JP4114467B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray detector and an X-ray CT apparatus, and in particular, an X-ray having a collimator for removing scattered X-rays in front of the X-ray detector in order to remove scattered X-rays from a subject. The present invention relates to a CT apparatus.
[0002]
[Prior art]
The X-ray CT apparatus emits X-rays from the X-ray tube while the X-ray tube and the X-ray detector rotate in the circumferential direction around the body axis of the subject, and a plurality of circular arrangements opposed to the X-ray CT apparatus. An X-ray dose transmitted through the subject is detected by a detector, and the signal is computer processed to form an image.
FIG. 6 shows the configuration of each part installed in the examination room and operation room of a conventional X-ray CT apparatus and a control block diagram. The X-ray CT apparatus places the subject 4 on the top 5, raises the bed 6, positions the subject 4 at the height of the rotation shaft 8, and places the top 5 on the gantry via the slide portion 5 a. Move it into 1. Then, the X-ray tube 2 and the detector 3 around the rotation axis 8 around the subject 4 are the ring frame of the rotating unit 7 in the gantry 1, and the gantry / bed according to the command from the host CPU 21. Under the control of the control device 20, X-rays are emitted while rotating. The rotation speed is 1 second or less in a full scan (360 degrees) of a high-class machine class. During this scan, for example, in the case of an abdominal slice, a high voltage is applied to the X-ray tube 2 by the high voltage generator set by the X-ray control device, and an X-ray dose of about 120 kV and 200 mAs is applied to the subject. 4 is irradiated. This X-ray beam is narrowed in a fan shape by a collimator at the exit of the X-ray tube 2 and partially hits the subject 4 and the transmitted X-ray enters the detector 3 and is processed as diagnostic image information. . High-speed continuous imaging in spiral scan mode (which is also called helical scan mode, where the top plate 5 is moved into the gantry 1 at a constant speed and the X-ray tube 2 and the detector 3 are continuously rotated) Since the movement of the plate 5 is performed during photographing, a wide range can be photographed in a short time, so that it is popular as the mainstream of CT. The output signal from the detector 3 is input to the amplifier of the data acquisition unit 22 (DAS) arranged at the lower part of the detector 3 and passes through circuits such as a sample hold circuit, a multiplexer, an A / D converter, and an interface. Stored in memory. The host CPU 21 which is the center of system control includes a microprocessor, storage means 21b such as a memory, a hard disk, and a magnetic disk, and peripheral devices such as an input device 21a provided with a keyboard. 23, the image display device 24, a scan control unit (not shown), and the like, data processing is performed, and an image is reconstructed by the high-speed arithmetic processing of the reconstruction device 23, and the image data is stored on the magnetic disk of the storage unit 21b. At the same time, it is displayed on the monitor as an analog signal by the image display device 24.
Recently, it has become possible to increase the image reconstruction processing speed more than 10 times faster than conventional methods, and it has a function that can display images like a movie at a speed of several frames per second in parallel during scanning. A fluoroscopy method has been put into practical use, and can be continuously observed in real time like conventional X-ray fluoroscopy.
[0003]
The detector 3 is usually provided with a collimator 9 for removing scattered X-rays on the front surface where X-rays transmitted through the subject 4 enter the detector 3. A plate-shaped X-ray shielding plate is formed in the direction. The collimator 9 for removing scattered X-rays is formed of a metal plate having a large X-ray absorption, for example, a lead plate, a tungsten plate, etc., and effectively produces scattered rays generated by the interaction between the subject 4 and the X-rays. Only a valid signal is taken into the detector 3 by being removed or reduced. The scattered radiation increases as the subject 4 is thicker, the irradiation field is larger, and the tube voltage is higher. The scattered radiation is detected by the detector 3 and decreases the contrast of the X-ray image.
[0004]
The detector 3 faithfully converts the intensity of the X-ray transmitted through the subject 4 into an electrical signal. (1) Semiconductor detector, (2) Scintillator + photoelectric conversion element detector, (3) Xe gas There are ionization chamber detectors. (1) and (2) are solid state detectors. (3) Since the X-ray capture efficiency is higher than that of the Xe gas ionization chamber detector and the S / N is good, (2) scintillator + photoelectric conversion element detector Is often used. On the other hand, (3) Xe gas ionization chamber detectors have little variation in the sensitivity of each channel, and the Xe gas absorption wavelength is changed by changing the Xe gas pressure and applied voltage. Have.
[0005]
However, the X-rays irradiated from the X-ray tube 2 are polychromatic X-rays in which energy is distributed with a value corresponding to the tube voltage as a maximum value, but only X-rays belonging to a specific region in such energy distribution. I couldn't shoot with Therefore, “comprising a plurality of scintillator elements arranged in the X-ray transmission direction and a plurality of light detection elements respectively facing the plurality of scintillator elements in a direction perpendicular to the X-ray transmission direction. "Characteristic X-ray detector" is disclosed (for example, see Patent Document 1).
FIG. 7 shows the external appearance of the above-described detector 3a. The x-ray transmission direction is indicated by j, the tomographic plane direction is i, and the direction perpendicular to the X-ray transmission direction (body axis direction) is indicated by k. FIG. 8 is a sectional view of the detection element in the jk direction. Show.
This detector 3a is provided with a scintillator assembly 100 in a direction (k) perpendicular to the X-ray transmission direction and a light detection element assembly 110 in a lateral direction close to the scintillator assembly 100, and the scintillator assembly 100 is arranged in the X-ray transmission direction. A plurality of scintillator elements 102 are stacked vertically, and correspondingly, a light detection element assembly 110 has light detection elements 112 arrayed vertically. By disposing the light detection element 112 beside the scintillator element 102, the influence on the subsequent scintillator element 102 in the X-ray transmission direction is eliminated.
FIG. 9 shows the relationship between the number of X-ray photons captured by each scintillator and the photon energy. The characteristic of the first-stage scintillator element 102 is indicated as Q ₁ , the characteristic of the next stage as Q ₂ , and the characteristic of the third stage as Q ₃ .
[0006]
[Patent Document 1]
JP 2001-174564 A (page 3, FIG. 5)
[0007]
[Problems to be solved by the invention]
Although the conventional X-ray CT apparatus is configured as described above, the detector 3 that converts the X-ray dose transmitted through the subject 4 into an electric signal has many elements of about 800 in the X-ray CT apparatus. Arranged. In order to improve the resolution, the number of detector elements is increased, and a plurality of detector elements are arrayed in the body axis direction of the subject 4 so that a plurality of tomographic planes can be obtained at one time. The detector 3 is configured two-dimensionally. Then, as in the above-mentioned published patent, when the detector 3 having different characteristics is formed in multiple stages in the X-ray transmission direction so as to be photographed using X-rays belonging to a specific region of the energy distribution, the detector However, there is a problem that it is very difficult to take out an electric signal. In addition, since many solid detectors are used, there is a problem that the manufacturing cost of the detector 3 is significantly increased.
[0008]
The present invention has been made in view of such circumstances, and is formed in multiple stages in the X-ray transmission direction with a detector having a structure that is easy to manufacture and does not cost much, and information on different X-ray energy regions. It is an object of the present invention to provide an X-ray CT apparatus capable of capturing the image data.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, an X-ray CT apparatus according to the present invention is a solid state detector having an X-ray tube and a collimator for removing scattered radiation on the front surface around the body axis of a subject placed on a bed. Radiates X-rays from the X-ray tube while rotating in the circumferential direction, detects the X-ray dose transmitted through the subject with a plurality of solid-state detectors arranged in a circle, and computer-processes the signals to regenerate the image. An X-ray CT apparatus comprising: a space in which the collimator is provided is sealed, Xe gas is sealed in to form an Xe detector using an Xe gas ionization chamber, and the solid state detector and the Xe detector are in the X-ray transmission direction. The image can be reconstructed with the signal from the double detector.
[0010]
In the X-ray CT apparatus of the present invention, the Xe detector and the solid state detector react to X-rays having different energy levels.
[0011]
Moreover, the X-ray CT apparatus of the present invention forms an array by arranging a plurality of the double detectors in a direction perpendicular to the X-ray transmission direction.
[0012]
The X-ray CT apparatus of the present invention is configured as described above, and a space provided with a collimator for removing scattered radiation is sealed with a plate made of a material having good X-ray permeability, and high-pressure Xe gas is contained inside. A Xe gas ionization chamber to form an Xe detector, and a solid state detector composed of a scintillator and a photodiode provided below the collimator is provided in the X-ray transmission direction behind the Xe detector, The image can be reconstructed with the signal from the double detector.
For this reason, the use of the Xe detector makes it easier to take out an electric signal and the cost does not increase significantly, rather than using a multi-stage solid state detector.
And the Xe detector consisting of the Xe gas ionization chamber has little variation in the sensitivity of each channel, and the Xe energy absorption wavelength captured by the first-stage detector differs by changing the Xe gas pressure and applied voltage. X-ray information with different energies can be obtained for the solid state detector, and an image due to the difference in energy can be reconstructed.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the X-ray CT apparatus of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing the configuration of the X-ray CT apparatus of the present invention.
The X-ray CT apparatus of the present invention includes an X-ray tube 2 that is attached to a rotating part 7 in a gantry 1 and rotates around a rotating shaft 8, a bed 6 that includes a top plate 5 on which a subject 4 is placed, A detector 30 having a double structure that is attached to the rotating unit 7 and rotates around the rotating shaft 8, a gantry / bed control device 20 that controls the rotating unit 7 and the bed 6, and an image processing unit (below the detector 30). A data collecting unit 22 arranged to collect signal data, an input device 21a and a storage means 21b, a host CPU 21 that controls the system, and a reconstruction device that reconstructs signals from the data collecting unit 22 and constructs an image 23 and an image display device 24) for displaying the reconstructed image.
[0014]
Next, each component of the X-ray CT apparatus will be described.
A high voltage of 80 to 140 kV is applied to the X-ray tube 2 from a high voltage generator mounted in the gantry 1 and miniaturized by high frequency inverter control, and tube current control and rotational drive control of the rotary anode are performed.
The couch 6 includes a top plate 5 at the top, and is configured by vertical and horizontal (sliding portions 5a) driving mechanisms, and performs alignment at the gantry opening by an operation switch of the gantry 1. When performing helical scanning, horizontal driving is performed remotely from the operation console, and during scanning, control is performed in synchronization with X-ray generation and data collection.
The detector 30 has a double structure of a collimator / Xe detector 30a provided in the upper part and a solid state detector 30b provided in the lower X-ray transmission direction.
FIG. 2 shows a schematic structure of the detector 30. The collimator and Xe detector 30a is set up in the horizontal direction of the X-ray shielding plate made of a lead plate or a tungsten plate in order to remove scattered radiation. On the upper surface, a plate made of an insulator made of a material having a good X-ray transparency is sealed with a plate 38a in which electrodes 41 are formed correspondingly between the collimators 32 by depositing Al or the like. The plate 38b made of a conductive material having good X-ray transparency such as Al is sealed. Further, the wall in the front-rear direction is supported and sealed by a metal fitting that supports the detector 30. A high-pressure Xe gas 31 is sealed in the space of the collimator 32, and a voltage is applied between the upper electrode 41 and the lower plate 38b. An ionization current due to incident X-rays is extracted from the electrode 41 to the outside.
The collimator / Xe detector 30a has both the function of removing scattered rays by the collimator 32 and the function of the Xe detector of the Xe gas ionization chamber, and can effectively use the collimator space. And the Xe detector has little variation in sensitivity of each channel, and the Xe gas pressure and applied voltage can be changed to change the absorption wavelength of the X-ray energy, which is different from the solid state detector 30b. X-ray information can be obtained.
The solid state detector 30b is conventionally used and is composed of a scintillator that emits light by transmitted X-rays and a photoelectric conversion element that converts light emitted from the scintillator into an electric signal, such as a photodiode, and has high X-ray capture efficiency. S / N is good. Here, it is arranged below the collimator and Xe detector 30a and is configured to coincide with the X-ray transmission direction.
The gantry / bed control device 20 controls the rotation unit 7, the X-ray tube 2, and the bed 6 in the gantry 1, and the rotation speed of the rotation unit 7, the imaging conditions supplied to the X-ray tube 2, and the bed 6. And the like, and the horizontal movement of the top plate 5 are controlled.
The image processing unit displays a data collection unit 22 that collects a signal from the detector 30 in the gantry 1, a reconstruction device 23 that reconstructs an image based on a signal from the data collection unit 22, and a reconstructed image. Image display device 24.
The output signals from the detector 30 (collimator / Xe detector 30a and solid state detector 30b) are circuits such as an amplifier, a sample hold circuit, a multiplexer, an A / D converter, and an interface of the data acquisition unit 22 (DAS). Is stored in the memory via. The host CPU 21 that is the center of system control includes a microprocessor, storage means 21b such as a memory, a hard disk, and a magnetic disk, and peripheral devices such as an input device 21a. The host CPU 21, the reconstruction device 23, and an image display Data processing is performed by the device 24 and a scan control unit (not shown), and an image is reconstructed by the high-speed arithmetic processing of the reconstruction device 23, and the image data is filed on a magnetic disk, and at the same time, an image display device 24, it is displayed on the monitor as an analog signal.
[0015]
Next, an embodiment of the detector 30 of the present X-ray CT apparatus will be described with reference to FIGS. FIG. 3 shows a cross-sectional structure of the detector 30 in the slice direction, and FIG. 4 shows the appearance of the collimator unit.
The collimator unit includes an X-ray shielding collimator 32 disposed between an arcuate main support 40 and a support plate 37, both ends of which are reinforced by support rods 42 in the front and rear direction in the body axis direction. A collimator 32 is inserted in the direction of incidence of the X-ray beam and fixed using a jig.
In the collimator / Xe detector 30 a, an insulating plate 38 a having a good X-ray permeability, in which an electrode 41 is formed on the inner side of the collimator portion, is bonded onto the main support portion 40 and the side support portion 36. A conductive plate 38b having good X-ray permeability is inserted and bonded to the grooves of the main support portion 40 and the side support portion 36 at the lower portion of the collimator portion to seal the space of the collimator portion. Then, the Xe gas 31 is injected at a high pressure from the end into the space of the collimator unit to form an Xe gas ionization chamber, the signal line 39 is wired to the outside, and ions generated by the transmission X-rays are ionized from the electrode 41 to the outside. Take out as a signal.
The solid state detector 30b is one module in which 8 to 30 pieces of optically bonded scintillators 34 and PDAs (photodiode arrays) 35 are arranged on a substrate 33 from a mechanical arrangement in manufacturing. Such a detector module is continuously arranged in a substantially arc shape on the circumference of the lower part of the main support part 40 and the side support part 36, and the lower part of the collimator and Xe detector 30a The signal lines 39 are wired to the outside. At this time, the position accuracy of the collimator 32 and the solid state detector 30b is set accurately, and the X-ray CT detector 30 is configured so that the detection sensitivity of each solid state detector 30b is uniform and maximized. . The detector 30 is attached to the rotating part 7 in the gantry 1 through the main support part 40.
[0016]
Next, the difference in the X-ray energy region in which X-ray photons are captured by the collimator / Xe detector 30a and the solid state detector 30b will be described with reference to FIG. FIG. 5A is a diagram showing the X-ray intensity S with respect to the wavelength irradiated from the X-ray tube 2. (B) shows the characteristic _{D 1} of the photon energy is captured by the collimator and Xe detector 30a, (c) shows the characteristic _{D 2} of photon energy is captured by the solid state detector 30a.
In the collimator / Xe detector 30a, as shown in FIG. 2, the Xe gas 31 is sealed at a high pressure, and the gas pressure of the Xe gas 31 and the voltage applied between the electrode 41 and the conductive plate 38b. the characteristic D ₁ of the photon energy is shifted varies. Characteristics D ₂ of the photon energy of the solid state detector 30b in comparison with which is substantially fixed as shown in (c). For this reason, the information from the X-ray image of the subject 4 with different energy regions is captured by separately reconstructing the signal from the collimator / Xe detector 30a and the signal from the solid-state detector 30b. be able to.
From the viewpoint of the attenuation of radiation, the human body can be considered as a two-component system consisting of bone and soft tissue, and the linear absorption coefficient of each component differs depending on the energy of the radiation. The thickness and further the weight of each component can be determined by measuring the amount of transmitted radiation of the subject 4 by alternately or simultaneously irradiating the human body with radiation having One of the methods is a bone density measurement method called DXA (Dual X-ray Absorption Memories). Different X-ray energy ranges to monitor bone growth in pediatric schoolchildren, assessment of maximum bone mass in young adulthood, screening and diagnosis of osteoporosis in regression, determination of drug effects, etc. X-ray CT diagnosis with a double detector is useful.
[0017]
In the above embodiment, the electrode 41 and the lower plate 38b are used as counter electrodes. However, without forming the electrode 41 on the plate 38a, another collimator plate is set up between the collimators 32 and insulated. It is good also as an electrode.
In the embodiment, the single-slice detector 30 has been described. However, a three-dimensional structure in which a plurality of double detectors 30 are arranged in a direction perpendicular to the X-ray transmission direction to form an array. It can be a detector. As a result, a multi-slice tomographic image can be obtained with two images having different energy regions in one scan.
[0018]
【The invention's effect】
The X-ray CT apparatus of the present invention is configured as described above, and a space provided with a collimator for removing scattered radiation is used as an Xe detector for an Xe gas ionization chamber, and has both a collimator function and a detector function. Since a conventional solid state detector composed of a scintillator and a photodiode is provided below, a different signal from each detector in the two stages can be taken in.
Then, by changing the Xe gas pressure and applied voltage of the Xe detector, the X-ray energy region captured by the first-stage Xe detector can be changed, and X-ray information different from the X-ray energy region captured by the solid state detector can be obtained. It is possible to reconstruct two images due to energy differences. In addition, by incorporating information with different X-ray energy regions, it is possible to obtain bone density measurement data.
And, it is easier to take out an electric signal and the cost is not significantly increased when a structure using a Xe detector is used rather than a structure having a multi-stage solid state detector.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of an X-ray CT apparatus of the present invention.
FIG. 2 is a diagram showing the principle of the detector of the X-ray CT apparatus of the present invention.
FIG. 3 is a view for explaining the structure of the detector of the X-ray CT apparatus of the present invention.
FIG. 4 is a diagram showing an entire structure of a collimator.
FIG. 5 is a diagram showing the wavelength used by each detector of the X-ray CT apparatus of the present invention.
FIG. 6 is a diagram showing a configuration of a conventional X-ray CT apparatus.
FIG. 7 is a diagram illustrating an appearance of a multi-slice detector.
FIG. 8 is a view showing a cross section of a detector in which scintillators are formed in three stages.
FIG. 9 is a diagram showing energy characteristics absorbed by each scintillator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Gantry 2 X-ray tube 3, 3a, 30 Detector 4 Subject 5 Top plate 5a Slide part 6 Bed 7 Rotating part 8 Rotating shaft 9 Collimator 20 Gantry 21 Host CPU
21a Input device 21b Storage means 22 Data collection unit 23 Reconstruction device 24 Image display device 30a Collimator / Xe detector 30b Solid state detector 31 Xe gas 32 Collimator 33 Substrate 34 Scintillator 35 PDA
36 side support portion 37 support plate 38a, 38b plate 39 signal line 40 main support portion 41 electrode 42 support rod 100 scintillator assembly 102 scintillator element 110 photodetection element assembly 112 photodetection element

Claims (3)

A solid-state detector equipped with an X-ray tube and a collimator for removing scattered radiation around the body axis of the subject placed on the bed emits X-rays from the X-ray tube while rotating in the circumferential direction. An X-ray CT apparatus that detects X-rays transmitted through a subject with a plurality of solid detectors arranged in a circle and reconstructs an image by computer processing of the signals, and is a space provided with the collimator The Xe gas is sealed and Xe gas ionized chamber is used as the Xe detector, the solid state detector and the Xe detector are provided in the X-ray transmission direction, and the image can be reconstructed with the signal from the double detector. An X-ray CT apparatus characterized by being configured as described above. The X-ray CT apparatus according to claim 1, wherein the Xe detector and the solid-state detector respond to X-rays having different energy levels. 2. The X-ray CT apparatus according to claim 1, wherein a plurality of the double detectors are arranged in a direction perpendicular to the X-ray transmission direction to form an array.

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