The present invention relates to an X-ray image intensifier.
As X-ray image intensifiers (to be referred to as "I.I."s hereinafter), a general-purpose single visual field type I.I. and a high-grade variable visual field type I.I. are frequently used. In general, an I.I. comprises a vacuum housing which includes a substantially cylindrical outer casing, and an X-ray entrance window and an X-ray exit window which are arranged to close two ends of the outer casing. In the vacuum housing, input and output surfaces are arranged along the entrance and exit windows, respectively, and a focusing electrode constituting an electronic lens is located between the input and output surfaces. The I.I.s are classified into the single visual field type and variable visual field type due to differences in the number and arrangement of focusing electrodes, and the like. In the case of a variable visual field type I.I., when a voltage distribution to the focusing electrodes is switched, an output visual field image can be enlarged like, a normal visual field, a second visual field, a third visual field,....
The input surface has a base and a phosphor screen formed on the base, and has an arcuated circular shape.
In U.S. Pat. No. 3,716,713, the thickness of the phosphor screen is increased from its center toward the periphery, and is maximized at the periphery.
According to an I.I. disclosed in Japanese Patent Disclosure No. 53-102663, the phosphor screen has the same arrangement as that in the above U.S. Pat. No., and the base has a mosaic structure having a large number of grooves for effecting a light guide function.
According to an I.I. disclosed in Japanese Patent Disclosure No. 59-207551, the thickness of the phosphor screen is decreased from its center toward the periphery, and X-ray optical path lengths passing through the phosphor screen are adjusted to be equal to each other at the center and the periphery of the phosphor screen.
In the I.I.s having the above-mentioned arrangements of the input surfaces, the characteristic of an image obtained at the output surface, in particular, a luminance distribution characteristic, is such that a luminance is high at the center of the image and is gradually decreased toward the periphery. Therefore, a luminance distribution curve obtained as a result of measurement along the diameter of an image becomes a quadratic curve. In the variable visual field I.I., the same luminance distribution characteristic is obtained either in a normal visual field operation or in an enlarged visual field operation.
The reason for the above-mentioned luminance distribution can be considered as follows.
In the I.I.s disclosed in U.S. Pat. No. 3,716,713 and Japanese Patent Disclosure No. 53-102663, in order to prolong an X-ray passage distance in the phosphor screen, which influences light emission, so as to compensate for a quantity of light emitted from the phosphor screen, the thickness of the peripheral portion of the phosphor screen is increased. However, a portion between the intermediate portion and periphery of the phosphor screen cannot provide a similar effect upon increase in thickness, and, to the contrary, the luminance of the periphery of an image is decreased. This is because an excessive increase in thickness at the peripheral portion of the phosphor screen does not contribute to light emission of the phosphor by means of X-rays but degrades a transmittance of X-rays.
In Japanese Patent Disclosure No. 59-207551, in order to obtain a constant passage distance of X-rays at respective positions in the phosphor screen, the thickness of the phosphor screen is decreased at a given rate from its center toward the periphery. However, in order to obtain a theoretical luminance, the phosphor screen must be formed to have a uniform structure and a uniform emission intensity distribution. If these conditions cannot be satisfied, the luminance at the peripheral portion of an image, in particular, an area shifted from the center of the image toward the periphery by a distance 80 to 95% of an effective image diameter, is considerably decreased as compared to the above two prior arts.
When the I.I.s having the above luminance distribution characteristic are used, the following problems are posed. In the distribution characteristic, the luminance at the center of an image is high and is decreased toward the periphery. When the I.I. is coupled to an optical system, a luminance difference between the center and the periphery of the image is emphasized due to an operation of the optical system. For this reason, a dark portion at the peripheral portion of the image has degraded discriminating ability of an object, and cannot be used for observing an object. Therefore, a virtual image area is decreased. When an object is observed upon clinical examination, a contour image of the object must be confirmed. However, when the effective image area is small as described above, the I.I. must be moved stepwise so that a portion to be observed is located at the center of the image. For this reason, the observation requires a long time, and an X-ray irradiation time is also prolonged. For example, when an observation is performed using a TV fluoroscopic imaging method, the entire object, i.e., the entire image, must be scanned, and this requires still more time.
In the enlarged visual field operation mode, e.g., in the second visual field operation mode, the luminance distribution characteristic of an output image is such that the center of an image is bright and the peripheral portion thereof is dark as in the normal visual field operation mode. In any visual field operation mode, an area of an input visual field is changed, but an image area which can be observed is almost not changed. For this reason, when the enlarged visual field operation is performed in order to microscopically observe the object after the contour image of the object is confirmed, the I.I. must be moved to locate the object at the center of image. If the object is a moving body, and is moved to the peripheral portion of an output image, the object cannot be discriminated since the luminance of the peripheral portion is low.
Since the luminance distribution characteristic is not changed in the enlarged visual field operation mode, a low luminance portion is moved upon switching of visual fields. The object is often out of sight upon switching of the visual fields, and the I.I. must be moved to confirm the object at that time. For example, upon clinical examination wherein a change in object must be immediately judged, such as blood vessel imaging, the lack of necessary data and the complicated operations as described above may cause serious problems.