X-ray or gamma ray systems or devices – Beam control – Antiscatter grid
Reexamination Certificate
2002-01-04
2004-09-21
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Antiscatter grid
Reexamination Certificate
active
06795528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radiographic apparatus, a radiographic method, and a computer-readable recording medium, which can use a scattered radiation removal grid for removing radiation scattered by and within an object when the object is radiographed.
2. Description of the Related Art
Radiographing such as X-ray imaging has been used in diagnosis in medical practice, and examination and test of industrial products, for example. In X-ray imaging, an object is exposed to X rays, and X rays transmitted through the object are used to photosensitize a film or a screen. An X-ray image is thus obtained. The X rays transmitted through the object contain X rays linearly transmitted through the object and X rays that have been scattered within the object and then transmitted therethrough (hereinafter referred to as “scattered X rays”). In X ray imaging, the scattered X rays degrade the contrast of an X ray image, thereby substantially affecting the quality of the X-ray image.
A scattered X ray removal grid (hereinafter simply referred to as a “grid”) is arranged parallel to the plane of imaging to efficiently remove the scattered X rays thereby improving the contrast of the X-ray image. The grid is formed of lead foils that are arranged so as to focus toward a focal point of an X-ray source, or lead foils that are arranged perpendicular to the imaging plane. The grid facilitates the transmission of X rays that have entered after being linearly transmitted through the object from the focal point of the X-ray source while blocking the X rays that are incident at a slant angle after being scattered within the object.
In the conventional X-ray imaging using a film/screen system with the grid, the frequency of stripe-like lead foils of grid (i.e., the reciprocal number of the pitch of the lead foils, also referred to as a “grid frequency” or a “grid density”) is typically set higher than a frequency component of an ordinary X-ray image (on the assumption that no grid is employed) so that a striped pattern, occurring due to the grid in the X-ray image, is inconspicuous. By moving the grid in parallel to the imaging plane, the contrast of the striped pattern occurring due to the grid is reduced to make the striped pattern less visible or invisible in the X-ray image.
Digital radiographic apparatuses have been used. The digital radiographic apparatus obtains X-ray image data by converting the X rays transmitted through the object into visible light rays proportional to the intensity of the X rays through a fluorescent material (scintillator), converting the visible light rays into electrical signals through a photoelectric conversion element comprising a plurality of pixels, and (analog-to-digital) converting the electrical signals by an AD converter. Digital radiographic apparatuses of a sort begin to be used which obtain X-ray image data by sensing the distribution of electric charges proportional to the intensity of X rays that result from directly absorbing (through photoelectric conversion) X rays transmitted through an object, converting the charges into electrical signals, and (analog-to-digital) converting the electrical signals through an AD converter.
The above-mentioned digital radiographic apparatuses present an X-ray absorption ratio higher than that of the conventional film/screen system, and is free from a mottle structure that degrades granularity of an image in the conventional film/screen system. An X-ray image obtained from the above-mentioned digital radiographic apparatus is a digital image. Therefore the X-ray image is not necessarily output with a predetermined tone scale characteristics the same as that used in a film/screen system. Observing an X-ray image on a monitor, a user may freely change the tone scale characteristics. The user is also free to perform image processing such as frequency analysis, frequency enhancement or suppression processing or the like on the resulting X-ray image, thereby improving image quality.
The digital radiographic apparatus using the above-mentioned scattered X ray removal grid suffers from a moire pattern (a striped pattern, here) which does not exist in the X-ray image in the conventional film/screen system. The moire pattern is generated on the basis of a difference between the pixel pitch of a sensor for sensing the X ray as an electrical signal and the pitch of the lead foils forming the grid (the reciprocal number of the grid density). Let Ny represent the Nyquist frequency of the sensor and Gy represent the grid frequency (the grid density), and the frequency of the moire pattern is expressed as 2 Ny−Gy.
For example, when the Nyquist frequency Ny is 2.5 lp/mm (the pixel pitch of the sensor is 200 &mgr;m) and the grid frequency Gy is 4.0 lp/mm (40 lines/cm), the frequency of the moire pattern is 1.0 lp/mm. Since this frequency component is the one appearing in the typical X-ray images, the moire pattern is distinctly visible, thereby substantially degrading the quality of the resulting X-ray images. Here, lp/mm stands for line pairs per mm. The resolving power of the radiographic apparatus is sometimes measured using a chart having lead lines. The resolving power is defined by referring to how many pairs of the presence and absence of the lead line are recognized per unit length.
With the advance of technology, the digital radiographic apparatus provides an X-ray absorption ratio and resolving power higher than those of the conventional film/screen radiographic apparatus using the film/screen system. Viewing the screen of a monitor, the user is free to change the tone scale of the X-ray image. Therefore the component of the moire pattern having some contrast, as opposed in the conventional X-ray imaging, becomes a problem. Accordingly, a movement method of the grid different from that in the conventional film/screen system and a radiographing method accounting for the movement method are needed to make the moire pattern removed or inconspicuous.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a radiographic apparatus, a radiographic method, and a computer-readable storage medium for acquiring a radiograph, wherein the probability that a moire pattern is not generated or, is inconspicuous in the radiograph is heightened.
In one aspect of the present invention, a radiographic apparatus for obtaining a radiograph of an object includes a grid movement controller for controlling a movement of a grid which can move reciprocatingly, an input unit for inputting a method parameter relating to a radiographic method, and an imaging controller for setting a movement parameter relating to the movement of the grid to be used by said grid movement controller based on the method parameter input by the input circuit.
In another aspect of the present invention, a radiographic method for obtaining a radiograph of an object, includes the steps of controlling a movement of a grid which can move reciprocatingly, inputting a method parameter relating to a radiographic method, and setting a movement parameter relating to the movement of the grid to be used in said grid movement controlling step based on the method parameter input in said inputting step.
Further objects, features, and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.
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Bruce David V.
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ho Allen C.
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