (relative value) of light-rays reaching photoelectric layer 10 can be given as: α/(β - α) × exp(-βT) × {exp[(β - α)T] - 1} When α = β, αT · exp(-αT) When X-ray absorption coefficient α is a measurement value of 4.4 × 10³ µm⁻¹ with respect to a monochromatic X-ray of 60 KeV, film thickness T of input fluorescent screen 8 is 300 µm⁻¹, and light-rays absorption coefficient β is changed from 1 × 103 µm⁻¹ to 5 × 10³ µm⁻¹, then the relative luminance can be plotted as shown in Fig. 9. In Fig. 9, if light-rays absorption coefficient β is changed from 3 × 103 µm⁻¹ to 2 × 10³ µm⁻¹, the luminance is increased by about 18%. When the light-rays absorption coefficient of input fluorescent screen 8 gradually decreases from central portion 8b toward peripheral portion 8c, i.e., the transmission coefficient of input fluorescent screen 8 gradually increases as it extends from central portion 8b toward peripheral portion 8c, as shown in Fig. 10, then the relative luminance can be made substantially uniform, as shown in Fig. 11. Since input fluorescent screen 8 is composed of elongated columnar crystals 8a arranged in the direction perpendicular to input fluorescent screen 8, the light-rays, excluding light-rays directed in the direction perpendicular to input fluorescent screen 8, is totally reflected by the inner surfaces of columnar crystals 8a or passes through gaps located between columnar crystals 8a, and is attenuated. That is, the light-rays absorption coefficient of input fluorescent screen 8 in the direction along input fluorescent screen 8 (the direction indicated by reference symbol B in Fig. 6) is smaller than that of the light-rays absorption coefficient in the direction perpendicular thereto (the direction indicated by reference symbol A in Fig. 6). Since diffusion of the light-rays, excluding the light-rays perpendicular to input fluorescent screen 8, can be reduced, the resolution can be improved. X-ray absorption coefficient α of input fluorescent screen 8 changes as the quality of the X-ray is changed. However, as shown in Fig. 12, it was discovered from the calculations of the above formula that changes in the X-ray absorption coefficient do not greatly influence the relation between light-rays absorption coefficient β and the relative luminance. Therefore, the output luminance of input fluorescent screen 8 is not greatly influenced by the quality of the X-ray. In addition, since the film thickness of input fluorescent screen 8 is substantially constant, the deposition can be easily controlled. Furthermore, since the light-rays absorption coefficient of input fluorescent screen 8 can be altered by changing the temperature of the substrate during deposition, input fluorescent screen 8 having the above arrangement can be easily manufactured by means of deposition. According to the above arrangement, even when input fluorescent screen 8 having a large film thickness type is used, the resolution and photoelectric sensitivity of the peripheral portion of input fluorescent screen 8 can be improved. Therefore, as shown in Fig. 8, the output luminance distribution can be corrected so as to be flat. At the same time, changes in the output luminance distribution, which are caused by changes in the quality of X-rays, can be minimized. Although input fluorescent screen 8 of the above embodiment is composed of columnar crystals 8a, it may also be composed of normal crystals 8d, as shown in Fig. 13. Fig. 14 shows a measuring device for measuring the light-rays transmittance. The device comprises monochromator 34 for emitting monochromatic light and photodetector 36 for detecting the monochromatic light-rays. The light-rays transmittance is obtained from the ratio representing the amount of light-rays which is transmitted when sample 38 is located on the path of the monochromatic light-rays, compared with the amount of light-rays transmitted when it is not on the path. The relation between the light-rays transmission coefficient and the light-rays transmittance can be given as: light-rays transmittance = exp(-AT) where A denotes the light-rays transmission coefficient, and 13 denotes the thickness of a sample.

Claims: An X-ray image intensifier comprising: a vacuum envelope (2) having an input window (4) for receiving X-rays; an input fluorescent screen (8) for converting X-rays received through said input window (4) into light-rays; a photoelectric layer (10) for converting the light-rays into electrons; electrode means (12, 16) constituting an electron lens for accelerating and converging the electrons; and an output fluorescent screen (14) for converting the electrons accelerated and converged by said electron lens into a visible image; wherein said input fluorescent screen (8) consists of crystals (8a) formed into elongated columnar shapes and arranged along a direction perpendicular to said input fluorescent screen (8), the diameters of said crystals (8a) being gradually increased from a central portion (8b) of said input fluorescent screen (8) toward a peripheral portion (8c) thereof, the lengths of said crystals (8a) being uniform throughout said central and peripheral portions (8b, 8c), the light-rays transmission coefficient of the peripheral portion of said input fluorescent screen (8) being thereby larger than that of the central portion thereof. The intensifier according to claim 1, characterized in that the light-rays transmission coefficient of said input fluorescent screen (8) with respect to light-rays in a direction along said input fluorescent screen (8) is smaller than that with respect to light-rays in a direction perpendicular to said input fluorescent screen (8). The intensifier according to claim 1, characterized in that said input fluorescent screen (8) essentially consists of activated cesium iodide. The intensifier according to claim 3, characterized in that said input fluorescent screen (8) essentially consists of sodium-activated cesium iodide. An input fluorescent screen (8) for an X-ray image intensifier, for converting X-rays incident on said X-ray image intensifier into light-rays and outputting the light, wherein it consists of crystals (8a) formed into elongated columnar shapes and arranged along a direction perpendicular to said input fluorescent screen (8), the diameters of said crystals (8a) being gradually increased from a central portion (8b) of said input fluorescent screen (8) toward a peripheral portion (8c) thereof, the light-rays transmission coefficient of the peripheral portion of said input fluorescent screen (8) being thereby larger than that of the central portion thereof. The screen according to claim 5, characterized in that the lengths of said crystals (8a) are uniform throughout said central and peripheral portions (8b, 8c).

Inventors: KUBO, HIROSHI C/O PATENT DIVISION, YOSHIDA, ATSUYA C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H01J29
IPC codes: H01J29