X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-10-29
2001-08-07
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06272198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a radiation image forming method and apparatus for obtaining at least either one of a volume signal and a tomographic image signal representing an image of an object. This invention particularly relates to a radiation image forming method and apparatus, wherein radiation image signals representing radiation images of an object are detected via scattered radiation removing means, which removes radiation scattered by the object, and at least either one of a volume signal and a tomographic image signal representing an image of an object, is obtained from the radiation image signals.
2. Description of the Prior Art
In the fields of medical images, systems for detecting two-dimensional radiation image signals (e.g., tomographic image signals), such as computed tomography scanners (CT scanners) and magnetic resonance imaging systems (MRI systems), have heretofore been used widely. (Such systems will hereinbelow be referred to as the radiation image detecting systems for two-dimensional images.)
Also, recently, research has been conducted to detect three-dimensional radiation image signals. As techniques for detecting three-dimensional radiation image signals, for example, helical CT and cone-beam CT have been proposed. (Such techniques are described in, for example, “Present State and Future of Cone-Beam CT Development,” Image Information (M), pp. 122-127, January 1988; and Japanese Unexamined Patent Publication No. 9(1997)-253079.)
With the cone-beam CT, a radiation source and a two-dimensional radiation detector are rotated around an object, radiation is irradiated from the radiation source to the object, and a three-dimensional radiation image signal (i.e., a volume signal) representing the object image is acquired from radiation image signals (projected image signals), which have been detected at respective positions of rotation by the radiation detector. The systems for detecting three-dimensional radiation image signals will hereinbelow be referred to as the radiation image detecting systems for three-dimensional images. Also, the aforesaid radiation image detecting systems for two-dimensional images and the radiation image detecting systems for three-dimensional images will hereinbelow be referred to simply as the radiation image detecting systems.
In the radiation image detecting systems described above, such that radiation scattered by an object may not be detected, a grid comprising a material impermeable to radiation, such as lead, and a material permeable to radiation, such as aluminum or wood, which are arrayed alternately at a small pitch of, e.g., 4.0 pieces/mm, is often located between the object and radiation detecting means, and image recording operation is performed in this state. The utilization of the grid is advantageous in that radiation scattered by the object does not impinge upon the radiation detecting means, and therefore radiation image signals with high contrast can be obtained.
However, in cases where an image recording operation is performed by utilizing the grid, a stripe-like grid pattern is detected together with the object image by the radiation detecting means. In the radiation image detecting systems described above, the volume signal or the tomographic image signal is obtained from the radiation image signals, which have been detected by the radiation detecting means. Therefore, in such cases, the volume signal or the tomographic image signal is obtained from the radiation image signals carrying the information of the object image and the grid pattern. As a result, an artifact due to the grid pattern occurs in the reconstructed three-dimensional image or the reconstructed tomographic image. Accordingly, the problems occur in that a correct image cannot be reconstructed, and an image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, cannot be obtained.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide a radiation image forming method for use in a radiation image detecting system, wherein at least either one of a volume signal and a tomographic image signal representing an image of an object is obtained from radiation image signals, which represent radiation images of the object and which have been detected via scattered radiation removing means, such as a grid, such that an artifact due to a pattern of the scattered radiation removing means may not occur in an image reproduced from the volume signal or the tomographic image signal.
Another object of the present invention is to provide an apparatus for carrying out the radiation image forming method.
The present invention provides a radiation image forming method, wherein at least either one of a volume signal and a tomographic image signal representing an image of an object is obtained from a plurality of radiation image signals, which represent radiation images of the object and which have been obtained by irradiating radiation from different directions to the object and detecting the radiation carrying image information of the object via scattered radiation removing means for removing radiation having been scattered by the object, the method comprising the steps of:
i) reducing image signal components representing a pattern of the scattered radiation removing means, which are contained in the radiation image signals, pattern-reduced radiation image signals, in which the image signal components representing the pattern of the scattered radiation removing means have been reduced, being thereby obtained, and
ii) obtaining at least either one of the volume signal and the tomographic image signal from the pattern-reduced radiation image signals.
The scattered radiation removing means acts to prevent the radiation, which has been scattered by the object, from impinging upon radiation detecting means. By way of example, the scattered radiation removing means may be a grid or a collimator for removing scattered radiation, which is employed in an ordinary X-ray image recording operation. The collimator is constituted of partitions, which are arrayed at predetermined intervals. The partitions absorb the scattered radiation, which travels in oblique directions due to the scattering, and transmit only the primary radiation, which travels straightly toward the object. The grid has basically the same structure as the structure of the collimator. In this specification, the scattered radiation removing means constituted of the partitions arrayed at large intervals (of, e.g., at least 1 mm) is referred to as the collimator, and the scattered radiation removing means constituted of the partitions arrayed at small intervals is referred to as the grid.
The reducing of the image signal components representing the pattern of the scattered radiation removing means, which are contained in the radiation image signals, may be performed by utilizing one of various techniques. For example, the pattern of the scattered radiation removing means is superposed as a linear pattern upon the image represented by the image signal, which is detected by the radiation detecting means. Therefore, one of various techniques for reducing the linear pattern may be employed. Specifically, a technique for performing spatial-domain filtering processing, frequency-domain filtering processing, morphology filtering processing, or the like, may be employed. With the spatial-domain filtering processing, frequency components of the image signal corresponding to the frequency of the linear pattern are reduced by utilizing a spatial-domain filter. (The spatial-domain filtering processing is described in, for example, Japanese Unexamined Patent Publication Nos. 3(1991)-114039 and 3(1991)-12785.) With the frequency-domain filtering processing, the image signal is transformed into the frequency domain with Fourier transform, or the like, frequency components corresponding to the frequency of the linear pattern are reduced, and thereafter inverse transform
Bruce David V.
Fuji Photo Film Co. , Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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