JP2011206249A - Method for capturing radiation image, and radiation image capturing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reconstructed image further suited to a diagnosis by appropriately suppressing an influence of artifact or the like generated by the body movement of a subject, without decreasing a diagnosis efficiency.SOLUTION: This radiation image capturing apparatus acquires radiation image signals for each predetermined rotating angle by revolving a radiation image detector around the subject, and generates a three-dimensional reconstructed image of the subject based on the radiation image signals for each rotating angle. This radiation image capturing apparatus detects the body movement of the subject and reduces a voxel resolution of the reconstructed image based on the detected body movement.

Description

  The present invention acquires a radiographic image signal for each predetermined rotation angle by rotating a radiographic image detector around the subject, and based on the radiographic image signal for each rotation angle, a three-dimensional reconstructed image of the subject The present invention relates to a radiographic image capturing method and apparatus for generating the image.

  Conventionally, a radiation source and a radiation image detector are arranged facing each other around the subject, and these sets are rotated around the subject to radiate radiation from various angles to capture radiation images. A radiation CT image capturing system that reconstructs a tomographic image using an angle radiation image and displays an arbitrary cross section is widely used in clinical practice.

  Here, in the radiation CT image capturing system, as described above, radiation is irradiated from a plurality of angles and a radiation image for each angle is captured. Therefore, a certain amount of time is required until all the radiation images are acquired.

  However, if the subject moves during this imaging time, there is a problem that artifacts occur when the radiation CT image is reconstructed. In addition, there is a problem that motion artifacts occur due to the subject's breathing or heartbeat.

  Therefore, for example, in Patent Document 1, for example, a sensor is installed in a subject to determine the amplitude and frequency of respiration, and based on the result, correction calculation that minimizes motion artifacts in the reconstructed image is performed in the image processing unit. Has been proposed to do.

JP 2008-73523 A Japanese Patent Laid-Open No. 59-200454

  However, Patent Document 1 proposes to correct motion artifacts, but no specific correction method has been proposed. For example, complicated calculation processing was performed as a correction method. Therefore, it takes time until the reconstructed image is generated and displayed, resulting in a decrease in diagnostic efficiency.

  In view of the above-described circumstances, the present invention can appropriately suppress the influence of artifacts and the like caused by body movement of a subject without causing a decrease in diagnosis efficiency, and provides a reconstructed image more suitable for diagnosis. An object of the present invention is to provide a radiographic imaging method and apparatus capable of performing the above.

  In the radiographic image capturing method of the present invention, a radiation image detector that generates a charge upon receiving irradiation of radiation that has passed through the subject and outputs a radiographic image signal representing the radiographic image of the subject circulates around the subject, In a radiographic image capturing method for obtaining a radiographic image signal for each predetermined rotation angle and generating a three-dimensional reconstructed image of the subject based on the radiographic image signal for each rotation angle, detecting the body movement of the subject, The voxel resolution of the reconstructed image is lowered based on the detected body movement.

  A radiographic imaging apparatus of the present invention generates a charge upon receiving radiation that has passed through a subject, and outputs a radiographic image signal representing a radiographic image of the subject, and the radiographic image detector around the subject And a reconstructing unit that acquires a radiation image signal for each predetermined rotation angle and generates a three-dimensional reconstructed image of the subject based on the radiation image signal for each rotation angle. In the radiographic imaging apparatus, the body motion detection unit that detects the body motion of the subject and the reconstruction unit reduce the voxel resolution of the reconstructed image based on the body motion detected by the body motion detection unit. It is characterized by.

  In the radiographic imaging apparatus of the present invention, the reconstruction unit lowers the voxel resolution of the reconstruction image when the magnitude of the body motion detected by the body motion detection unit is equal to or greater than a predetermined threshold. can do.

  In addition, the reconstruction unit can lower the voxel resolution of the reconstructed image when the magnitude of the body motion detected by the body motion detection unit is 1 voxel or more.

  Further, the reconstruction unit can lower the voxel resolution only in the direction in which the magnitude of the body motion is equal to or greater than a predetermined threshold among the directions of the body motion detected by the body motion detection unit.

  According to the radiographic image capturing method and apparatus of the present invention, the radiographic image detector is rotated around the subject to acquire radiographic image signals for each predetermined rotation angle, and based on the radiographic image signals for each rotation angle. In the radiographic image capturing method and apparatus for generating a three-dimensional reconstructed image of a subject, the body motion of the subject is detected, and the voxel resolution of the reconstructed image is reduced based on the detected body motion. Without performing complicated calculation processing, it is possible to appropriately suppress influences such as artifacts generated by the body movement of the subject, and to provide a reconstructed image more suitable for diagnosis.

  Further, in the radiographic image capturing method and apparatus, when the voxel resolution is reduced only in the direction in which the magnitude of the body motion is equal to or greater than a predetermined threshold among the detected body motion directions, there is no body motion. The influence of the artifact can be appropriately suppressed without reducing the voxel resolution unnecessarily to the direction and reducing the contrast.

1 is a schematic configuration diagram of a radiation CT image capturing system using an embodiment of a radiation image capturing apparatus of the present invention. The block diagram which shows the internal structure of a radiation detection part and a computer in the radiation CT imaging system using one Embodiment of the radiographic imaging apparatus of this invention. The figure which shows the test subject where the acceleration sensor is installed The flowchart for demonstrating the effect | action of one Embodiment of the radiographic imaging apparatus of this invention. The figure for demonstrating the zero point width | variety of the signal intensity distribution of a reconstructed image, and a half value width The figure which shows the result of having measured the change of the zero point width | variety of the signal intensity distribution of the reconstruction image with respect to u-axis offset amount according to a to-be-photographed object's deviation, and a half value width. Diagram for explaining u-axis offset The figure which shows the simulation result of the reconstructed image at the time of giving a vibration of 2 to 16 voxels to the subject The figure which shows the simulation result of the reconstruction image when not giving vibration to a subject The figure which shows an example of a subject's movement The figure which shows an example at the time of reducing voxel resolution only about the positive direction of z direction The figure which shows an example at the time of reducing voxel resolution about the positive direction of az direction, x direction, and the positive and negative direction of y direction

  A radiation CT image capturing system using an embodiment of a radiation image capturing apparatus of the present invention will be described below with reference to the drawings. First, a schematic configuration of the entire radiation CT image capturing system will be described. FIG. 1 is a diagram showing a schematic configuration of the present radiation CT image capturing system.

  As shown in FIG. 1, the present radiation CT image imaging system is connected to an imaging apparatus 1 that captures a radiographic image of a subject P, a bed 22 that is a support base for supporting the subject P, and the imaging apparatus 1. A computer 30 that controls the imaging apparatus 1 and processes a radiographic image signal obtained by imaging, a monitor 31 connected to the computer 30, and a body motion detection unit 40 that detects the body motion of the subject P. And.

  The imaging apparatus 1 includes a radiation source 10 that emits conical radiation, a radiation detection unit 11 that detects radiation emitted from the radiation source 10, a radiation source 10, and a radiation detection unit 11 that face each end. A C-arm 12 that holds them, a rotation drive unit 15 that rotates the C-arm 12, and an arm 20 that holds the rotation drive unit 15.

  The C arm 12 is attached to the rotation drive unit 15 so as to be able to rotate 360 ° around the rotation axis C. The arm 20 includes a movable portion 20a and is held by a base portion 21 that is movably installed with respect to the ceiling. The C-arm 12 can be moved to a wide range of positions in the photographing room by moving the base 21, and the rotation direction (rotation axis angle) can be changed by moving the movable part 20a of the arm 20. Has been.

  The radiation source 10 and the radiation detection unit 11 are disposed to face each other with the rotation axis C interposed therebetween. When performing radiation CT image capturing, the positional relationship between the rotation axis C, the radiation source 10 and the radiation detection unit 11 is as follows. In a fixed state, the C-arm 12 is rotated 360 ° by the rotation driving unit 15.

  FIG. 2 is a block diagram illustrating a schematic configuration inside the radiation detection unit 11 and the computer 30.

  As illustrated in FIG. 2, the radiation detection unit 11 generates a charge upon receiving radiation transmitted through the subject P, and outputs a radiation image signal representing a radiation image of the subject P. And a signal processing unit 11b that performs predetermined signal processing on the radiation image signal output from the image detector 11a.

  The radiological image detector 11a can repeatedly perform recording and reading of radiographic images, and a so-called direct type radiographic image detector that directly receives radiation and generates charges may be used. Alternatively, a so-called indirect radiation image detector that converts radiation once into visible light and converts the visible light into a charge signal may be used. Further, as a radiation image signal readout method, it is desirable to use a so-called TFT readout method in which a radiation image signal is read out by turning on and off a TFT (thin film transistor) switch. You may make it use not only other things.

  The signal processing unit 11b includes an amplifier unit including a charge amplifier that converts the charge signal read from the radiation image detector 11a into a voltage signal, and an AD conversion unit that converts the voltage signal output from the amplifier unit into a digital signal. Etc.

  The computer 30 includes a central processing unit (CPU) and a storage device such as a semiconductor memory, a hard disk, and an SSD. The hardware includes a reconstruction unit 30a, a display signal generation unit 30b, a body movement acquisition unit 30c, And the imaging | photography control part 30d is comprised.

  The reconstruction unit 30a reconstructs a radiation CT image signal representing a three-dimensional radiation image of the subject P based on the radiation image signal at each imaging angle. Here, the reconstruction unit 30a changes the voxel resolution when generating the reconstructed image based on the body motion of the subject P acquired by the body motion acquisition unit 30c. The operation will be described in detail later.

  The display signal generation unit 30 b generates a display control signal based on the radiation CT image signal output from the reconstruction unit 30 a and outputs the display control signal to the monitor 31.

  The body motion acquisition unit 30c acquires a signal representing the body motion acquired by the body motion detection unit 40, and outputs body motion information to the reconstruction unit 30a based on the signal.

  The imaging control unit 30d drives and controls the rotation speed of the C arm 12 by the rotation driving unit 15 and the irradiation timing of the radiation emitted from the radiation source 10.

  The body motion detection unit 40 detects the body motion of the subject P, and specifically uses acceleration sensors 40a to 40d in the present embodiment. For example, as shown in FIG. 3, the acceleration sensors 40 a to 40 d are installed so as to have a target positional relationship with respect to the center axis of the body of the subject P. In the present embodiment, the acceleration sensor is used to detect the body movement of the subject P. However, the present invention is not limited to this. For example, other sensors such as an infrared sensor, The body movement of the subject P may be detected based on a laser displacement meter or an image obtained by photographing the subject P with a camera or the like. Further, when detecting the pulsation of the heart or the blood vessel in the body, an ultrasonic sensor or the like may be used.

  Next, the operation of this radiation CT image capturing system will be described with reference to the flowchart shown in FIG.

  First, the subject P is laid on the bed 22, and the radiation source 10 and the radiation detection unit 11 are arranged at symmetrical positions with the rotational axis C being the approximate center of the subject P's body. Thus, positioning of the C arm 12 is performed. The movement of the C-arm 12 is performed based on the operation of the computer 30 by the photographer.

  Then, imaging conditions such as an imaging region are input by the photographer in the computer 30 (S10). Then, when an imaging start instruction is input by the operator, the imaging control unit 30d, based on the imaging conditions set and input as described above, the radiation irradiation dose condition, the C-arm 12 rotation speed condition, and the like. And a control signal according to the irradiation dose condition is output to the radiation source 10 and a control signal according to the rotation speed condition is output to the rotation drive unit 15 that drives the C-arm 12.

  Based on these control signals, the radiation source 10 and the rotation drive unit 15 are driven and controlled to take a radiographic image of the subject P (S12). Specifically, the C arm 12 is rotated by the rotation drive unit 15 based on the input control signal, and the radiation source 10 and the radiation detection unit 11 rotate integrally around the rotation axis C passing through the subject P. And radiation is emitted from the radiation source 10 based on the input control signal.

  Then, at every predetermined rotation angle of the C-arm 12, the conical radiation passing through the subject P is exposed to the radiation image detector 11a and the charge signal recorded in the radiation image detector 11a is read, A plurality of radiographic image signals obtained by photographing the subject P from different angles are sequentially read out.

  The read radiographic image signal as described above is converted into a voltage signal in the amplifier unit of the signal processing unit 11b, converted into a digital signal in the AD conversion unit, and then converted into a reconfiguration unit 30a of the computer 30. Is output.

  On the other hand, during the radiographing of the subject P as described above, the body motion of the subject P is detected by the acceleration sensors 40a to 40d installed in the subject P. Specifically, the x direction, the y direction, and the z direction are preset in each of the acceleration sensors 40a to 40d, and the movement amount of the subject P in each direction is detected by each sensor. The x direction, the y direction, and the z direction set in the acceleration sensors 40a to 40d are the x direction, the y direction, and the z direction that are coordinate axes used when generating the radiation CT image signal in the reconstruction unit 30a. Assume that they are set in the same direction.

  Then, the movement amounts in the x direction, the y direction, and the z direction detected by the respective acceleration sensors 40a to 40d are acquired by the body motion acquisition unit 30c of the computer 30. In addition, the output of the movement amount from the acceleration sensors 40a to 40d to the body movement acquisition unit 30c may be output as a wireless signal, or may be output via a wire.

  The body motion acquisition unit 30c calculates an average value of the movement amount in each direction based on the movement amounts in the x direction, the y direction, and the z direction detected by the acceleration sensors 40a to 40d, respectively. It is determined whether the average value ax of the movement amount in the direction, the average value ay of the movement amount in the y direction, and the average value az of the movement amount in the z direction are each greater than a preset threshold value (S14).

  Here, in this embodiment, 1 voxel is used as the threshold value. The reason why the threshold is set to 1 voxel will be described below. Here, one voxel is one unit of voxel resolution at the time of reconstruction, and is a size determined by an imaging region and an imaging mode (for example, a high resolution mode, a low resolution mode, etc.).

  First, an experiment was conducted to examine the influence of blurring on the radiation CT image due to the body movement of the subject P. Specifically, ceramic particles having a diameter of 100 μm were photographed as subjects and the influence of the offset amount was confirmed. Here, the offset amount of the radiological image detector 11a is changed to correspond to the body movement. The offset amount is a movement amount of the installation position with respect to the rotation center of the radiation image detector 11a.

  A subject is placed at a position substantially in the middle between the radiation source 10 and the radiation detection unit 11, and while rotating the C arm 12 360 degrees with respect to the subject, a radiation image is acquired every time, and a total of 360 radiation images are obtained. Was taken. Then, the offset amount of the radiation image detector 11a was changed, and the radiation image was reconstructed by the Feldkamp method. The resolution of the radiation image detector 11a was 50 μm per pixel, and reconstruction was performed at a voxel resolution of 25 μm.

  And the signal intensity distribution of the reconstructed image acquired as mentioned above was acquired, and the half value width in the signal intensity distribution was acquired. In addition, as shown in FIG. 5, a half value width means the width | variety of signal strength distribution when signal strength becomes 1/2 of the maximum value.

As a result, as shown in FIG. 6, it was found that when the offset amount on the radiation image detector 11a exceeds ± 1 pixel, that is, ± 50 μm, the image size cannot be appropriately captured. Considering the relative movement of the subject and the radiation image detector 11a, the above results show that the object is affected by movement of ± 1 voxel, that is, ± 25 μm.
Next, an experiment for examining the influence of artifacts on the radiation CT image due to the body movement of the subject P was performed. Specifically, similarly to the above, ceramic particles having a diameter of 100 μm were used as a subject, the u-axis offset amount when the subject was vibrated by a sine wave was calculated, and the u-axis offset amount was given as a reconstruction parameter. The reconstructed image was generated by simulation experiments. Note that the u-axis offset amount is a size obtained by projecting the shift amount of the subject onto the radiation image detector, as shown in FIG. Therefore, if the shift amount in FIG. 7 is x1, the u-axis offset amount is x1 cos θ.

  FIG. 8 shows a simulation result when the amplitude of vibration applied to the subject is changed from 2 voxels to 16 voxels. For comparison, FIG. 9 shows a simulation result when the subject is not vibrated. As shown in FIG. 8, it can be seen that streak-like streak artifacts gradually appear when the amplitude exceeds 2 voxels (± 1 voxel).

  From the experimental results as described above, one voxel is used as the threshold value for switching the voxel resolution. However, the threshold value is not limited to one voxel, and may be changed as appropriate according to the required image quality.

  If at least one of the average values ax, ay, and az of the movement amounts in each direction is larger than one voxel in S14 (S14, YES), the body motion acquisition unit 30c instructs the resolution to decrease ( (Hereinafter, referred to as resolution switching signal) is output to the reconstruction unit 30a (S16). On the other hand, when the average values ax, ay, and az of the movement amounts in each direction are all equal to or less than 1 voxel in S14 (S14, NO), the body motion acquisition unit 30c sends a resolution switching signal to the reconstruction unit 30a. Do not output.

  Then, the reconstruction unit 30a reconstructs a radiation CT image signal representing a three-dimensional radiation CT image using, for example, the Feldkamp method based on the input radiation image signal at each imaging angle. The voxel resolution is switched according to the presence / absence of a resolution switching signal from the motion acquisition unit 30c. Specifically, switching is performed so that the second voxel resolution when receiving the resolution switching signal is lower than the first voxel resolution when receiving no resolution switching signal from the body motion acquisition unit 30c (S16). ). For example, the switching is performed so that the second resolution is about 1/2 to 1/8 of the first resolution, and more preferably 1/4 to 1/8.

  Note that the voxel resolution is lowered when the movement amount is larger than the predetermined threshold. The contrast is lowered when the voxel resolution is lowered. This is because it becomes smaller.

  The change in the voxel resolution is, for example, when the average value ax, ay, or az of the movement amount is larger than one voxel, the voxel resolution is lowered in all directions of the x direction, the y direction, and the z direction. Alternatively, the voxel resolution may be lowered only in a direction in which the magnitude of the movement amount average values ax, ay, and az is larger than a predetermined threshold (1 voxel).

  For example, as shown in FIG. 10, when the subject P moves only in the positive direction in the z direction and the movement amount is larger than a predetermined threshold, the voxel of the voxel in the positive direction in the z direction is shown in FIG. 11. The length may be increased to lower the voxel resolution only in the z direction.

  Alternatively, as shown in FIG. 12, the length of the voxel is increased only in the positive direction in the z direction, and the length of the voxel is increased in both the positive and negative directions in the x direction and the y direction so that the voxel resolution is lowered. It may be. That is, the length of the voxel may not be extended only in the direction opposite to the moving direction.

  Then, the reconstruction unit 30a generates a radiation CT image signal by performing reconstruction at the first voxel resolution or the second voxel resolution according to the resolution switching signal, and displays the radiation CT image signal as a display signal generation unit 30b. (S18).

  The display signal generation unit 30 b generates a display control signal based on the input radiation CT image signal and outputs the display control signal to the monitor 31. The monitor 31 displays a three-dimensional radiation CT image of the subject P based on the input display control signal.

  In the description of the above embodiment, the two resolutions of the first resolution and the second resolution are switched according to the movement amount of the subject P. For example, two or more threshold values are set stepwise. You may make it set and switch 3 or more resolutions in steps according to the movement amount of the test subject P.

DESCRIPTION OF SYMBOLS 1 Imaging device 10 Radiation source 11 Radiation detection part 11a Radiation image detector 11b Signal processing part 12 C arm 15 Rotation drive part 30 Computer 30a Reconstruction part 30b Display signal generation part 30c Body movement acquisition part 30d Imaging control part 31 Monitor 40 Body Motion detector 40a-40d Acceleration sensor

Claims (5)

The radiation image detector that generates a charge upon receiving radiation that has passed through the subject and outputs a radiation image signal that represents the radiation image of the subject circulates around the subject, so that the predetermined rotation angle is increased. In a radiographic image capturing method of acquiring a radiographic image signal and generating a three-dimensional reconstructed image of the subject based on the radiographic image signal for each rotation angle,
Detecting the movement of the subject,
A radiographic image capturing method, wherein the voxel resolution of the reconstructed image is lowered based on the detected body movement. A radiation image detector that generates a charge upon receiving radiation that has passed through the subject and outputs a radiation image signal representing a radiation image of the subject, and a rotation drive unit that circulates the radiation image detector around the subject And a reconstruction unit that obtains the radiation image signal for each predetermined rotation angle and generates a three-dimensional reconstruction image of the subject based on the radiation image signal for each rotation angle In
A body motion detector for detecting the body motion of the subject;
The radiographic imaging apparatus, wherein the reconstruction unit lowers the voxel resolution of the reconstructed image based on the body motion detected by the body motion detection unit.   The reconstructing unit reduces the voxel resolution of the reconstructed image when the size of the body motion detected by the body motion detection unit is equal to or greater than a predetermined threshold. Radiographic imaging device.   4. The reconstruction unit according to claim 3, wherein the reconstruction unit lowers the voxel resolution of the reconstruction image when the size of the body motion detected by the body motion detection unit is 1 voxel or more. Radiation imaging device.   The reconstructing unit lowers the voxel resolution only in a direction in which the magnitude of body movement is equal to or more than the predetermined threshold among the body movement directions detected by the body movement detection unit. The radiographic imaging device according to any one of claims 2 to 4.