JP5523715B2 - X-ray CT apparatus and image processing apparatus - Google Patents

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus and an image processing apparatus, and more particularly to processing of an image obtained by dual energy imaging.

  Conventionally, a predetermined substance is obtained by weighting and subtracting two images obtained by performing dual energy imaging of a subject with an X-ray CT apparatus and having different X-ray tube voltage (energy distribution of irradiated X-rays). Substance-suppressed image in which the components of the substance are suppressed (This is also called a substance-enhanced image because components of other substances are emphasized by suppressing the components of a given substance. There is known an image processing method for obtaining (see, for example, Patent Document 1, FIG. 5 and the like).

  The substance suppression image obtained in this way is used for image diagnosis as it is, or used as an image from which another image is obtained.

JP 2008-154784 A

  However, in the above image processing method, weighting subtraction processing is performed using the same weighting coefficient for each pixel, and therefore the influence of the quality of the X-ray beam (beam) irradiated on the subject varies depending on the spatial position. Is not eliminated, leaving room for improving the accuracy of the desired substance suppression images.

  In view of the above circumstances, the present invention provides an X-ray CT apparatus and an image processing apparatus capable of accurately obtaining a substance-suppressed image of a subject based on two images having different tube voltages obtained by dual energy imaging. The purpose is to provide.

  In a first aspect, the present invention includes an X-ray tube and an X-ray detector, and the subject is subjected to a first tube voltage and a second tube voltage different from the first tube voltage. An imaging unit that collects projection data by X-ray CT imaging, a first image based on projection data based on the first tube voltage, and a second image based on projection data based on the second tube voltage are image-reproduced. An image is obtained by performing weighted subtraction processing on the pixel values of the corresponding pixels in the first and second images using different weighting coefficients depending on the position of the pixel. An X-ray CT apparatus comprising weighted subtraction means is provided.

  In a second aspect, the present invention provides the X-ray beam generated from the X-ray tube, in which the imaging means has an X-ray filter that spatially changes the quality of the X-ray passing therethrough. Is provided to the subject via the X-ray filter. The X-ray CT apparatus of the first aspect is provided.

  In a third aspect, the present invention provides an X-ray filter in which the imaging unit has a plurality of X-ray filters having different X-ray absorption characteristics, and the X-ray beam is selected from the plurality of X-ray filters. The X-ray CT apparatus according to the second aspect is provided, wherein the subject is irradiated via a weight, and the weighted subtracting means performs weighted subtraction using a weighting factor corresponding to the selected X-ray filter. .

  In a fourth aspect, the present invention is the first aspect according to the first aspect, wherein the weighting factor changes according to a distance from a center of an image reconstruction area in the first and second images to the corresponding pixel. An X-ray CT apparatus according to any one of the three viewpoints is provided.

  In a fifth aspect, the present invention provides the X-ray CT apparatus according to the fourth aspect, wherein the weighting factor is larger in the peripheral part than in the central part of the image reconstruction area.

  In a sixth aspect, the present invention provides that the X-ray detector has a plurality of detection element arrays in the body axis direction of the subject, and the first and second images are obtained from the subject. From the first aspect to the fifth aspect, an image of a predetermined position with respect to the X-ray detector in the body axis direction, wherein the weighted subtraction means performs a weighted subtraction process using a weighting factor corresponding to the predetermined position. An X-ray CT apparatus according to any one of the aspects is provided.

  In a seventh aspect, the present invention provides the weight coefficient in the pixel at the same position as the corresponding pixel of the image obtained when X-ray CT imaging is performed with the first tube voltage using the imaging unit. The first pixel value corresponding to a predetermined substance and the predetermined pixel in a pixel at the same position as the corresponding pixel of an image obtained when X-ray CT imaging is performed with the second tube voltage using the imaging unit. An X-ray CT apparatus according to any one of the first to fifth aspects is provided based on a ratio with a second pixel value corresponding to a substance.

  In an eighth aspect, the present invention provides the first substance-suppressed image in which the weighted subtracting unit suppresses the component of the first substance by the weighted subtraction process using the weighting coefficient corresponding to the first substance. And obtaining a second substance suppression image in which a component of the second substance is suppressed by weighted subtraction processing using a weighting factor according to the second substance, and the first substance suppression image and the There is provided an X-ray CT apparatus according to any one of the first to seventh aspects, further comprising weighted addition means for obtaining a third image by weighted addition with a second substance suppression image.

  In a ninth aspect, the present invention provides the X-ray CT according to any one of the first to eighth aspects, wherein one of the first and second substances is water and the other is iodine. Providing equipment.

  In a tenth aspect, the present invention provides an imaging device having an X-ray tube and an X-ray detector, wherein the subject is subjected to a first tube voltage and a second tube voltage different from the first tube voltage. Image receiving means for receiving a first image based on the first tube voltage and a second image based on the second tube voltage, which are reconstructed based on projection data acquired by X-ray CT imaging; An image comprising weighted subtracting means for obtaining an image by subjecting pixel values of pixels corresponding to each other in the first and second images to weighted subtraction using different weighting coefficients according to the positions of the pixels. A processing device is provided.

  According to the present invention, two images having different tube voltages can be weighted and subtracted by using an appropriate weighting coefficient corresponding to the position of each corresponding pixel, and the X-ray beam quality is spatial. It is possible to reduce the influence of being different depending on the position and to obtain the substance suppression image of the subject with high accuracy.

It is a figure which shows the structure of the X-ray CT apparatus which concerns on Example 1 of this invention. It is a figure when the principal part of the imaging system of a scanning gantry is seen in the z direction. It is a figure when the principal part of the imaging system of a scanning gantry is seen in the x direction. It is a figure which shows the structure of a central processing unit. It is a figure which shows the process of the data (data) process for producing | generating the image equivalent to a desired tube voltage. It is a figure which shows each slice (slice) position of the image to be reconfigure | reconstructed. It is a figure which shows the spectrum (spectrum) of the X-ray by a predetermined | prescribed tube voltage. It is a figure which shows the relationship between the X-ray photon energy (x-ray photon energy) and X-ray absorption coefficient in a predetermined substance. It is a figure which shows the relationship between the distance from the center of an image reconstruction area | region, and a target image, and a dual energy ratio. It is a figure which shows the example of a measurement of the dual energy ratio of the iodine in the center part of a tomogram. It is a figure which shows the example of a measurement of the dual energy ratio of the iodine in the peripheral part of a tomogram. It is a figure which shows the relationship between the position of the z direction of a tomogram, and a dual energy ratio. It is a figure which shows the change with respect to the position Z of the z direction of the tomogram in the dual energy ratio of iodine.

It is a flow figure which shows an example of operation | movement of the X-ray CT apparatus which concerns on Example 1 of this invention. It is a figure which shows the structure of the image processing apparatus which concerns on Example 2 of this invention.

  Embodiments of the present invention will be described below. Note that the present invention is not limited thereby.

  An X-ray CT apparatus according to Embodiment 1 will be described. This X-ray CT apparatus collects projection data by dual energy imaging, reconstructs two tomographic images having different tube voltages of the X-ray tube based on the collected projection data, and weights these two tomographic images. By subtraction processing (also called weighted subtraction processing), two substance density images (substance suppression images) representing the density distribution of another substance are obtained by suppressing the component of a predetermined substance, and weighted addition of these two substance density images A tomographic image corresponding to a desired tube voltage is generated by processing (weighted addition processing). The feature is that a weighting coefficient (also referred to as a weighting coefficient) used for weighting subtraction processing for obtaining these substance density images is changed according to various conditions including pixel positions and the like, and a highly accurate substance density image is generated. There is.

  FIG. 1 is a diagram illustrating a configuration of an X-ray CT apparatus 100 according to the first embodiment.

  The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry (imaging means) 20.

  The operation console 1 is acquired by an input device 2 that receives input from an operator, a central processing unit 3 that performs control of each unit for performing dual energy imaging, data processing for generating an image, and the like, and a scanning gantry 20. A data collection buffer (buffer) 5 that collects the data, a monitor 6 that displays an image, and a storage device 7 that stores a program, data, and the like.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and put into and out of the opening B of the scanning gantry 20. The cradle 12 is moved up and down and horizontally moved by a motor built in the imaging table 10. Here, the body axis direction of the subject 40, that is, the linear movement direction of the cradle 12 is the z direction, the vertical direction is the y direction, and the horizontal direction perpendicular to the z direction and the y direction is the x direction.

  The scanning gantry 20 includes a rotating unit 15 and a main body 20a that rotatably supports the rotating unit 15. The rotating unit 15 includes an X-ray tube 21, an X-ray controller 22 that controls the X-ray tube 21, and a collimator (collimate) that shapes and collimates the X-ray beam Xb generated from the X-ray tube 21. collimator) 23, an X-ray filter unit 28 for adjusting the quality and intensity of the X-ray beam Xb shaped by the collimator 23, an X-ray detector 24 for detecting the X-ray beam Xb transmitted through the subject 40, The DAS (Data Acquisition System) 25 (also referred to as a data acquisition device) that converts the output of the X-ray detector 24 into projection data and collects it, and the X-ray controller 22, collimator 23, DAS 25, and X-ray filter unit 28 are controlled. A rotating unit controller 26 is mounted. The main body 20 a includes a control controller 29 that communicates control signals and the like with the operation console 1 and the imaging table 10. The rotating part 15 and the main body part 20 a are electrically connected via a slip ring 30.

  The configuration of the main part of the imaging system in the scanning gantry 20 will be described.

  2 is a diagram when the main part of the imaging system is viewed in the z direction, and FIG. 3 is a diagram when the main part of the imaging system is viewed in the x direction.

  As shown in FIG. 2, the X-ray tube 21 and the X-ray detector 24 are arranged to face each other so as to sandwich the opening B of the scanning gantry 20.

  As shown in FIG. 3, the X-ray tube 21 includes a cathode filament 21c and a rotating anode 21a. Electrons e emitted from the cathode filament 21c are generated by a tube voltage applied between the electrodes. It is accelerated and collides with the rotating anode 21a to generate an X-ray beam Xb.

  As shown in FIGS. 2 and 3, the X-ray detector 24 is a multi-row detector having a plurality of detection element arrays 24r in which a plurality of X-ray detection elements 24s are arranged in the channel direction CH in the z direction which is the slice direction. It is. In the present embodiment, as shown in FIG. 3, the X-ray detector 24 has 64 detection element arrays having detection element array numbers j of 32A,..., 1A, 1B,. 24r. Each detection element array 24r includes X-ray detection elements 24s for 1000 channels in which the channel number i is 1 to 1000 in the channel direction CH, as shown in FIG.

  As shown in FIGS. 2 and 3, the collimator 23 and the X-ray filter unit 28 are disposed between the X-ray tube 21 and the opening B.

  The collimator 23 collimates the X-ray beam Xb generated by the X-ray tube 21 in the channel direction and the slice direction (z direction) and shapes it into a fan beam shape.

  The X-ray filter unit 28 includes a beam forming X-ray filter that spatially adjusts the quality and intensity of the X-ray beam Xb shaped by the collimator 23. The beam forming X-ray filter has a concave shape in which the central portion in the channel direction of the X-ray beam Xb is the thinnest and becomes thicker as it approaches the peripheral portion, and the peripheral portion absorbs X-rays more than the central portion. Designed to do. This prevents excessive X-rays from being irradiated on the body surface of the subject having a circular or elliptical cross-sectional shape.

  In this embodiment, the X-ray filter unit 28 has a plurality of beam forming X-ray filters 28L, 28M, and 28S having different spatial X-ray absorption characteristics. Then, any one of the beam forming X-ray filters selected based on the size of the subject is inserted into the irradiation path of the X-ray beam Xb. The X-ray filter included in the X-ray filter unit 28 is not limited to the beam forming X-ray filter.

  After the X-ray beam Xb generated from the X-ray tube 21 is shaped by the collimator 23, the X-ray filter unit 28 spatially adjusts the quality and intensity of the X-ray beam Xb, and the X-ray beam Xb is applied to the subject 40 conveyed into the opening B. Irradiated. The X-ray beam Xb that has passed through the subject is detected by the X-ray detector 24.

  In addition, when the rotating unit 15 is rotated around the rotation center ISO while irradiating the X-ray beam Xb from the X-ray tube 21, a region irradiated with the X-ray beam from all directions is the imaging field of view SFOV. The image reconstruction area of the tomographic image is an area included in the imaging field of view SFOV with the rotation center ISO as the center.

  FIG. 4 is a diagram showing the configuration of the central processing unit 3. The central processing unit 3 includes an imaging condition setting unit 31, a scan control unit 32, an image reconstruction unit 33, a weighting subtraction unit 34, a weighting addition unit 35, and a display control unit 36. Note that these components are realized by the functions of the central processing unit 3.

  The shooting condition setting unit 31 sets shooting conditions for dual energy shooting in accordance with an input from the operator. As imaging conditions, for example, a high tube voltage HV (first tube voltage) and a low tube voltage LV (second tube voltage), which are set tube voltages in dual energy imaging, and types of beam forming X-ray filters used for imaging FL, imaging range, slice thickness, scan one rotation time, etc. The set tube voltage is, for example, a high tube voltage HV of 140 kV and a low tube voltage LV of 80 kV.

  The scan control unit 32 controls the scanning gantry 2 and the imaging table 10 so as to execute dual energy imaging according to the set imaging conditions. For example, by setting the tube voltage of the X-ray tube 21 to a high tube voltage HV (or a low tube voltage), collecting projection data of a plurality of views corresponding to a view angle that is a predetermined rotation angle, The scanning gantry 2 and the imaging table 10 are controlled so that the tube voltage of the X-ray tube 21 is set to a low tube voltage LV (or a high tube voltage) and projection data of a plurality of views corresponding to the same view angle is collected. Alternatively, projection data of a plurality of views corresponding to a predetermined view angle is collected for each tube voltage while repeatedly switching the tube voltage of the X-ray tube 21 between a high tube voltage HV and a low tube voltage LV in units of one or several views. Thus, the scanning gantry 2 and the imaging table 10 are controlled. Thereby, a plurality of views of projection data PHV (view, j, i) corresponding to a predetermined view angle due to a high tube voltage and a plurality of views of projection data PLV (view corresponding to a predetermined view angle due to a low tube voltage) , j, i) are collected. Here, view is a view number (view = 1, 2,..., About 2000), j is a detection element row number (j = 32A,..., 1A, 1B,..., 32B), and i is a channel number (i = 1). , 2,..., 1000). When the latter method is used, the projection data of the missing view may be supplemented by weighting and adding the collected projection data in the view direction. The predetermined view angle is, for example, 2π [rad] or the fan angle α [rad] of the π + X-ray beam Xb.

  The image reconstruction unit 33 reconstructs a high tube voltage tomogram (first image) and a low tube voltage tomogram (second image) based on projection data collected by dual energy imaging. . That is, as shown in FIG. 5, a tomographic image GHV (x, y, z) with a high tube voltage is reconstructed based on projection data PHV (view, j, i) with a high tube voltage, and with a low tube voltage. Based on the projection data PLV (view, j, i), a tomographic image GLV (x, y, z) having a low tube voltage is reconstructed. In this embodiment, as shown in FIG. 6, each of the z-directions in the passage region of the X-ray beam Xb irradiated from the X-ray tube 21 to the X-ray detector 24 on the rotation center (axis) ISO of the rotating unit 15. For slices SL1 to SL64 having positions z1 to z64 as slice positions, a high tube voltage tomogram GHV (x, y, z) and a low tube voltage tomogram GLV (x, y, z) (z = z1, z2,..., z64) are reconstructed. For image reconstruction, for example, a filtered back-projection method or the like is used.

  The weighted subtracting unit 34 obtains two different material density images by performing weighted subtraction on the high tube voltage tomogram and the low tube voltage tomogram. In other words, as shown in FIG. 5, the tomographic image GHV (x, y, z) having a high tube voltage and the tomographic image GLV (x, y, z) having a low tube voltage are subjected to weighted subtraction processing, so that the subject 40 Water density image W (x, y, z) (first substance suppression image) representing the density distribution of water and iodine density image Io (x, y, z) representing the density distribution of iodine in the subject 40 ( 2nd substance suppression image). In this embodiment, a water density image W (x, y, z) and an iodine density image Io (x, y, z) (z = z1, z2,..., Z64) are obtained for each of the slices SL1 to SL64. .

  Here, the weighting subtraction processing by the weighting subtraction unit 34 will be described in detail.

  FIG. 7 is a diagram showing an X-ray spectrum (energy distribution) with a predetermined tube voltage. In this figure, SP80 indicates the X-ray spectrum when the tube voltage is 80 kV, and SP140 indicates the X-ray spectrum when the tube voltage is 140 kV. FIG. 8 is a diagram showing the relationship between the X-ray photon energy and the X-ray absorption coefficient in a predetermined substance. In this figure, μa represents the X-ray absorption coefficient of the substance a, and μb represents the X-ray absorption coefficient of the substance b.

  The X-ray spectrum varies depending on the tube voltage of the X-ray tube that generates the X-ray, as shown in FIG. Further, as shown in FIG. 8, the X-ray absorption coefficient of the substance changes according to the X-ray photon energy, and the change curve differs depending on the kind of elements constituting the substance or a combination thereof.

  On the other hand, the projection data or tomographic image of the subject can be approximately expressed by the density distributions of two different substances and their X-ray absorption coefficients.

  Based on this approximate model, a water density image W (x, y, z) and an iodine density image Io (x, y, z) can be obtained. In other words, two images GHV (x, y, z) and GLV (x, y, z) with different tube voltages are pixels corresponding to iodine in order to suppress the iodine component and emphasize the water component. By performing weighted subtraction processing so that the value becomes zero, a water density image W (x, y, z) representing the water density distribution can be obtained. Similarly, two images GHV (x, y, z) and GLV (x, y, z) with different tube voltages are used for the pixel corresponding to water to suppress the water component and emphasize the iodine component. By performing weighted subtraction processing so that the pixel value becomes zero, an iodine density image Io (x, y, z) representing the density distribution of iodine can be obtained.

Specifically, for example, a weighted subtraction process is performed according to the following formula.

  Here, kw is a conversion coefficient for converting the pixel value W (x, y, z) of the water density image into a value representing the density [mg / ml] of water, and kio is the pixel value (x of the iodine density image Io). , y, z) is a conversion coefficient for converting iodine density [mg / ml], Rio is the dual energy ratio of iodine between the high tube voltage HV and the low tube voltage LV, and Rw is the high tube voltage HV. And the dual energy ratio of water at low tube voltage LV.

  In Equation 1 representing weighted subtraction processing for obtaining the water density image W (x, y, z), the weighting coefficient to be multiplied by the pixel value GLV (x, y, z) of the tomographic image of the low tube voltage is kw. The weighting factor for multiplying the pixel value GHV (x, y, z) of the tomographic image with a high tube voltage is kw · Rio. Further, in Equation 2 representing the weighted subtraction process for obtaining the iodine density image Io (x, y, z), the weighting coefficient to be multiplied by the pixel value GLV (x, y, z) of the tomographic image of the low tube voltage is kio. The weighting factor for multiplying the pixel value of the high tube voltage tomogram GHV (x, y, z) is kio · Rw.

  Furthermore, the dual energy ratio Rio is a pixel value (CT value) GCio, LV (corresponding to iodine on a tomographic image GCLV (x, y, z) obtained when X-rays are taken with a low tube voltage LV. x, y, z) is a pixel value GCio, HV (x, y, x, y) corresponding to iodine on a tomographic image GCHV (x, y, z) obtained when X-rays are taken with a high tube voltage HV. The value obtained by dividing by z) (pixel value ratio). Further, the dual energy ratio Rw is obtained by calculating the pixel value GCw, LV (x, y, z) corresponding to water on the tomographic image GCLV (x, y, z) on the tomographic image GCHV (x, y, z). The pixel value GCw, 140 (x, y, z) corresponding to water is divided into values.

  These dual energy ratios R change according to the tube voltage of the X-ray tube and the type of the substance, but actually, in addition to these, they also change according to other conditions as shown below.

  First, the dual energy ratio R changes according to the position of the pixel in the tomographic image, and particularly, as shown in FIG. 9A, the pixel g (x , y, z) has a feature of changing according to the distance r. That is, if the positions of the pixels having the same dual energy ratio R are connected by a line, as shown in FIG. 9B, the distribution is represented as a distribution close to a concentric circle centering on the center ISO of the image reconstruction area. This is because the quality of the X-ray beam Xb irradiated to the subject 40 changes in the channel direction CH, or the X-ray transmission path of the beam forming X-ray filter changes in the channel direction CH. Conceivable.

  The X-ray beam Xb generated from the X-ray tube 21 becomes a soft X-ray beam Xs at the center of the channel direction CH as shown in FIG. 2 due to the structure of the rotating anode 21a constituting the X-ray tube 21. It is known that a hard X-ray beam Xh is generated at the periphery in the channel direction CH. A soft X-ray beam is an X-ray beam with a large amount of low X-ray energy components, that is, an X-ray effective energy is relatively low, and a hard X-ray beam is a high X-ray energy component, that is, an X-ray beam. An X-ray beam having a relatively high effective energy. In addition, the beam forming X-ray filter has a large amount of X-ray absorption with low X-ray photon energy, and the closer to the periphery in the channel direction CH, the thicker the filter is, and the amount of X-ray absorption increases. It is considered that this tendency is further increased in the X-ray beam Xb that has passed through the formed X-ray filter.

  As an example, FIG. 10 shows a measurement example of the dual energy ratio Rio of iodine in the central part of the tomographic images of tube voltages of 80 kV and 140 kV when photographing a phantom in which iodine is arranged in the central part or the peripheral part. FIG. 11 shows an example of measurement of the dual energy ratio Rio of iodine in the peripheral part of the tomographic images of voltages 80 kV and 140 kV. In this measurement example, the dual energy ratio Rio of iodine is 1.70 at the central portion (r = 0 cm) of the tomographic image, 1.80 at the peripheral portion (r = 25 cm) of the tomographic image, and dual when the distance r increases. It can be seen that the energy ratio Rio also tends to increase.

  Secondly, the dual energy ratio R has a characteristic that it changes according to the type of beam forming X-ray filter used for imaging, that is, its shape and X-ray absorption characteristics. This is because, if the type of the beam forming X-ray filter is different, the spatial X-ray absorption characteristics of the filter are also different, so that the quality of the X-ray beam Xb that has passed through the beam forming X-ray filter also changes spatially. .

  Third, as shown in FIG. 12, the dual energy ratio R has a feature that it changes in accordance with the position Z in the z direction with respect to the X-ray detector 24 of the tomographic image to be subjected to the weighting subtraction process. The X-ray beam Xb generated from the X-ray tube 21 is soft at the peripheral portion near the cathode filament 21c in the z direction, as shown in FIG. 3, due to the structure of the rotating anode 21a constituting the X-ray tube 21. It is known that the X-ray beam Xs becomes a hard X-ray beam Xh in the peripheral portion near the rotary anode 21a in the z direction. The dependency of the dual energy ratio R on the position Z in the z direction of the tomographic image is considered to be due to the difference in the quality of the X-ray beam Xb in the z direction.

  As an example, FIG. 13 shows a change in the z-direction position Z of the tomographic image in the dual energy ratio Rio of iodine. As shown in this example, it is understood that the dual energy ratio R tends to increase as the position Z in the z direction of the tomographic image approaches from the filament cathode 21c side to the rotating anode 21a side.

  In this embodiment, the water density image W (x, y, z) and the element density image Io (x, y, z) of the subject 40 are obtained with high accuracy in consideration of these characteristics regarding the dual energy ratio R. Take the following configuration.

  The storage device 7 is a combination of a material type M, tube voltages HV and LV, a beam forming X-ray filter type FL, and a position Z in the z direction of a tomographic image (a slice thickness may be added thereto). Separately, information RT representing the relationship between the coordinates (x, y) of each pixel in the tomographic image or the distance r from the center ISO of the image reconstruction area to each pixel in the tomographic image and the dual energy ratio R is stored.

  Options for each element constituting the combination include, for example, substance M = water, iodine, tube voltage HV, LV = (140 kV, 80 kV), (120 kV, 60 kV), beam forming X-ray filter type FL = 28L, 28M , 28S, the position Z of the tomographic image in the z direction Z = z1, z2,.

  The information RT for each combination is, for example, information representing a pixel in a tomographic image and a dual energy ratio R at that pixel in association with each pixel. Further, for example, the dual energy ratio R is information that is defined as increasing continuously or stepwise from the center of the tomogram to the periphery. That is, the weighting coefficient used for the weighted subtraction process between the low tube voltage tomogram and the high tube voltage tomogram is set so that the peripheral portion is larger than the central portion of the tomogram. The format of the dual energy ratio information RT may be a mathematical expression or a table.

  Note that the value of the dual energy ratio R is a tomographic image obtained by actually photographing a phantom in which substances such as iodine and water are arranged at respective positions in the xy plane using the scanning gantry 20. It can be obtained from the pixel value of or a simulation based on theoretical calculation. The information RT can be determined based on the value thus obtained. At this time, it may be determined using a calculation method such as polynomial approximation.

  The weighting subtracting unit 34 includes the type M of the substance to be suppressed, the set tube voltages HV and LV, the type FL of the beam forming X-ray filter used for imaging, and the tomographic image to be subjected to the weighting subtraction process. The information RT (M, HV, LV, FL, Z) of the dual energy ratio corresponding to the combination with the position Z in the z direction is read from the storage device 7. Next, for each pixel corresponding to each other between the high tube voltage cut-off image GHV (x, y, z) and the low tube voltage tomogram GLV (x, y, z), the position of the pixel is determined. The dual energy ratio R is specified based on the dual energy ratio information RT (M, HV, LV, FL, Z) read out earlier. Then, the specified dual energy ratio R is substituted into the weighted subtraction formulas shown in the above formulas 1 and 2, and the pixel values of corresponding pixels are weighted and subtracted. That is, the weighting subtraction unit 34 corresponds to the material type M, tube voltage HV, LV, X-ray filter type FL, tomographic position Z in the z direction, and pixel coordinates (x, y) or distance r. A weighted subtraction process is performed using a weighting coefficient.

  The weighted addition unit 35 performs a weighted addition process on two different material density images to obtain a tomographic image (third image) corresponding to another tube voltage. That is, as shown in FIG. 5, a tomographic image corresponding to a desired tube voltage NV is obtained by performing weighted addition processing on the water density image W (x, y, z) and the iodine density image Io (x, y, z). Find GNV (x, y, z). In this embodiment, a tomographic image GNV (x, y, z) (z = z1, z2,..., Z64) corresponding to the tube voltage NV is obtained for each of the slices SL1 to SL64.

The tomographic image GNV (x, y, z) corresponding to the tube voltage NV can be obtained by performing weighted addition processing according to the following formula, for example.

  Here, keV1 is the effective energy of X-rays by the tube voltage NV, μw (keV1) is the X-ray absorption coefficient of water at the effective energy keV1, μio (keV1) is the X-ray absorption coefficient of iodine at the effective energy keV1, and kc is the tube This is a change coefficient for changing the pixel value of the tomographic image corresponding to the voltage NV to a value representing the CT value. As is well known, the CT value is a value indicating the degree of X-ray absorption of a substance, the CT value of air is represented by -1000 [HU], and the CT value of water is represented by 0 [HU].

  In Equation 5 representing weighted addition processing for obtaining a tomographic image GNV (x, y, z) corresponding to the tube voltage NV, the weighting coefficient to be multiplied with the water density image W (x, y, z) is kc · μw (keV1 ) And the weighting coefficient to be multiplied with the iodine density image Io (x, y, z) is kc · μio (keV1).

  The display control unit 36 includes a high tube voltage tomogram GHV (x, y, z), a low tube voltage tomogram GLV (x, y, z), a water density image W (x, y, z), and an iodine density. Control is performed to display the image Io (x, y, z), the tomographic image GNV (x, y, z) corresponding to the tube voltage NV, etc. on the screen of the monitor 6 according to the operator's request or automatically. .

  Now, the operation of the X-ray CT apparatus according to the first embodiment of the present invention will be described.

  FIG. 14 is a flowchart showing an example of the operation of the X-ray CT apparatus according to Embodiment 1 of the present invention.

  In step ST1, shooting conditions are set. Here, the imaging condition setting unit 31 sets imaging conditions necessary for dual energy imaging based on information input by the operator via the input device 2.

  In step ST2, dual energy imaging is performed according to the set imaging conditions. Here, the scan control unit 32 controls the scanning gantry 20 and the imaging table 10 according to the imaging conditions to execute dual energy imaging, and uses projection data PLV (view, j, i) with a low tube voltage and a high tube voltage. Projection data PHV (view, j, i) is collected.

  In step ST3, an image reconstruction process is performed. Here, for each of the slices SL1 to SL64, the image reconstruction unit 33 generates a tomographic image GLV (x, y, z) with a low tube voltage based on the projection data PLV (view, j, i) with a low tube voltage. While reconstructing an image, a tomographic image GHV (x, y, z) with a high tube voltage is reconstructed based on projection data PHV (view, j, i) with a high tube voltage.

  In step ST4, a weighted subtraction process is performed. Here, the weighting subtractor 34 sets the type M of the substance to be suppressed, the set high tube voltage HV and low tube voltage LV, the type FL of the beam forming X-ray filter used for imaging, and the weighted subtraction. Dual energy ratio information RT (M, HV, LV, FL, Z) corresponding to the combination of the tomographic image to be processed and the position Z in the z direction is read from the storage device 7. Next, for each pixel corresponding to each other between the high tube voltage tomogram GHV (x, y, z) and the low tube voltage tomogram GLV (x, y, z), a dual corresponding to the position of the pixel is obtained. The energy ratio is specified based on the previously read dual energy ratio information RT (M, HV, LV, FL, Z). Then, the pixel values of the corresponding pixels are subjected to weighted subtraction according to Equation 1 or Equation 2. Thereby, the water density image W (x, y, z) and the iodine density image Io (x, y, z) are obtained for each of the slices SL1 to SL64.

  In step ST5, a weighted addition process is performed. Here, the weight addition unit 35 performs the weight addition process on the water density image W (x, y, z) and the iodine density image Io (x, y, z) according to Equation 5 for each of the slices SL1 to SL64. Thus, a tomographic image GNV (x, y, z) corresponding to the desired tube voltage NV is obtained.

  In step ST <b> 6, the display control unit 36 controls the monitor 6 to display the tomographic image GNV (x, y, z) corresponding to the obtained tube voltage NV on the screen of the monitor 6.

  Thus, according to the X-ray CT apparatus 100 according to the first embodiment, a high tube voltage tomogram GHV (x, y, z) and a low tube voltage tomogram GLV (x, y, z) are obtained. Weighting and subtraction processing can be performed using an appropriate weighting factor in consideration of the dual energy ratio R according to various conditions including the pixel position, and the quality of the X-ray beam Xb irradiated to the subject is a spatial position. Therefore, the influence of the difference is reduced, the substance density image such as the water density image W (x, y, z) and iodine density image Io (x, y, z) of the subject, and further, the tube voltage corresponding to the desired tube voltage. The tomographic image GNV (x, y, z) can be obtained with high accuracy.

  An image processing apparatus according to the second embodiment will be described. This image processing apparatus receives a low tube voltage tomogram and a high tube voltage tomogram obtained by dual energy imaging from the X-ray CT apparatus 100, and performs weighted subtraction processing of these two tomograms having different tube voltages. To obtain a water density image representing the water density distribution of the subject and an iodine density image representing the density distribution of iodine of the subject, and a tomographic image corresponding to a desired tube voltage by weighted addition processing of these two substance density images Is generated.

  FIG. 15 is a diagram illustrating the configuration of the image processing apparatus 102 according to the second embodiment.

  The image processing apparatus 102 includes a workstation and the like, and functions thereof include a data receiving unit (image receiving unit) 50, a weighting subtracting unit 34, a weighting adding unit 35, a display control unit 36, and the like. Have

  The data receiving unit 50 generates a high tube voltage tomogram GHV (x, y, z) and a low tube voltage tomogram GLV (x, y, z) obtained from the X-ray CT apparatus 100 by dual energy imaging. receive.

  Further, the data receiving unit 50 receives information for specifying a weighting coefficient used for the weighted subtraction process by the weighted subtracting unit 34 from the X-ray CT apparatus 100. In this embodiment, such information includes the type M of the substance to be suppressed, the set tube voltages HV and LV, the type FL of the beam forming X-ray filter used for imaging, and the weighted subtraction process. Dual energy ratio information RT (M, HV, LV, FL, Z) corresponding to the combination with the position Z in the z direction of the target tomographic image is received.

  The weighting subtractor 34 determines the position of each pixel corresponding to each other between the high tube voltage tomogram GHV (x, y, z) and the low tube voltage tomogram GLV (x, y, z). The dual energy ratio R corresponding to is specified based on the dual energy ratio information RT (M, HV, LV, FL, Z) read out earlier. Then, the pixel values of the corresponding pixels are subjected to a weighted subtraction process according to Equation 1 or Equation 2 described in the first embodiment. Thereby, the water density image W (x, y, z) and the iodine density image Io (x, y, z) are obtained.

  The weighted addition unit 35 performs a weighted addition process on the water density image W (x, y, z) and the iodine density image Io (x, y, z) according to Equation 5 described in the first embodiment, thereby obtaining a desired value. A tomographic image GNV (x, y, z) corresponding to the tube voltage NV is obtained.

  The display control unit 36 controls a monitor (not shown) included in the image processing apparatus 102 to display a tomographic image GNV (x, y, z) corresponding to the obtained tube voltage NV on the screen of the monitor.

  As described above, according to the image processing apparatus 102 according to the second embodiment, similarly to the X-ray CT apparatus 100 according to the first embodiment, the influence of the X-ray beam quality depending on the spatial position is reduced. Thus, a substance density image of the subject and a tomographic image corresponding to a desired tube voltage can be obtained with high accuracy. Further, such an image can be generated at a place different from the place where the X-ray CT apparatus is installed, and the degree of freedom in handling these images is expanded.

  Conventionally, as another method for generating a substance density image of a subject, new projection data is obtained by weighting and subtracting projection data with a high tube voltage and projection data with a low tube voltage obtained by dual energy imaging. A method for generating and reconstructing a material density image based on the new projection data is known. In addition, when this method is used, a method of performing correction in the projection data space is also known in order to reduce the influence of the quality of the X-ray beam Xb irradiated on the subject depending on the spatial position. However, this method requires a large amount of data processing, puts a burden on the computer, and cannot obtain a target image at high speed.

  On the other hand, as in the first and second embodiments, a high tube voltage tomogram and a low tube voltage tomogram are once reconstructed from the projection data, and these are weighted and subtracted to subject the substance density image of the subject. Is a relatively small amount of data processing, and a target image can be obtained at high speed. However, in this method, no method has been established so far to reduce the influence of the quality of the X-ray beam Xb irradiated on the subject depending on the spatial position.

  Therefore, the X-ray CT apparatus 100 according to the first embodiment and the image processing apparatus 102 according to the second embodiment exhibit an excellent effect that the substance density image of the subject can be obtained “accurately” and “at high speed”. To do.

  Various modifications can be made to the first and second embodiments.

  In the first embodiment, dual energy imaging is performed using a single X-ray tube, but dual energy imaging may be performed using a plurality of X-ray tubes having different X-ray irradiation directions. In this case, a high tube voltage may be set for the first X-ray tube, and a low tube voltage may be set for the second X-ray tube.

  In the first embodiment, the scanning method in dual energy imaging is an axial scan, but this may be a helical scan.

  In the first and second embodiments, the water density image and the iodine density image are used as the substance density image of the subject to be subjected to the weighted subtraction process. However, the present invention is not limited to this combination. For example, a combination of a water density image and a calcium density image, or a combination of an iodine density image and a calcium density image may be used.

  In Examples 1 and 2, a tomographic image corresponding to a desired tube voltage is obtained. However, since the material density image alone is a useful material for image diagnosis, the material density image is not executed without performing the above-described weighted addition processing. You may only ask for it.

  In the first embodiment, dual energy ratio information RT is stored, and weighted subtraction processing is performed using a value obtained by multiplying the read dual energy ratio R by a predetermined coefficient as a weighting coefficient. However, information on weighting factors determined based on the dual energy ratio R may be stored, and weighted subtraction processing may be performed using the read weighting factors.

  Similarly, in the second embodiment, dual energy ratio information RT is received from the X-ray CT apparatus, and weighted subtraction processing is performed using a value obtained by multiplying the dual energy ratio R by a predetermined coefficient as a weighting coefficient. However, information on a weighting factor determined based on the dual energy ratio R may be received, and weighted subtraction processing may be performed using the weighting factor.

  In the first and second embodiments, it is preferable that one of the two set tube voltages in the dual energy imaging is set as a settable minimum tube voltage and the other is set as a settable maximum tube voltage. As a result, the degree of energy separation is increased and a tomographic image with a good SN ratio is obtained. As a temporary guide, one of the set tube voltages is set to 60 kV or more and 100 kV or less, and the other is set to 120 kV or more and 160 kV or less.

  In the first and second embodiments, the weighting factor includes the type M of the substance to be suppressed, the set high tube voltage HV and the low tube voltage LV, and the type FL of the beam forming X-ray filter used for imaging. And the position Z in the z direction of the tomographic image to be subjected to weighted subtraction processing, but either or both of the beam forming X-ray filter type FL and the position Z in the z direction of the tomographic image are changed. It may be in a form not considered.

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Storage device 10 Imaging table 12 Cradle 15 Rotating unit 20 Scanning gantry 20a Main unit 21 X-ray tube 22 X-ray controller 23 Collimator 24 X-ray detector 25 DAS
DESCRIPTION OF SYMBOLS 26 Rotation part controller 28 X-ray filter part 28L, 28M, 28S Beam forming X-ray filter 29 Control controller 30 Slip ring 31 Imaging condition setting part 32 Scan control part 33 Image reconstruction part 34 Weighting subtraction part 35 Weighting addition part 36 Display control Unit 40 subject 50 data receiving unit 100 X-ray CT apparatus 102 image processing apparatus B opening

Claims (10)

An X-ray tube and an X-ray detector are provided, and the subject is subjected to X-ray CT imaging with a first tube voltage and a second tube voltage different from the first tube voltage to collect projection data Photographing means to
Image reconstruction means for reconstructing a first image based on projection data based on the first tube voltage and a second image based on projection data based on the second tube voltage;
A weighting factor that changes pixel values of corresponding pixels in the first and second images according to a distance from a center of an image reconstruction area in the first and second images to the corresponding pixel. An X-ray CT apparatus comprising weighted subtracting means for obtaining an image by performing weighted subtracting processing.   The X-ray CT apparatus according to claim 1, wherein the weighting coefficient is larger in a peripheral portion than in a central portion of the image reconstruction area.   The imaging means has an X-ray filter that spatially changes the quality of X-rays passing therethrough, and an X-ray beam generated from the X-ray tube is applied to the subject via the X-ray filter. The X-ray CT apparatus according to claim 1 or 2 for irradiation. The imaging unit includes a plurality of X-ray filters having different X-ray absorption characteristics, and irradiates the subject with the X-ray beam via an X-ray filter selected from the plurality of X-ray filters. And
The X-ray CT apparatus according to claim 3, wherein the weighting subtraction unit performs weighting subtraction processing using a weighting factor corresponding to the selected X-ray filter. The X-ray detector has a plurality of detection element arrays in the body axis direction of the subject,
The first and second images are images of predetermined positions with respect to the X-ray detector in the body axis direction of the subject,
5. The X-ray CT apparatus according to claim 1, wherein the weighting subtraction unit performs weighting subtraction processing using a weighting coefficient corresponding to the predetermined position.   The weighting factor is a first pixel corresponding to the predetermined substance in a pixel at the same position as the corresponding pixel of an image obtained when X-ray CT imaging is performed with the first tube voltage using the imaging unit. And a second pixel value corresponding to the predetermined substance in a pixel at the same position as the corresponding pixel of an image obtained when X-ray CT imaging is performed with the second tube voltage using the imaging unit The X-ray CT apparatus according to any one of claims 1 to 4, based on a ratio of: The weighting subtraction means obtains a first substance suppression image in which a component of the first substance is suppressed by weighting subtraction processing using a weighting factor corresponding to the first substance, and according to the second substance. A second substance-suppressed image in which a component of the second substance is suppressed by a weighted subtraction process using a weighting factor;
The weight addition means which calculates | requires a 3rd image by carrying out the weight addition of the said 1st substance suppression image and the said 2nd substance suppression image is further provided with X of any one of Claims 1-6. Line CT device.   The X-ray CT apparatus according to claim 1, wherein one of the first and second substances is water and the other is iodine. Projection acquired by X-ray CT imaging of a subject with a first tube voltage and a second tube voltage different from the first tube voltage by an imaging means having an X-ray tube and an X-ray detector. Image receiving means for receiving a first image based on the first tube voltage and a second image based on the second tube voltage reconstructed based on data;
A weighting factor that changes pixel values of corresponding pixels in the first and second images according to a distance from a center of an image reconstruction area in the first and second images to the corresponding pixel. An image processing apparatus comprising weighted subtracting means for obtaining an image by performing weighted subtracting processing.   A program for causing a computer to function as the image processing apparatus according to claim 9.

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