X-ray CT apparatus and method of controlling the same

Abstract

To provide an X-ray CT apparatus capable of accurately aligning the body thickness center of a subject with the rotation center of a scanner, and to provide a control method thereof.SOLUTION: The X-ray CT apparatus includes: a bed on which a subject is placed; an X-ray source that irradiates the subject with X-rays; an X-ray detector that detects X-rays transmitted through the subject; a scanner that rotates the X-ray source and the X-ray detector around the subject; and an image generation unit that generates a tomographic image using projection images at a plurality of projection angles acquired by the scanner; a height direction error calculation unit that calculates a height direction error between the body thickness center of the subject and a rotation center of the scanner for each position in a body axis direction of the subject based on a scan plan image acquired in advance; and a bed control unit that controls the height of the bed based on the height direction error from the start to the end of the scan.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an X-ray CT (Computed Tomography) apparatus that images tomographic images of a subject.

Background Art

[0002] An X-ray CT apparatus is an apparatus that generates a tomographic image of a subject using projection images at a plurality of projection angles obtained by a scanner that rotates an X-ray source that irradiates X-rays and an X-ray detector that detects X-rays around the subject. The generated tomographic image is used for image diagnosis of the subject as a medical image. Since the tomographic image generated by the X-ray CT apparatus has high spatial resolution at the rotation center of the scanner, it is desirable that the region of interest during image diagnosis be aligned with the rotation center.

[0003] Patent Document 1 discloses controlling a stretcher on which a subject is placed so that a region of interest designated by an operator on a pre-captured image is located at the rotation center of the scanner from the start to the end of the scan.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in Patent Document 1, there is insufficient consideration for the body thickness of the subject. Since the region of interest designated by the operator does not necessarily lie at the center of the body thickness of the subject, the image quality of the generated tomographic image deteriorates and the unnecessary exposure of the subject increases.

[0006] Therefore, an object of the present invention is to provide an X-ray CT apparatus and a control method thereof capable of accurately aligning the center of the body thickness of a subject with the rotation center of a scanner.

Means for Solving the Problem

[0007] In order to achieve the above object, the present invention provides an X-ray CT apparatus including a bed on which a subject is placed, an X-ray source that irradiates the subject with X-rays, an X-ray detector that detects X-rays transmitted through the subject, a scanner that rotates the X-ray source and the X-ray detector around the subject, and an image generation unit that generates a tomographic image using projection images at a plurality of projection angles acquired by the scanner. The X-ray CT apparatus further includes a height direction error calculation unit that calculates a height direction error between the body thickness center of the subject and the rotation center of the scanner for each position in the body axis direction of the subject based on a scan planning image acquired in advance, and a bed control unit that controls the height of the bed based on the height direction error from the start to the end of the scan.

[0008] The present invention also provides a control method for an X-ray CT apparatus including a bed on which a subject is placed, an X-ray source that irradiates the subject with X-rays, an X-ray detector that detects X-rays transmitted through the subject, a scanner that rotates the X-ray source and the X-ray detector around the subject, and an image generation unit that generates a tomographic image using projection images at a plurality of projection angles acquired by the scanner. The control method further includes a height direction error calculation step of calculating a height direction error between the body thickness center of the subject and the rotation center of the scanner for each position in the body axis direction of the subject based on a scan planning image acquired in advance, and a bed control step of controlling the height of the bed based on the height direction error from the start to the end of the scan.

Advantages of the Invention

[0009] According to the present invention, it is possible to provide an X-ray CT apparatus and a control method thereof capable of accurately aligning the body thickness center of a subject with the rotation center of a scanner.

Brief Description of the Drawings

[0010]

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Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the X-ray CT apparatus according to the present invention will be described with reference to the accompanying drawings. The X-ray CT apparatus is an apparatus that generates a tomographic image of a subject using projection images at a plurality of projection angles obtained by repeatedly irradiating the subject with X-rays and detecting the X-rays transmitted through the subject.

Examples

[0012] The overall configuration of the X-ray CT apparatus according to Example 1 will be described with reference to FIG. 1. The X-ray CT apparatus includes a scanner 100 and an operation unit 120. The scanner 100 is installed in an imaging room surrounded by a shielding material that shields X-rays, and the operation unit 120 is installed in an operation room outside the imaging room. The body axis direction of the subject 10 is defined as the Z-axis, the horizontal direction orthogonal to the Z-axis is defined as the X-axis, and the vertical direction orthogonal to the Z-axis is defined as the Y-axis.

[0013] The scanner 100 includes an X-ray source 101, a rotating plate 102, a collimator 103, an X-ray detector 106, a data collection unit 107, a bed 105, a rotating plate control unit 108, a bed control unit 109, an X-ray control unit 110, and a high voltage generation unit 111. The X-ray source 101 is a device that irradiates the subject 10 placed on the bed 105 with X-rays, for example, an X-ray tube device. The collimator 103 is a device that limits the irradiation range of X-rays. The rotating plate 102 includes an opening 104 into which the subject 10 placed on the bed 105 enters, and mounts the X-ray source 101 and the X-ray detector 106, and rotates the X-ray source 101 and the X-ray detector 106 around the subject 10.

[0014] The X-ray detector 106 is disposed opposite to the X-ray source 101, includes a plurality of detection elements that detect X-rays transmitted through the subject 10, and is a device that detects the spatial distribution of X-rays. The detection elements of the X-ray detector 106 are arranged two-dimensionally in the rotation direction and the rotation axis direction of the rotating plate 102. The data collection unit 107 is a device that collects the spatial distribution of X-rays detected by the X-ray detector 106 as digital data.

[0015] The rotating plate control unit 108 is a device that controls the rotation and inclination of the rotating plate 102. The bed control unit 109 is a device that controls the up / down, front / back, left / right movement of the bed 105. The high voltage generation unit 111 is a device that generates a high voltage applied to the X-ray source 101. The X-ray control unit 110 is a device that controls the output of the high voltage generation unit 111. The rotating plate control unit 108, the bed control unit 109, and the X-ray control unit 110 are, for example, an MPU (Micro-Processing Unit) or the like.

[0016] The operation unit 120 includes an input unit 121, an image generation unit 122, a display unit 125, a storage unit 123, and a system control unit 124. The input unit 121 is a device used for inputting inspection data such as the name of the subject 10, the inspection date and time, and imaging conditions, and is, for example, a keyboard, a pointing device, a touch panel, or the like. The image generation unit 122 is a device that generates tomographic images using the digital data collected by the data collection unit 107 or generates multi-slice images using a plurality of tomographic images, and is, for example, an MPU, a GPU (Graphics Processing Unit), or the like. The display unit 125 is a device that displays the tomographic images and the like generated by the image generation unit 122, and is, for example, a liquid crystal display, a touch panel, or the like. The storage unit 123 is a device that stores the digital data collected by the data collection unit 107, the tomographic images generated by the image generation unit 122, the programs executed by the system control unit 124, the data used by the programs, and the like, and is, for example, an HDD (Hard Disk Drive), an SSD (Solid State Drive), or the like. The system control unit 124 is a device that controls each unit such as the rotating plate control unit 108, the bed control unit 109, and the X-ray control unit 110, and is, for example, a CPU (Central Processing Unit).

[0017] Based on the imaging conditions set via the input unit 121, the high voltage generator 111 generates the tube voltage, which is the high voltage applied to the X-ray source 101, so that X-rays corresponding to the imaging conditions are irradiated from the X-ray source 101 to the subject 10. The X-ray detector 106 detects the X-rays irradiated from the X-ray source 101 and transmitted through the subject 10 with a large number of detection elements, and acquires the spatial distribution of the transmitted X-rays. The rotating plate 102 is controlled by the rotating plate control unit 108 and rotates based on the imaging conditions input from the input unit 121, particularly the rotation speed and the like. The bed 105 is controlled by the bed control unit 109 and moves the subject 10 into the imaging field, which is the range where the transmitted X-rays are detected, by moving relative to the rotating plate 102.

[0018] The irradiation of X-rays by the X-ray source 101 and the detection of X-rays by the X-ray detector 106 are repeated along with the rotation of the rotating plate 102, so that projection data, which is a projection image of the subject 10, is measured at a plurality of projection angles. The projection data is associated with a view representing each projection angle, a channel number which is the detection element number of the X-ray detector 106, and a column number. The measured projection data is transmitted to the image generation unit 122. The image generation unit 122 generates a tomographic image by performing back-projection processing on the plurality of projection data. The generated tomographic image is displayed on the display unit 125 as a medical image or stored in the storage unit 123. Which of the plurality of generated medical images is to be displayed on the display unit 125 is specified by operating the input unit 121.

[0019] An example of the functional blocks of Example 1 will be described with reference to FIG. 2. These functional blocks may be configured by dedicated hardware using an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or the like, or may be configured by software operating on the system control unit 124. In the following description, the case where the functional blocks of Example 1 are configured by software will be described.

[0020] In Example 1, a height direction error calculation unit 201 and a lateral direction error calculation unit 202 are provided. Each unit will be described below.

[0021] The height direction error calculation unit 201 calculates the height direction error between the body thickness center of the subject 10 and the rotation center of the scanner 100 based on the scan plan image acquired prior to the generation of the tomographic image. The height direction error is calculated for each position in the body axis direction of the subject 10. A body thickness calculation function 203 stored in the storage unit 123 may be used for the calculation of the height direction error.

[0022] The body thickness calculation function 203 will be described with reference to FIG. 3. The body thickness calculation function 203 is a function representing the relationship between the projection value P of the projection image 301 and the body thickness T of the subject 10, and is, for example, expressed by the following equation.

[0023] T = a·P + b … (Equation 1) Here, a and b are coefficients, and are obtained by linearly approximating a graph plotted with the projection value P obtained from each of the plurality of projection images 301 and the body thickness T measured from each of the plurality of tomographic images 302 by the least squares method.

[0024] A graph plotted with the projection value P and the body thickness T is created for each position of the subject 10 in the body axis direction. The position of the subject 10 in the body axis direction is normalized using characteristic parts such as the cervical vertebra, diaphragm, and pelvis extracted from the projection image 301.

[0025] The projection value P is calculated from a projection value profile 303 obtained for each position in the body axis direction from the projection image 301, and may be, for example, the maximum value of the projection value profile 303 or the average value at the center of the X-axis. The projection value profile 303 is also obtained by projecting the pixel values of the tomographic image 302 in the Y-axis direction in order.

[0026] The body thickness calculation function 203 is not limited to a linear function such as (Equation 1), and may be a quadratic function or the like. Also, each time a new tomographic image 302 is acquired, the body thickness calculation function 203 may be updated. Return to the description of FIG. 2.

[0027] The lateral error calculation unit 202 calculates the lateral error between the midline of the subject 10 and the rotation center of the scanner 100 based on the scan plan image. The lateral error is calculated for each position of the subject 10 in the body axis direction.

[0028] Using FIG. 4, an example of the processing flow of Example 1 will be described step by step.

[0029] (S401) Prior to generating a tomographic image used for image diagnosis, a scan plan image is acquired. The scan plan image may be a projection image 301 or a low-dose tomographic image. The low-dose tomographic image is a tomographic image acquired with an irradiation dose lower than that when acquiring the tomographic image used for image diagnosis.

[0030] (S402) Based on the scan plan image acquired in S401, the height direction error calculation unit 201 calculates the height direction error between the body thickness center of the subject 10 and the rotation center of the scanner 100. Also, the lateral direction error calculation unit 202 calculates the lateral direction error between the midline of the subject 10 and the rotation center of the scanner 100 based on the scan plan image. Note that the calculation of the lateral direction error is not essential.

[0031] Using FIG. 5, an example of the flow of the calculation process of the height direction error and the lateral direction error in S402 will be described step by step.

[0032] (S501) When the scan plan image acquired in S401 is the projection image 301, the height direction error calculation unit 201 and the lateral direction error calculation unit 202 acquire a projection value profile 303 for each position in the body axis direction of the subject 10.

[0033] (S502) The height direction error calculation unit 201 calculates a projection value P from the projection value profile 303 acquired in S501, and inputs the projection value P to the body thickness calculation function 203 to calculate the body thickness T of the subject 10. When a pacemaker or the like is implanted in the subject 10, the projection image 301 includes a metal region, and a projection value P that does not correspond to the body thickness T is calculated. Therefore, the metal region is removed prior to the calculation of the projection value P.

[0034] With reference to FIG. 6, the removal of the metal region will be described. The metal region 601 included in the projection image 301A is extracted by threshold processing or the like on the pixel values. Note that the extraction of the metal region 601 is not limited to threshold processing, and for example, extraction by AI (Artificial Intelligence) may be used. Then, by replacing the pixel values of the extracted metal region 601 with the average value of the pixel values around the metal region 601, the metal region 601 included in the projection image 301A becomes a replacement region 602 as shown in the projection image 301B. That is, the metal region 601 is removed, and the body thickness T is accurately calculated. Return to the description of FIG. 5.

[0035] (S503) Based on the body thickness T calculated in S502, the height direction error calculation unit 201 calculates the height direction error.

[0036] With reference to FIG. 7, the calculation of the height direction error Δy will be described. The height direction error Δy is calculated, for example, by the following formula.

[0037] Δy = H_C - (H_B + T / 2) …(Equation 2) Here, H_C is the height from the rotation center 700 of the scanner 100 to the floor 702 of the imaging room, H_B is the height from the upper surface of the bed 105 to the floor 702 of the imaging room, and T / 2 is the height from the body thickness center 701 to the upper surface of the bed 105. When Δy > 0, it is necessary to raise the bed 105, and when Δy < 0, it is necessary to lower the bed 105. Return to the description of FIG. 5.

[0038] (S504) Based on the projection value profile 303 acquired in S501, the lateral direction error calculation unit 202 calculates the lateral direction error for each position in the body axis direction of the subject 10.

[0039] Using FIG. 8, the calculation of the lateral error Δx will be described. The lateral error calculation unit 202 obtains the midpoint 801 between the ends of the body region of the subject 10 based on the projection value profile 303 for each position in the body axis direction of the subject 10. The ends of the body region of the subject 10 are extracted by threshold processing using the projection values of air. Since the midpoint 801 between the ends overlaps with the midline of the subject 10, the distance between the midpoint 801 between the ends and the rotation center axis 800 is calculated as the lateral error Δx.

[0040] According to the processing flow described with reference to FIG. 5, the height direction error Δy and the lateral error Δx for each position in the body axis direction of the subject 10 are calculated based on the projection image 301 for each position. Since it is common to obtain the projection image 301 prior to imaging the tomographic image, by calculating the height direction error Δy and the lateral error Δx based on the projection image 301, additional imaging can be avoided. Note that the calculation of the lateral error Δx is not essential. Also, the calculation of the height direction error Δy and the lateral error Δx is not limited to the processing flow illustrated in FIG. 5.

[0041] Using FIG. 9, another example of the processing flow for calculating the height direction error and the lateral error in S402 will be described step by step.

[0042] (S901) When the scan plan image acquired in S401 is a low-dose tomographic image, the height direction error calculation unit 201 and the lateral error calculation unit 202 extract the body region of the subject 10 from the low-dose tomographic image. For example, threshold processing is used for the extraction of the body region, and the body region is extracted for each position in the body axis direction of the subject 10.

[0043] (S902) The height direction error calculation unit 201 and the lateral error calculation unit 202 calculate the centroid of the body region 1000 extracted in S901.

[0044] (S903) Based on the coordinates of the center of gravity calculated in S901, the height direction error calculation unit 201 calculates the height direction error, and the lateral direction error calculation unit 202 calculates the lateral direction error. In FIG. 10, the body region 1000 of the subject 10 placed on the bed 105 and its center of gravity 1001 are illustrated, and the height direction error Δy and the lateral direction error Δx between the rotation center 700 of the scanner 100 and the center of gravity 1001 are shown.

[0045] According to the processing flow described with reference to FIG. 9, based on the low-dose tomographic images for each position in the body axis direction of the subject 10, the height direction error Δy and the lateral direction error Δx for each position are calculated. The height direction error Δy and the lateral direction error Δx calculated based on the low-dose tomographic images are obtained more accurately. In FIG. 11, six points indicated by the height direction error Δy and the lateral direction error Δx calculated for each position in the body axis direction of the subject 10 are illustrated. Return to the description of FIG. 4.

[0046] (S403) With the rotation of the rotating plate 102 and the movement of the bed 105 in the body axis direction of the subject 10, the X-ray irradiation from the X-ray source 101 and the X-ray detection by the X-ray detector 106 are repeated, whereby the subject 10 is scanned. Projection data at a plurality of projection angles is acquired by scanning the subject 10.

[0047] During the period from the start to the end of the scan of the subject 10, the bed control unit 109 controls the height of the bed 105 based on the height direction error Δy calculated in S402. By controlling the height of the bed 105 based on the height direction error Δy, the center of the body thickness of the subject 10 can be aligned with the rotation center of the scanner 100 during the scan.

[0048] Furthermore, in addition to controlling the height of the bed 105, the bed control unit 109 may control the bed 105 in the lateral direction, that is, in the X-axis direction, based on the lateral direction error Δx calculated in S402. By controlling the bed 105 in the lateral direction based on the lateral direction error Δx, the midline of the subject 10 can be aligned with the rotation center of the scanner 100 during the scan.

[0049] (S404) The image generation unit 122 generates a tomographic image of the subject 10 using the projection data at a plurality of projection angles acquired in S403. Since the tomographic images are generated for each position of the subject 10 in the body axis direction, the image generation unit 122 may generate a three-dimensional image of the subject 10 using the plurality of generated tomographic images.

[0050] Note that as shown in FIG. 12, the center of gravity 1001 of the body region of the subject 10 is moved to the rotation center 700 of the scanner 100 in the plurality of generated tomographic images. Therefore, if the plurality of generated tomographic images 302 are directly used for the generation of the three-dimensional image, a three-dimensional image that does not match the actual shape of the subject 10 may be generated.

[0051] Therefore, the image generation unit 122 may generate a three-dimensional image after adding the height direction error Δy and the lateral direction error Δx calculated for each position of the subject 10 in the body axis direction in S402 to each of the plurality of tomographic images. By adding the height direction error Δy and the lateral direction error Δx to each of the tomographic images, a three-dimensional image that matches the actual shape of the subject 10 can be generated.

[0052] According to the processing flow described with reference to FIG. 4, the subject 10 can be scanned while accurately aligning the body thickness center 701 of the subject 10 with the rotation center 700 of the scanner 100. As a result, it is possible to suppress a decrease in the image quality of the generated tomographic image and unnecessary exposure of the subject 10.

[0053] The embodiments of the present invention have been described above. The present invention is not limited to the above embodiments, and components can be modified and embodied without departing from the gist of the invention. Also, a plurality of components disclosed in the above embodiments may be appropriately combined. Furthermore, some components may be deleted from all the components shown in the above embodiments.

Explanation of Reference Numerals

[0054] 10: Subject, 100: Scanner, 101: X-ray source, 102: Rotating plate, 103: Collimator, 104: Aperture, 105: Bed, 106: X-ray detector, 107: Data acquisition unit, 108: Rotating plate control unit, 109: Bed control unit, 110: X-ray control unit, 111: High voltage generation unit, 120: Operation unit, 121: Input unit, 122: Image generation unit, 123: Memory unit, 124: System control unit, 125: Display unit, 201: Height direction error calculation unit, 202: Lateral direction error calculation unit, 203: Body thickness calculation function, 301: Projection image, 302: Tomographic image, 303: Projection value profile, 601: Metal region, 602: Replacement region, 700: Rotation center, 701: Body thickness center, 702: Floor, 800: Rotation center axis, 801: Midpoint between ends, 1000: Body region, 1001: Center of gravity.

Claims

1. An X-ray CT apparatus comprising a bed on which a subject is placed, an X-ray source that irradiates the subject with X-rays, an X-ray detector that detects X-rays transmitted through the subject, a scanner that rotates the X-ray source and the X-ray detector around the subject, and an image generation unit that generates a tomographic image using projection images at a plurality of projection angles acquired by the scanner, a height direction error calculation unit that calculates a height direction error between the center of the thickness of the subject and the rotation center of the scanner for each position in the body axis direction of the subject based on a scan plan image acquired in advance, and further comprising a bed control unit that controls the height of the bed based on the height direction error from the start to the end of the scan. The X-ray CT apparatus is characterized by this.

2. The X-ray CT apparatus according to claim 1, further comprising a storage unit that stores a thickness calculation function for calculating the thickness of the subject by inputting projection values, wherein the height direction error calculation unit calculates the height direction error based on the thickness obtained by inputting the projection values of the projection image into the thickness calculation function when the scan plan image is a projection image. The X-ray CT apparatus is characterized by this.

3. The X-ray CT apparatus according to claim 2, wherein the height direction error calculation unit removes a metal region included in the projection image and then obtains the thickness. The X-ray CT apparatus is characterized by this.

4. The X-ray CT apparatus according to claim 1, further comprising a lateral direction error calculation unit that calculates a lateral direction error between the midline of the subject and the rotation center of the scanner for each position in the body axis direction of the subject based on the scan plan image, wherein the bed control unit controls the bed based on the height direction error and the lateral direction error. The X-ray CT apparatus is characterized by this.

5. The X-ray CT apparatus according to claim 4, The X-ray CT apparatus is characterized in that when the scan planned image is a projection image, the lateral error calculation unit calculates the lateral error using the midpoint between the ends of the body region of the subject obtained based on the projection value profile of the projection image.

6. The X-ray CT apparatus according to claim 4, when the scan planned image is a low-dose tomographic image, the body region of the subject is extracted from the low-dose tomographic image and the center of gravity of the body region is calculated, based on the coordinates of the center of gravity, the height direction error calculation unit calculates the height direction error, and the lateral error calculation unit calculates the lateral error, characterized in that it is an X-ray CT apparatus.

7. The X-ray CT apparatus according to claim 4, further comprising an image generation unit that generates a three-dimensional image from the tomographic image using the height direction error and the lateral direction error, characterized in that it is an X-ray CT apparatus.

8. A control method for an X-ray CT apparatus, comprising a bed on which a subject is placed, an X-ray source that irradiates the subject with X-rays, an X-ray detector that detects X-rays transmitted through the subject, a scanner that rotates the X-ray source and the X-ray detector around the subject, and an image generation unit that generates a tomographic image using projection images at a plurality of projection angles acquired by the scanner, a height direction error calculation step of calculating a height direction error between the body thickness center of the subject and the rotation center of the scanner for each position in the body axis direction of the subject based on a scan planned image acquired in advance, and a bed control step of controlling the height of the bed based on the height direction error from the start to the end of the scan, characterized in that it is a control method for an X-ray CT apparatus.

Images