JP2014042662A - X-ray computerized tomographic device and scan control program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To limit scanning to a region of interest and reconfigure an image by controlling a collimator according to a relative position of the region of interest with respect to the center of a field of view and a view angle.SOLUTION: An X-ray computerized tomographic device 1 includes: an X-ray tube 101; a high voltage generation part 109; an X-ray detection part 103; a projection data collection part 104 for collecting projection data; a reconstruction part 114 for reconstructing an image; a collimator 121 disposed on the front face of the X-ray tube 101 whose opening width and opening center are variable; and a control part 110 for controlling the collimator 121, the high voltage generation part 109, the projection data collection part 104, and the reconstruction part 114 for performing main scanning for the analyte and pre-scanning repeated before the main scanning to obtain the main scanning start point. The X-ray irradiation range in the main scanning is the whole of a field of view and the X-ray irradiation range in the pre-scanning is part of the field of view.

Description

  Embodiments described herein relate generally to an X-ray computed tomography apparatus and a scan control program that scan an image with a limited field of view.

  Conventionally, in contrast medium imaging, it is important to scan at the timing when the contrast medium administered to the subject flows into the slice of interest (hereinafter referred to as the main scan). For this reason, scanning (hereinafter referred to as pre-scanning) and image reconstruction at a lower dose than the main scanning dose are repeated before the main scanning. The pre-scan refers to a scan in a site where the blood flow is upstream from the site of interest. The operator visually recognizes the contrast agent concentration (also referred to as the degree of shadow) in the region of interest (hereinafter referred to as ROI) in the reconstructed image, and when the contrast agent concentration has increased to some extent, A scan trigger is entered. The main scan is a scan of a region of interest. When a trigger is input, the X-ray computed tomography apparatus stops pre-scanning and starts main scanning.

  In recent years, a so-called real prep scan technique for automating the start timing of the main scan has been widely used. In the real prep scan, the average of a plurality of CT values in the ROI (hereinafter referred to as the ROI average CT value) is compared with a predetermined threshold value from the tomographic image data of the subject obtained in the prescan. The prescan is stopped at a time when the ROI average CT value exceeds a predetermined threshold (hereinafter referred to as a threshold excess time). The X-ray computed tomography apparatus starts a main scan after a predetermined time has elapsed from the threshold excess time.

  The ROI in the prescan in the real prep scan is, for example, a part of blood vessels into which a contrast agent flows. However, the pre-scan is not limited to the ROI, and the subject is imaged in the same field of view (Field Of View: hereinafter referred to as FOV) as the main scan. For this reason, there is a problem that unnecessary exposure of the subject to the non-ROI occurs. Further, the conventional collimator has a problem that scanning cannot be limited to an arbitrary ROI in the FOV. Even if the scan can be limited to the ROI to avoid unnecessary exposure to the non-ROI, the center of the ROI does not always coincide with the FOV center, so that the conventional reconstruction algorithm cannot reconstruct the image. is there.

  The purpose is to reconstruct an image by scanning only in the region of interest by controlling the collimator in accordance with the relative position of the region of interest with respect to the center of the imaging field and the view angle in the real prep scan.

  An X-ray computed tomography apparatus according to this embodiment includes an X-ray tube that generates X-rays, a high-voltage generation unit that generates a high voltage for applying a high voltage to the X-ray tube, and the X-ray tube An X-ray detection unit that detects X-rays generated from the subject and transmitted through the subject, a projection data collection unit that collects projection data via the X-ray detection unit, and an image based on the projection data A reconstruction unit to be configured; a collimator provided on the front surface of the X-ray tube; the aperture width and the aperture center being variable; the main scan for the subject; and the main scan for determining the start time of the main scan X-ray irradiation in the main scan, comprising: a control unit that controls the collimator, the high voltage generation unit, the projection data collection unit, and the reconstruction unit in order to perform a prescan repeated before Range is imaged A whole, X-rays irradiation range in the pre-scan, said a portion of the imaging field of view, characterized by.

FIG. 1 is a configuration diagram showing the configuration of the X-ray computed tomography apparatus according to the first embodiment. FIG. 2 is a diagram illustrating an example of the FOV in the ROI setting scan and the main scan and the ROI that is a range in which X-rays are limitedly emitted in the pre-scan according to the first embodiment. FIG. 3 is a diagram illustrating an example of a schematic diagram schematically showing the relative positions of the X-ray tube and the collimator with respect to the ROI in the pre-scan for a plurality of view angles according to the first embodiment. FIG. 4 is a diagram illustrating an example of collimating generated X-rays in order to irradiate the ROI with limited X-rays at the 0 ° view angle in the pre-scan according to the first embodiment. FIG. 5 is a schematic diagram showing an example of an outline of a position related to ROI and an X-ray tube position at a view angle ω in the coordinate system with the center of the FOV as the origin according to the first embodiment. FIG. 6 is a flowchart illustrating an example of a processing procedure according to the ROI-limited X-ray irradiation function according to the first embodiment. FIG. 7 is a configuration diagram showing the configuration of the X-ray computed tomography apparatus according to the second embodiment. FIG. 8 is a diagram illustrating a cross-sectional image of a subject at a certain view angle together with an X-ray irradiation range in an X-ray tube, a collimator, and a pre-scan according to the second embodiment. FIG. 9 is a diagram showing the dependence of the X-ray transmission intensity on the channel in the pre-scan X-ray irradiation range in FIG. 8 according to the second embodiment. FIG. 10 is a diagram illustrating the dependency of the X-ray transmission intensity on the channel when the ROI center coincides with the FOV center according to the second embodiment. FIG. 11 is a CT value distribution diagram showing the CT value distribution of the ROI setting image together with the ROI in the pre-scan according to the second embodiment. FIG. 12 is a conceptual diagram showing an example of the concept of attenuation correction and beam hardening correction for projection data collected in the pre-scan in the CT value distribution diagram in FIG. 11 according to the second embodiment. FIG. 13 is a diagram illustrating an example of a superimposed image displayed during pre-scanning according to the second embodiment. FIG. 14 is a flowchart illustrating an example of a procedure for executing the projection data correction process according to the second embodiment. FIG. 15 is a flowchart illustrating an example of a procedure for executing threshold correction processing according to the second embodiment. FIG. 16 relates to the second embodiment, and relates to correcting the scan start threshold based on the first average value regarding the ROI setting image, the second average value regarding the plurality of pre-scan images, and the predetermined standard value. It is a figure which shows an example of the change of CT value during a scan. FIG. 17 is a flowchart illustrating an example of a procedure for executing an in-ROI image update process according to the second embodiment. FIG. 18 is a diagram illustrating an example of the superimposed image together with the ROI in the superimposed image according to the second embodiment.

  Hereinafter, an embodiment of an X-ray computed tomography apparatus 1 (Computed Tomography) will be described with reference to the drawings. In the X-ray computed tomography apparatus 1, the X-ray tube 101 and the X-ray detection unit 103 are integrally rotated with a Rotate / Rotate-Type that rotates around the subject, and a large number of X-ray detections arrayed in a ring shape. There are various types such as Stationary / Rotate-Type in which the element is fixed and only the X-ray tube 101 rotates around the subject, and any type is applicable to the present embodiment. Further, in order to reconstruct an image, projection data for 360 ° around the subject and projection data for 180 ° + fan angle are required for the half scan method. The present embodiment can be applied to any reconfiguration method. In addition, the mechanism for changing incident X-rays into electric charges is based on an indirect conversion type in which X-rays are converted into light by a phosphor such as a scintillator, and the light is further converted into electric charges by a photoelectric conversion element such as a photodiode, and The generation of electron-hole pairs in a semiconductor such as selenium and the transfer to the electrodes, that is, the direct conversion type utilizing a photoconductive phenomenon, are the mainstream. Any of these methods may be adopted as the X-ray detection element. Furthermore, in recent years, the so-called multi-tube type X-ray computed tomography apparatus in which a plurality of pairs of the X-ray tube 101 and the X-ray detection unit 103 are mounted on the rotating ring 102 has been commercialized, and the development of peripheral technology thereof has progressed. Is progressing. In the present embodiment, either a conventional single-tube X-ray computed tomography apparatus or a multi-tube X-ray computed tomography apparatus can be applied. Here, a single tube type will be described.

  In the following description, components having substantially the same function and configuration are denoted by the same reference numerals, and redundant description will be given only when necessary.

(First embodiment)
FIG. 1 is a diagram showing a configuration of an X-ray computed tomography apparatus 1 according to the first embodiment. The X-ray computed tomography apparatus 1 according to the first embodiment includes a gantry 100, a preprocessing unit 106, a reconstruction unit 114, an input unit 115, a display unit 116, and a control unit 110.

  The gantry 100 houses a rotation support mechanism. The rotation support mechanism includes a rotation ring 102, a ring support mechanism that supports the rotation ring 102 so as to be rotatable about the rotation axis Z, and a gantry drive unit 107 (electric motor) that drives the rotation of the ring. Mounted on the rotating ring 102 are an X-ray tube 101, a high voltage generation unit 109, and an area detector (hereinafter referred to as an X-ray detection unit 103) also referred to as a two-dimensional array type or a multi-row type. .

  The high voltage generator 109 generates a high voltage to be applied to the X-ray tube 101 (hereinafter referred to as tube voltage) and a plurality of currents to be supplied to the X-ray tube 101 (hereinafter referred to as tube current). Occur. The high voltage generation unit 109 generates a tube voltage and a tube current in accordance with a control signal input from the control unit 110 described later via the slip ring 108.

  The X-ray tube 101 radiates X-rays from the focal point of X-rays in response to application of tube voltage and supply of tube current from the high voltage generator 109. When the tube currents supplied by the high voltage generation unit 109 are different, the X-ray tube 101 generates X-rays having a plurality of energy spectra respectively corresponding to the plurality of tube currents. Hereinafter, for the sake of simplicity, it is assumed that there are two types of tube currents, a main scan tube current (hereinafter referred to as a main scan tube current) and a prescan tube current (hereinafter referred to as a prescan tube current). ). The prescan tube current is smaller than the main scan tube current. As a result, the X-ray dose generated by the pre-scan tube current is lower than the dose generated by the main scan tube current. The main scan and the pre-scan will be described in detail later in the input unit 115.

  The collimator 121 is attached to the X-ray emission window on the front surface of the X-ray tube 101. The collimator 121 has a plurality of collimator plates. The plurality of collimator plates shape X-rays emitted from the X-ray focal point into, for example, a cone beam shape (pyramidal shape). Specifically, the plurality of collimator plates are driven by the control unit 110 described later in order to obtain projection data having a preset slice thickness. Furthermore, at least two collimator plates (hereinafter, the two collimator plates are referred to as blade A and blade B, respectively) among the plurality of collimator plates have an opening width corresponding to the fan angle and a center of the opening width (hereinafter referred to as the following). Are called independently under the control of the control unit 110. Specifically, the blade A and the blade B (collectively referred to as fan angle direction collimators) are regions of interest (Region Of Interest: hereinafter) whose origin is the center of the imaging field of view (Field of View: hereinafter referred to as FOV). In order to adjust the aperture width and the aperture center based on the position of ROI) and the view angle, they are independently driven by the control of the control unit 110. The blade A and the blade B are made of lead, for example.

  The FOV is indicated by the dotted line 112 in FIG. The X axis is a straight line perpendicular to the rotation axis Z and upward in the vertical direction. The Y axis is a straight line orthogonal to the X axis and the rotation axis Z. Hereinafter, for the sake of simplicity, it is assumed that the center of the FOV (hereinafter referred to as the FOV center) is on the rotation axis.

  The X-ray detection unit 103 is attached at a position and an angle facing the X-ray tube 101 across the rotation axis Z. The X-ray detection unit 103 has a plurality of X-ray detection elements. Here, it is assumed that a single X-ray detection element constitutes a single channel. The plurality of channels are perpendicular to the rotation axis Z and centered on the focal point of the radiated X-ray, and the arc direction (channel) having a radius from this center to the center of the light receiving portion of the X-ray detection element for one channel. Direction) and the slice direction. The two-dimensional array is configured by arranging a plurality of channels arranged one-dimensionally along the channel direction in a plurality of rows in the slice direction. The X-ray detection unit 103 having such a two-dimensional X-ray detection element array may be configured by arranging a plurality of modules arranged in a one-dimensional shape in a substantially arc direction in a plurality of rows in the slice direction. The X-ray detection unit 103 may be composed of a plurality of modules in which a plurality of X-ray detection elements are arranged in a line. At this time, the modules are arranged one-dimensionally in a substantially arc direction along the channel direction.

  During imaging or scanning, the subject P is placed on the top 120 and inserted into a cylindrical imaging region 111 between the X-ray tube 101 and the X-ray detection unit 103. A projection data collection unit 104 (so-called data collection circuit) called DAS (Data Acquisition System: hereinafter referred to as DAS) is connected to the output of the X-ray detection unit 103.

  The projection data acquisition unit 104 includes an IV converter that converts the current signal of each channel of the X-ray detection unit 103 into a voltage, and periodically integrates the voltage signal in synchronization with the X-ray exposure cycle. An integrator, an amplifier that amplifies the output signal of the integrator, and an analog / digital converter that converts the output signal of the amplifier into a digital signal are attached to each channel. Data (pure raw data) output from the projection data collection unit 104 is transmitted to the preprocessing unit 106 via the non-contact data transmission unit 105 using magnetic transmission / reception or optical transmission / reception. The projection data collection unit 104 changes the integration interval in the integrator according to the scan under the control of the control unit 110 described later.

  The preprocessing unit 106 performs preprocessing on the pure raw data output from the projection data collection unit 104. The preprocessing includes, for example, sensitivity non-uniformity correction processing between channels, X-ray strong absorber, processing for correcting signal signal drop or signal loss due to extreme metal intensity mainly. Data immediately before reconstruction processing output from the pre-processing unit 106 (referred to as raw data or projection data, here referred to as projection data) is data representing a view angle (hereinafter referred to as projection data). And is stored in a projection data storage unit (not shown) provided with a magnetic disk, a magneto-optical disk, or a semiconductor memory. That is, the preprocessing unit 106 generates projection data for each of a plurality of channels in association with view angle data.

  The projection data is a set of data values corresponding to the intensity of X-rays that have passed through the subject. Here, for convenience of explanation, a set of projection data over all channels having the same view angle collected almost simultaneously in one shot is referred to as a projection data set. In addition, the view angle is expressed as an angle in a range of 360 ° with each position of the circular orbit around which the X-ray tube 101 circulates about the rotation axis Z as the top of the circular orbit vertically upward from the rotation axis Z being 0 °. It is a thing. The projection data for each channel of the projection data set is identified by the view angle, cone angle, and channel number.

  The reconstruction unit 114 reconstructs a substantially cylindrical three-dimensional image by the Feldkamp method or the cone beam reconstruction method based on a projection data set whose view angle is within a range of 360 ° or 180 ° + fan angle. It has a function. The reconstruction unit 114 has a function of reconstructing a two-dimensional image (tomographic image) by, for example, a fan beam reconstruction method (also referred to as a fan beam convolution back projection method) or a filtered back projection method. The Feldkamp method is a reconstruction method when a projection ray intersects the reconstruction surface like a cone beam. The Feldkamp method is an approximate image reconstruction method in which convolution is performed by assuming that the cone angle is small, and processing is performed as a fan projection beam, and back projection is processed along a ray at the time of scanning. The cone beam reconstruction method is a reconstruction method that corrects projection data in accordance with the angle of the ray with respect to the reconstruction surface, as a method that suppresses cone angle errors more than the Feldkamp method. The reconstruction unit 114 selects a projection data set used for reconstruction in accordance with the scan under the control of the control unit 110 described later.

  An interface (not shown) connects the X-ray computed tomography apparatus 1 to an electronic communication line (hereinafter referred to as a network). A radiation department information management system (not shown) and a hospital information system (not shown) are connected to the network.

  The display unit 116 displays medical images reconstructed by the reconstruction unit 114, conditions set for X-ray computed tomography, and the like.

  The input unit 115 captures various instructions, commands, information, selections, and settings from the operator into the X-ray computed tomography apparatus 1. The various instructions, commands, information, selections, and settings that are taken in are output to the control unit 110, which will be described later. Although not shown, the input unit 115 includes a trackball, a switch button, a mouse, a keyboard, and the like for setting a region of interest (ROI).

  The input unit 115 generates a region of interest to be described later with respect to the displayed scanogram, which is generated by the scan for determining the scan start position and scan conditions for the subject (hereinafter referred to as scanano scan). A scan range and a scan position of a scan (hereinafter referred to as ROI setting scan) and a main scan are input. The ROI setting scan is a scan for generating a reconstructed image (hereinafter referred to as a region-of-interest setting image) for setting a blood vessel or ROI for detecting a contrast agent. The main scan is a scan executed at the timing when the contrast agent administered to the subject flows into the slice of interest. In the ROI setting scan and the main scan, the X-ray irradiation range corresponds to the entire FOV.

  The input unit 115 inputs the ROI by the operator to the region-of-interest setting image obtained by the ROI setting scan. The input ROI is used for pre-scanning. The pre-scan is a scan that is performed on the input ROI after the ROI setting scan and at a dose lower than the dose in the main scan before the main scan. In the pre-scan, the collimator 121 is controlled by the control unit 110 described later so that X-rays are irradiated with the ROI as an X-ray irradiation range over all view angles. Control of the collimator 121 and the like in the pre-scan will be described in the control unit 110 described later. FIG. 2 is a diagram illustrating an example of the FOV in the ROI setting scan and the main scan and the ROI that is the X-ray irradiation range in the pre-scan. In the pre-scan, X-rays are emitted only for the ROI that is a part of the FOV over all view angles. The X-ray irradiation range in the pre-scan is narrower than the X-ray irradiation range in the ROI setting scan and the main scan.

  The input unit 115 inputs a threshold value related to the start of the main scan (hereinafter referred to as a scan start threshold value) in a so-called real prep scan that automates the start timing of the main scan. Note that the input unit 115 may input an operation related to the start of the main scan in order to generate a trigger signal related to the start of the main scan.

  The input unit 115 detects the coordinates of the cursor displayed on the display screen, and outputs the detected coordinates to the control unit 110. Note that the input unit 115 may be a touch panel provided so as to cover the display screen. In this case, the input unit 115 detects the coordinates instructed by the touch reading principle such as an electromagnetic induction type, an electromagnetic distortion type, and a pressure sensitive type, and outputs the detected coordinates to the control unit 110.

  The control unit 110 functions as the center of the X-ray computed tomography apparatus 1. The control unit 110 includes a CPU and a memory (not shown). The control unit 110 controls the high voltage generation unit 109, the gantry 100, and the like for X-ray computed tomography based on examination schedule data and a control program stored in a memory (not shown). Specifically, the control unit 110 temporarily displays an operator's instruction and the like sent from an input unit 115 described later, a radiation department information management system (not shown), a hospital information system (not shown), and the like. Store in memory not shown. The control unit 110 controls the high voltage generation unit 109, the gantry 100, and the like based on these pieces of information temporarily stored in the memory. The control unit 110 reads a control program for executing predetermined image generation / display and the like from a storage unit (not shown), develops it on its own memory, and executes calculations / processings and the like regarding various processes.

  The control unit 110 performs high-voltage generation unit 109, in order to execute imaging for determining the scan start position and imaging conditions for the subject (hereinafter referred to as scano imaging) for the subject. The collimator 121, the projection data collection unit 104, and the reconstruction unit 114 are controlled. The control unit 110 controls the display unit 116 in order to display a scanogram for the subject generated by scanography.

  The control unit 110 controls the high voltage generation unit 109, the collimator 121, the projection data collection unit 104, and the reconstruction unit 114 in order to execute the ROI setting scan. The tube current in the ROI setting scan is the same as the main scan tube current. The controller 110 controls the collimator 121 to emit X-rays over the entire FOV in the ROI setting scan. In the ROI setting scan, X-rays can be irradiated over the entire FOV.

  The control unit 110 controls the high voltage generation unit 109, the collimator 121, the projection data collection unit 104, and the reconstruction unit 114 in order to execute prescan. Specifically, the control unit 110 controls the high voltage generation unit 109 in the pre-scan in order to generate X-rays corresponding to a lower dose than the ROI setting scan and the main scan. The tube current supplied from the high voltage generation unit 109 in the pre-scan is smaller than the tube current supplied from the high voltage generation unit 109 in the ROI setting scan and the main scan. The control unit 110 controls the collimator 121 in order to irradiate X-rays limited to the ROI over all view angles in the pre-scan. More specifically, the control unit 110 controls the fan angle collimator based on the relative position of the ROI with respect to the FOV center and the view angle. The fan angle collimator drives each of the two collimator plates independently under the control of the control unit 110. By driving each of the two collimator plates independently, the opening width and the opening center are adjusted in accordance with the relative position of the ROI with respect to the FOV center and the view angle.

  FIG. 3 is a diagram showing an example of a schematic diagram schematically showing the relative positions of the X-ray tube 101 and the collimator 121 with respect to the ROI in the pre-scan for a plurality of view angles. FIG. 3 a shows that the X-ray tube 101 and the collimator 121 are positioned at a view angle of 0 °. In FIG. 3, a ′ represents an X-ray irradiation range corresponding to the position a in FIG. 3. FIG. 3 b shows that the X-ray tube 101 and the collimator 121 are positioned at a 90 ° view angle. B 'in FIG. 3 indicates an X-ray irradiation range corresponding to the position b in FIG. FIG. 3c shows that the X-ray tube 101 and the collimator 121 are positioned at a view angle of 180 °. C ′ in FIG. 3 indicates an X-ray irradiation range corresponding to the position c in FIG. FIG. 3d shows that the X-ray tube 101 and the collimator 121 are located at a viewing angle of 270 °. D ′ in FIG. 3 indicates an X-ray irradiation range corresponding to the position d in FIG.

  FIG. 4 is a diagram illustrating an example in which generated X-rays are collimated in order to perform limited X-ray irradiation on the ROI at a 0 ° view angle in the pre-scan. As shown in FIG. 4, at a 0 ° view angle, the projection data set used for image reconstruction is not a set of projection data for a plurality of channels on which X-rays transmitted through the ROI are incident, but X-ray detection. A set of projection data covering the entire area of the unit 103.

  In the following paragraphs, the control of the collimator 121 by the control unit 110 will be described in detail. FIG. 5 is a schematic diagram showing an example of an outline of the center position of the ROI and the position of the X-ray tube at the view angle ω in the coordinate system with the FOV center as the origin (hereinafter referred to as the FOV coordinate system). The position of the X-ray tube with respect to the FOV center is specified by the distance from the FOV center (Rc + Rx) and the angle from the X axis (view angle). The ROI is specified by the center coordinates (x, y) of the ROI with the FOV center as the origin and the radius r of the ROI. The position of the fan angle direction collimator in the collimator 121 is specified by the distance Rc from the FOV center and the angle (view angle) from the X axis. Note that Rx is the distance between the X-ray focal point and the fan angle collimator. Hereinafter, for description, the two collimator plates in the fan angle collimator are referred to as blade A and blade B, respectively. The opening width and the opening center are determined by the relative positions of the blade A and the blade B with respect to the FOV center.

Specifically, the control unit 110 determines the radius r of the ROI, the distance Rc from the FOV center to the fan angle collimator, the distance Rx from the X-ray focal point to the fan angle collimator, the view angle ω, and the center of the ROI. Based on the coordinates (x, y), the positions of blade A and blade B in the FOV coordinate system are determined. The control unit 110 determines the position (P _A ) of the blade A by the function A having r, Rc, Rx, ω, x, and y as input values. The control unit 110 determines the position (P _B ) of the blade B by a function B having r, Rc, Rx, ω, x, and y as input values. The control unit 110 stores the functions A and B in a memory (not shown).

  Note that the control unit 110 stores a correspondence table A of the position of the blade A with respect to r, Rc, Rx, ω, x, y instead of the function A, and r, Rc, Rx instead of the function B in a memory (not shown). , Ω, x, y may be stored a correspondence table B of the position of the blade B. In any case, the control unit 110 determines the positions of the blade A and the blade B based on r, Rc, Rx, ω, x, and y. The control unit 110 controls the fan angle collimator to move the blade A and the blade B to the determined positions according to the view angle ω. As a result, in the pre-scan, it is possible to irradiate X-rays limitedly to the ROI or blood vessel that is a part of the FOV.

  The control unit 110 calculates an average CT value (or pixel value) in the ROI (hereinafter referred to as an average value in the ROI) in each of the images reconstructed repeatedly in the pre-scan. The control unit 110 compares the calculated in-ROI average value with the scan start threshold value input via the input unit 115. The control unit 110 stops the pre-scanning when the ROI average value exceeds the scan start threshold (hereinafter referred to as the excess time). The control unit 110 includes a collimator 121, a high voltage generation unit 109, a projection data collection unit 104, and a reconstruction unit in order to execute a main scan that irradiates X-rays over the entire FOV after a predetermined time has elapsed since the excess point. 114.

(ROI limited X-ray irradiation function)
The ROI-limited X-ray irradiation function is a function for limitedly irradiating X-rays to a preset ROI in the pre-scan in the real prep scan. Hereinafter, processing according to the ROI limited X-ray irradiation function in real prep scanning (hereinafter referred to as ROI limited X-ray irradiation processing) will be described.

  FIG. 6 shows a process for executing a main scan when a pre-scan in a real prep scan is performed by irradiating a predetermined ROI with X-rays limitedly and the average CT value in the ROI reaches the scan start threshold. It is a flowchart which shows an example of the procedure of.

  Scano imaging is performed on the subject. Based on the image generated by the scano imaging, the start position of the ROI setting scan and the main scan, the imaging conditions, and the like are input via the input unit 115. At the input position, the ROI setting scan is executed (step Sa1). A blood vessel or ROI for detecting a contrast agent and a scan start threshold are set for the image (region of interest setting image) reconstructed by the ROI setting scan (step Sa2). In response to the injection of the contrast medium into the subject, the pre-scan in the real prep scan is started (step Sa3).

  Based on the set ROI and view angle, the collimator 121, the X-ray tube 101, the projection data generation unit 104, and the reconstruction unit 114 are controlled so that X-rays are emitted only in the ROI. A plurality of projection data sets respectively corresponding to a plurality of view angles used in the configuration are collected. By using the collected plurality of projection data sets, the image being pre-scanned is reconstructed (step Sa4). In the image reconstructed during the pre-scan, the average value within ROI is calculated (step Sa5). Until the average value within ROI exceeds the scan start threshold value, the processes of step Sa5 and step Sa6 are repeated (step Sa6). When the ROI average value exceeds the scan start threshold value, the main scan is executed after a predetermined time has elapsed from the point of excess (step Sa7).

According to the configuration described above, the following effects can be obtained.
According to the X-ray computed tomography apparatus 1 in the present embodiment, the collimator 121 can be controlled in order to irradiate X-rays limited to a preset ROI in the pre-scan in the real prep scan. In addition, the collimator 121 in the X-ray computed tomography apparatus 1 has a fan angle direction collimator for irradiating X-rays limited to the ROI. The fan angle direction collimator has a plurality of blades that are independently driven under the control of the control unit 110 with respect to the opening width corresponding to the fan angle and the opening center. By controlling these, X-ray irradiation can be performed only in the ROI of the subject in the pre-scan, and the exposure dose to the non-ROI (non-region of interest) of the subject can be reduced.

(Second Embodiment)
The difference from the first embodiment is that the projection data by the pre-scan is corrected based on the image (region of interest setting image) reconstructed by the ROI setting scan.

  FIG. 7 is a configuration diagram showing the configuration of the X-ray computed tomography apparatus according to the second embodiment.

  The projection data collection unit 117 collects ROI setting scan projection data related to the ROI setting scan. The projection data collection unit 117 collects prescan projection data related to prescan.

The reconstruction unit 114 reconstructs a region-of-interest setting image (hereinafter referred to as an ROI setting image) based on the ROI setting scan projection data. Note that the reconstruction unit 114 may reconstruct an image related to prescan based on the prescan projection data. The projection data correction unit 117 performs two different corrections. As the first correction, the projection data correction unit 117 corrects the pre-scan projection data based on the view angle and the position of the fan angle direction collimator. More specifically, the projection data correction unit 117 applies X-rays irradiated to the subject with respect to pre-scan projection data corresponding to outputs from each of the plurality of X-ray detection elements regarding the X-ray irradiation range in the pre-scan. Correction for making the dose (hereinafter referred to as incident dose) the same is executed. Specifically, the projection data correction unit 117 provides a correspondence table of correction values for a function (hereinafter referred to as a correction function) for making the incident dose the same, or for the view angle and the position of the fan angle direction collimator. Store in a memory (not shown). The input values of the correction function are the position of the blade A (P _A ) and the position of the blade B (P _B ).

The projection data correction unit 117 uses the correction function determined by the position of the blade A (P _A ) and the position of the blade B (P _B ) or the correction value determined by the correspondence table, and outputs the pre-scan projection data for each channel. to correct. FIG. 8 is a diagram illustrating an example of an X-ray irradiation range related to pre-scan projection data in which the first correction is performed in the pre-scan. FIG. 9 is a diagram illustrating an example of the concept relating to the first correction with respect to FIG. 8. In FIG. 9, when the ROI is an object having a uniform X-ray attenuation coefficient, such as air, and X-rays are irradiated only to the ROI, the X-ray intensity related to the ROI differs for each channel (for example, I ₀ ′ ≠ I ₀ ). This phenomenon is caused by the fact that the center of ROI does not coincide with the center of FOV. The correction value for correcting this deviation is calculated by a correction function in which the position of blade _A (P _A ) and the position of blade B (P _B ) are substituted. The prescan projection data on which the first correction has been executed is output to the preprocessing unit 106.

The correction value calculated by the projection data correction unit 117 may be a correction value determined by the correspondence table based on the position of the blade A (P _A ) and the position of the blade B (P _B ).

  The pre-processing unit 106 generates projection data obtained by pre-processing the pre-scan projection data on which the first correction has been executed (hereinafter referred to as pre-processed projection data).

  As a second correction, the projection data correction unit 117 performs pre-processed projection data (pre-processed pre-processed data) for each ray and view angle connecting the X-ray focal point and a plurality of channels based on the ROI setting image. Beam hardening correction is performed on the scan projection data. Specifically, the projection data correction unit 117 specifies a region corresponding to the bone in the ROI setting image based on a predetermined CT value. The projection data correction unit 117 estimates the degree of the quality hardening in the pre-processed projection data for each view angle and ray using the CT value of the specified region. For example, the projection data correction unit 117 stores a correspondence table of the correction amount of the quality hardening with respect to the X-ray transmission length. The projection data correction unit 117 determines the X-ray transmission length of the specified area based on the CT value, view angle, and ray of the specified area. The projection data correction unit 117 determines the correction amount for the quality hardening based on the determined X-ray transmission length and the correspondence table. The projection data correction unit 117 corrects the pre-processed projection data based on the determined correction amount of the quality hardening. The projection data correction unit 117 outputs the pre-processed projection data corrected for the line quality hardening to the reconstruction unit 114.

  Note that the projection data correction unit 117 may further specify a region corresponding to water based on a predetermined CT value range in the ROI setting image. At this time, the projection data correction unit 117 performs beam hardening correction (hereinafter referred to as attenuation correction) on the basis of the post-processed projection data. That is, the projection data correction unit 117 corrects the pre-processed projection data in order to recover the attenuation of X-rays that have passed through the region corresponding to water.

  FIG. 11 is a diagram illustrating an example of the CT value distribution of the ROI setting image. The shading of each cell in FIG. 11 indicates the magnitude of the CT value. That is, a plurality of dark-colored pixels (3 rows 10 columns, 4 rows 9 columns, 4 rows 10 columns, 4 rows 11 columns, 5 rows 8 columns, 5 rows 9 columns, 5 rows 10 columns, 5 rows 11 in FIG. Column, 5 rows 12 columns, 6 rows 8 columns, 6 rows 9 columns, 6 rows 10 columns, 6 rows 11 columns, 6 rows 12 columns, 7 rows 8 columns, 7 rows 9 columns, 7 rows 10 columns, 7 rows 11 Column, 7 rows 12 columns, 8 rows 9 columns, 8 rows 10 columns, 8 rows 11 columns, 9 rows 10 columns, 10 rows 13 columns, 10 rows 14 columns, 10 rows 15 columns, 11 rows 13 columns, 11 rows 14 Column, 11 rows and 15 columns, 12 rows and 13 columns, 12 rows and 14 columns, and 12 rows and 15 columns) indicate CT values larger than other pixels. The projection data correction unit 117 identifies the region corresponding to the bone in FIG. In FIG. 11, the region corresponding to the bone is the 32 pixels. Note that the projection data correction unit 117 may specify a region corresponding to water in FIG. In FIG. 11, the region corresponding to water is a pixel excluding the 32 pixels and pixels included in the ROI. In FIG. 11, white pixels correspond to air.

  FIG. 12 shows a region to which beam hardening correction is applied and a region to which attenuation correction is applied in the ROI setting image in FIG. In FIG. 12, attenuation correction and beam hardening correction are performed on the pre-processed projection data related to the X-ray irradiation range in the pre-scan. In addition, in FIG. 12, the ray from the direction of 45 ° diagonally to the ROI (the direction from the 1st row 19th column to the 19th row 1st column) does not pass through the pixels in the dark hue region in FIG. The effect of ning is reduced. The cross section in the ROI setting scan and the cross section in the pre-scan are the same. The FOV in the ROI setting scan and the FOV in the pre-scan are the same. The difference between the ROI setting scan and the pre-scan is that the X-ray irradiation range is different.

  The reconstruction unit 114 reconstructs an image (hereinafter referred to as a pre-scan image) based on the pre-processed projection data on which the second correction has been performed.

  The threshold correction unit 118 is based on the average value of CT values in the ROI in the ROI setting image by the ROI setting scan (hereinafter referred to as the first average value) and the average value of the CT values in the ROI in the pre-scan image. The scan start threshold is corrected. Specifically, the threshold correction unit 118 calculates an average value (first average value) of CT values (or pixel values) in the ROI of the ROI setting image. When the prescan is a continuous scan, the threshold correction unit 118 calculates an average value (hereinafter referred to as a second average value) of CT values in the ROI of the first prescan image. Note that the threshold correction unit 118 calculates the second average value in the ROI of the first image even when the pre-scan is an intermittent scan. Note that the threshold correction unit 118 may calculate the second average value using an image related to prescan instead of the prescan image.

  The threshold correction unit 118 calculates the absolute value of the difference between the first average value and the second average value. The threshold correction unit 118 compares the absolute value with a predetermined standard value stored in a memory (not shown). If the absolute value exceeds a predetermined standard value, the threshold correction unit 118 determines a value obtained by subtracting the absolute value from the scan start threshold as a new scan start threshold. The new scan start threshold value is output to the control unit 110 as a threshold value for starting the main scan.

  The image generation unit 113 generates a superimposed image in which the non-ROI portion of the ROI setting image is superimposed on the non-ROI portion of the pre-scan image. Specifically, the image generation unit 113 reconstructs the ROI pixel value of the superimposed image every time the prescan image is reconstructed until the prescan is stopped after the superimposed image is generated. The pixel value in the ROI of the pre-scan image is updated.

  The display unit 116 displays the superimposed image and the updated superimposed image. FIG. 13 is a diagram illustrating an example of a superimposed image displayed during pre-scanning according to the second embodiment.

(Projection data correction function)
The projection data correction function corrects the pre-scan projection data based on the view angle and the position of the fan angle collimator and the correction function (or correspondence table) (first correction), and after pre-processing based on the ROI setting image This function performs the second correction on the projection data. Hereinafter, processing according to the projection data correction function (hereinafter referred to as projection data correction processing) will be described.

FIG. 14 is a flowchart illustrating an example of a procedure for executing the projection data correction process.
Scano imaging is performed on the subject. Based on an image generated by scano imaging, the ROI setting scan and main scan start positions, imaging conditions, and the like are input via the input unit 115. The ROI setting image is reconstructed by the ROI setting scan executed at the input position (step Sb1). The ROI is set for the ROI setting image via the input unit 115 (step Sb2). Based on the ROI setting image, the correction amount of the quality hardening corresponding to the view angle and the ray is determined (step Sb3).

  The pre-scan is started when the contrast medium is injected into the subject (step Sb4). Pre-scan projection data is generated by the projection data collection unit 104 (DAS) (step Sb5). Based on the view angle and the position of the fan angle collimator and the correction function (or correspondence table), the first correction is executed on the pre-scan projection data (step Sb6). Pre-processing is performed on the pre-scan projection data subjected to the first correction, and post-processed projection data is generated (step Sb7). The second correction is executed based on the pre-processed post-projection data and the ray quality hardening correction amount having the same view angle and ray (step Sb8). A pre-scan image is generated based on the pre-processed projection data on which the second correction has been executed (step Sb9).

(Threshold correction function)
The threshold correction function is a function that corrects the scan start threshold based on the absolute value when the absolute value of the difference value between the first average value and the second average value exceeds a predetermined standard value. Specifically, the threshold correction function is a function that determines a value obtained by subtracting the absolute value from the scan start threshold as a new scan start threshold. Hereinafter, processing according to the threshold correction function (hereinafter referred to as threshold correction processing) will be described.

FIG. 15 is a flowchart illustrating an example of a procedure for executing threshold correction processing.
The ROI setting image is reconstructed by the ROI setting scan (step Sc1). Via the input unit 115, the ROI setting image and the scan start threshold for the start of the main scan are set for the ROI setting image (step Sc2). A first average value for the ROI setting image is calculated (step Sc3). The pre-scan is started with the injection of the contrast medium into the subject (step Sc4). A pre-scan image is reconstructed based on the pre-scan projection data for which the second correction has been performed (step Sc5). A pre-scan image is reconstructed based on the pre-scan projection data on which the first and second corrections have been performed (step Sc5).

  Based on the first pre-scan image, a second average value is calculated (step Sc6). The absolute value of the difference between the first average value and the second average value is calculated (step Sc7). When the absolute value exceeds a predetermined standard value (step Sc8), the scan start threshold is corrected using the absolute value (step Sc9). When the average value within ROI in the pre-scan image has reached the corrected scan start threshold, the main scan is executed (step Sc10). When the absolute value is equal to or smaller than the predetermined standard value (step Sc8), the main scan is executed when the average value within the ROI in the pre-scan image has reached the scan start threshold (step Sc11).

  FIG. 16 shows the change in the CT value during pre-scan in relation to correcting the scan start threshold based on the first average value for the ROI setting image, the second average value for the first pre-scan image, and the predetermined standard value. It is a figure which shows an example.

  In FIG. 16, when the imaging a of the ROI setting scan is executed, the first average value b is calculated. Next, a scan start threshold c is set at time d. A pre-scan is started at a time point e after a fixed time has elapsed since the injection of the contrast medium into the subject. A second average value f is calculated based on the first prescan image. The absolute value | b−f | of the value obtained by subtracting the second average value f from the first average value b is calculated. The absolute value | b−f | is the reconstruction of the first pre-scan image based on the pre-scan projection data in which X-ray irradiation is performed only in the ROI in the pre-scan and the first and second corrections are performed. caused by. Subsequently, the scan start threshold c is corrected by subtracting the absolute value | b−f | from the scan start threshold c (g). At the time point (h) when the ROI average value in the pre-scan image exceeds the corrected scan start threshold value, the pre-scan is stopped. After the elapse of a predetermined time I from the excess time point h, a main scan for irradiating X-rays over the entire FOV is started (J).

(Image update function within ROI)
The in-ROI image update function is a function for updating the superimposed image by replacing the pixel value in the ROI of the superimposed image with the pixel value in the ROI in the pre-scan image when the pre-scan image is reconstructed. is there. Hereinafter, processing according to the ROI image update function (hereinafter referred to as ROI image update processing) will be described.

FIG. 17 is a flowchart illustrating an example of a procedure for executing the in-ROI image update process.
When the subject is placed on the top 120, scanography is started. A scan range and a scan position in the main scan and the ROI setting scan are set. The ROI setting scan is executed for the FOV (step Sd1). The ROI is set in the ROI setting image reconstructed by the ROI setting scan (step Sd2). The ROI setting image is displayed on the display unit 116.

  Pre-scanning is started (step Sd3). A pre-scan image is reconstructed based on the pre-scan projection data subjected to the first and second corrections (step Sd4). A superimposed image is generated by superimposing the non-ROI image of the ROI setting image on the non-ROI of the pre-scan image (step Sd5). The generated superimposed image is displayed on the monitor of the display unit 116 (step Sd6).

  The superimposed image in FIG. 18 is displayed during the prescan. The pixel value in the ROI indicated by the dotted line in the superimposed image in FIG. 18 is the pixel value in the ROI of the prescan image reconstructed by the prescan.

  If the prescan is not completed (step Sd7), a prescan image corresponding to the next prescan is reconstructed (step Sd8). The image in the ROI of the superimposed image is updated to the image in the ROI in the prescan image corresponding to the next prescan (step Sd9). Until the pre-scan is finished, the processing from step Sd6 to step Sd9 is repeated.

According to the configuration described above, the following effects can be obtained.
According to the X-ray computed tomography apparatus 1 in the present embodiment, the collimator 121 can be controlled in order to irradiate X-rays limited to a preset ROI in the pre-scan in the real prep scan. Thereby, it is possible to reduce the exposure dose to the subject in the pre-scan. In addition, according to the X-ray computed tomography apparatus 1, correction (first correction) for making the incident dose the same can be performed on the prescan projection data collected in the prescan. . With the first correction, any reconstruction method such as the parallel beam reconstruction method and the fan beam reconstruction method can be used even when the reconstruction center is not the center of the FOV. Further, according to the present X-ray computed tomography apparatus 1, in order to remove the contribution of the X-ray attenuation coefficient in the non-ROI contributing to the pre-scan projection data, the pre-scan projection data is corrected (attenuation correction and Beam hardening correction: second correction). Furthermore, according to the X-ray computed tomography apparatus 1, image reconstruction can be executed based on pre-scan projection data in which the first and second corrections have been executed over all channels and all view angles. . As a result, in any of the reconstruction methods such as the parallel beam reconstruction method and the fan beam reconstruction method, it is possible to generate a prescan image with improved image quality in the ROI in the prescan.

  According to the X-ray computed tomography apparatus 1 in the present embodiment, the scan start threshold is corrected based on the first average value regarding the ROI setting image, the second average value regarding the pre-scan image, and the predetermined standard value. be able to. Thereby, when there is no influence of the contrast agent, when the difference between the ROI average value of the pre-scan image and the ROI average value of the ROI setting image is equal to or larger than a predetermined standard value, a preset scan start threshold value is set. It can be corrected. Thus, even when the in-ROI average value of the pre-scan image reconstructed in the pre-scan is different from the in-ROI average value of the ROI setting image, it is possible to maintain a predetermined accuracy with respect to the scan start timing.

  According to the X-ray computed tomography apparatus 1 in the present embodiment, when the prescan image is reconstructed during the prescan, the pixel value in the non-ROI in the prescan image is changed to the pixel value in the non-ROI of the ROI setting image. A superimposed image in which pixel values are superimposed can be generated. In other words, the ROI in the superimposed image is an image of the ROI in the pre-scan image. Further, the non-ROI in the superimposed image is a non-ROI image in the ROI setting image. In addition, during the prescan, the ROI in the superimposed image is updated for each prescan to the ROI image in the prescan image corresponding to the prescan. Thereby, it is possible to maintain a predetermined image quality while reducing the exposure of the subject to the non-ROI, and provide a contrast agent monitoring image (superimposed image) to the operator.

  Note that each function according to each embodiment can also be realized by installing a program for executing the processing in a computer such as a workstation and developing the program on a memory. At this time, a program capable of causing the computer to execute the method is stored in a storage medium such as a magnetic disk (floppy (registered trademark) disk, hard disk, etc.), an optical disk (CD-ROM, DVD, etc.), or a semiconductor memory. It can also be distributed.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... X-ray computed tomography apparatus, 100 ... Gantry, 101 ... X-ray tube, 102 ... Rotating ring, 103 ... X-ray detection part, 104 ... Projection data collection part (data collection circuit: DAS), 105 ... Non-contact data Transmission unit 106... Pre-processing unit 107. Stand drive unit 108. Slip ring 109 109 High voltage generation unit 110 Control unit 111 Imaging region 112 Imaging field of view (FOV) 113 Image generation unit , 114 ... reconstruction unit, 115 ... input unit, 116 ... display unit, 117 ... projection data correction unit, 118 ... threshold correction unit, 120 ... top plate, 121 ... collimator

Claims (8)

An X-ray tube that generates X-rays;
A high voltage generator for generating a high voltage for applying a high voltage to the X-ray tube;
An X-ray detector that detects X-rays generated from the X-ray tube and transmitted through the subject;
A projection data collection unit for collecting projection data via the X-ray detection unit;
A reconstruction unit for reconstructing an image based on the projection data;
A collimator provided on the front surface of the X-ray tube and having an opening width and an opening center variable;
The collimator, the high voltage generation unit, the projection data collection unit, and the like are used to execute a main scan for the subject and a pre-scan that is repeated before the main scan to determine a start time of the main scan. A control unit for controlling the reconfiguration unit,
The X-ray irradiation range in the main scan is the entire imaging visual field,
The X-ray irradiation range in the pre-scan is a part of the imaging field;
X-ray computed tomography apparatus. A portion of the imaging field is a region of interest,
The projection data collection unit collects prescan projection data related to the prescan,
The reconstruction unit reconstructs an image related to the prescan based on the prescan projection data,
The control unit includes the collimator and the collimator to execute the main scan with the time when a predetermined time has elapsed from the time when the average pixel value in the region of interest of the image related to the pre-scan reaches a predetermined threshold as the start time. Controlling the high voltage generation unit, the projection data collection unit, and the reconstruction unit;
The X-ray computed tomography apparatus according to claim 1. The projection data collection unit collects region-of-interest setting scan projection data related to a region-of-interest setting scan for obtaining a region-of-interest setting image in which the region of interest is set,
The reconstruction unit reconstructs the region-of-interest setting image based on the region-of-interest setting scan projection data,
The control unit controls the collimator, the high voltage generation unit, the projection data collection unit, and the reconstruction unit to execute the region-of-interest setting scan before the pre-scan;
The X-ray computed tomography apparatus according to claim 2. A threshold correction unit that corrects the predetermined threshold based on an average pixel value in the region of interest in the region-of-interest setting image and an average pixel value in the region of interest of the pre-scan image;
The X-ray computed tomography apparatus according to claim 3. A projection data correction unit for correcting the pre-scan projection data based on the region-of-interest setting image;
The reconstructing unit reconstructs a prescan image based on the corrected prescan projection data;
The X-ray computed tomography apparatus according to claim 3. The projection data correction unit
Further correcting the pre-scan projection data based on a relative position and a view angle of the region of interest with respect to the center of the imaging field;
The X-ray computed tomography apparatus according to claim 5. An image generator that generates a superimposed image in which the image of the non-interesting region in the region-of-interest setting image is superimposed on the non-interesting region of the pre-scan image;
A display unit for displaying the superimposed image;
The X-ray computed tomography apparatus according to claim 3. On the computer,
A control function for controlling a collimator in order to execute a main scan for a subject and a pre-scan repeated before the main scan in order to determine a start time of the main scan;
The X-ray irradiation range in the main scan is the entire imaging visual field,
The X-ray irradiation range in the pre-scan is a part of the imaging field;
A scan control program characterized by.

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