JP2002306467A - X-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus which can obtain a tomographic image of a high picture quality by using an X-ray detector wherein X-ray quantum noises and an afterglow can be reduced. SOLUTION: This X-ray CT apparatus is equipped with an X-ray source, the X-ray detector which is arranged to be confronted with the X-ray source, a rotating disk, and an image reconstituting means. The rotating disk holds the X-ray source and the X-ray detector, and rotates around an examee. The image reconstituting means reconstitutes a tomographic image of the examee based on the strength of the X-ray detected by the X-ray detector. The X-ray detector is equipped with at least two kinds of detectors of which the characteristics in the slicing direction are different. Those at least two kinds of X-ray detectors having different characteristics in the slicing direction is a combination of solid detectors which are constituted of a scintillator and a light detector, or a combination of the solid detector 1 and an ion chamber detector 2 in which a xenon gas is enclosed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an X-ray CT apparatus, and more particularly to an X-ray CT apparatus suitable for obtaining high-quality tomographic images using an X-ray detector capable of reducing X-ray quantum noise and afterglow.

[0002]

2. Description of the Related Art An X-ray CT apparatus irradiates a subject with a fan-shaped X-ray beam from an X-ray tube and detects X-rays transmitted through the subject at a position facing the X-ray tube. The detected data is subjected to image processing to obtain a tomographic image of the subject.

The X-ray detector is composed of a group of hundreds of detection elements arranged in an arc shape, and is arranged opposite to an X-ray tube with a subject interposed therebetween. The number of X-ray paths distributed radially corresponding to the number of the X-ray tubes is formed, and the X-ray tube and the X-ray detector are integrally rotated by a rotating disk to rotate at least 180 degrees or more around the subject, and at a certain angle. First, the transmitted X-ray of the subject is detected.

Further, in this X-ray CT apparatus,
Spiral scans called helical scans and spiral scans are characterized by features such as "scanning over a wide range in a short time" and "obtaining continuous data in the body axis direction, which makes it possible to generate three-dimensional images." CT has spread rapidly. The spiral CT significantly reduces the time required for multi-layer imaging over a wide range by actively moving the imaging position during imaging, and enables three-dimensional CT imaging. The measurement data is collected by continuously rotating the tube and the X-ray detector around the subject.

An X-ray detector used in such an X-ray CT apparatus includes an ionization chamber detector filled with xenon gas for detecting X-rays by ionization of a gas, and a scintillator which absorbs and emits X-rays. A solid state detector or the like in which a light detector that converts light emitted from the scintillator into an electric signal is used.

[0006]

As described above, the X-ray C
The X-ray detector of the T apparatus uses an ionization chamber detector and a solid state detector, and these detectors have advantages and disadvantages in the following two points.

(1) X-ray detection efficiency The X-ray detection efficiency indicates how much a detector can detect X-rays transmitted through a subject, and is an SN ratio (signal to noise ratio) of a CT image.
tio: signal-to-noise ratio). If the X-ray detection efficiency is low, the influence of the quantum noise inherent in the X-rays appears on the tomographic image as graininess, deteriorating the image quality.

(2) Afterglow This is peculiar to solid-state detectors, and refers to a phenomenon in which the scintillator continues to emit light even after X-rays are cut off, which causes a reduction in time resolution.

As described above, the X-ray detector of the X-ray CT apparatus only needs to satisfy both of the above two points, but the ionization chamber detector filled with xenon gas has a problem in terms of X-ray utilization efficiency. Thus, the solid state detector has an essential problem in terms of afterglow. That is, the ionization chamber detector filled with xenon gas does not have a problem with the afterglow characteristic unlike the solid-state detector, but has a low X-ray utilization efficiency. Efforts have been made to increase the utilization efficiency of X-rays incident on the detector by increasing the sealed gas pressure, but the utilization efficiency is at most 60% of the X-ray dose incident on the detector. Only about.
With an X-ray efficiency of about 60%, no further improvement in image quality can be expected.

On the other hand, a solid state detector has a high X-ray utilization efficiency of 90% or more, but a scintillator of about 0.01% when X-rays are incident 100 ms after the X-rays are cut off even if the afterglow is small. Is emitted. The effect of this afterglow appears on the image as an artifact called a dark band at a place where the X-ray absorption coefficient changes abruptly, for example, around a bone, and the diagnostic ability is reduced. As a countermeasure, scintillators with smaller afterglow have been developed, and afterglow correction processing for image reconstruction has been performed.
While it is desired to reduce the scan time, that is, to increase the scanner rotation speed, such as seconds,..., There is a limit to the increase in the rotation speed that can be handled in terms of the afterglow. Attempts to reduce this afterglow decrease the X-ray detection sensitivity. Conversely, increasing the sensitivity increases the afterglow, and there is a trade-off between these sensitivities and afterglow.

As described above, the X-ray detectors used in the current X-ray CT apparatus are not optimal, and each has its advantages and disadvantages.

An object of the present invention is to provide an X-ray CT apparatus capable of obtaining a high-quality tomographic image using an X-ray detector capable of reducing X-ray quantum noise and afterglow.

[0013]

An object of the present invention is to provide an X-ray source,
An X-ray detector arranged opposite to the X-ray source, a rotating disk holding the X-ray source and the X-ray detector and driven to rotate around a subject, An X-ray CT apparatus comprising: image reconstruction means for reconstructing a tomographic image of the subject based on the detected X-ray intensity; wherein the X-ray detector has at least two types having different characteristics in a slice direction. Is achieved by providing a detector of

At least two different characteristics in the slice direction
The X-ray detectors of more than one kind may be a combination of solid state detectors composed of a scintillator and a photodetector, or a combination of a solid state detector and an ionization chamber detector filled with xenon gas.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a diagram showing a configuration of an X-ray CT apparatus according to the present invention. The X-ray CT apparatus according to the present invention includes an X-ray source 3 for generating X-rays, and an object 6 irradiated from the X-ray source 3.
X-ray detector 1 and X-ray detector 2 for detecting X-rays transmitted through
A rotating disk 4 that holds the X-ray source 3, the X-ray detector 1 and the X-ray detector 2 and rotates around the subject 6, a table 5 on which the subject 6 is placed, It comprises image reconstruction means (not shown) for reconstructing a tomographic image of the subject based on the X-ray dose detected by the X-ray detector 1 and the X-ray detector 2. The arrow shown in FIG. 1 indicates the direction in which the X-ray source 3, the X-ray detector 1, and the X-ray detector 2 rotate when the rotating disk 4 rotates. The feature of the present invention resides in that the X-ray detectors 1 and 2 having different characteristics are arranged in the slice direction as shown in FIG. 1, and as an example in FIG. A detector using the detector 1B is arranged, and a detector using the ionization chamber detector 2 is arranged as the X-ray detector 2. The X-ray detector 1 is a scattered X-ray that is generated by scattered X-rays radiated to a position in front of the solid state detector 1B by the subject 6 or the like.
The configuration is such that a grid 1A for removing scattered X-rays for removing lines is arranged.

The grid 1A for removing scattered X-rays is made of a metal plate having a large X-ray absorption coefficient, for example, molybdenum. The solid state detector 1B includes a scintillator that absorbs X-rays and emits light, and a photodetector that detects the amount of light emitted from the scintillator, for example, a silicon photodiode.

The element pitch of the grid 1A and the solid state detector 1B is about 1 mm. The ionization chamber detector 2 as the X-ray detector 2 includes an electrode plate and a gas sealed between the electrode plates, for example, a xenon gas. This ionization chamber detector 2
Since the number of ions generated by the ionization of the xenon gas differs depending on the X-ray dose of the incident X-ray, X-ray can be detected.

Since the ionization chamber detector 2 has the same function as the scattered radiation removal grid 1A inside thereof, another scattered radiation removal grid is provided outside the ionization chamber detector 2. In the embodiment of FIG. 1, the distance from the X-ray source 3 is the same as that of the grid 1A because there is no need to provide the grid, but the embodiment is not limited to this.

Using the X-ray CT apparatus having such a configuration,
To obtain a tomographic image with reduced effects of quantum noise and afterglow,
X-ray detectors 1 with different characteristics for any tomographic plane
And the X-ray detector 2 perform imaging. At this time, the effect of afterglow on the true image data is superimposed on the image obtained by the solid state detector 1B, and the effect of quantum noise is superimposed on the true image data on the image obtained by the ionization chamber detector 2. Have been. Therefore, by taking the average of both data, the influence of each can be reduced by half, and the image quality can be improved. In addition, since the influence of quantum noise and afterglow is significant depending on the imaging region, the image quality can be improved by weighting each data for each imaging region. Furthermore, since a part without bone such as soft tissue is hardly affected by afterglow, such a part is photographed with the solid state detector 1 and a part where the bone with great influence of afterglow is complicated is ionized with the ionization chamber detector 2. , And the solid-state detector 1B and the ionization chamber detector 2 may be selectively used according to the site.

In the embodiment of FIG. 1 described above, the X-ray detector 1 and the X-ray detector 2 are combined with a solid-state detector and an ionization chamber detector, but the present invention is not limited to this. Instead, a combination of solid state detectors having different characteristics may be used. For example, the object of the present invention can be achieved even if solid state detectors using scintillators having different afterglow characteristics are arranged in the slice direction. That is, by taking into account the difference in the afterglow characteristics of each scintillator, the detected measurement data is subjected to afterglow correction processing (for example, Japanese Patent Application Laid-Open No. 6-343629) to reconstruct an image, and thus the afterglow is reduced. The effect can be reduced. When the solid state detectors having different characteristics are arranged in the slice direction, the grid for removing scattered X-rays may be lengthened in the slice direction and shared by the two solid state detectors.

In the above embodiment, two types of X-ray detectors are arranged in the slice direction.
The type is not limited to two as long as the object of the present invention is achieved.

[0023]

As described above, according to the present invention, X-rays are detected by at least two types of X-ray detectors having different characteristics, and a tomographic image is reconstructed using data measured by the detectors. Therefore, it is possible to obtain an X-ray CT apparatus for high-quality images in which the effects of quantum noise and afterglow are reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an X-ray CT apparatus according to the present invention.

[Explanation of symbols] 1 X-ray detector, 1A Grid for removing scattered radiation, 1B solid-state detector 2 X-ray detector (ionization chamber detector), 3 X-ray source,
4 rotating discs, 5 tables, 6 subjects

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G01T 1/20 G01T 1/20 G

Claims (3)

[Claims] 1. An X-ray source, an X-ray detector arranged opposite to the X-ray source, and a rotary member that holds the X-ray source and the X-ray detector and is driven to rotate around a subject. A disk and the X
Image reconstruction means for reconstructing a tomographic image of the subject based on the intensity of the X-rays detected by the X-ray detector.
The X-ray CT apparatus according to the T apparatus, wherein the X-ray detector has at least two types of detectors having different characteristics in a slice direction. 2. The X-ray CT according to claim 1, wherein the at least two types of X-ray detectors having different characteristics in the slice direction are solid-state detectors including a scintillator and a photodetector. apparatus. 3. The X-ray detector according to claim 1, wherein the at least two types of X-ray detectors having different characteristics in the slice direction are a solid state detector composed of a scintillator and a photodetector, and an ionization chamber detector filled with xenon gas. An X-ray CT apparatus comprising: