X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-07-28
2002-07-09
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06418186
ABSTRACT:
TECHNICAL FIELD
The present invention relates to X-ray imaging apparatus (X-ray image pickup device) and more particularly, to a technology effectively applicable to multilayer detector type X-ray tomograph and method for reconstructing a tomogram of a subject from X-ray images detected by helically rotating an X-ray generating source and a multilayer X-ray detector around the subject.
BACKGROUND ART
A conventional multilayer detector type X-ray tomograph is comprised of a scan driver including an X-ray generating source and a multilayer detector, a bed including a mechanism for supporting a subject and moving it along a body-axis direction of the subject, control means for controlling the scan driver and the bed, data collection means for collecting projection data detected by the multilayer detector, reconstruction operation means for reconstructing X-ray absorption coefficient distribution of the subject from the collected projection data, display means for imaging and displaying the reconstructed X-ray absorption coefficient distribution, and a console for an operator to set values necessary for the multilayer detector type X-ray tomograph or to input commands.
DISCLOSURE OF INVENTION
Hereinafter, in the present specification hereinafter, the multilayer detector means has the meaning of a detector in which many X-ray detection elements are arranged along the rotation direction and in the rotary axis direction of the scan driver that is, in the form of a matrix which forms part of the cylindrical surface. Most X-ray tomograph put into practice at present have been so-called single-layer detector type X-ray tomograph, which has many detection elements arranged along the rotation direction on an arc. Contrarily, the multilayer detector has detectors of this type (single-layer) stacked to form layers also in the rotary axis direction and is discriminated by being called the multilayer type.
Next, a conventional imaging method by the multilayer detector type X-ray tomograph will be, described. In the multilayer detector type X-ray tomograph, too, an imaging method comparable to the method which has hitherto been known as the helical scan method is employed. More particularly, the scan driver is continuously rotated and at the same time, the bed carrying a subject is moved in the rotary axis direction; and during this operation, X-rays are projected from the X-ray generating source and X-rays transmitting through the subject are measured as projection data (X-ray images) by means of the multilayer detector. In other words, the X-ray generating source and the multilayer detector moved on orbits which are helical relative to the subject. The moving speed (moving distance per one rotation of the scan driver) of the bed and the aperture width of the detection element in the rotary axis direction (moving direction of bed) at that time were important parameters for determining spatial resolution of a reconstructed image in the rotary axis direction.
On the other hand, in the conventional single-layer detector type X-ray tomograph, the scan was carried out usually by making the moving speed of the bed nearly equal to the aperture width of the detection element in the rotary axis direction. Therefore, if the aperture width of the detection element in the rotary axis direction remains unchanged, spatial resolution nearly equal to that in usual imaging method using the single-layer detector type X-ray tomograph could be obtained with the multilayer detector type X-ray tomograph even when the moving speed of the bed approximates [the aperture width]×[the number of detector layers]. Accordingly, by forming the detector to the multilayer type, throughput could be improved.
As a reconstruction operation method of reconstructing X-ray absorption coefficient distribution in the subject, that is, a reconstructed image from projection data measured as above by the multilayer detector type X-ray tomograph, various operation methods have already been proposed and as a most fundamental multilayer detector type X-ray tomograph, an apparatus described in, for example, JP-A-4-343836 (hereinafter referred to as “literature 1”) may be referred to. In the multilayer detector type X-ray tomograph described in the literature 1, projection data measured by the multilayer detector are approximately considered as projection data measured by detectors stacked up single-layer to effect reconstruction. Measurement using the multilayer detector greatly differs from that based on the single-layer detector in that projection is slanted. Namely, in the measurement using the single-layer detector, all projected X-ray beams passed through a plane containing the X-ray generating source
1
and being vertical to the rotary axis. On the other hand, in the measurement using the multilayer detector, most projections were not vertical to the rotary axis but were oblique thereto. Therefore, the more the number of layers of the detector and the larger the arrangement pitch between detector layers in the rotary axis direction, the larger the inclination becomes.
In the reconstruction operation method described in the literature 1, however, this inclination of projection was not taken into consideration. Therefore, as far as the conventional reconstruction operation method was used, actually measured X-lay beam transmitting paths differed from X-ray beam transmitting paths presupposed approximately for the reconstruction operation and as a result, there arose a problem that, for example, a partial volume artifact is generated to degrade the quality of a reconstructed image. As a method for solving the above problem, a multilayer detector type X-ray tomograph using a reconstruction operation method described in, for example, JP-A-8-322831 (hereinafter referred to as “literature 2”) has been available. In the reconstruction operation method in the apparatus, the two closest X-ray beams were determined in respect of individual pixels on a reconstruction cross section and weighting complying with distances between these beams and the reconstrucntion cross section was effected to conduct subsequent inverse projection. In this manner, in the reconstruction operation method described in the literature 2, the reconstruction operation taking the inclination of projection into account was performed to thereby obtain a reconstructed image of good picture quality.
The present inventor has studied the prior arts to find the following problems. Even in the conventional multilayer detector type X-ray tomograph described in the literature 2, there arose as below. A first problem resided in the amount of operations necessary for determining the two closest X-ray beams in respect of the individual pixels on the reconstruction cross section and a second problem resided in continuity of projection.
Firstly, the first problem will be described. In utilizing the multilayer detector type X-ray tomograph, convenience will be met by the ability to suitably select the moving speed of the bed and the aperture width of the detector in the rotary axis direction depending on the condition of a subject, a region of interest in diagnosis or sought image quality or spatial resolution. Then, the combination of the two closest X-ray beams in respect of the individual pixels on the reconstruction cross section changes in various ways depending on the moving speed of the bed and the aperture width of the detector in the rotary axis direction. This combination may be calculated every operation or may be calculated in advance and held in a table. But in the former case, a problem of a drastic increase in computational amount arose. On the other hand, the latter case faced a problem of a drastic increase in table capacity.
Next, the second problem will be described. As described above, when the two closest X-ray beams were determined in respect of the individual pixels on the reconstruction cross section, there arose a problem that continuity of projection data to be inversely projected between pixels is impaired. For example, it is assumed
Kawai Hiroyuki
Ueda Ken
Antonelli Terry Stout & Kraus LLP
Bruce David V.
Hitachi Medical Corporation
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