JP2010243470A - Ct device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a sectional image with a wide dynamic range using a radiation detector for output of a transmission image with a plurality of color components. <P>SOLUTION: The CT device includes an X-ray detector 3 having a color X-ray II3a for detecting a radioactive ray and converting to a color visible light image, and a color camera 3b for photographing a color visible light image to output transmission image data for each color component, in which the sectional image of an object 5 to be inspected is reconfigured using monochrome transmission image data that transmission image data for each color component obtained by the color X-ray II3a at a plurality scanning positions is added mutually by an image synthesizing section 9e. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a computed tomography apparatus (hereinafter referred to as a CT (Computed Tomography) apparatus) that captures a cross-sectional image of a subject.

  A CT apparatus called a so-called RR (Rotate Rotate) method (third generation method) in a conventional CT apparatus irradiates a subject with radiation (X-rays) generated from a radiation source, and irradiates the subject with radiation light. Radiation having a plurality of one-dimensional or two-dimensional detection channels that are rotated relative to the radiation at a rotation axis that intersects the direction of the axis, and that transmits radiation from the subject at each predetermined rotational position during one rotation. This is detected by a detector, and a cross-sectional image or three-dimensional data of the subject is obtained (tomographic imaging) from the detector output.

  FIG. 7 is a schematic diagram (front view) showing a configuration of a conventional CT apparatus described in Patent Document 1. As shown in FIG. An X-ray tube 101 and an X-ray detector 103 that detects a pyramid-shaped X-ray beam 102 generated from the X-ray tube 101 with a two-dimensional resolution are arranged to face each other, and are placed on a table 104 so as to enter the X-ray beam 102. A transmission image (transmission image data) of the placed subject 105 is obtained.

  The table 104 is disposed on the rotation / elevation mechanism 107, and when taking a cross-sectional image of the subject 105, transmission images are displayed in a plurality of directions while the table 104 is rotated once by the rotation / elevation mechanism 107 with respect to the rotation axis RA. Get (referred to as scanning). The plurality of transmission images are processed by the control processing unit 108 to obtain cross-sectional images (one or more) of the subject 105.

  As the X-ray detector 103, an X-ray image intensifier (hereinafter referred to as X-ray II) 103a that converts an intensity distribution image of the X-ray beam 102 into a visible light image, and a visible image obtained by photographing the visible light image (see FIG. An X-ray detector 103 comprising a camera 103b that outputs as transmitted image data) is used.

  In the CT apparatus described above, the X-ray detector 103 outputs a single-color transmission image, while a radiation detector that outputs transmission images of a plurality of color components is known. As a first example, a radiation detector using a color X-ray image intensifier (abbreviated as color II (registered trademark) hereinafter referred to as color X-ray II) and a color camera is disclosed in Patent Document 2 and the like. Are known.

  Color X-ray II converts the radiation (X-ray) distribution incident on the scintillator layer on the input surface into an electron distribution, accelerates the electrons to form an image on the output surface, and causes the color scintillator layer on the output surface to emit light. It converts to a visible light image. The color scintillator on the output surface emits light in colors (multiple colors), but the emission characteristic curve is different for each color component (R, G, B: red, green, blue). That is, there is a characteristic that the sensitivity is higher in the order of R, G, and B with respect to the amount of incident electrons.

  The color camera captures the converted color visible light image and outputs a transmission image for each color component (R, G, B).

  As a second example, a radiation detector using a color scintillator and a color camera is known from Patent Document 3 and the like.

  In this method, radiation (X-rays) is incident on a color scintillator layer on the input surface to emit light, thereby converting the radiation distribution into a visible light image and photographing the visible light image with a color camera. . The color scintillator emits light in color, but the emission characteristic curve is different for each color component (R, G, B: red, green, blue). That is, there is a characteristic that the sensitivity is higher in the order of R, G, and B with respect to the amount of incident radiation. The color camera captures the converted color visible light image and outputs a transmission image for each color component (R, G, B).

  When the transmission images of the above-mentioned multiple color components are observed in color display, the low transmittance part (low radiation part) is highly sensitive in red and details can be observed well, and the high transmittance part (high radiation part) is saturated in red. However, details can be observed well in blue with low sensitivity. That is, a radiation detector with a wide dynamic range is possible with this configuration.

  FIG. 8 is a graph showing an example of a detection characteristic curve of a radiation detector that outputs transmission images of a plurality of color components. The horizontal axis represents the incident X-ray dose to one pixel, and the vertical axis represents the output of one pixel. The input range for each color component R, G, B until the output reaches the saturation level from the noise level is the dynamic range. The dynamic range of the transmitted color image is a region of the logical sum of the dynamic ranges of the respective color components, and is increased as compared with the case of a single color.

  The radiation detector that outputs a transmission image of a plurality of color components described above is effectively used in a radiographic inspection apparatus that displays a color transmission image, but a CT apparatus using this radiation detector has not yet been realized. Not.

JP 2002-62268 A JP 2006-179424 A JP 2003-202382 A

  In the case of using a radiation detector that outputs a transmission image of a plurality of color components having a wide dynamic range in a conventional CT apparatus, a cross-sectional image is reconstructed from each transmission image of each color component, and then the respective cross-sectional images are synthesized. However, it is not a cross-sectional image with a wide dynamic range.

  The reason for this is that in cross-sectional image reconstruction, if there is a saturated portion or a portion below the noise level in the transmission image data used for reconstruction, the entire cross-sectional image is broken (remarkably deteriorated), so that the cross-sections of all color components This is because the dynamic range is narrowed to the logical product area (A in FIG. 8) of the dynamic range of each color component as a condition that the image is not broken. That is, when taking a cross section, it is necessary to adjust the X-ray condition and the geometric condition so that the incident X-ray dose falls within the narrow dynamic range A. That is, a cross-sectional image with a narrow dynamic range is taken. That is.

  Therefore, in order to widen the dynamic range of the cross-sectional image, it is preferable to reconstruct the single-color transmission image having the maximum dynamic range by combining the R, G, and B transmission images before reconstruction. It is done.

  For example, as disclosed in Patent Document 3, a method of creating a monochromatic transmission image from R, G, and B transmission images employs an R image for a dark portion, a B image for a bright portion, and a G image for an intermediate portion. There is a method for synthesizing a monochrome image. However, this method does not use all data without waste. For example, the G image and the B image are discarded in a dark part, and the S / N ratio (signal / noise) of the image decreases accordingly, and the dynamic range becomes narrow. In addition, the seam of R, G, B may adversely affect the image.

  An object of the present invention is to provide a CT apparatus that reconstructs a cross-sectional image with a wide dynamic range using a radiation detector that outputs transmission images of a plurality of color components.

  In order to achieve the above object, a CT apparatus according to claim 1 of the present invention detects a radiation source that emits radiation toward a subject placed on a table, and radiation that has passed through the subject. Radiation detecting means for outputting as transmission image data, scanning means for relatively scanning the table and the radiation so as to change the direction of transmission with respect to the subject, and a cross-sectional image of the subject from the transmission image data In the CT apparatus having the reconstruction means for reconstructing, the radiation detection means detects the radiation and converts the visible light image into a color visible light image and the visible light image of the color for each color component. An image synthesizing unit that produces monochromatic transmission image data obtained by adding the transmission image data for each color component to each other, and the reconstruction unit includes a plurality of reconstruction units. Reconstructing a cross-sectional image of the subject using the monochromatic transmission image data obtained by adding the transmission image data for each color component obtained by the radiation detection unit at the scanning position to each other by the image synthesis unit. And

  With this configuration, it is possible to synthesize a monochromatic transmission image with an expanded dynamic range by adding and synthesizing transmission images for color components having different sensitivities to radiation, and reconstructing the monochromatic transmission image to widen the dynamic range. A cross-sectional image can be reconstructed.

  A CT apparatus according to a second aspect of the present invention is the CT apparatus according to the first aspect, wherein non-linearity correction means adds a non-linearity correction to the single-color transmission image data by an inverse function of a detection characteristic function of the single-color transmission image data. The reconstructing means reconstructs using the monochromatic transmission image data after the nonlinearity correction is performed.

  With this configuration, transmission images for each color component with different sensitivities to radiation are added, and then nonlinear correction is applied to synthesize a monochrome transmission image with an expanded dynamic range weighted with differential sensitivity. A cross-sectional image with a wide dynamic range can be reconstructed by reconstructing the transmission image.

  A CT apparatus according to a third aspect of the present invention is the CT apparatus according to the first aspect, wherein the nonlinearity correction is performed by adding a nonlinearity correction to the transmission image data for each color component by an inverse function of each detection characteristic function. And the image composition means adds the transmission image data for each color component after the nonlinearity correction is added to each other.

  With this configuration, transmission images for each color component with different sensitivities to radiation can be added after nonlinearity correction, and a monochrome transmission image with an expanded dynamic range can be synthesized, and this monochrome transmission image is reconstructed. Can reconstruct a cross-sectional image with a wide dynamic range.

  The CT apparatus according to a fourth aspect of the present invention is the CT apparatus according to the third aspect, wherein the addition is performed by multiplying a weight proportional to the square of the slope of each detection characteristic function for each color component. Is the gist.

  With this configuration, the transmission range for each color component with different sensitivity to radiation is added with nonlinearity correction and then multiplied by the square of the slope of the detection characteristic function (differential sensitivity) to maximize the dynamic range. A monochromatic transmission image that is widened can be synthesized, and the monochromatic transmission image can be reconstructed to reconstruct a cross-sectional image with a wide dynamic range.

  The CT apparatus according to claim 5 of the present invention is the CT apparatus according to any one of claims 1 to 4, wherein the addition is performed by multiplying a weight inversely proportional to the square of the detector noise for each color component. It is a summary.

  With this configuration, when the detector noise differs by color component, an addition multiplied by a weight that is inversely proportional to the square of the detector noise is added, so that a monochrome transmission image with an expanded dynamic range can be synthesized, and this monochrome transmission image can be regenerated. It is possible to reconstruct a cross-sectional image having a wide dynamic range.

  A CT apparatus according to a sixth aspect of the present invention is the CT apparatus according to any one of the first to fifth aspects, wherein the radiation-visible light converting means is color X-ray II, and the imaging means is red, green, blue. The gist of the present invention is that it is a color camera that outputs transmission image data of the three color components.

  With this configuration, a transmission image is obtained for each of the three color components of red, green, and blue having different sensitivities to radiation.

  A CT apparatus according to a seventh aspect of the present invention is the CT apparatus according to any one of the first to fifth aspects, wherein the radiation-visible light converting means is a color scintillator, and the imaging means is red, green, or blue. The gist of the present invention is that it is a color camera that outputs transmission image data of one color component.

  With this configuration, a transmission image is obtained for each of the three color components of red, green, and blue having different sensitivities to radiation.

  According to the present invention, it is possible to provide a CT apparatus that reconstructs a cross-sectional image with a wide dynamic range using a radiation detector that outputs transmission images of a plurality of color components.

The schematic diagram (front view) which showed the structure of CT apparatus which concerns on 1st embodiment of this invention. FIG. 5 is a flowchart of calibration of detection characteristics prior to tomography according to the first embodiment. The graph which shows the example of the detection characteristic function obtained by the calibration which concerns on 1st embodiment. FIG. 3 is a flowchart of tomography according to the first embodiment. The graph of the detection characteristic function for demonstrating the effect of 1st and 2nd embodiment. The flowchart of tomography which concerns on 2nd embodiment. The schematic diagram (front view) which shows the structure of the conventional CT apparatus. The graph which shows the example of the detection characteristic curve of the radiation detector which outputs the permeation | transmission image of a several color component.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Configuration of the first embodiment of the present invention)
The configuration of the first embodiment of the present invention will be described below with reference to FIG.

  FIG. 1 is a schematic view (front view) showing a configuration of a CT apparatus according to the first embodiment of the present invention.

  X-rays that detect a pyramidal X-ray beam (radiation) 2 that is a part of the X-rays emitted from the X-ray tube (radiation source) 1 and the X-ray focal point F of the X-ray tube 1 with two-dimensional resolution. The X-ray beam 2 transmitted through the subject 5 placed on the table 4 so as to enter the X-ray beam 2 is disposed by facing the detector (radiation detection means) 3 by the X-ray detector 3. Detected and output as a transmission image (transmission image data).

  The table 4 is disposed on a rotation / elevation mechanism (scanning means) 6, and is rotated by a rotation / elevation mechanism 6 with respect to a rotation axis RA perpendicular to the direction of the X-ray optical axis L that is the center line of the X-ray beam 2. And is moved (lifted / lowered) in the z direction parallel to the rotation axis RA.

  Further, the shift mechanism 7 moves the rotating / raising / lowering mechanism 6 and the support column 8 in the y direction parallel to the X-ray optical axis L, thereby bringing the rotation axis RA and the X-ray detector 3 closer to the X-ray tube 1. Alternatively, the imaging distance FCD (Focus to rotation Center Distance) and the detection distance FDD (Focus to Detector Distance) are changed.

  Here, the shift mechanism 7 is used to change the imaging magnification (= FDD / FCD) according to the purpose. The z movement of the rotation / lifting mechanism 6 moves the target portion of the subject 5 to the height of the X-ray beam 2. Used to match. The rotation of the rotation / elevating mechanism 6 is used to obtain a transmission image at a plurality of rotational positions by rotating the subject 5 with respect to the X-ray beam 2 when taking a cross-sectional image.

  The X-ray detector 3 includes a color X-ray image intensifier (abbreviated as color II (registered trademark), hereinafter referred to as color X-ray II) (radiation-visible light conversion means) 3a and a color camera (imaging means). 3b.

  The color X-ray II3a is obtained by converting the incident radiation (X-ray) distribution into a photoelectron distribution by the scintillator layer and the photoelectric layer of the input surface 3aa, and accelerating the photoelectrons to form an image on the output surface 3ab. The 3ab color scintillator layer emits light and converts it into a color (multicolor) visible light image. The color scintillator layer of the output surface 3ab has a different emission characteristic curve for each color component (R, G, B: red, green, blue). That is, there is a characteristic that the sensitivity is higher in the order of R, G, and B with respect to the amount of incident electrons.

  The color camera 3b captures the converted color visible light image and outputs a transmission image (transmission image data) for each color component (R, G, B) as digital data.

  In addition, as a component, each mechanism unit (rotation / lifting mechanism 6 and shift mechanism 7) is controlled, and a control processing unit 9 that processes a transmission image from the X-ray detector 3, a processing result, and the like are displayed. There are a display unit 9a, an X-ray control unit (not shown) for controlling the X-ray tube 1, and the like.

  The control processing unit 9 is a normal computer and includes a CPU, a memory, a disk (nonvolatile memory), a display unit 9a, an input unit (keyboard, mouse, etc.) 9b, a mechanism control board, an interface, and the like.

  The control processing unit 9 receives the signals of the operation positions output from the mechanism units 6 and 7 by the mechanism control board and controls the mechanism units 6 and 7 to align the subject, scan (tomographic scanning), etc. In addition to the above, a transmission image collection command pulse or the like is sent to the X-ray detector 3.

  Further, the control processing unit 9 collects and stores transmission images from the X-ray detector 3 at the time of tomography, creates a cross-sectional image of the subject by reconstruction processing, and displays the cross-sectional image on the display unit 9a.

  In addition, the control processing unit 9 issues a command to an X-ray control unit (not shown), specifies tube voltage and tube current, and instructs X-ray emission and stop. The tube voltage and tube current can be changed according to the subject.

  As shown in FIG. 1, the control processing unit 9 reads software and functions as a functional block for the CPU to function, a detection characteristic calibration unit 9c for calibrating the detection characteristic of the X-ray detector 3, and a tomographic scan. 9d, an image composition unit (image composition means) 9e for synthesizing a monochromatic transmission image from transmission images for each color component obtained by scanning, and nonlinearity of the synthesized monochromatic transmission image or transmission image for each color component. A non-linearity correction unit (non-linearity correction unit) 9f for correction, a reconstruction unit (reconstruction unit) 9g for creating a cross-sectional image of the subject from the monochromatic transmission image, and the like are provided.

(Operation of the first embodiment)
The operation will be described with reference to FIGS.

  FIG. 2 is a flowchart of detection characteristic calibration prior to tomography according to the first embodiment.

  In step S1, the detection characteristic calibration unit 9c sets the tube voltage / tube current and the detection distance FDD to initial values, and starts emitting X-rays.

  In step S2, an air transmission image without a subject is collected and stored in a memory in the control processing unit 9 together with the imaging conditions at that time. In step S3, it is determined whether or not the photographing has been completed for a predetermined range of the tube current. If not, the process proceeds to step S4, and the tube current (or tube voltage, detection distance FDD) is changed to a value that has not been photographed. Further, returning to step S2, an air transmission image is collected and stored together with the photographing conditions.

  If it is determined in step S3 that the imaging is completed, the X-ray emission is stopped and the process proceeds to step S5.

In step S5, the detection characteristic calibration unit 9c first obtains the average brightness R, G, B at the center of each of the obtained air transmission images for each color component, and a table (graph) for the normalized tube current I. In this manner, detection characteristic functions R _I (I), G _I (I), and B _I (I) are created. Here, the standardized tube current is a tube current obtained by replacing a tube voltage and a detection distance FDD with a change in the tube current. Since the tube current and the X-ray dose are proportional, a detection characteristic function for the X-ray dose is obtained.

  FIG. 3 is a graph showing an example of a detection characteristic function obtained by calibration.

Detecting characteristic calibration section 9c is further detected characteristic function _{R I (I), G I} (I), with respect to _B I (I), wherein,
T _I (I) = b1 · R _I (I) + b2 · G _I (I) + b3 · B _I (I) (1)
Thus, the weights b1, b2, and b3 are multiplied and added to obtain a detection characteristic function T _I (I) of monochromatic transmission image data. Here, b1, b2, and b3 are predetermined constants, and are each set to 1 here.

Next, the detection characteristic calibrating unit 9c obtains, from the detection characteristic functions R _I (I), G _I (I), B _I (I), and T _I (I), an inverse function of the detection characteristic function, I _R ( _R), determined I _{G (G),} I B and _{(B), I T (T} ), R, G, B, for each first step of T (e.g. 0,1,2, ... as in) table control The data is stored in the memory in the processing unit 9. The inverse function of this detection characteristic function is a nonlinearity correction function.

  In the present embodiment, the detection characteristics need not be frequently calibrated, and may be performed when the CT apparatus is manufactured, when the X-ray detector 3 is replaced, or when a periodic inspection is performed.

  Next, FIG. 4 is a flowchart of tomography according to the first embodiment.

  In step S10, scanning (tomographic scanning) is performed as follows. When the operator places the subject 5 on the table 4, sets the imaging conditions, and inputs a command to start scanning, the scan control unit 9 d emits X-rays and rotates the subject 5 at predetermined intervals. Transmission images (transmission image data) for each color component (R, G, B) at a plurality of rotation positions are collected over one rotation and stored in a memory in the control processing unit 9.

In step S11, the image composition unit 9e composes a monochrome transmission image (monochromatic transmission image data). The composition of the monochromatic transmission image is obtained by using an equation for transmission images R, G, and B for each color component at the same rotational position.
T = b1, R + b2, G + b3, B (2)
, The weights b1, b2, and b3 are multiplied and added to obtain a monochrome transmission image T. Here, addition is performed for each pixel, and b1, b2, and b3 are predetermined constants, and are each set to 1 here. Expression (2) is calculated for the transmission images at all rotational positions, and a monochrome transmission image T is obtained for each.

In step S12, the non-linearity correction unit 9f corrects the non-linearity with respect to the monochromatic transmission image T. The non-linearity correction is performed using the stored non-linearity correction function I _T (T) for each pixel of the monochrome transmission image T,
I _T = I _T (T) (3)
This is done by calculating Nonlinearity correction is performed on the monochromatic transmission images T at all rotational positions.

In step S13, each performs preprocessing such as logarithmic conversion or air corrected for reconstruction unit 9g monochromatic transmission image I _T after nonlinearity corrected for each rotation position.

In step S14, to reconstruct a sectional image of the subject 5 (one or plural) using monochromatic transmission image I _T pretreatment after each rotational position. The reconstruction is the same as in the prior art, and uses a filtered back projection method (FBP (Filtered Back Projection) method) or the like.

(Effects of the first embodiment)
FIG. 5 is a graph of detection characteristic functions for explaining the effects of the first and second embodiments.

Referring to FIG. 5, in the first embodiment, the transmission image R, G, B for each color component is simply added to synthesize a monochrome transmission image T, and then the monochrome transmission image I _{T to} which nonlinearity correction is applied. However, the error is equivalent to the case where the transmission images R, G, and B are each corrected for nonlinearity and then weighted with the slope (differential sensitivity) of the detection characteristic function and added and averaged. This is the formula,
I _T (T) = {(dR _I / dI) · I _R (R) + (dG _I / dI) · I _G (G) + (dB _I / dI) · I _B (B)} / (dR _I / DI + dG _I / dI + dB _I / dI) (4)
(Proof is described later). The values I _R (R), I _G (G), and I _B (B) after correcting the nonlinearity of R, G, and B are the slopes dR _I / dI, dG _I / dI, and dB _I / dI of the detection characteristic function. By adding by weighting, a monochromatic transmission image with few errors can be obtained. This is because pixels with the same X-ray dose I have higher differential sensitivity and higher detail information as the color component with a larger slope of the detection characteristic function increases, so the error is increased by adding a higher weight to the color component with higher accuracy. This is because it is possible to synthesize a monochromatic transmission image with a reduced SNR, an improved SN ratio, and a wide dynamic range. Referring to FIG. 5, when I is small, the weight of R is large. As I increases, R is saturated, so the weight of G is large. When I is much larger, G is saturated, so the weight of B is growing. As a result, the color component having a small saturation inclination can be reduced in weight. Also, the weight can be changed smoothly and continuously, and no seam or the like occurs.

  Therefore, according to the first embodiment, using a radiation detector that outputs transmission images of a plurality of color components, the transmission images R, G, and B for each color component having different sensitivities to radiation are added to obtain a single color. After synthesizing the transmission image T, by adding nonlinearity correction, a monochromatic transmission image after nonlinearity correction, which is weighted by the slope (differential sensitivity) of the detection characteristic function to improve the SN ratio and widen the dynamic range, is obtained. can get.

  As a result, a monochromatic transmission image with a wide dynamic range can be obtained, and this can be used to reconstruct a cross-sectional image with a wide dynamic range.

<Proof of Formula (4)>
After the transmission images R, G, and B are simply added to synthesize a monochrome transmission image T, nonlinearity correction is applied (the left side of equation (4)), after each of the transmission images R, G, and B is corrected for nonlinearity. It is proved that the error is equivalent to the weighted average of the slope of the detection characteristic function (the right side of equation (4)).

Referring to FIG. 5, first, when there is no error (detector noise), each value I _T (T), I _R (R), I _G (G), and I _B (B) after nonlinearity correction is Since all are the same value, the arithmetic mean using arbitrary a1, a2, a3,
_{I = (a1 · I R (} R) + a2 · I G (G) + a3 · I B (B)) / (a1 + a2 + a3) ...... (5)
The value I obtained in (1) agrees with I _T (T).

Next, it is assumed that detector noises σ _R , σ _G , and σ _B (rms values: root mean square) of normal distribution exist in _R , _G , and _B. Consider the spread of noise after nonlinearity correction.

First, the noise σ1 (rms value) when nonlinearity correction is performed after synthesis is
σ1 = √ (σ _R ² + σ _G ² + σ _B ² ) / (dT _I / dI)
= √ (σ _R ² + σ _G ² + σ _B ² ) / (dR _I / dI + dG _I / dI + dB _I / dI) (6)
It becomes. On the other hand, the noise σ2 (rms value) in the case of averaging after nonlinearity correction is
σ2 = √ {a1 ² · σ _R ² / (dR _I / dI) ² + a2 ² · σ _G ² / (dG _I / dI) ² + a3 ² · σ _B ² / (dB _I / dI) ² } / (a1 + a2 + a3 ) (7)
It becomes. here,
a1 = dR _I / dI, a2 = dG _I / dI, a3 = dB _I / dI (8)
Σ2 = σ1. <> End.

(Modification of the first embodiment)
In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention. The following modifications can be applied in combination.

(Modification 1)
In the first embodiment, the combination weight coefficients b1, b2, and b3 are set to 1, respectively, but may not be 1. For example, R, G, and detector noise sigma _R of the normal distribution in _B, σ _G, σ B _{if (rms} value) there may be selected to be inversely proportional to the square of detector noise. That is, the formula,
b1: b2: b3 = 1 / σ _R ² : 1 / σ _G ² : 1 / σ _B ² (9)
Thus, when there is a difference in the detector noise of the per-color transmission images, it is possible to synthesize a single-color transmission image with little error (good SN ratio and wide dynamic range).

(Modification 2)
In the first embodiment, when nonlinearity is small, nonlinearity correction can be omitted. However, nonlinearity is evaluated after logarithmic transformation. That is, even if it is not a straight line on a linear graph (brightness T and X-ray dose I), nonlinearity is small if it is close to a straight line on a logarithmically transformed graph (LOG (T) and LOG (I)).

For example, if the detection characteristic is
T = I ^γ (10)
In this case, when γ is 0.5, the detection characteristic function is a curved curve on the linear graph, but on the logarithmically transformed graph,
LOG (T) = γ · LOG (I) (11)
Thus, the slope becomes a straight line with γ and there is no nonlinearity.

(Modification 3)
In the first embodiment, the logarithmic conversion (S13) is performed after the synthesis of the monochrome transmission image (S11) and the nonlinearity correction (S12), but the order is not limited to this. For example, the order of logarithmic conversion, monochromatic transmission image synthesis, and non-linearity correction may be used in the order of monochromatic transmission image synthesis, logarithmic conversion, and non-linearity correction.

(Configuration of the second embodiment of the present invention)
The configuration of the second embodiment of the present invention is the same as that of the first embodiment of FIG.

(Operation of the second embodiment)
With the operation of the second embodiment, the detection characteristic calibration prior to tomography is almost the same as in the first embodiment described with reference to FIGS. 2 and 3, but the slope of the detection characteristic function can be obtained. The only difference is that it is added. Only the additional points will be described below.

In step S5 of FIG. 2, the detection characteristic calibration unit 9c obtains the nonlinearity correction functions I _R (R), I _G (G), I _B (B), and I _T (T), and then R _I (I ), G _I (I), B _I (I) The respective slopes (differential sensitivities) dR _I / dI, dG _I / dI, dB _I / dI are set for each step of R, G, B (for example, 0, (1, 2,...) Is obtained as a table and stored in the memory in the control processing unit 9. Hereinafter, the stored table is described as a function of R, G, and B as dR _I / dI (R), dG _I / dI (G), and dB _I / dI (B).

  Next, the operation of tomography will be described with reference to FIG. FIG. 6 is a flowchart of tomography according to the second embodiment.

  In step S20, scanning (tomographic scanning) is performed as follows. When the operator places the subject 5 on the table 4, sets the imaging conditions, and inputs a command to start scanning, the scan control unit 9 d emits X-rays and rotates the subject 5 at predetermined intervals. Transmission images (transmission image data) for each color component (R, G, B) at a plurality of rotation positions are collected over one rotation and stored in a memory in the control processing unit 9.

In step S21, the nonlinearity correcting unit 9f corrects the nonlinearity for each of the transmitted images R, G, and B for each color component. Nonlinearity correction is performed using the stored nonlinearity correction functions I _R (R), I _G (G), and I _B (B) for each pixel of the transmission images R, G, and B.
_{I R = I R (R)} ,
I _G = I _G (G),
I _B = I _B (B) (12)
This is done by calculating Non-linearity correction is performed on the transmission images for all color components at all rotational positions.

In step S22, the image synthesis unit 9e synthesizes a monochrome transmission image (monochromatic transmission image data). The composition of the monochromatic transmission image is obtained by using the following equation for transmission images I _R , I _G , and I _B after nonlinearity correction for each color component at the same rotational position:
_{I T = a1 · I R +} a2 · I G + a3 · I B ...... (13)
The sums by multiplying the weights a1, a2, a3, to obtain a monochromatic transmission image _{I T.} Here, the addition is performed for each pixel, it performs the calculation of equation (13) for all rotational positions, respectively to obtain a monochromatic transmission image I _T.

In equation (13), the weights a1, a2, and a3 are
a1: a2: a3 = {dR _I / dI (R)} ² : {dG _I / dI (G)} ² : {dB _I / dI (B)} ² (14)
As shown, the weights are proportional to the squares of the gradients (differential sensitivity) of the detected characteristic functions in R, G, and B, respectively.

In step S23, each performs preprocessing such as logarithmic conversion or air corrected for reconstruction unit 9g monochromatic transmission image I _T after the synthesis of each rotating position.

In step S24, to reconstruct a sectional image of the subject 5 (one or plural) using monochromatic transmission image I _T pretreatment after each rotational position. The reconstruction is the same as in the prior art, and uses a filtered back projection method (FBP (Filtered Back Projection) method) or the like.

(Effect of the second embodiment)
According to the second embodiment, the slope (differential sensitivity) of the detection characteristic function with respect to the values I _R (R), I _G (G), and I _B (B) after the nonlinearity correction of R, G, B is performed. squared ^{_{(dR I / dI) 2,}} (dG I / dI) 2, the transmission image (dB I / ^dI) by adding with a weight proportional to ^the error is minimum (SN ratio is the highest) can get. The reason is as follows.

  FIG. 5 is a graph of detection characteristic functions for explaining the effects of the first and second embodiments.

Referring to FIG. 5, first, when there is no error (detector noise), the values I _R (R), I _G (G), and I _B (B) after nonlinearity correction are all the same value.

Next, it is assumed that detector noises σ _R , σ _G , and σ _B (rms values) of normal distribution exist in _R , _G , and _B. Consider the spread of noise after nonlinearity correction. The noises σ _IR , σ _IG , and σ _IB (rms values) that spread to I _R (R), I _G (G), and I _B (B) are:
_{σ IR = σ R / {dR} I / dI (R)},
σ _IG = σ _G / {dG _I / dI (G)},
σ _IB = σ _B / {dB _I / dI (B)} (15)
(See FIG. 5). Where the theory,
“When the same amount is measured multiple times and there is an error in the normal distribution, the error is minimized when averaged by multiplying the square of each measurement error (rms value) by a weight that is inversely proportional to the square.”
...... (M1)
Is used as the weights a1, a2, and a3 for I _R (R), I _G (G), and I _B (B), respectively,
a1: a2: a3 =
{DR _I / dI (R)} ² / σ _R ² : {dG _I / dI (G)} ² / σ _G ² : {dB _I / dI (B)} ² / σ _B ² (16)
When the weights of these ratios are averaged, the SN ratio is maximized. If σ _R , σ _G , and σ _B are equal in equation (16), equation (14) is obtained.

  Therefore, according to the second embodiment, after using the radiation detector that outputs transmission images of a plurality of color components, the transmission images of the color components having different sensitivities to radiation are corrected for nonlinearity, respectively. By adding a weight proportional to the square of the slope (differential sensitivity) of the detection characteristic function and adding the result, a monochromatic transmission image having a maximum dynamic range with a high S / N ratio can be obtained.

  As a result, a monochromatic transmission image with a wide dynamic range can be obtained, and this can be used to reconstruct a cross-sectional image with a wide dynamic range.

<About Theory (M1)>
Briefly described.
For example, assume that one quantity is measured once in the first time to obtain a value I1 and an error σ1. Next, assume that the same measurement is performed four times for the second time and averaged to obtain a value I2 and an error σ2.

At this time, since σ2 is 1 / √4 times σ1, according to the theory, if the first measurement and the second measurement are averaged by weighting (weight) at 1: 4, the error should be minimized. In fact, this average is
I = (I1 + I2 · 4) / 5
This is the same as averaging the same measurement five times with the same weight, and it is guaranteed that I can be obtained with the minimum error. <> End

(Modification of the second embodiment)
In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention. The following modifications can be applied in combination.

(Modification 1)
In the second embodiment, the value after nonlinearity correction for each color component is added with the weight of Expression (14), but instead, the value is added with the weight of Expression (16). Also good. Thus, the weights that are inversely proportional to the square of the detector noise for each color component are multiplied and combined. In this case, when the detector noise is different in color components, a monochrome transmission image with the smallest error (maximum SN ratio and wide dynamic range) can be obtained.

(Modification 2)
In the second embodiment, logarithmic conversion (S23) is performed after nonlinearity correction (S21) and monochromatic transmission image synthesis (S22), but the order is not limited to this. For example, the order of logarithmic conversion, nonlinearity correction, and monochromatic transmission image synthesis may be used, or the order of nonlinearity correction, logarithmic conversion, and monochromatic transmission image synthesis may be used.

(Common modification of the first embodiment and the second embodiment)
The following modifications can be applied in combination.

(Modification 1)
In the first or second embodiment, when the detection characteristic functions R _I (I), G _I (I), and B _I (I) are obtained in the calibration of the detection characteristic function, smoothing (high frequency component) is performed in the I direction. If it is obtained by performing (removal), the measurement error at the time of calibration is averaged and the error of nonlinearity correction is reduced.

(Modification 2)
In the first or second embodiment, the nonlinear correction function and the slope of the detection characteristic function are stored as a table, but instead, a fitting curve is obtained as an equation for the graph obtained by measurement, and is stored as an equation. Also good.

(Modification 3)
In the first or second embodiment, the color camera 3b is used to output a transmission image as digital data, but may be converted to digital data by the control processing unit 9 as an analog output.

(Modification 4)
In the first or second embodiment, the radiation detector constituted by the color X-ray II (radiation visible light converting means) 3a and the color camera (imaging means) 3b is used, but is not limited thereto. For example, a plate having a color scintillator layer may be used as a radiation-visible light conversion means, a scintillator layer and a photoelectric layer are provided on the input surface of a microchannel plate for amplifying electrons, and a color scintillator layer is provided on the output surface. May be used.

(Modification 5)
In the first or second embodiment, the X-ray detector 3 outputs a transmission image of three color components (red, green, and blue), but the color components may not be red, green, and blue. The output may be two color components or four or more color components. Even in this case, a monochrome transmission image can be synthesized by the same processing.

(Modification 6)
In the first or second embodiment, a normal scan for rotating the subject once is performed, but a half scan, an offset scan, or a helical scan may be performed.

  In the first or second embodiment, the table 4 is rotated with respect to the X-ray beam 2. However, the rotation may be relative, and the X-ray tube 1 and the X-ray detector 3 are rotated together. By doing so, the X-ray beam 2 may be rotated.

  In the first or second embodiment, the RR type CT apparatus is taken as an example, but any method may be used as long as the table and the radiation are scanned relatively so as to change the transmission direction. The present invention can be applied to all CT apparatuses using a radiation detector that outputs transmission images of a plurality of color components.

(Modification 7)
In the first or second embodiment, a CT apparatus in which the rotation axis RA intersects perpendicularly with the direction of the X-ray optical axis L is taken as an example, but the present invention is a so-called inclined CT apparatus (special feature) This can also be applied to the open 2005-106515.

  The present invention can also be applied to a tomographic apparatus (also called a laminograph or a tomosynthesis apparatus) that captures a cross-sectional image.

(Modification 8)
In the present invention, not only X-rays but also radiation that is transmissive to the subject such as γ rays and microwaves can be used depending on the subject.

DESCRIPTION OF SYMBOLS 1 ... X-ray tube, 2 ... X-ray beam, 3 ... X-ray detector, 3a ... Color X-ray II, 3aa ... Input surface, 3ab ... Output surface, 3b ... Color camera, 4 ... Table, 5 ... Subject, DESCRIPTION OF SYMBOLS 6 ... Rotation / lifting mechanism, 7 ... Shift mechanism, 8 ... Strut, 9 ... Control processing part, 9a ... Display part, 9b ... Input part, 9c ... Detection characteristic calibration part, 9d ... Scan control part, 9e ... Image composition part , 9f: nonlinearity correction unit, 9g: reconstruction unit,
DESCRIPTION OF SYMBOLS 101 ... X-ray tube, 102 ... X-ray beam, 103 ... X-ray detector, 103a ... X-ray II, 103b ... Camera, 104 ... Table, 105 ... Subject, 107 ... Rotation / lifting mechanism, 108 ... Control processing part

Claims (7)

A radiation source that emits radiation toward the subject placed on the table; radiation detection means that detects the radiation transmitted through the subject and outputs it as transmission image data; and the direction of transmission with respect to the subject In a CT apparatus having scanning means for relatively scanning the table and the radiation so as to change the position, and reconstruction means for reconstructing a cross-sectional image of the subject from the transmission image data,
The radiation detection means comprises a radiation-visible light conversion means for detecting radiation and converting it into a color visible light image, and an imaging means for photographing the color visible light image and outputting transmission image data for each color component,
Image synthesis means for producing monochromatic transmission image data obtained by adding the transmission image data for each color component to each other;
The reconstruction means uses the monochromatic transmission image data obtained by adding the transmission image data for each of the color components obtained by the radiation detection means at a plurality of scanning positions to each other by the image synthesis means. CT apparatus characterized by reconfiguring. The CT apparatus according to claim 1,
Non-linearity correction means for applying non-linearity correction to the single-color transmission image data with an inverse function of the detection characteristic function of the single-color transmission image data,
The CT apparatus according to claim 1, wherein the reconstruction means reconstructs using the monochromatic transmission image data after the nonlinearity correction is applied. The CT apparatus according to claim 1,
Non-linearity correction means for applying non-linearity correction to the transmission image data for each color component by an inverse function of each detection characteristic function,
The CT apparatus according to claim 1, wherein the image synthesizing unit adds the transmission image data for each color component after the nonlinearity correction is performed. The CT apparatus according to claim 3,
The CT apparatus is an addition in which the addition is performed by multiplying a weight proportional to the square of the slope of each detection characteristic function for each color component. The CT apparatus according to any one of claims 1 to 4,
The CT apparatus, wherein the addition is an addition obtained by multiplying the square of the detector noise for each color component by a weight inversely proportional to the square. The CT apparatus according to any one of claims 1 to 5,
The CT apparatus, which is a color camera that outputs the transmission image data of three color components of red, green, and blue. The CT apparatus according to any one of claims 1 to 5,
The CT apparatus, wherein the radiation-visible light conversion means is a color scintillator, and the imaging means is a color camera that outputs transmission image data of three color components of red, green, and blue.

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