JP5295661B2 - Tomographic image processing apparatus, X-ray CT apparatus, and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To separate a substance with a small X-ray absorptance with high accuracy in a tomographic image which is obtained by X-ray CT (Computerized Tomography) photographing. <P>SOLUTION: Pixel values are shift-converted so that the pixel values corresponding to air becomes zero in two kinds of tomographic images g80 (x, y) and g140 (x, y) which are different in X-ray tube voltage in photographing (step S107), the ratio r'(x, y) of the converted pixel values of the corresponding pixels between the two kinds of tomographic images is obtained (step S108). Consequently, the ratio of the pixel values in the substance with the small X-ray absorptance is separated from the ratio of pixel values in another substance. Then, the substance is separated by determining the threshold of the ratio r'(x, y) of the converted pixel values (step S109). <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a tomographic image processing apparatus for processing a tomographic image obtained by X-ray CT (Computed Tomography) imaging of a subject, an X-ray CT apparatus, and a program therefor.

Dual energy (dual energy) is used as a material separation method in CT tomograms of subjects.
(energy) photographing is known (for example, see Patent Document 1). This method uses the characteristics of the substance that the X-ray absorption rate of the substance differs depending not only on the type of substance but also on the energy of the X-ray, and two types of X-rays having different energy distributions. X-ray CT imaging is performed, and the CT values of corresponding pixels are compared between the obtained two types of tomographic images to separate materials in the tomographic image.
JP 2008-125900 A

  As a specific example of the above method, for example, the method proposed in Japanese Patent Application No. 2007-292955 can be considered. In this method, first, a subject is imaged with dual energy. That is, X-ray CT imaging is performed with the first X-ray tube voltage and the second X-ray tube voltage, and two types of tomographic images representing the same part are obtained. Next, a CT value ratio is calculated for each corresponding pixel between these two types of tomographic images. Then, the substance in the tomographic image is specified based on the calculated CT value ratio. It is known that this CT value ratio takes a specific value depending on the type of substance. Therefore, if the relationship between the type of substance and the ratio of CT values is obtained in advance, substance separation can be performed by threshold value determination of the ratio of CT values. As is well known, the CT value is an index of the X-ray absorption rate of a substance, and is defined such that a value determined as air is −1000 and a value determined as water is zero.

  By the way, in dual energy imaging, for example, imaging is performed by changing the X-ray tube voltage within a range of 80 kV to 140 kV. In this case, for example, the CT value of iodine, which is the main component of an iodine-based contrast agent, varies between about +600 and +300, and the CT value of calcium, which is the main component of bone, is about +1500 to +1000. It changes with. That is, the ratio of the CT value in iodine (here, the value obtained by dividing the CT value at the tube voltage of 80 kV by the CT value at the tube voltage of 140 kV) is about 2.0 at the maximum, and the ratio of the CT value in calcium is the maximum Is about 1.5. As described above, substances having a large X-ray absorption rate such as iodine and calcium have a difference in CT value ratio, and therefore these substances can be separated with high accuracy.

  On the other hand, for example, the CT value of xenon (Xenon) used as a contrast agent for pulmonary function test is the same as that described above, with a xenon dilution gas (gas diluted with xenon mixed with air) of 30% concentration. In this case, it changes in the range of about -900 to -945. That is, the ratio of CT values in xenon is about 0.95 at the minimum. Xenon has a relatively large atomic number, but it is a gas at room temperature and its spatial density is lower than that of a liquid or solid. Therefore, the ratio of CT values and the CT value are close to those of air, and separation from air becomes difficult. As described above, in a substance having a small X-ray absorption rate, the ratio of CT values and the CT value itself are values close to those of air, so that it is difficult to accurately separate such substances.

  In view of the above circumstances, the present invention provides a tomographic image processing apparatus, an X-ray CT apparatus capable of separating a substance having a low X-ray absorption rate with high accuracy in a tomographic image obtained by X-ray CT imaging, and The purpose is to provide a program for this purpose.

  In a first aspect, the present invention relates to a first tomogram obtained by X-ray CT imaging of a subject with a first X-ray tube voltage, and the subject to the first X-ray tube voltage. The pixel value in the second tomographic image obtained by X-ray CT imaging with a different second X-ray tube voltage is changed to a pixel value defined so that the pixel value corresponding to air is substantially zero. A tomography comprising pixel value conversion means for conversion and pixel value ratio calculation means for calculating a ratio of the converted pixel values in the corresponding pixels between the first tomographic image and the second tomographic image. An image processing apparatus is provided.

  In a second aspect, the present invention provides a CT in which the pixel values of the first and second tomographic images are defined such that a pixel value corresponding to water is zero and a pixel value corresponding to air is −1000. The tomographic image processing apparatus according to the first aspect, wherein the pixel value conversion unit adds 1000 to the original pixel value.

  In a third aspect, the present invention further includes pixel area specifying means for specifying a pixel area representing a predetermined substance in the first or second tomogram based on the ratio of the converted pixel values. A tomographic image processing apparatus according to the first aspect or the second aspect is provided.

  In a fourth aspect, the present invention is the third aspect, further comprising display means for displaying the first or second tomographic image so that the specified pixel region is represented by a predetermined color or contrast. A tomographic image processing apparatus according to the above aspect is provided.

  In a fifth aspect, the present invention further comprises target substance input means for inputting a target substance to be specified on the first or second tomographic image, wherein the pixel value conversion means is configured to input the input target substance. The tomographic image processing apparatus according to the third aspect or the fourth aspect is provided, wherein the pixel value is converted when the includes a predetermined substance, and the pixel area specifying unit specifies a pixel area representing the predetermined substance. .

  In a sixth aspect, the present invention provides the above-described first method for specifying the concentration of the predetermined substance in the pixel based on the pixel value of the pixel included in the pixel region representing the predetermined substance or the converted pixel value. A tomographic image processing apparatus according to the fifth aspect is provided.

  In a seventh aspect, the present invention provides the tomographic image processing apparatus according to the fifth aspect or the sixth aspect, wherein the predetermined substance is xenon or a gas containing xenon.

  In an eighth aspect, the present invention further comprises reference material input means for inputting a reference material defined such that the corresponding pixel value is substantially zero, wherein the pixel value conversion means is configured to input the input reference. Provided is a tomographic image processing apparatus according to any one of the first to fourth aspects, wherein the pixel value is converted when the substance is air.

  In a ninth aspect, the present invention provides an X-ray CT image of a subject with a first X-ray tube voltage to obtain a first tomographic image, and the first X-ray tube voltage on the subject. Imaging means for obtaining a second tomographic image by X-ray CT imaging with a different second X-ray tube voltage, and pixel values corresponding to air are substantially equal to pixel values in the first and second tomographic images. A ratio of the converted pixel value in the corresponding pixel is calculated between the pixel value converting means for converting the pixel value to be defined to be zero and the first tomographic image and the second tomographic image. An X-ray CT apparatus comprising a pixel value ratio calculating means for providing a pixel value ratio is provided.

  In a tenth aspect, the present invention relates to a first tomographic image obtained by X-ray CT imaging of a subject with a first X-ray tube voltage, and the subject to the first subject. The pixel value corresponding to the air becomes substantially zero as the pixel value in the second tomographic image obtained by X-ray CT imaging with a second X-ray tube voltage different from the X-ray tube voltage of A pixel value ratio for calculating a ratio of the converted pixel value in the corresponding pixel between the pixel value converting means for converting to a prescribed pixel value and the first tomographic image and the second tomographic image. A program for functioning as a calculation means is provided.

  According to the present invention, after converting pixel values in two types of tomographic images having different X-ray tube voltages at the time of imaging into pixel values defined so that the pixel value corresponding to air is substantially zero, Since the ratio of the converted pixel value in the corresponding pixel is obtained between these two types of tomographic images, the ratio of the pixel value in the substance having a small X-ray absorption rate is determined from the ratio of the pixel value in the other substance. In a tomographic image obtained by X-ray CT imaging, a substance having a small X-ray absorption rate can be separated with high accuracy.

Embodiments according to the present invention will be described below with reference to the drawings.
(First embodiment)

  FIG. 1 is a block diagram showing an X-ray CT apparatus 100 according to an embodiment of the present invention. The X-ray CT apparatus 100 includes an operation console 1, an imaging table 8, and a scanning gantry 9.

  The operation console 1 includes an input device 2, a central processing unit 3, a control interface 4, a data collection buffer 5, and a monitor 6. The input device 2 accepts operator instructions and information. The central processing unit 3 executes a scan control process, an image reconstruction process, a pixel value conversion process, a pixel value ratio calculation process, and the like. The control interface 4 outputs a control signal and the like to the imaging table 8 and the scanning gantry 9 under the control of the central processing unit 3. The data collection buffer 5 collects data acquired by the scanning gantry 9. The monitor 6 displays an operation screen, a tomographic image, and the like.

  The imaging table 8 moves the placed subject in the body axis direction (hereinafter referred to as z direction) and conveys it to the cavity of the scanning gantry 9.

  The scanning gantry 9 has a cavity including an imaging space, and includes a rotating unit 7 that rotates around the cavity. The rotation unit 7 includes an X-ray controller 10, an X-ray tube 11, a collimator 12, an X-ray detector 13, a data collection unit 14, and a rotation controller 15. The X-ray controller 10 controls the tube voltage and tube current of the X-ray tube 11 and the on / off of X-ray irradiation. The collimator 12 shapes the X-ray beam emitted from the X-ray tube 11. The X-ray detector 13 is a so-called multi-row detector having a plurality of detector rows arranged in the channel direction in the slice direction, and each detection device detects a plurality of detector rows. A signal corresponding to the X-ray intensity is output. The data collection unit 14 performs A / D (analog-digital) conversion on the output signal of the X-ray detector 13 to collect projection data of the subject and sends it to the data collection buffer 5. The rotation controller 15 controls the rotation speed of the rotation unit 7, on / off of rotation, and the like. The X-ray tube 11 and the X-ray detector 13 are disposed to face each other with the cavity portion interposed therebetween.

  The central processing unit 3 is an example of a pixel value conversion unit and a pixel area specifying unit in the present invention. Further, the central processing unit 3 and the scanning gantry 9 are examples of photographing means in the present invention. The central processing unit 3 and the monitor 6 are examples of display means in the present invention. The input device 2 is an example of a target substance input unit and a reference substance input unit in the present invention.

  FIG. 2 is a flowchart showing a flow of tomographic material separation processing in the X-ray CT apparatus 100 according to the first embodiment.

  In step S101, dual energy imaging is performed. Here, it is assumed that a contrast medium is administered to a subject and the chest is subjected to dual energy imaging. Specifically, first, as an assumption, the subject is inhaled with a xenon dilution gas (mixed gas of xenon and air) having a concentration of 30%, and an iodinated contrast agent is injected into the blood vessel. In this state, the chest of the subject is scanned to collect first projection data when the X-ray tube voltage is 80 kV and second projection data when the X-ray tube voltage is 140 kV. Then, image reconstruction processing is performed based on the first projection data to generate a first tomographic image g80 (x, y) having an X-ray tube voltage of 80 kV at the time of imaging, and second projection data Based on the image reconstruction processing, a second tomographic image g140 (x, y) having an X-ray tube voltage of 140 kV at the time of imaging is generated. Note that the pixel values of the first tomographic image g80 (x, y) and the second tomographic image g140 (x, y) are CT values.

  3A is a diagram illustrating an example of a first tomographic image g80 (x, y), and FIG. 3B is a diagram illustrating an example of a second tomographic image g140 (x, y). In each tomogram shown in FIG. 3, coordinates (x1, y1) are normal lung regions, coordinates (x2, y2) are lung disease regions, coordinates (x3, y3) are spinal cord regions, and coordinates (x4, y4) are Located in the spine area. In the normal region of the lungs, ventilation is performed well, so the aspirated xenon dilution gas is filled. On the other hand, in the diseased area of the lungs, ventilation is poor, so the xenon concentration is low, for example, almost filled with air. Since there is a large blood flow in the spinal cord region, iodine, which is the main component of the iodine-based contrast agent, is concentrated. In the spine region, calcium which is the main component of bone concentrates.

  Therefore, in the first tomographic image g80 (x, y), the pixel value g80 (x1, y1) has a CT value corresponding to a xenon dilution gas having a concentration of 30% when the X-ray tube voltage is taken at 80 kV. For example, -900. Similarly, the pixel value g80 (x2, y2) is a CT value corresponding to air when the X-ray tube voltage is taken at 80 kV, for example -1000, and the pixel value g80 (x3, y3) is the X-ray tube. The CT value corresponding to iodine when imaged at a voltage of 80 kV, for example +600, and the pixel value g80 (x4, y4) is the CT value corresponding to calcium when imaged at an X-ray tube voltage of 80 kV, for example, +1500.

  In the second tomographic image g140 (x, y), the pixel value g140 (x1, y1) is a CT value corresponding to a xenon-diluted gas having a concentration of 30% when the X-ray tube voltage is taken at 140 kV. For example, -945. Similarly, the pixel value g140 (x2, y2) is a CT value corresponding to air when the X-ray tube voltage is taken at 140 kV, for example -1000, and the pixel value g140 (x3, y3) is the X-ray tube. The CT value corresponding to iodine when imaged at a voltage of 140 kV, for example +300, and the pixel value g140 (x4, y4) is the CT value corresponding to calcium when imaged at an X-ray tube voltage of 140 kV, for example, +1000.

  In step S102, the target substance is input. That is, the operator inputs, as a target substance, a substance that is desired to be separated and specified in the tomographic image.

  FIG. 4 is a diagram showing an example of the screen of the monitor 6 displaying the target substance input screen. Specifically, for example, as shown in FIG. 4, the first tomographic image g80 (x, y) and the second tomographic image g140 (x, y) are arranged side by side, and the target substance candidate is selected. A screen SC1 on which buttons CB are arranged is displayed on the monitor 6. The selection button BC is a set of two types of substances that are generally compared, and here, a button B1 for selecting iodine and calcium as target substances, and air and xenon as target substances. And a button B2 for the purpose. The operator selects and inputs a target substance by pressing any button with the GUI while viewing this screen.

  In step S103, it is determined which one of the first combination of iodine and calcium and the second combination of air and xenon is selected as the target substance. In this determination, if it is determined that the first combination has been selected, the process proceeds to step S104, and substance separation of iodine and calcium is performed. On the other hand, in this determination, when it is determined that the second combination is selected, the process proceeds to step S106, and the substance separation of air and xenon is performed.

  In step S104, the ratio of pixel values between tomographic images is calculated. Specifically, a pixel value ratio r (x, y) for each corresponding pixel between the first tomographic image g80 (x, y) and the second tomographic image g140 (x, y), that is, The ratio of CT values is calculated. This is expressed by the following equation.

      r (x, y) = g80 (x, y) / g140 (x, y) (Formula 1)

  FIG. 3 (3) is a diagram showing an example of a ratio image having the pixel value ratio r (x, y) as a pixel value. In this ratio image r (x, y), as shown in FIG. 3 (3), r (x1, y1) = − 900 / −945 = 0.95, and r (x2, y2) = − 1000 / -1000 = 1, r (x3, y3) = + 600 / + 300 = 2, and r (x4, y4) = + 1500 / + 1000 = 1.5.

  In step S105, substance separation of iodine and calcium is performed. That is, the pixel area Io and calcium representing iodine in the first tomographic image g80 (x, y) or the second tomographic image g140 (x, y) are determined by the threshold value determination of the pixel value calculated in step S104. Each pixel area Ca to be represented is specified.

  Here, the relationship between the type of substance and the ratio of pixel values will be described.

  FIG. 5 is a diagram illustrating a range of ratios of pixel values corresponding to each substance. In the graph shown in FIG. 5, the vertical axis is the pixel value (CT value) L-HU of the tomographic image when the X-ray tube voltage is taken at 80 kV, and the horizontal axis is when the X-ray tube voltage is taken at 140 kV. The tomographic image pixel value (CT value) H-HU. Each region divided so as to be sandwiched by straight lines in the graph indicates a range of pixel value ratios (L-HU / H-HU) determined according to the type of substance. The ratio of pixel values is determined according to the type of substance, but various noises such as X-ray absorption by a tissue other than the target substance and a background component on the detection signal of the X-ray detector 13. Considering (noise), it is determined as a range with a certain width. The range of the ratio of pixel values corresponding to iodine (L-HU / H-HU) is, for example, 1.7 to 2.0, and the ratio of pixel values corresponding to calcium (L-HU / H-HU). The range is 1.4 to 1.6, for example. The range of the pixel value ratio corresponding to air (L-HU / H-HU) is, for example, 0.95 to 1.05, and the ratio of pixel values corresponding to xenon or xenon dilution gas (L-HU). The range of (HU / H-HU) is, for example, 0.90 to 1.00. Note that the ratio of pixel values corresponding to the xenon dilution gas is constant regardless of the concentration.

  Thus, since the range of the ratio of pixel values corresponding to a substance having a relatively high X-ray absorption rate such as iodine or calcium is separated from that of other substances, separation of these substances is easy. On the other hand, the range of pixel value ratios corresponding to substances having a relatively low X-ray absorption rate such as air and xenon (xenon dilution gas) have overlapping portions, so that these substances should be separated accurately. Is difficult. Even if the pixel values themselves are compared, the difference in pixel values between these substances is small, and the difference may be buried in noise. Therefore, when the second combination of air and xenon having a low X-ray absorption rate is selected as the target substance, the pixel value is converted to differentiate the ratio of the pixel value to the substance having a low X-ray absorption rate. Separation is performed.

  Accordingly, in step S105, for example, the pixel value ratio of all pixels of the first tomographic image g80 (x, y) or the second tomographic image g140 (x, y) is 1.7 ≦ r ( A pixel having x, y) ≦ 2.0 is identified as one of the pixel regions Io representing iodine, and a pixel having a pixel value ratio of 1.4 ≦ r (x, y) ≦ 1.6 is determined. It is specified as one of the pixel areas Ca representing calcium.

  For example, the pixel at the coordinate (x3, y3) has a pixel value ratio r (x3, y3) = 2 as shown in FIG. 5, and therefore this pixel is one of the pixel regions Io representing iodine. Is identified. Further, as shown in FIG. 5, the pixel at the coordinates (x4, y4) has a pixel value ratio r (x4, y4) = 1.5. Therefore, this pixel is one of the pixel areas Ca representing calcium. To be identified. On the other hand, the pixel at the coordinates (x1, y1) has a pixel value ratio r (x1, y1) = 0.95, and the pixel at the coordinates (x2, y2) has a pixel value ratio r (x2, y2) = Since it is 1, the pixel area is not specified for these pixels.

  When such threshold determination of the pixel value ratio is executed for each pixel to be determined, the process proceeds to step S111.

  The pixels to be processed in steps S104 and S105 may be all pixels in the tomographic image, but it is efficient to use pixels excluding pixels that are clearly considered to represent substances other than iodine and calcium. For example, a pixel having a pixel value, that is, a CT value smaller than zero, represents a substance having an X-ray absorption coefficient smaller than that of water, and is considered a substance other than iodine or calcium. Therefore, the processing target may be limited to pixels of 0 ≦ g80 (x, y) or 0 ≦ g140 (x, y).

  In step S106, candidate pixel areas for air and xenon are identified. Specifically, for example, a pixel having a pixel value of −1000 ≦ g80 (x, y) ≦ −890 is specified as a candidate for a pixel region of air and xenon. This is because air and xenon have a small X-ray absorption rate, and their pixel values, that is, CT values, are considered to fall within the predetermined range as described above. A pixel having a pixel value of −890 <g80 (x, y) has a large pixel value, that is, a CT value, and therefore is highly likely to be another substance. Thus, for example, a pixel at coordinates (x1, y1) has a pixel value g80 (x1, y1) = − 900, and a pixel at coordinates (x2, y2) has a pixel value g80 (x2, y2) = − 1000. As such, these pixels are identified as candidates for air and xenon pixel regions. On the other hand, the pixel with coordinates (x3, y3) has a pixel value g80 (x3, y3) = + 600, and the pixel with coordinates (x4, y4) has a pixel value g80 (x4, y4) = + 1500. Are excluded from the air and xenon pixel region candidates.

  In step S107, the pixel value is converted. That is, at least the air and xenon pixel area candidate pixel values in the first tomographic image g80 (x, y) and the second tomographic image g140 (x, y) are set to zero. It converts into the pixel value prescribed | regulated. That is, pixel value conversion is performed such that the reference material that defines the corresponding pixel value to be zero is changed from water to air. Specifically, according to the following equation, 1000 is added to the original pixel value for conversion, and the first converted tomographic image g′80 (x, y) and the second converted tomographic image g′140 ( x, y).

g′80 (x, y) = g80 (x, y) +1000 (Formula 2-1)
g′140 (x, y) = g140 (x, y) +1000 (Formula 2-2)

  FIG. 6 (1) shows an example of the first converted tomogram g′80 (x, y), and FIG. 6 (2) shows an example of the second converted tomogram g ′ 140 (x, y). is there.

  In the first converted tomogram g′80 (x, y), the pixel value g′80 (x1, y1) = − 900 + 1000 = + 100 and the pixel value g′80 (x2, y2) = − 1000 + 1000 = 0. Pixel value g′80 (x3, y3) = + 600 + 1000 = + 1600, and pixel value g′80 (x4, y4) = + 1500 + 1000 = + 2500.

  Similarly, in the second converted tomographic image g′140 (x, y), the converted pixel value g′140 (x1, y1) = − 945 + 1000 = + 55, and the converted pixel value g′140 (x2, y2). ) = − 1000 + 1000 = 0, converted pixel value g′140 (x3, y3) = + 300 + 1000 = + 1300, and converted pixel value g′140 (x4, y4) = + 1000.

  In step S108, a ratio of converted pixel values between converted tomograms is calculated. That is, at least the air and xenon pixel area candidates are handled between the first converted tomogram g′80 (x, y) and the second converted tomogram g′140 (x, y). A ratio r ′ (x, y) of converted pixel values is calculated for each pixel to be processed. This is expressed by the following equation.

      r ′ (x, y) = g′80 (x, y) / g′140 (x, y) (Formula 3)

  FIG. 6 (3) is a diagram illustrating an example of a ratio image in which the ratio r ′ (x, y) of the converted pixel values is a pixel value. In this ratio image r ′ (x, y), as shown in FIG. 6 (3), r ′ (x1, y1) = + 100 / + 55 = 1.82, and r ′ (x2, y2) = 0 / 0 = NA.

  In step S109, air and xenon are separated. That is, in the first converted tomogram g′80 (x, y) or the second converted tomogram g′140 (x, y) by the threshold determination of the ratio of the converted pixel values calculated in step 108. The pixel area Air representing air and the pixel area Xe representing xenon (xenon dilution gas) are respectively identified.

  FIG. 7 is a diagram illustrating a range of ratios of converted pixel values corresponding to xenon. In the graph shown in FIG. 7, the vertical axis represents a converted pixel value (CT value + 1000) L′−HU of a tomographic image taken at an X-ray tube voltage of 80 kV, and the horizontal axis represents an image at an X-ray tube voltage of 140 kV. The converted pixel value (CT value + 1000) H′−HU of the tomographic image at that time. A region divided so as to be sandwiched by straight lines in the graph indicates the range of the ratio of the converted pixel value corresponding to xenon (L′−HU / H′−HU). The range of the ratio (L′−HU / H′−HU) of converted pixel values corresponding to xenon is, for example, 1.8 to 1.9.

  On the other hand, the range of the ratio (L′−HU / H′−HU) of converted pixel values corresponding to air is not determined. Here, since the pixel value is originally converted so that the pixel value corresponding to air becomes zero, the converted pixel value H′-HU corresponding to air takes a value near zero, and the ratio of the converted pixel values This is because (L'-HU / H'-HU) diverges.

  In this way, the ratio of the converted pixel value corresponding to xenon and the ratio of the converted pixel value corresponding to air are differentiated, so air and xenon are accurately separated based on the ratio of the converted pixel value. It becomes possible to do.

  Therefore, in step S109, among the candidate pixel areas of air and xenon, for example, a pixel having a ratio of converted pixel values of 1.8 ≦ r ′ (x, y) ≦ 1.9 is selected as a pixel representing xenon. The region Xe is specified, and the remaining pixels are specified as a pixel region Air representing air.

  In step S110, the pixel region Xe representing xenon is further separated by xenon density. Specifically, the converted pixel value g′80 (x, y) or g′140 (x, y) or the pixel value before conversion for each pixel in the pixel region Xe representing xenon specified in step S109. Separate by performing threshold determination of g80 (x, y) or g140 (x, y). For example, as shown in FIG. 7, a pixel having a converted pixel value of 5 <g′80 (x, y) ≦ 40 is set as one of pixel regions Xe10 representing xenon dilution gas having a concentration of 0% to 10%. A pixel having a converted pixel value of 40 <g′80 (x, y) ≦ 75 is specified as one of the pixel regions Xe20 representing a xenon dilution gas having a concentration of 10% to 20%, and the converted pixel A pixel having a value of 75 <g′80 (x, y) ≦ 110 is specified as one of the pixel regions Xe30 representing a xenon dilution gas having a concentration of 20% to 30%.

  As shown in FIG. 7, the pixel at the coordinates (x1, y1) has a converted pixel value ratio r ′ (x1, y1) = 1.82, and the converted pixel value g′80 (x1, y1) = Since it is 100, it is specified here as one of the pixel regions Xe30 of the xenon dilution gas having a concentration of 20% to 30%.

  In step S111, a substance separation image is displayed. That is, the first tomographic image g80 (x, y) or the second tomographic image g140 (the second tomographic image g140 (so that the pixel region specified in step S105 or step S109 + S110) is represented by a color corresponding to the type of the pixel region. A substance separation image in which colors are superimposed on x, y) is displayed on the screen of the monitor 6.

  FIG. 8 is a diagram illustrating a first example of the screen of the monitor 6 displaying the substance separation image, and FIG. 9 is a diagram illustrating a second example of the screen of the monitor 6 displaying the substance separation image.

  When the combination of the target substances is iodine and calcium, for example, as shown in FIG. 8, the iodine pixel area Io and the calcium pixel area Ca are separated on the tomographic image, and the substance separation images ga (x, The screen SC2 including y) is displayed.

When the combination of the target substances is air and xenon, for example, as shown in FIG. 9, on the tomographic image, the air pixel region Air, the xenon dilution gas Xe10 having a concentration of 0 to 10%, and the concentration of 10 to 20% The xenon dilution gas Xe20 and the xenon dilution gas Xe30 having a concentration of 20 to 30% are separated, and a screen SC3 including the color-separated substance separation images gb (x, y) is displayed.
(Second Embodiment)

  In this embodiment, a reference material that defines a pixel value as zero is input, pixel values of each tomographic image are converted as necessary, and a distribution of pixel value ratios between tomographic images, or between converted tomographic images The tomographic image is displayed so that the distribution of the ratio of the converted pixel values in can be understood.

  FIG. 10 is a flowchart showing the flow of pixel value ratio distribution image generation processing in the X-ray CT apparatus 100 according to the second embodiment.

  In step S201, dual energy imaging is performed as in step S101. As a result, a first tomographic image g80 (x, y) having an X-ray tube voltage of 80 kV at the time of imaging is generated, and image reconstruction processing is performed based on the second projection data. A second tomographic image g140 (x, y) having a tube voltage of 140 kV is generated.

  In step S202, a reference substance is input. That is, the operator inputs a substance for which the pixel value is zero as a reference substance.

  FIG. 11 is a diagram illustrating an example of a screen of the monitor 6 that displays a reference substance input screen. Specifically, for example, as shown in FIG. 11, the first tomographic image g80 (x, y) and the second tomographic image g140 (x, y) are arranged side by side, and selection of a reference material candidate is selected. A screen SC3 on which the button KB is arranged is displayed on the monitor 6. The reference substance candidate selection button KB includes a button B3 for selecting water as the reference substance and a button B4 for selecting air as the reference substance. Then, the operator presses a selection button via the input device 2 while viewing this screen, and selects and inputs one reference substance from these candidates.

  In step S203, it is determined whether the reference substance is water or air. In this determination, if it is determined that the reference material is water, the process proceeds to step S204, and if it is determined that the reference material is air, the process proceeds to step S206.

  In step S204, as in step S104, the ratio of pixel values between tomographic images is calculated. Thus, the pixel value ratio r (x, y), that is, the CT value ratio is calculated.

  In step S205, the first tomographic image g80 (x, y) or the second tomographic image g140 is displayed so that each pixel on the tomographic image is represented by a color corresponding to the pixel value ratio r (x, y). A pixel value ratio distribution image in which colors are superimposed on (x, y) is displayed on the screen of the monitor 6, and then the process proceeds to step S209.

  FIG. 12 is a diagram illustrating an example of a screen of the monitor 6 displaying the pixel value ratio distribution image. For example, as shown in FIG. 12, the color of the pixel (x, y) is closer to blue as the pixel value ratio r (x, y) of the pixel is smaller, and closer to red as the pixel is larger. A screen SC5 including the pixel value ratio distribution image gc (x, y) is displayed.

  In step S206, pixel values are converted in the same manner as in step S107. That is, the pixel values in the first tomographic image g80 (x, y) and the second tomographic image g140 (x, y) are converted into pixel values that are defined so that the pixel value corresponding to air is zero. . That is, pixel value conversion is performed such that the reference material that defines the corresponding pixel value to be zero is changed from water to air. As a result, a first converted tomographic image g′80 (x, y) and a second converted tomographic image g′140 (x, y) are generated.

  In step S207, the ratio of the converted pixel values between the converted tomograms is calculated as in step S108. Thereby, the ratio r ′ (x, y) of the converted pixel values is calculated.

  In step S208, the first tomographic image g80 (x, y) or the second tomographic image g80 (x, y) is displayed so that each pixel on the tomographic image is represented by a color corresponding to the ratio r ′ (x, y) of the converted pixel value. The converted pixel value ratio distribution image in which the color is superimposed on the tomographic image g140 (x, y) is displayed on the screen of the monitor 6.

  FIG. 13 is a diagram illustrating an example of the screen of the monitor 6 displaying the converted pixel value ratio distribution image. For example, as shown in FIG. 13, the color of the pixel (x, y) becomes closer to blue as the ratio r ′ (x, y) of the converted pixel value of the pixel is smaller, and the color closer to red as it is larger. A screen SC6 including the converted pixel value ratio distribution image gd (x, y) is displayed.

  In step S209, it is determined whether or not to reselect the reference material according to the input from the operator. If the determination is affirmative, the process returns to step S201. If the determination is negative, the process ends.

  As described above, according to each of the above embodiments, the pixel values in two types of tomographic images having different X-ray tube voltages at the time of imaging are defined so that the pixel value corresponding to air is zero as necessary. Since the ratio of the pixel value in the corresponding pixel is obtained between these two types of tomographic images after conversion into the pixel value, the ratio of the pixel value in the substance having a small X-ray absorption rate is changed to the pixel in the other substance. It is possible to separate a substance having a small X-ray absorption rate with high accuracy in a tomographic image obtained by X-ray CT imaging.

  The present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

  For example, the present invention can also be applied to an adaptive weighted difference method proposed in Japanese Patent Application No. 2007-292955.

  Further, when displaying the material separation image, the contrast of the specified pixel region of the first tomographic image g80 (x, y) or the second tomographic image g140 (x, y) is compared with other regions. May be displayed so as to be lower.

  For xenon, more accurate xenon concentration can be calculated by calculating not only using the ratio of converted pixel values but also using CT values which are pixel values before conversion.

  Further, xenon may not be separated according to density, but xenon having a predetermined density or more may be separated as a single pixel area.

  Unlike the X-ray CT apparatus according to the above embodiment, the imaging unit is not provided, and the pixel values in the first tomographic image and the second tomographic image are defined so that the pixel value corresponding to air is zero. A pixel value conversion unit that converts the pixel value into a pixel value and a pixel value ratio calculation unit that calculates a ratio of the converted pixel value in the corresponding pixel between the first tomographic image and the second tomographic image. A tomographic image processing apparatus such as a station is also an embodiment of the present invention.

  A computer that converts pixel values in the first tomographic image and the second tomographic image into pixel values that are defined so that a pixel value corresponding to air is zero; A program for functioning as a pixel value ratio calculating means for calculating a ratio of converted pixel values in corresponding pixels between the tomographic image and the second tomographic image is also an embodiment of the present invention.

1 is a block diagram showing an X-ray CT apparatus according to a first embodiment. It is a flowchart which shows the flow of the tomographic material separation process in the X-ray CT apparatus by 1st Embodiment. It is a figure which shows an example of two types of tomographic images obtained by dual energy imaging | photography, and its ratio image. It is a figure which shows an example of the screen which displayed the input screen of the target substance. It is a figure which shows the correspondence of the kind of substance, and the ratio of a pixel value. It is a figure which shows an example of two types of converted tomograms obtained by conversion of pixel values and their ratio images. It is a figure which shows the correspondence of the density | concentration of a xenon, and the ratio of the converted pixel value. It is a figure which shows the 1st example of the screen which displayed the substance separation image. It is a figure which shows the 2nd example of the screen which displayed the substance separation image. It is a flowchart which shows the flow of the pixel value ratio distribution image display process in the X-ray CT apparatus by 2nd Embodiment. It is a figure which shows an example of the screen which displayed the input screen of the reference material. It is a figure which shows an example of the screen which displayed the pixel value ratio distribution image. It is a figure which shows an example of the screen which displayed the converted pixel value ratio distribution image.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 X-ray CT apparatus 1 Operation console 2 Input device 3 Central processing unit 4 Control interface 5 Data acquisition buffer 6 Monitor 7 Rotating part 8 Imaging table 9 Scanning gantry 10 X-ray controller 11 X-ray tube 12 Collimator 13 X-ray detector 14 Data Collection unit 15 Rotation controller

Claims (10)

A first tomographic image obtained by X-ray CT imaging of the subject with a first X-ray tube voltage, and a second X-ray tube voltage different from the first X-ray tube voltage of the subject. For each pixel with the second tomographic image obtained by X-ray CT imaging, an index of the X-ray absorption rate of the substance corresponding to the pixel is represented, and the value corresponding to water is substantially 0 (zero). ) Represents the index of the X-ray absorption rate of the substance corresponding to the pixel, and the value corresponding to air is defined to be substantially 0 (zero). Calculating means for calculating a second pixel value to be performed;
In the first or second tomographic image, a ratio of the second pixel value in pixels corresponding to each other between the first tomographic image and the second tomographic image, and the ratio and the kind of material A tomographic image processing apparatus comprising: specifying means for specifying a pixel region representing a first substance having an X-ray absorption rate close to air based on the correspondence relationship between   The tomographic image processing apparatus according to claim 1, wherein the first pixel value is a CT value defined such that a value corresponding to air is −1000.   The tomographic image processing apparatus according to claim 2, wherein the calculation unit calculates the second pixel value by adding 1000 to the first pixel value. When the substance to be specified is the first substance, the specifying means determines the second pixel value of the pixels corresponding to each other between the first tomographic image and the second tomographic image. Based on the ratio, a pixel region representing the first material is specified, and when the material to be specified is a second material different from the first material, the first regions in the pixels corresponding to each other. based on a ratio of one pixel value, determining the pixel region representing the material of the second tomographic image processing apparatus according to any one of claims 1 to 3.   The tomographic image processing apparatus according to claim 1, wherein the first substance is a gas containing a predetermined gas different from air.   The said specifying means specifies the density | concentration of the said predetermined gas in this pixel based on the said 1st pixel value or the said 2nd pixel value in the pixel showing the gas containing the said predetermined gas. Tomographic image processing equipment. The first substance is a plurality of substances including air and a gas containing xenon,
The tomographic image processing apparatus according to claim 4, wherein the specifying unit separately specifies a pixel region representing air and a pixel region representing gas containing xenon. The second substance is a plurality of substances, and contains iodine and calcium.
The tomographic image processing apparatus according to claim 4, wherein the specifying unit separately specifies a pixel region representing iodine and a pixel region representing calcium.   An X-ray CT apparatus comprising the tomographic image processing apparatus according to any one of claims 1 to 8.   A program for causing a computer to function as the tomographic image processing apparatus according to any one of claims 1 to 8.

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