JP2009018024A - Radiation ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radiation CT apparatus capable of controlling the quantity of focal shift according to conditions for radiography. <P>SOLUTION: The radiation CT apparatus includes a radiation source for applying radiation to a subject; a radiation detector disposed facing the radiation source for detecting the radiation penetrating the subject; a scanner with the radiation source and the radiation detector mounted therein to rotate around the subject; an image reconstruction device for reconstructing the tomogram of the subject based on the quantity of penetrating radiation detected by the radiation detector; an image display device for displaying the tomogram reconstructed by the image reconstruction device; a control table to be used for setting the conditions for radiography; and a radiation focal shift device for shifting the focus of the radiation in the direction of rotation of the scanner or in the direction of the rotation axis. The radiation CT apparatus further includes a radiation focal shift quantity control device for controlling the quantity of focal shift of radiation based on the set conditions for radiography. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a radiation CT apparatus, and more particularly, to a radiation CT apparatus capable of improving spatial resolution and reducing artifacts by arbitrarily shifting a radiation focal position.

  A radiation CT apparatus reconstructs and displays a tomographic image in a subject based on projection data acquired by imaging, and provides it for image diagnosis. In particular, an X-ray CT apparatus is widely used. The configuration was detected by a radiation source that irradiates the subject with radiation, a radiation detector that detects the amount of radiation that has passed through the subject as projection data, and a scanner that is equipped with both and rotates around the subject. It comprises a device for reconstructing a tomographic image from projection data at a plurality of angles and a display device.

  The performance required for the radiation CT apparatus includes high spatial resolution, high concentration resolution, and high temporal resolution. In order to improve the spatial resolution, the element pitch of the detector may be made finer, but there is a limit to the subdivision of the elements. As another method for improving the spatial resolution, there is a technique called channel offset for shifting the relative position of the detector element and the radiation focus in the scanner rotation direction (hereinafter referred to as channel direction). By using channel offset, projection data from a certain direction and projection data in the direction opposite to that direction (hereinafter referred to as opposing data) become different X-ray paths, and spatial resolution can be reduced in the same way as subdivision of element pitch. It can be improved.

  As a method of realizing the channel offset, there are a method of shifting the detector in the channel direction and a method of shifting the radiation focus position in the channel direction. While the former requires time to adjust the position of the detector and it is difficult to obtain the required accuracy, the latter can easily shift the focal position by deflecting the electron beam with a magnetic field or electric field in the X-ray source Therefore, the required accuracy can be obtained.

As a technique for further utilizing the deflection of the electron beam, there is a flying focal spot described in Patent Document 1. Flying focal spot is a technology that periodically shifts the focal position during scanner rotation, while the focal position is fixed at channel offset, and improves the sampling density during measurement by shifting the radiation path. A high spatial resolution is obtained. The focal position shift direction is not limited to the channel direction, and may be the body axis direction of the subject (hereinafter referred to as the column direction). By shifting in the column direction, the spatial resolution in the body axis direction of the subject is improved.

JP 2000-287960 A.

  However, in the above-mentioned document, no consideration has been given to controlling the focus shift amount according to the photographing conditions.

  Therefore, an object of the present invention is to provide a radiation CT apparatus capable of obtaining an image having an appropriate spatial resolution by controlling a focus shift amount according to an imaging condition.

  In order to solve the above-described problems, the present invention is configured as follows.

  A radiation source that irradiates the subject with radiation; a radiation detector that is disposed opposite to the radiation source and that detects radiation transmitted through the subject; and the radiation source and the radiation detector are mounted around the subject. A rotating scanner, an image reconstruction device for reconstructing a tomographic image of a subject based on the amount of transmitted radiation detected by the radiation detector, and an image display device for displaying the tomographic image reconstructed by the image reconstruction device; In a radiation CT apparatus comprising: a console used to set imaging conditions for imaging the subject; and a radiation focus shift device that shifts the focal point of the radiation in a rotation direction or a rotation axis direction of the scanner. A radiation focus shift amount control device that controls the shift amount of the radiation focus based on imaging conditions set via the console.

  As described above, according to the present invention, since the focus shift amount is controlled according to the photographing conditions, an image having an appropriate spatial resolution can be acquired.

  An X-ray CT apparatus to which the present invention is applied will be described with reference to the drawings.

  FIG. 1 is an overall configuration diagram of an X-ray CT apparatus 1 to which the present invention is applied. This apparatus includes a scan gantry unit 100 and an operation console 120.

  The scan gantry unit 100 includes an X-ray tube 101, a rotating disk 102, a collimator 103, an X-ray detector 106, a data collection device 107, a bed 105, a gantry control device 108, a bed control device 109, An X-ray control device 110. The X-ray tube 101 is an apparatus that irradiates a subject placed on a bed 105 with X-rays. The collimator 103 is a device that controls the radiation direction of X-rays emitted from the X-ray tube 101. The X-ray detector 106 is a device that detects the X-rays that are disposed to face the X-ray tube 101 and pass through the subject. The rotating disk 102 includes an opening 104 through which a subject placed on a bed 105 enters, and is equipped with an X-ray tube 101 and an X-ray detector 106, and rotates around the subject. The data collection device 107 is a device that converts the X-rays detected by the X-ray detector 106 into a predetermined signal. The gantry control device 108 is a device that controls the rotation of the rotary disk 102. The bed control device 109 is a device that controls the vertical movement of the bed 105. The X-ray control device 110 is a device that controls the output to the X-ray tube 101.

  The console 120 includes an input device 121, an image arithmetic device 122, a display device 125, a storage device 123, and a system control device 124. The input device 121 is a device for inputting a subject name, examination date and time, imaging conditions, and the like. The image computation device 122 is a device that performs CT processing on the measurement data sent from the data collection device 107 and performs CT image reconstruction. The display device 125 is a device that displays the CT image created by the image calculation device 122. The storage device 123 is a device that stores data collected by the data collection device 107 and image data of a CT image created by the image calculation device 122. The system control device 124 is a device that controls these devices, the gantry control device 108, the bed control device 109, and the X-ray control device 110.

  FIG. 2 shows the internal configuration of the X-ray tube 101. FIG. 2 is a view of the X-ray tube 101 viewed from the X-ray detector 106 side. The X-ray tube 101 includes a cathode 201 for generating an electron beam 204, an anode 202 to which a predetermined voltage (for example, 120 kV) is applied to the cathode 201 and irradiated with the electron beam 204, and an electron beam deflection for deflecting the electron beam 204. Means 203 are provided. A point where the electron beam 204 generated from the cathode 201 collides with the anode 202 is an X-ray focal point 205, and X-rays are emitted from the X-ray focal point 205.

  The electron beam deflecting means 203 is a device for generating a magnetic field or an electric field. By changing the intensity and direction of the generated magnetic field or electric field, the deflection width and direction of the electron beam 204 are controlled, and the position of the X-ray focal point 205 is changed. Shift. An X-ray focal point 205A in FIG. 2 is a case where no magnetic field or electric field is generated from the electron beam deflecting means 203. An X-ray focal point 205B in the figure is a case where a magnetic field perpendicular to the paper surface or an electric field in the vertical direction of the paper surface is generated from the electron beam deflecting means 203 and the electron beam 204 is shifted in the channel direction. The position of the X-ray focal point 205 can be shifted in the column direction by generating a magnetic field in the vertical direction on the paper surface or an electric field perpendicular to the paper surface from the electron beam deflection means 203. In FIG. 2, the electron beam deflecting means 203 is shown so as to be arranged in the X-ray tube 101. However, if a magnetic field or an electric field can be applied on the trajectory of the electron beam 204, the electron beam deflecting means 203 is provided. It may be arranged outside the X-ray tube 101.

  In FIG. 2, the channel offset is made possible by setting the intensity of the magnetic field perpendicular to the paper surface generated from the electron beam deflection means 203 or the intensity of the electric field in the vertical direction of the paper surface to a value corresponding to the required deflection width. The value of the magnetic field or electric field is determined by the system controller 124 using a method described later based on the imaging conditions input via the console 121 and output to the X-ray tube 101 via the X-ray controller 110. The

The channel offset amount Doff that improves the sampling density is expressed as follows using the integer Z.

Doff = (2Z + 1) / 4 (Equation 1)

In (Equation 1), if Z = 0, Doff = 0.25, and this value is preferably used as the channel offset amount.

  A flying focal spot can be obtained by periodically changing the magnetic field or electric field generated from the electron beam deflection means 203. Note that there are flying focal spots (FFS) which are a combination of the channel direction and the column direction, and are hereinafter referred to as a channel direction FFS, a column direction FFS, and a channel column direction FFS, respectively.

  By superimposing a periodically changing magnetic field or electric field on the magnetic field or electric field to be channel offset, the channel offset and the flying focal spot can be combined. FIG. 3 shows the X-ray focal point position when the channel offset and the channel direction FFS are combined, with the anode 202 in FIG. 2 enlarged. By generating a predetermined magnetic field or electric field from the electron beam deflecting means 203, the channel is offset from the X-ray focal point 205A to the X-ray focal point 205B. Furthermore, by superimposing a periodically changing magnetic field or electric field from the electron beam deflection means 203, the positions are alternately shifted to the positions of the X-ray focal points 205C and 205D.

The channel offset amount Doff_ffs for improving the sampling density when using the channel direction FFS is expressed as follows using the integer Z.

Doff_ffs = (2Z + 1) / 8 (Formula 2)

In (Expression 2), if Z = 0, Doff_ffs = 0.125, and this value is preferably used as the channel offset amount when the channel direction FFS is used. That is, the preferred channel offset amount differs depending on whether or not the channel direction FFS is used. In addition, when the opposite data does not exist when the channel direction FFS is used and the channel offset amount is 0.125, or when the channel offset is 0.125 when the channel direction FFS is not used, data sampling is not evenly spaced. As a result, the spatial resolution is lower than when the channel offset amount is set to 0.25 without using the channel direction FFS. Therefore, by changing the channel offset amount according to the set imaging conditions, it is possible to perform imaging without reducing the spatial resolution.

  The method for determining the channel offset amount according to the present invention will be described below with reference to FIGS.

  In the first method, as shown in FIG. 4, when the channel direction FFS is used, the channel offset is set to 0.125 channel, and in other cases, the channel offset is set to 0.25 channel.

  As shown in FIG. 5, the second method sets the channel offset to 0.125 channel when the channel direction FFS or the channel column direction FFS is used, and sets the channel offset to 0.25 channel otherwise. In this case, it means that FFS in the channel direction is included.

  The third method is to set the channel offset to 0.125 when the channel direction FFS is used as shown in Figure 6 and the circular scan or spiral scan is low pitch, otherwise the channel offset is set to 0.25 channel To do. Here, the low pitch is a helical pitch when the number of views that can be used for image reconstruction can be collected over 360 degrees. In this case, the channel direction FFS is used, and the reconstruction view width can be used one or more times.

  The X-ray focal position shift amount for realizing the desired channel offset amount according to the present invention will be described below. Assuming that the X-ray focal position shift amount is x (mm), x is approximately expressed as follows.

X ≒ c ・ d ・ SOD / SDD (Equation 3)
Here, the distance between the X-ray focal point and the imaging center is SOD (mm), the distance between the X-ray focal point and the X-ray detector is SDD (mm), and the distance between the X-ray detector (center position) and the imaging center is The distance was DOD (mm), the detector element channel direction size was d (mm), and the channel offset amount was c (channel).

  From the foregoing description of various embodiments of the present invention, it is evident that the objects of the invention are attained. While the present invention has been described and illustrated in detail, they are intended for purposes of illustration and illustration only and are not intended to be limiting. In this embodiment, a tomographic apparatus using X-rays is used, but the present invention is not limited to this, and the present invention can also be applied to a tomographic apparatus using gamma rays or light. Thus, the gist of the present invention is limited only by the claims.

The figure for demonstrating the whole structure of this invention. The figure for demonstrating the internal structure of the X-ray tube used for this invention. The figure for demonstrating a X-ray focus position when channel offset and channel direction FFS are combined. The figure which shows the 1st method of determining channel offset amount. The figure which shows the 2nd method of determining channel offset amount. The figure which shows the 3rd method of determining channel offset amount.

Explanation of symbols

  1 X-ray CT system, 100 scan gantry unit, 101 X-ray tube, 102 rotating disk, 103 collimator, 104 aperture, 105 bed, 106 X-ray detector, 107 data collection device, 108 gantry control device, 109 bed control device , 110 X-ray control device, 120 console, 121 input device, 122 image calculation device, 123 storage device, 124 system control device, 125 display device, 201 cathode, 202 anode, 203 electron beam deflection means, 204 electron beam, 205 X-ray focus

Claims (1)

A radiation source that irradiates the subject with radiation; a radiation detector that is disposed opposite to the radiation source and that detects radiation transmitted through the subject; and the radiation source and the radiation detector are mounted around the subject. A rotating scanner, an image reconstruction device for reconstructing a tomographic image of a subject based on the amount of transmitted radiation detected by the radiation detector, and an image display device for displaying the tomographic image reconstructed by the image reconstruction device; In a radiation CT apparatus comprising: a console used to set imaging conditions for imaging the subject; and a radiation focus shift device that shifts the focal point of the radiation in a rotation direction or a rotation axis direction of the scanner.
A radiation focus shift amount control device that controls the shift amount of the radiation focus based on imaging conditions set via the console;
A radiation CT apparatus further comprising:

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