JP4647345B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT apparatus (Computed Tomography) and the like. More specifically, the present invention relates to an X-ray CT apparatus that performs dynamic imaging and corrects image misalignment.

  Conventionally, dynamic imaging using an X-ray CT apparatus has been performed in perfusion analysis and the like. Dynamic imaging is an imaging method in which a contrast medium is rapidly injected into a subject and a change in CT value with time is obtained to perform dynamic observation. In dynamic imaging in the X-ray CT apparatus, the bed position is fixed and changes over time in CT values in the same section are collected.

  For perfusion analysis, SPECT (single photon tomography) using radioisotope (RI), PET (positron emission tomography), and Xe-CT using xenon gas have been performed. CT-Perfusion (CT perfusion imaging) using an iodine-based contrast agent, which is advantageous in terms of the convenience of examination and the diffusion rate of the device compared with the method, is rapidly spreading as a new method for perfusion analysis . For example, cerebral perfusion imaging is clinically applied to acute cerebral infarction and the like.

In general, in perfusion imaging, image displacement may occur due to body movement (movement of a subject during imaging).
Therefore, in order to prevent degradation of the analysis accuracy due to the image misalignment, the analysis is performed after removing the misaligned image, and the image in the removed time phase is interpolated based on the previous and subsequent time phase images. There is a method (hereinafter referred to as “prior art 1”).
There is also a method of calculating a correlation coefficient and performing alignment so that the correlation coefficient is maximized (hereinafter referred to as “conventional technology 2”).

In brain perfusion imaging, a continuous scan is usually performed on the same cross section for about 40 to 50 seconds. Therefore, the local exposure dose becomes large. For example, in the case of general imaging conditions, tube voltage 80 kV, tube current 150 to 200 mA, gantry rotation speed 1 second / rotation, total imaging time 40 to 50 seconds, the local exposure dose is 500 to 700 mGy. This local exposure dose corresponds to 10 or more simple head CT scans.
One method for reducing the exposure dose is intermittent scanning. Intermittent scanning is a technique for reducing the total exposure time by performing imaging while repeating exposure and pause. Some facilities have already introduced CT-Perfusion that performs intermittent imaging at intervals of about 1 second (hereinafter referred to as “Prior Art 3”).

  In order to reduce the exposure dose, there is a method of scanning at a low tube current and a low gantry rotation speed. The total local exposure dose in CT-Perfusion is proportional to the total number of scans. If the gantry rotation speed is reduced while keeping the total imaging time constant, the total number of scans is reduced, so that the exposure dose can be reduced. For example, when imaging is performed with a total imaging time of 45 seconds, a tube current of 100 mA, and a gantry rotation speed of 1.5 seconds / 1 rotation, the exposure dose can be reduced to 2/3 (hereinafter referred to as “Prior Art 4”). .)

  In order to reduce the exposure dose, there is a method of improving the S / N ratio (Signal to Noise ratio) by using an image filter that performs imaging with a low tube current and removes noise while maintaining resolution. According to this method, the tube current can be reduced to about 1/4 of the normal, and the exposure dose can be reduced to about 1/4 (see, for example, [Non-Patent Document 1]).

  There is also a technique called fractional scanning. In the fractional scan, projection data (X-ray transmission data) is collected in all directions of 360 degrees in the first round scan, and the projection angle is 0 in the second, sixth, tenth,. Projection data is collected in the range of degrees to 90 degrees, and projection data is collected in the range of projection angles of 90 degrees to 180 degrees in the third, seventh, eleventh,. In the eyes, the eighth lap, the twelfth lap,..., Projection data is collected within a projection angle range of 180 to 270 degrees, and in the fifth, ninth, thirteenth,. Projection data is collected in an angle range of 270 to 360 degrees, and projection data at a projection angle that is not collected in the second and subsequent rounds is used in the first round of projection data and the second and subsequent rounds. Obtained by performing an interpolation process based on the projection data. According to the fractional scan, the exposure dose can be reduced to ¼ in the time phase after the second round (see, for example, [Non-Patent Document 2]).

  In addition, an image alignment apparatus using difference processing has been proposed. In the alignment apparatus using this difference processing, a perspective projection method and a three-dimensional affine transformation are used to create a two-dimensional projection image having a high correlation with the two-dimensional transmission image from the three-dimensional transmission image. Alignment is performed (for example, refer to [Patent Document 1]).

Sasaki et al., INNERVISION (18.5) 2003 separate volume appendix, 11-13) Jiang Hsieh, etc. Med. Phys. 31 (5) 2004, 1254-1257 JP 2003-153082 A

  However, since “Prior Art 1” performs analysis by removing an image in which body motion has occurred, there is a problem in that the time resolution of the removed portion is reduced. Also. When body motion occurs in a plurality of continuous time phases, there is a problem that the interpolation accuracy is deteriorated and the reliability of the analysis result is lowered.

  Further, in “Prior Art 2”, the positional deviation correction of the image is performed so that the correlation coefficient is maximized. Therefore, unlike the “Prior Art 1”, the time resolution is not sacrificed, and the reliability of the analysis result is improved. Will not be damaged. However, in general, a method using a correlation coefficient has a problem that the calculation time increases because it requires repeated calculation. In the case of a test that is frequently applied to acute cerebral infarction as in the case of cerebral perfusion analysis, the speed of the test is required, so that the increase in the calculation time is a serious adverse effect.

  In “Prior Art 3” or “Prior Art 4”, the exposure dose is reduced by reducing the X-ray irradiation time or by reducing the number of gantry rotations, but the time resolution is lowered. There is a problem.

  In addition, in “Prior Art 5”, the exposure dose is reduced by image processing after data collection. Taking a picture with a small tube current, the mAs value (tube current time product) is reduced. The increasing X-ray quantum noise is suppressed by a special image filter. However, since the S / N ratio is lowered when the projection data is collected by reducing the mAs value, there is a problem that the reliability of the analysis result is lowered and artifacts are likely to occur.

  In “Prior Art 6”, in the second and subsequent scans, only some directions are scanned, and the number of projection data is small, so the reliability of the analysis result is reduced and artifacts are generated. There is a problem that it becomes easy.

Thus, “Prior Art 1” and “Prior Art 2” relate to body motion correction technology. “Prior Art 3” to “Prior Art 6” relate to exposure reduction techniques.
However, the data reliability decreases in “Prior Art 1”, “Prior Art 5”, and “Prior Art 6”. In addition, the “conventional technique 2” increases the computation time. In addition, the time resolution is lowered in “Prior Art 3” and “Prior Art 4”.

  The present invention has been made in view of the above-described problems, and quickly corrects the positional deviation of an image caused by body movement, and sacrifices the time resolution and the S / N ratio at the time of projection data collection. An object of the present invention is to provide an X-ray CT apparatus that can reduce the exposure dose without any problems.

In order to achieve the above-described object, the X-ray CT apparatus of the present invention captures while scanning a subject, collects projection data for a plurality of time phases, performs image reconstruction processing on the projection data, An X-ray CT apparatus for creating a reconstructed image for a plurality of time phases, wherein a calculation means for calculating an image moment of the reconstructed image and a position shift based on a difference in the image moment at each time phase a positional deviation detecting means for determining whether, on the reconstructed image is determined that there is the positional deviation, the position deviation correction means for performing a positional deviation correcting process, after the correction of the positional deviation, the reconstructed image An X-ray CT apparatus comprising: time direction addition averaging means for performing addition averaging in the time direction .

  Further, the X-ray CT apparatus performs imaging at a dose of 1 / N of a normal dose, and the time direction addition averaging means takes an average of N or more reconstructed images obtained by image reconstruction processing. A reconstructed image of the target time phase may be created.

Detection of the positional deviation of the reconstructed images can be performed based on the difference image moment of the reconstructed image. For example, by creating a target region binarized image (mask image) of the reconstructed image, obtaining a centroid and inertial principal axis (inclination angle of inertial equivalent ellipse), and detecting a deviation from the reference centroid and reference inertial principal axis, A misalignment of the reconstructed image can be detected.

Further, positional deviation correction, intends row for the reconstructed image obtained by detecting the reconstructed image or positional deviation.

Also, averaging process in the time direction, intends row after positional deviation correcting process in the reconstructed image.

An X-ray CT apparatus according to a preferred embodiment counts the number of consecutive time phases that have caused a positional shift, and determines whether or not the number of consecutive times has reached a predetermined number (for example, twice). The X-ray CT apparatus performs interpolation processing using the reconstructed images of the preceding and succeeding time phases when the number of consecutive times does not reach the predetermined number and the time phase in which the positional deviation has occurred is not continuous. Create a reconstructed image of the phase. The X-ray CT apparatus performs weighted addition using a reconstructed image of a plurality of time phases when the number of consecutive times reaches a predetermined number and the time phase where the positional deviation has occurred is continuous. to create a reconstructed image of the time phase.
It should be noted that the weight of the weighted addition is preferably determined based on the size of the positional deviation or the time phase difference.
In addition, it is desirable to set the weight of the time phase when the subject moves in the direction horizontal to the imaging section to be larger than the weight of the time phase when the subject moves in the direction perpendicular to the imaging section.

The X-ray CT apparatus according to the preferred embodiment of the present invention corrects misregistration for a reconstructed image in which misregistration is detected, so that it is not necessary to repeatedly perform calculations for all reconstructed images . In addition, the X-ray CT apparatus according to a preferred embodiment performs imaging with a dose smaller than usual in each time phase, and performs addition averaging in the time direction on projection data or reconstructed images of a plurality of time phases, to create a reconstructed image that put in the time phase. Therefore, the processing time required for body motion correction can be shortened, the quality of the captured image can be improved, and the exposure dose can be reduced without sacrificing the S / N ratio and time resolution.
Further, X-ray CT apparatus according to an exemplary embodiment, depending on the occurrence of positional displacement, the positional deviation correcting process based on the difference or the like of the size Ige image moment of processing load, and interpolation processing load is small, the Since it is appropriately selected and executed, the processing load can be reduced and the speed of processing can be improved while maintaining the positional deviation correction accuracy.

  According to the present invention, it is possible to quickly correct a positional deviation of an image caused by body movement, and to reduce an exposure dose without sacrificing time resolution, S / N ratio at the time of collecting projection data, or the like. An X-ray CT apparatus can be provided.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of an X-ray CT apparatus and the like according to the present invention will be described in detail with reference to the accompanying drawings. In the following description and the accompanying drawings, the same reference numerals are given to components having substantially the same functional configuration, and redundant description will be omitted.

First, the configuration of the X-ray CT apparatus 1 according to the first embodiment of the present invention will be described with reference to FIG.
FIG. 1 is a block diagram showing the configuration of the X-ray CT apparatus 1.
As shown in FIG. 1, the X-ray CT apparatus 1 includes a scanner unit 3, a scanner control unit 5, an arithmetic processing unit 7, an image display unit 9, and the like.

  The X-ray CT apparatus 1 is an apparatus that images a subject 25 as a subject and outputs a captured image, such as an X-ray CT apparatus used in a medical institution. The X-ray CT apparatus 1 controls the scanner unit 3 with the scanner control unit 5, images the subject 25 with the scanner unit 3, performs image processing with the arithmetic processing unit 7, and outputs a captured image with the image display unit 9. (Display, print, etc.).

The scanner unit 3 is an apparatus that images the subject 25 and collects X-ray transmission data.
The scanner unit 3 includes a gantry 11. In the gantry 11, the X-ray source 13, the collimator 15, and the detector array 17 are provided at positions facing each other with the subject 25 interposed therebetween.

The X-ray source 13 is a device that irradiates the subject 25 with X-rays 21.
The detector array 17 is composed of a plurality of detector elements 19. The detector array 17 is a device that detects X-rays 21 that have passed through a subject 25 on a bed (not shown). The detector elements 19 are arranged in one row or a plurality of parallel rows. Each detector element 19 generates an electric signal indicating the intensity of the incident X-ray beam, that is, the degree of attenuation when the X-ray beam passes through the subject 25.
Projection data is collected by rotating the gantry 11 around the rotation center 23 in a state where the X-ray source 21 is irradiated with the X-ray 21.

  The scanner control unit 5 is a device that controls the gantry 11, the X-ray source 13, and the like of the scanner unit 3, and sends projection data acquired by the scanner unit 3 to the arithmetic processing unit 7. The scanner control unit 5 includes an X-ray control unit 27, a gantry control unit 29, a DAS 31 (Data Acquisition System), and the like.

The X-ray control means 27 is a device that controls X-ray irradiation of the X-ray source 13.
The gantry control means 29 is a device that controls the operation of the gantry 11.
The DAS 31 is a device that converts an analog signal sent from the detector array 17 into a digital signal.

The arithmetic processing unit 7 is an arithmetic processing device such as a computer. The arithmetic processing unit 7 includes a reconstruction unit 33, a positional deviation detection unit 35, a positional deviation correction unit 37, an addition averaging unit 39, a storage unit 41, a temporary storage unit 43, an input unit 45, and the like.
The arithmetic processing unit 7 reconstructs the projection data digitized by the reconstruction unit 33 and stores it in the storage unit 41. Further, the arithmetic processing unit 7 detects a positional shift of the reconstructed image or projection data caused by the body movement of the subject 25 by the positional shift detection unit 35, and the positional shift correction unit 37 detects the reconstructed image or the projection data. The positional deviation is corrected, and the addition averaging means 39 adds and averages projection data and reconstructed images in a plurality of rounds in the time axis direction.
Note that the reconstruction unit 33 and the like may be configured independently of the arithmetic processing unit 7.

The storage unit 41 is a storage device such as a hard disk.
The temporary storage unit 43 is a device that temporarily stores programs, data, and the like, and is, for example, a memory such as a RAM (Random Access Memory).
The input unit 45 is a device that inputs operation instructions, data, and the like, and is, for example, a keyboard, a mouse, or the like.
The arithmetic processing unit 7 includes at least one of a digital signal processor (DSP), a micro processor unit (MPU), and a central processing unit (CPU) (none of which is shown).

The image display unit 9 is a display device such as a display.
The image display unit 9 may be configured integrally with the arithmetic processing unit 7 or may be configured independently.

Next, the operation of the X-ray CT apparatus 1 according to the first embodiment of the present invention will be described with reference to FIGS.
FIG. 2 is a flowchart showing the operation of the X-ray CT apparatus 1 in the first embodiment.
FIG. 3 is a diagram showing a flow of operation of the X-ray CT apparatus 1 in the first embodiment.

In the scanner unit 3, the X-ray CT apparatus 1 performs X-ray irradiation on the subject 25 and collects projection data 51 (step 101).
As for X-ray irradiation, it is desirable to irradiate with a dose 1 / N times the normal dose in order to reduce the exposure dose. In this case, the effect of reducing the exposure dose is improved as N is increased, but N is preferably about 2 to 3.

The X-ray CT apparatus 1 performs image reconstruction by the reconstruction unit 33 of the arithmetic processing unit 7 based on the projection data collected by the scanner unit 3, and creates a reconstructed image 53 (tomographic image data) (step 102). ).
The X-ray CT apparatus 1 determines whether or not there is a displacement of the reconstructed image by the displacement detection means 35 of the arithmetic processing unit 7 with respect to the reconstructed image created by the reconstruction means 33 (step 103). ), When the positional deviation of the reconstructed image occurs (Yes in step 103), the positional deviation correction of the reconstructed image is performed by the positional deviation correcting means 37 of the arithmetic processing unit 7 (step 104).
The process for detecting misalignment of the reconstructed image (step 103) and the process for correcting misalignment of the reconstructed image (step 104) will be described later.
The X-ray CT apparatus 1 performs addition averaging in the time axis direction on the reconstructed image obtained by the processing so far (step 105).
Note that the averaging process (step 105) of the reconstructed image in the time axis direction will be described later.

Here, with reference to FIG. 4, the reconstructed image misregistration detection process (FIG. 2: step 103) and the reconstructed image misregistration correction process (FIG. 2: step 104) will be described.
FIG. 4 is a flowchart showing the operation of the X-ray CT apparatus 1 in the reconstructed image misalignment detection process (step 103) and the reconstructed image misalignment correction process (step 104).

  The X-ray CT apparatus 1 binarizes the reconstructed image 53 (step 151). In the binarization processing, for example, in the reconstructed image, the pixel value of a pixel having a pixel value greater than or equal to a predetermined threshold is set to “1”, and the pixel value of a pixel having a pixel value less than the predetermined threshold is set to “0”. This is a replacement process. The threshold value may be any value that allows separation of the biological tissue to be analyzed and room air. For example, the threshold value may be set to “−200”.

The X-ray CT apparatus 1 performs a labeling process on the image binarized in step 151 (step 152).
The labeling process is a process of assigning the same number (label) to each connected component (all connected pixels) and assigning another label to different connected components. For example, when adjacent pixels are “1”, these pixels are connected. Note that the value assigned as the label is, for example, a serial number from “50”.

The X-ray CT apparatus 1 searches for the maximum connected component (step 153), and extracts the maximum connected component (step 154).
The search process for the maximum connected component is a process of scanning the entire labeled image, counting the number of pixels for each label value, and selecting the connected component having the label value with the largest number of pixels.
The maximum connected component extraction process is a process of leaving the pixel having the label value with the largest number of pixels selected in step 153 and replacing the pixel values of pixels other than the pixel with “0”. With this process, the pixel value of an unnecessary area such as a bed where the pixel value may be replaced with “1” is replaced with “0” in the binarization process.

The X-ray CT apparatus 1 tracks the contour of the largest connected component and extracts the outermost contour of the largest connected component (step 155).
The maximum connected component contour tracking process scans rightward laterally from the upper left corner pixel of the image where only the maximum connected component was left in step 154, and starts with the first non-zero pixel value encountered. Is a process of tracing the contour in the counterclockwise direction and ending the tracking when returning to the start point.
It is desirable to substitute a value different from the label value, for example, “1” for the pixel value of the pixel on the contour line.

The X-ray CT apparatus 1 fills the inside of the contour calculated in the process of step 155 and creates a target region binarized image (step 156).
As a process for filling the inside of the contour line, a conventional seed fill algorithm (a process for filling the inside of the closed region with one point inside the closed region as a starting point) or the like can be applied.
In the present embodiment, the closed region is the contour line obtained in step 155, and the points inside the closed region are pixels with the label value of the maximum connected component obtained in step 154. If one pixel with this label value is detected and seed fill processing is performed using this pixel as a starting point, the inside of the contour line can be filled.

The X-ray CT apparatus 1 calculates the center of gravity (Xc, Yc) and inertial principal axis θ (inclination angle of inertial equivalent ellipse) in the target region binarized image created in step 156 (step 157).
The center of gravity (Xc, Yc) and the inertial principal axis θ can be obtained according to [Equation 1].
However, I (x, y) is a pixel value at coordinates (x, y). In the target binarized image, I (x, y) = 1. Further, a, b, c in [Formula 1] are represented by [Formula 2].

The X-ray CT apparatus 1 calculates the difference between the center of gravity (Xc, Yc) and inertia principal axis θ in the target region binarized image calculated in step 157 and the reference center of gravity (X ₀ , Y ₀ ) and reference inertia principal axis θ ₀ . ΔXc = | Xc−Xc ₀ |, ΔYc = | Yc−Yc ₀ |, Δθ = | θ−θ ₀ | are calculated (step 158).

The reference centroid (X ₀ , Y ₀ ) can be set as appropriate.
When the reference centroid (X ₀ , Y ₀ ) is set to about half the size (number of pixels) of the reconstructed image, for example, when the size of the reconstructed image is 512 × 512 pixels, the reference centroid (X ₀ , Y ₀ ) = (255, 255).
Further, the reference centroid (X ₀ , Y ₀ ) may be the centroid of the target region binarized image in any specific time phase obtained by dynamic imaging, for example, the first of a series of reconstructed images obtained by dynamic imaging. The centroid of the target area binarized image in the time phase may be the reference centroid (X ₀ , Y ₀ ).

The reference inertia main axis θ ₀ can be set as appropriate, and can be set to θ / 2 rad, for example.
The reference inertia main axis θ ₀ may be a reference inertia main axis of the target region binarized image in any specific time phase obtained by dynamic imaging. For example, the reference inertia main axis θ ₀ the principal axis of inertia of the target image binarized image in the time phase may be used as the reference principal axis theta _0.

  The X-ray CT apparatus 1 determines whether or not the difference from the reference value calculated in step 158, ΔXc, ΔYc, Δθ is larger than a predetermined allowable value (step 159), and if it is larger than the predetermined value The reconstructed image of the time phase to be processed is determined to be misaligned (Yes in step 159, FIG. 2: Yes in step 103), and the misalignment is corrected by translation, rotational movement, etc. with respect to the reconstructed image. (Step 160, FIG. 2: Step 104).

In step 159, if at least one of ΔXc> εx (allowable value), ΔYc> εy (allowable value), and Δθ> εθ (allowable value), there is a displacement of the reconstructed image (Yes in step 159). FIG. 2: Yes in step 103).
Further, the positional deviation correction by the parallel movement and the rotational movement in Step 160 can be performed according to [Equation 3].
However, (x, y) represents arbitrary coordinates in the reconstructed image before the positional deviation correction, and (x1, y1) represents arbitrary coordinates in the reconstructed image after the positional deviation correction.

  The X-ray CT apparatus 1 performs position shift correction on a series of reconstructed images obtained by dynamic imaging by performing the above processing steps (steps 151 to 160) on all the reconstructed images 53. Do.

Here, the averaging process (FIG. 2: step 105) in the time axis direction of the reconstructed image will be described.
In order to obtain an S / N ratio on a reconstructed image that is equal to or higher than that obtained when taking a normal dose, the average of N or more reconstructed images is added. It is desirable to take.
Hereinafter, a case will be described in which the averaging process is performed using a total of three reconstructed images of an arbitrary time phase t _n , a time phase t _n−1 before and after that, and a time phase t _{n + 1} .
_{Assuming that} the reconstructed image at the time phase t _n is P (t _n ), the reconstructed image P ′ (t _n ) after the averaging process in the time direction is expressed as [Equation 4].

  W is a weight function and is expressed as [Formula 5].

FIG. 5 is a graph showing the relationship between W (Δt) (weight) and Δt (time phase difference).
W (Δt) is a weight determined depending on the time phase difference. W (Δt) can be determined by, for example, the graph shown in FIG.
FIG. 6 is a graph showing the relationship between W (Δr) (weight) and Δr (center of gravity misalignment).
W (Δr) is a weight determined depending on the displacement of the center of gravity. W (Δr) can be determined by, for example, the graph shown in FIG. Δr is expressed as [Formula 6].

FIG. 7 is a graph showing the relationship between W (Δθ) (weight) and Δθ (shift of inertia main axis).
W (Δθ) is a weight determined depending on the deviation of the principal axis of inertia. W (Δθ) can be determined by, for example, the graph shown in FIG.

K1 (= W (Δr = 0)) in FIG. 6 and k2 (= W (Δθ = 0)) in FIG. 7 are real numbers between 0 and 1, and can be arbitrarily designated by the operator.
Since the positional deviation when the subject moves in the horizontal direction on the imaging section can be corrected with high accuracy, k1 and k2 are desirably set to values close to 1.
As for the positional deviation when the subject moves in the direction perpendicular to the imaging section, the correction accuracy tends to be slightly lowered, so k1 and k2 are set to values smaller than 1 depending on the magnitude of the positional deviation. It is desirable.

  In the present embodiment, the method of taking the addition average from the three reconstructed images has been described. However, the number of addition averages is not limited to three, and the addition average may be taken from any number. It is realized in the same way.

  Through the above process, the X-ray CT apparatus 1 collects projection data for a plurality of time phases, performs image reconstruction, creates a reconstructed image, and shifts the position of the reconstructed image based on the difference in image moments. , And a positional shift correction is performed on the reconstructed image of the time phase in which the misalignment is detected, and a reconstructed image of the target time phase is created by adding and averaging a plurality of reconstructed images of the time phase.

As described above, in the first embodiment, the X-ray CT apparatus corrects misalignment for the reconstructed image in the time phase where misalignment is detected, so there is no need to repeatedly perform calculations for all reconstructed images. In addition, the X-ray CT apparatus captures a reconstructed image in each time phase by performing imaging with a dose smaller than usual in each time phase, and performing averaging in the time direction for the reconstructed images of a plurality of time phases. create.
Accordingly, the processing time required for body motion correction can be shortened, the quality of the captured image can be improved, and the exposure dose can be reduced without sacrificing the S / N ratio, time resolution, and the like.

  In general, an X-ray CT apparatus acquires projection data and sequentially performs image reconstruction. That is, projection data acquisition and image reconstruction processing are incorporated into the X-ray CT apparatus as a series of processing. The X-ray CT apparatus according to the first embodiment performs a misalignment detection process and a misalignment correction process on a reconstructed image after performing a series of processes of acquiring projection data and reconstructing an image. Therefore, the above-described misregistration detection processing function and misregistration correction processing function can be easily implemented in an existing X-ray CT apparatus.

Next, the operation of the X-ray CT apparatus 1 according to the second embodiment of the present invention will be described with reference to FIGS.
FIG. 8 is a flowchart showing the operation of the X-ray CT apparatus 1 in the second embodiment.
FIG. 9 is a diagram showing an operation flow of the X-ray CT apparatus 1 in the second embodiment.

In the scanner unit 3, the X-ray CT apparatus 1 performs X-ray irradiation on the subject 25 and collects projection data 51 (step 201).
As for X-ray irradiation, it is desirable to irradiate with a dose 1 / N times the normal dose in order to reduce the exposure dose. In this case, the effect of reducing the exposure dose is improved as N is increased, but N is preferably about 2 to 3.

The X-ray CT apparatus 1 determines whether or not there is a displacement of the projection data by the displacement detection means 35 of the arithmetic processing unit 7 with respect to the collected projection data (step 202), and the displacement of the projection data is determined. If this occurs (Yes in step 202), the positional deviation correction means 37 of the arithmetic processing unit 7 corrects the positional deviation of the projection data (step 203).
The projection data misalignment detection process (step 202) and the projection data misalignment correction process (step 203) will be described later.
The X-ray CT apparatus 1 performs addition averaging in the time axis direction with respect to a series of projection data groups obtained by the processing so far (step 204).
The addition averaging process (step 204) of the projection data in the time axis direction will be described later.
The X-ray CT apparatus 1 performs image reconstruction by the reconstruction unit 33 of the arithmetic processing unit 7 based on the projection data obtained in step 204, and creates a reconstructed image 53 (tomographic image data) (step 205). ).

  Here, with reference to FIG. 10 to FIG. 14, the projection data misalignment detection process (FIG. 8: step 202) and the projection data misalignment correction process (FIG. 8: step 203) will be described.

10 and 11 are diagrams showing projection data in the time phase t and the time phase t ′, respectively.
10 and 11, the horizontal axis indicates the channel 61 (Channel: C), and the vertical axis indicates the projection angle 62 (View: V).

12 and 13 are diagrams showing a profile of projection data at a projection angle 62 = φ.
12 and 13, the horizontal axis indicates the channel 61 (Channel: C), and the vertical axis indicates the signal intensity 67 (Signal: S).
FIG. 14 is a diagram showing the direction of body movement of the subject 25.

Point 63 and point 64 indicate the rising position and falling position of the profile curve at time phase t and projection angle 62 = φ, respectively. That is, points 63 and 64 indicate the position of the boundary between the subject 25 and room air at time phase t.
Point 65 and point 66 indicate the rising position and falling position of the profile curve at time phase t ′ and projection angle 62 = φ, respectively. That is, the points 65 and 66 indicate the positions of the boundaries between the subject 25 and the room air at the time phase t ′.

  When there is no body movement of the subject 25, when the projection data profiles in the channel 61 direction are compared at the same projection angle 62 = φ, the boundary between the subject 25 and the room air in the time phase t and the time phase t ′ The positions of the channels 61 coincide. That is, the point 63 (a) and the point 65 (a ′) match, and the point 64 (b) and the point 66 (b ′) match.

  On the other hand, when there is body movement of the subject 25, for example, when the subject 25 moves in the direction of the arrow 71 from the position at the time phase t in the time phase t ', that is, from the X-ray source 13 to the detector array. When the subject moves in a direction perpendicular to the axis toward 17, the profile of the projection data at the time phase t is shown by the profile curve 68 in FIG. 12, whereas the profile of the projection data at the time phase t ′ is This is indicated by a profile curve 69 in FIG. Accordingly, the rising position and the falling position of the profile curve 68 at the time phase t are the points 63 (a) and 64 (b), whereas the rising position and the falling position of the profile curve 69 at the time phase t ′. Are point 65-1 (a ′) and point 66-1 (b ′).

  Further, when there is a body movement of the subject 25, for example, in the time phase t ′, when the subject 25 moves in the direction of the arrow 72 from the position in the time phase t, that is, from the X-ray source 13 to the detector array. When the subject moves in a direction parallel to the axis toward 17, the profile of the projection data at the time phase t is shown by the profile curve 68 in FIG. 13, whereas the profile of the projection data at the time phase t ′ is This is indicated by a profile curve 70 in FIG. Accordingly, the rising position and the falling position of the profile curve 68 at the time phase t are the points 63 (a) and 64 (b), whereas the rising position and the falling position of the profile curve 70 at the time phase t ′. Are point 65-2 (a ′) and point 66-2 (b ′).

  The X-ray CT apparatus 1 has a predetermined difference (Δa = | a−a ′ |, Δb = | b−b ′ |) between the rising position and the falling position of the profile curve at time phase t and time phase t ′. If the allowable value is exceeded, it is determined that there is a positional deviation of the projection data (FIG. 8: Yes in Step 202). Otherwise, it is determined that there is no positional deviation of the projection data (FIG. 8: No in Step 202). Note that if at least one of Δa> εa (allowable value) and Δb> εb (allowable value), it may be determined that there is a positional deviation of the projection data.

When the X-ray CT apparatus 1 determines that there is a positional deviation of the projection data (FIG. 8: Yes in step 202), the X-ray CT apparatus 1 corrects the positional deviation of the projection data by [Equation 7] (FIG. 8: step 203).
S (C) indicates the signal value at the channel 61 position C before the correction of the positional deviation of the projection data, and S1 (C) indicates the signal value at the channel 61 position C after the correction of the positional deviation of the projection data.

  The X-ray CT apparatus 1 performs position shift correction on a series of projection data obtained by dynamic imaging by performing the above-described processing on all projection data.

Here, the addition averaging process (FIG. 8: step 204) about the time-axis direction of projection data is demonstrated.
In order to obtain an S / N ratio on a reconstructed image that is equal to or higher than that obtained when shooting with a normal dose, the average of projection data for N or more rounds is added. It is desirable to take.
Hereinafter, a case will be described in which addition averaging processing is performed using projection data of a total of three time phases of an arbitrary time phase t _n , a time phase t _n−1 before and after that, and a time phase t _{n + 1} .
_{Assuming that} the projection data in the time phase t _n is R (t _n ), the projection data R ′ (t _n ) after the averaging process in the time direction is expressed as [Equation 8].

  w is a weighting function and is expressed as [Equation 9].

FIG. 15 is a graph showing the relationship between w (Δt) (weight) and Δt (time phase difference).
w (Δt) is a weight determined depending on the time phase difference. For example, w (Δt) can be determined by the graph shown in FIG.
FIG. 16 is a graph showing the relationship between w (l) (weight) and l (deviation amount based on body movement in a direction horizontal to the axis from the X-ray tube toward the detector).
w (l) is a weight determined depending on the positional deviation in the direction of the arrow 72 (FIG. 14). W (l) can be determined by the graph shown in FIG. 16, for example. In addition, l is expressed as [Formula 10].

FIG. 17 is a graph showing the relationship between W (m) (weight) and m (deviation amount based on body movement in a direction perpendicular to the axis from the X-ray tube toward the detector).
W (m) is a weight determined depending on the deviation in the direction of the arrow 71 (FIG. 14). W (m) can be determined by, for example, the graph shown in FIG. Note that m is expressed as [Formula 11].

K3 (= w (1 = 0)) in FIG. 16 and k4 (= w (m = 0)) in FIG. 17 are real numbers between 0 and 1, and can be arbitrarily designated by the operator.
When the positional deviation is relatively small, k3 and k4 are desirably set to values close to 1.
When the positional deviation is relatively large, k3 and k4 are desirably set to a value smaller than 1 depending on the magnitude of the positional deviation.

In [Equation 8], for projection data R (t _n−1 ) and projection data R (t _{n + 1} ) before and after time phase t _n , data for one round may be used, or for half image For example, for R (t _n-1 ), projection data at a projection angle of 180 ° to 360 ° is used, and for R (t _{n + 1} ), a projection angle of 0 ° to 180 ° is used. Projection data may be used.

In the present embodiment, a method has been described in which an addition average is calculated from projection data of a total of three time phases, that is, an arbitrary time phase t _n , a time phase t _n−1 before and after that, and a time phase t _{n + 1.} The number of time phases to be taken is not limited to three, and the same can be realized in the case of taking an averaging from an arbitrary number of time phases.

  Through the above process, the X-ray CT apparatus 1 collects projection data for a plurality of time phases, detects the positional deviation of the projection data based on the profile of the projection data, and detects the positional deviation of the temporal phase. Is corrected, and the projection data of the target time phase is created by averaging the projection data of multiple time phases, and the reconstructed image is created by performing image reconstruction processing on a series of projection data groups To do.

As described above, in the second embodiment, the X-ray CT apparatus corrects misalignment with respect to the projection data at the time phase in which misalignment is detected, so there is no need to repeatedly perform calculations for all projection data. The X-ray CT apparatus performs imaging with a dose smaller than usual in each time phase, and creates projection data in each time phase by performing averaging of the projection data in a plurality of time phases in the time direction.
Accordingly, the processing time required for body motion correction can be shortened, the quality of the captured image can be improved, and the exposure dose can be reduced without sacrificing the S / N ratio, time resolution, and the like.

  In addition, the X-ray CT apparatus of the second embodiment performs misalignment detection processing based on the rising position and falling position of the profile of the projection data. That is, the X-ray CT apparatus performs a positional deviation detection process and a positional deviation correction process on the projection data before image reconstruction. Therefore, the X-ray CT apparatus does not need to perform the calculation process related to the image moment (see [Formula 1] and [Formula 2]) as in the first embodiment described above, and the process for the reconstructed image. The burden can be reduced.

Next, the operation of the X-ray CT apparatus 1 according to the third embodiment of the present invention will be described with reference to FIGS.
FIG. 18 is a flowchart showing the operation of the X-ray CT apparatus 1 in the third embodiment.
FIG. 19 is a diagram illustrating an operation flow of the X-ray CT apparatus 1 according to the third embodiment.

In the scanner unit 3, the X-ray CT apparatus 1 performs X-ray irradiation on the subject 25 and collects projection data 51 (step 301).
As for X-ray irradiation, it is desirable to irradiate with a dose 1 / N times the normal dose in order to reduce the exposure dose. In this case, the effect of reducing the exposure dose is improved as N is increased, but N is preferably about 2 to 3.

The X-ray CT apparatus 1 performs image reconstruction by the reconstruction unit 33 of the arithmetic processing unit 7 based on the projection data collected by the scanner unit 3, and creates a reconstructed image 53 (tomographic image data) (step 302). ).
The X-ray CT apparatus 1 determines whether or not there is a misalignment of the reconstructed image by the misalignment detecting means 35 of the arithmetic processing unit 7 with respect to the reconstructed image created by the reconstructing means 33 (step 303). ) If the positional deviation of the reconstructed image has occurred (Yes in step 303), it is determined whether the positional deviation of the reconstructed image has occurred in the preceding and succeeding time phases or only in that time phase. (Step 304).

When the positional deviation of the reconstructed image occurs in the preceding and following time phases, that is, when the positional deviation of the reconstructed image occurs continuously (the time during which the positional deviation of the reconstructed image has occurred is a predetermined time. In such a case (Yes in Step 304), the X-ray CT apparatus 1 corrects the misalignment of the reconstructed image by the misalignment correcting unit 37 of the arithmetic processing unit 7 (Step 305).
When the position shift of the reconstructed image occurs only in the time phase, that is, when the position shift of the reconstructed image is not continuous (the time when the position shift of the reconstructed image occurs is less than a predetermined time. In such a case (No in step 304), the X-ray CT apparatus 1 performs interpolation processing using the reconstructed images of the time phases before and after the time phase, and creates a reconstructed image in the time phase (step 306). ).
The X-ray CT apparatus 1 performs addition averaging in the time axis direction on the reconstructed image obtained by the processing so far (step 307).

  It should be noted that with respect to the process for detecting misalignment of the reconstructed image (step 303), the process for correcting misalignment of the reconstructed image (step 305), and the averaging process for the reconstructed image in the time axis direction (step 307), The same technique as that described in the description of the embodiment can be used.

  Through the above process, the X-ray CT apparatus 1 collects projection data for a plurality of time phases, performs image reconstruction, creates a reconstructed image, and shifts the position of the reconstructed image based on the difference in image moments. When the positional deviation of the reconstructed image is detected continuously, the positional deviation correction is performed on the reconstructed image at the time phase when the positional deviation is detected, and the positional deviation of the reconstructed image occurs sporadically. If there is, the interpolation processing is performed using the reconstructed images of the time phases before and after the time phase, and a reconstructed image of the time phase is created. Further, the X-ray CT apparatus 1 adds and averages a plurality of time phase reconstruction images to create a target time phase reconstruction image.

  As described above, in the X-ray CT apparatus of the third embodiment, as in the first embodiment, the X-ray CT apparatus performs imaging with a dose smaller than usual in each time phase, and a plurality of time phases. A reconstructed image in each time phase is created by averaging the reconstructed images in the time direction. Accordingly, the processing time required for body motion correction can be shortened, the quality of the captured image can be improved, and the exposure dose can be reduced without sacrificing the S / N ratio, time resolution, and the like. Further, the positional deviation detection processing function and the positional deviation correction processing function can be easily implemented in an existing X-ray CT apparatus.

  In the third embodiment, the X-ray CT apparatus includes a positional deviation correction process based on a difference in image moment, which has a relatively large processing burden, and an interpolation process, which has a small processing burden, according to the degree of occurrence of the positional deviation. Therefore, the processing load can be reduced and the speed of processing can be improved while maintaining the positional deviation correction accuracy.

Next, the operation of the X-ray CT apparatus 1 according to the fourth embodiment of the present invention will be described with reference to FIGS.
FIG. 20 is a flowchart showing the operation of the X-ray CT apparatus 1 in the fourth embodiment.
FIG. 21 is a diagram showing a flow of operation of the X-ray CT apparatus 1 in the fourth embodiment.

The X-ray CT apparatus 1 performs X-ray irradiation on the subject 25 in the scanner unit 3 and collects projection data 51 (step 401).
As for X-ray irradiation, it is desirable to irradiate with a dose 1 / N times the normal dose in order to reduce the exposure dose. In this case, the effect of reducing the exposure dose is improved as N is increased, but N is preferably about 2 to 3.

  The X-ray CT apparatus 1 determines whether or not there is a positional deviation of the projection data by the positional deviation detection means 35 of the arithmetic processing unit 7 with respect to the collected projection data (step 402), and the positional deviation of the projection data. If this occurs (Yes in step 402), it is determined whether or not the positional deviation of the projection data occurs in the preceding and succeeding time phases or only in the time phase concerned (step 403).

When the positional deviation of the projection data occurs also in the preceding and following time phases, that is, when the positional deviation of the projection data occurs continuously (the time when the positional deviation of the projection data occurs is a predetermined time or more. In some cases (Yes in step 403), the X-ray CT apparatus 1 corrects misalignment of projection data by the misalignment correcting means 37 of the arithmetic processing unit 7 (step 404).
When the positional deviation of the projection data occurs only in the time phase, that is, when the positional deviation of the projection data is not continuous (such as when the positional deviation of the projection data is less than a predetermined time). (No in step 403), the X-ray CT apparatus 1 performs an interpolation process using the projection data of the time phase before and after the time phase, and creates projection data in the time phase (step 405).
The X-ray CT apparatus 1 performs addition averaging in the time axis direction with respect to a series of projection data groups obtained by the processing so far (step 406).
The X-ray CT apparatus 1 performs image reconstruction by the reconstruction unit 33 of the arithmetic processing unit 7 based on the projection data obtained in step 406, and creates a reconstruction image 53 (tomographic image data) (step 407). ).

  It should be noted that the projection data misregistration detection process (step 402), the projection data misregistration correction process (step 404), and the projection data time axis direction averaging process (step 406) are the second embodiment. The same method as that described in the explanation can be used.

  Through the above process, the X-ray CT apparatus 1 collects projection data for a plurality of time phases, detects the positional deviation of the projection data based on the profile of the projection data, and the positional deviation of the projection data continuously occurs. If the positional deviation of the projection data is sporadically generated, the temporal phase projection data before and after the temporal phase are used. Interpolation processing is performed to create projection data for the time phase concerned. In addition, the X-ray CT apparatus 1 adds and averages a plurality of time-phase projection data to create target time-phase projection data, performs image reconstruction processing on a series of projection data groups, and reconstructs an image. Create

  As described above, in the X-ray CT apparatus of the fourth embodiment, as in the second embodiment, the X-ray CT apparatus performs imaging with a dose smaller than usual in each time phase, and a plurality of time phases. Projection data in each time phase is created by performing averaging in the time direction for the projection data. Accordingly, the processing time required for body motion correction can be shortened, the quality of the captured image can be improved, and the exposure dose can be reduced without sacrificing the S / N ratio, time resolution, and the like. In addition, it is not necessary to perform a calculation process related to the image moment (see [Formula 1] and [Formula 2]), and the processing load on the reconstructed image can be reduced.

  In the fourth embodiment, the X-ray CT apparatus includes a misalignment correction process based on a profile of projection data with a relatively large processing load and an interpolation process with a small processing load according to the degree of occurrence of misalignment. Therefore, the processing load can be reduced and the speed of processing can be improved while maintaining the positional deviation correction accuracy.

  In addition, about operation control of processing in the above-described first to fourth embodiments, various processes, and the like, a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), An electronic computer such as a computer having a storage device (such as a hard disk) or an input / output device (such as a media reader) can be used.

Programs necessary for various processes / controls, data necessary for program execution, input data, etc. are stored in a storage device, and the CPU calls and executes these in the work memory area on the RAM when the process is executed. The above-mentioned various functions can be realized by performing operation control and comparison operation, hardware and software operation control, and the like.
Also, a program or the like necessary for various processes and controls may be held and distributed on a recording medium such as a CD-ROM, or the program can be transmitted and received via a communication line.

  Further, the configuration of the arithmetic processing unit 7 of the X-ray CT apparatus 1 is not limited as long as it can perform arithmetic processing and storage processing of digital data or the like. The arithmetic processing unit 7 may be configured separately from the main body of the X-ray CT apparatus 1 using a general personal computer or the like.

  The preferred embodiments of the X-ray CT apparatus and the like according to the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to such examples. It will be apparent to those skilled in the art that various changes or modifications can be conceived within the scope of the technical idea disclosed in the present application, and these are naturally within the technical scope of the present invention. Understood.

Block diagram showing the configuration of the X-ray CT apparatus 1 Flow chart showing operation of X-ray CT apparatus 1 (first embodiment) The figure which shows the flow of operation | movement of X-ray CT apparatus 1 (1st Embodiment). A flowchart showing the operation of the X-ray CT apparatus 1 in the detection process of the positional deviation of the reconstructed image and the positional deviation correction process of the reconstructed image. Graph showing the relationship between W (Δt) (weight) and Δt (time difference) Graph showing the relationship between W (Δr) (weight) and Δr (center of gravity misalignment) Graph showing the relationship between W (Δθ) (weight) and Δθ (shift of inertia main axis) Flow chart showing operation of X-ray CT apparatus 1 (second embodiment) The figure which shows the flow of operation | movement of X-ray CT apparatus 1 (2nd Embodiment). The figure which shows the projection data in the time phase t The figure which shows the projection data in time phase t ' The figure which shows the profile of the projection data in the projection angle 62 = phi The figure which shows the profile of the projection data in the projection angle 62 = phi The figure which shows the direction of the body movement of the subject 25 A graph showing the relationship between w (Δt) (weight) and Δt (time phase difference) Graph showing the relationship between w (l) (weight) and l (deviation amount based on body movement in the direction horizontal to the axis from the X-ray tube toward the detector) Graph showing the relationship between W (m) (weight) and m (deviation amount based on body movement in a direction perpendicular to the axis from the X-ray tube toward the detector) Flow chart showing operation of X-ray CT apparatus 1 (third embodiment) The figure which shows the flow of operation | movement of X-ray CT apparatus 1 (3rd Embodiment). Flow chart showing operation of X-ray CT apparatus 1 (fourth embodiment) The figure which shows the flow of operation | movement of X-ray CT apparatus 1 (4th Embodiment).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ......... X-ray CT apparatus 3 ......... Scanner part 5 ......... Scanner control part 7 ......... Calculation processing part 9 ......... Image display part 11 ......... Gantry 13 ......... X-ray source 15 ... ... collimator 17 ... detector array 19 ... detector element 21 ... X-ray 23 ... center of rotation 27 ... X-ray control means 29 ... ... gantry control means 31 ... DAS
33 ......... Reconstruction means 35 ......... Position deviation detection means 37 ......... Position deviation correction means 39 ......... Addition averaging means 41 ......... Storage means 43 ......... Temporary storage means 45 ......... Input means 51 ……… Projection data 53 ……… Reconstructed image 61 ……… Channel 62 ……… Projection angle 67 ……… Signal intensity

Claims (7)

An X-ray CT apparatus that scans a subject and collects projection data for a plurality of time phases, performs image reconstruction processing on the projection data, and creates a reconstructed image for the plurality of time phases. There,
Calculating means for calculating an image moment of the reconstructed image;
A misregistration detection means for determining the presence or absence of misregistration based on the difference of the image moment in each time phase ;
On the reconstructed image is determined that there is the positional deviation, the position deviation correction means for performing a positional deviation correcting process,
A time direction addition averaging means for performing addition averaging in the time direction on the reconstructed image after the positional deviation correction processing;
An X-ray CT apparatus comprising:   The imaging is imaging at a dose of 1 / N of the normal dose,
  2. The X-ray CT apparatus according to claim 1, wherein the time direction addition averaging means takes an addition average of N or more reconstructed images obtained by image reconstruction processing.   A target area binarized image creating means for creating a target area binarized image of the reconstructed image;
  The calculation means calculates the center of gravity and the inertial principal axis of the target region using the target region binarized image,
  2. The position shift detection unit detects a position shift of the reconstructed image by detecting a difference between the calculated center of gravity and inertial principal axis and a reference center of gravity and reference inertial principal axis. X-ray CT system. The X-ray CT apparatus according to claim 1, further comprising weighted addition means for creating a reconstructed image of a target time phase by weighted addition of reconstructed images for a plurality of time phases. The counter further includes a determination unit that counts the number of times that the time phase in which the positional deviation has occurred continues, and determines whether the number of times reaches a predetermined number,
The misregistration correction means includes
When the determination unit determines that the number of times does not reach the predetermined number, the interpolation process is performed using the reconstruction images of the preceding and succeeding time phases to create a reconstruction image of the target time phase,
When the determination unit determines that the number of times has reached the predetermined number, the reconstructed image for a plurality of time phases is weighted and added to create a reconstructed image of the target time phase. The X-ray CT apparatus according to claim 1. Depending on the size and time phase difference of the positional deviation, X-rays CT apparatus according to claim 4 or claim 5, characterized in that it comprises a weight determining means for determining the weight of the weighted addition.   The weight determination means is characterized in that the weight of the time phase when the subject moves in the direction horizontal to the imaging section is set larger than the weight of the time phase when the subject moves in the direction perpendicular to the imaging section. The X-ray CT apparatus according to claim 6.

Images