JP2011136002A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To adjust relationships between a substance and a pixel value in tomographic images in generating tomographic images at a plurality of positions in a z-direction in an X-ray CT apparatus. <P>SOLUTION: The X-ray CT apparatus includes a data collector 20 for collecting first and second projection data by executing dual energy radiography through conventional scanning in a scanning range, and a generator 53 for generating tomographic images at a plurality of positions in the z-direction by executing a process M of acquiring monochromatic tomographic images corresponding to images captured by the radiography with X rays of desired effective X ray energy based on the first and second projection data. The generator 53 changes conditions for the process M, in which the substantially effective X-ray energy of the generated images can be adjusted, according to the positions of the generated images in the z-direction. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus, and more particularly, to an X-ray CT apparatus including an X-ray tube that emits X-rays having different quality in the body axis direction of a subject.

  Conventionally, in many X-ray tubes provided in an X-ray CT apparatus, an anode and a cathode are arranged in the body axis direction (z direction) of a subject (see, for example, Patent Document 1, FIG. 2, FIG. 5, etc.). ).

JP 2003-111754 A

The quality of X-rays (X-ray spectrum) emitted from such an X-ray tube becomes non-uniform in the z direction due to the heel effect. In other words, on the anode side, the distance through which X-rays pass through the anode is long, so that the effective X-ray energy (energy) of X-rays shifts to the higher side and becomes relatively “hard”. In this case, since the distance through which the X-ray passes through the anode is short, the effective X-ray energy of the X-ray does not shift to the higher side, and becomes relatively “soft”. Such non-uniformity of the X-ray quality becomes more significant as the cone angle, which is the angle in the z direction of the X-ray beam, increases. For this reason, for example, an X-ray beam whose width in X-ray detectors (coverage) in the z direction exceeds 100 mm, such as 64 or more detector rows, particularly 256 rows or 320 rows, is a wide cone. (Wide
In the cone X-ray CT apparatus, the difference in X-ray quality cannot be ignored. In particular, the substance (excluding water and air) and pixel values (CT values) in the tomographic images at both ends of the scan range in the z direction ) Cannot be ignored. The CT value of water alone is adjusted to be kept uniform in the z direction, but other substances such as bone (calcium) and contrast agent (iodine) have different CT values due to differences in X-ray quality. End up. For example, a conventional scan (also called an axial scan) or a cine scan is continuously performed in the z direction, and MPR (multi plane) of the xz plane or yz plane is performed.
reconstruction) image, oblique image, etc., the tomographic image on the anode side scanned with hard X-rays and the tomographic image on the cathode side scanned with soft X-rays are adjacent to each other. Discontinuities in the z direction are noticeable and become banding artifacts.

  For this reason, when generating images at a plurality of positions in the body axis direction of a subject, an X-ray CT apparatus capable of adjusting the relationship between substances and pixel values in these images is desired.

  The first aspect of the invention is an X-ray tube that emits X-rays having different radiation qualities in the body axis direction of a subject, a multi-row detector, and dual energy ( dual energy), and the first projection data (data) of the first X-ray tube voltage by the X-ray and the X-ray of the second X-ray tube voltage different from the first X-ray tube voltage Data collection means for collecting second projection data, and processing for obtaining an image obtained when imaging is performed with X-rays having desired effective X-ray energy based on the first and second projection data And generating means for generating images at a plurality of positions in the body axis direction, wherein the generating means sets the processing conditions under which the substantial effective X-ray energy of the generated image can be adjusted. , At the position of the body axis direction of the image to be generated Flip to provide an X-ray CT apparatus varied.

  The invention of the second aspect is the relationship between each position in the region in the body axis direction scanned by one conventional scan and the condition to be used when generating an image of the position, When the image at each position in the image is generated by the processing, the image processing apparatus further includes a storage unit that stores a relationship determined so that the pixel value of the predetermined substance in the image is constant, and the generation unit includes: An X-ray CT apparatus according to the first aspect of the present invention that determines the condition with reference to the stored relationship is provided.

  The invention of the third aspect further includes setting means for setting the desired effective X-ray energy based on an input by an operator, and the storage means is configured to store the effective X-ray energy for a plurality of effective X-ray energies. The relationship to be referred to when the line energy is the desired effective X-ray energy is stored, and the generation means has the same effective as the desired effective X-ray energy among the plurality of stored relationships. There is provided an X-ray CT apparatus according to the second aspect, which refers to a relationship corresponding to X-ray energy, a relationship corresponding to effective X-ray energy in the vicinity thereof, or a relationship obtained by interpolation using the relationship.

  According to a fourth aspect of the present invention, in the first and second weighted subtraction processes, the processing is different for each of the plurality of positions with respect to the first and second projection data corresponding to the slice at the position. To obtain two types of projection data, image reconstruction processing is performed on each of the two types of projection data to obtain first and second images, and weighted addition processing is performed on the first and second images. X in any one of the first to third aspects, wherein the condition is a weight used for the first and second weighted subtraction processes and / or the weighted addition process. A line CT apparatus is provided.

  According to a fifth aspect of the invention, in the processing, for each of the plurality of positions, the first and second projection data corresponding to the slice at the position are subjected to image reconstruction processing to obtain two types of images. The first and second weighted subtraction processes that are different from each other are performed on the two types of images to obtain the first and second images, and the weighted addition process is performed on the first and second images. X-ray CT according to any one of the first to third aspects, wherein the condition is a weight used for the first and second weighted subtraction processes and / or the weighted addition process. Providing equipment.

  The invention according to a sixth aspect provides the X-ray according to any one of the first to fifth aspects, wherein the predetermined substance is at least one of water, fat, iodine, and calcium. A CT apparatus is provided.

  The invention according to a seventh aspect is the first aspect according to the first aspect, wherein the relationship is determined based on a pixel value of an image obtained by conventional scanning of a phantom including the predetermined substance. An X-ray CT apparatus according to any one of the six aspects is provided.

  According to an eighth aspect of the present invention, the scan range is a range wider than the region in the body axis direction scanned by one conventional scan, and the data collection means is configured so that the region is adjacent to the body axis direction or Provided is an X-ray CT apparatus according to any one of the first to seventh aspects, in which a plurality of conventional scans that are partially overlapped are performed.

  The invention according to the ninth aspect is the eighth aspect, further comprising display means for reconstructing and displaying an image along a predetermined direction different from the body axis direction based on the generated image. An X-ray CT apparatus is provided.

  According to the X-ray CT apparatus of the invention of the above aspect, with the above-described configuration, a process for obtaining an image obtained when imaging is performed with X-rays having a desired effective X-ray energy, and the body axis direction of the subject In addition, an image of a plurality of positions in the image is generated, and further, the processing conditions capable of adjusting the substantial effective X-ray energy of the generated image are changed according to the position of the generated image in the body axis direction. When generating images at a plurality of positions in the body axis direction, the relationship between substances and pixel values in these images can be adjusted.

It is a figure which shows the structure of the X-ray CT apparatus by this embodiment. It is a figure which shows the schematic structure of the X-ray tube in this embodiment, and the spatial nonuniformity of X-ray quality. It is a mechanism block diagram functionally representing the configuration of the X-ray CT apparatus according to the present embodiment. It is a figure which shows roughly the process which produces | generates a monochromatic (monochromatic) tomogram. It is a figure which shows an example of the phantom used for imaging | photography for calculating | requiring process condition relationship. It is a figure for demonstrating the imaging | photography method for calculating | requiring process condition relationship. It is a figure which shows an example of the pixel value of the iodine aqueous solution in the tomographic image of each coordinate position of az direction. It is a figure which shows an example of the change of CT value which is a pixel value of the iodine aqueous solution of the monochrome tomographic image of the coordinate position B64 of az direction. It is a figure which shows an example of a processing condition relationship. It is a flowchart (flowchart) which shows the flow of a process in the X-ray CT apparatus of this embodiment. It is a figure which shows an example of the scanning range set to the subject. It is a flowchart which shows the production | generation process of a monochrome matic tomogram. An example of the MPR image of the xz plane displayed by this embodiment is shown. An example of the MPR image of xz surface displayed by the conventional method is shown.

  Embodiments of the invention will be described below.

  FIG. 1 is a diagram showing the configuration of the X-ray CT apparatus according to the present embodiment.

  The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20. The imaging table 10 and the scanning gantry 20 are examples of data collection means in the invention.

  The operation console 1 collects data acquired by the input device 2 that receives input from the operator, a central processing unit 3 that controls various parts for scanning the subject 40 and performs various calculations, and the scanning gantry 20. A data collection buffer (buffer) 5, a monitor 6 for displaying images, and a storage device 7 for storing programs and data are provided.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and carried 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 horizontal 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 support unit 16 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 23 that collimates the X-rays 81 generated from the X-ray tube 21. An X-ray detector 24 that detects X-rays 81 irradiated from the X-ray tube 21 and transmitted through the subject 40, and a data acquisition device (DAS;) that converts and collects the output of the X-ray detector 24 into projection data. Data Acquisition System) 25 and a rotating unit controller 26 for controlling the X-ray controller 22, the collimator 23, and the DAS 25 are mounted. The X-ray detector 24 is, for example, a multi-row detector in which 256 detector rows having a width in the z direction of 0.625 [mm] are arranged in the z direction. The support unit 16 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 support part 16 are electrically connected via a slip ring 30.

  FIG. 2 is a diagram showing a schematic configuration of the X-ray tube and non-uniformity in the z direction of the X-ray quality in the present embodiment. As shown in FIG. 2, the X-ray tube 21 has an anode 21a and a cathode 21c, which are arranged at a predetermined distance in the z direction. The electron beam emitted from the cathode 21c collides with the anode 21a, and emits an X-ray beam 81 that spreads in the channel direction and the z direction from the focal point 21f. The quality of the X-ray beam 81 is such that the X-ray beam becomes hard (shifted to a higher effective X-ray energy) because the distance through which the X-ray passes through the anode on the anode 21a side is long, and the X-ray beam 81 passes through the anode inside the anode X Since the distance through which the line passes is short, the X-ray becomes soft (the effective X-ray energy does not shift to the higher side). This is called the X-ray quality heel effect. Note that the scanning region R in the z direction scanned when the X-ray tube 21 and the X-ray detector are rotated around the rotation axis IC and one conventional scan is performed is the X-direction scan region R on the rotation axis IC. This is a region having a width d of the line 81 in the z direction. The Z coordinate positions where the tomographic images are generated in the scan region R are A1 to AN from the center on the anode side, and B1 to BN from the center on the cathode side.

  FIG. 3 is a mechanism block diagram functionally showing the configuration of the X-ray CT apparatus according to the present embodiment. In this figure, the main part of the operation console is functionally represented.

  The operation console 1 of the X-ray CT apparatus 100 includes a scan condition setting unit 51, a scan control unit 52, a monochromatic tomographic image generation unit (generation unit) 53, a target effective X-ray energy setting unit 54 (setting unit), and a processing condition storage. A display unit (storage unit) 55 and a display control unit (display unit) 56.

  The scan condition setting unit 51 sets the scan condition based on the input by the operator, but here sets at least the scan range SR in the z direction, that is, the scan start position Zs and the scan end position Zf.

  The scan control unit 52 controls the imaging table 10 and the scanning gantry 20 to perform dual energy imaging by conventional scanning on the scan range SR of the subject 40. For example, a conventional scan is performed while alternately switching the X-ray tube voltage between the first X-ray tube voltage and the second X-ray tube voltage, which are different from each other, in units of one view or several views. Collect projection data. Alternatively, the X-ray tube voltage is set to the first X-ray tube voltage, the conventional scan is performed once, projection data for a predetermined view angle is collected, and the X-ray tube voltage is switched to the second X-ray tube voltage. A conventional scan is performed once, and projection data for a predetermined view angle is collected. As a result, the first X-ray tube voltage projection data by the X-ray of the first X-ray tube voltage and the second X-ray tube voltage projection data by the X-ray of the second X-ray tube voltage are collected. When the scan range SR exceeds the scan area R in the z direction that can be scanned by one conventional scan, such a conventional scan is performed a plurality of times, that is, the scan area is adjacent to the z direction or partly over. It is performed at a plurality of positions in the z direction so as to wrap. The first and second X-ray tube voltages are 140 [kVp] and 80 [kVp], for example. The predetermined view angle is, for example, a fan angle α [rad] or 2π [rad] of a π + X-ray beam.

  The monochromatic tomographic image generation unit 53 generates a monochromatic tomographic image for each position in the scan range SR based on the first and second X-ray tube voltage projection data corresponding to the position. The monochromatic tomographic image is a tomographic image equivalent to a tomographic image obtained when the subject 40 is imaged with X-rays having a predetermined effective X-ray energy closer to a monochromatic X-ray, and is affected by beam hardening. It is a reduced tomogram. The effective X-ray energy of the tomographic image can be freely changed by changing the processing conditions in the generation process of the monochromatic tomographic image.

  Here, a process for generating a monochrome tomographic image will be described.

  FIG. 4 is a diagram schematically showing a process for generating a monochromatic tomographic image. As shown in FIG. 4, first, predetermined preprocessing including logarithmic conversion, radiation hardening correction, X-ray detector sensitivity correction, and the like is performed on the first and second X-ray tube voltage projection data PHV and PLV. I do. Next, a predetermined reconstruction function is superimposed on the preprocessed first and second X-ray tube voltage projection data PHV and PLV. Different first and second higher-order weighted addition processes are performed on the first and second X-ray tube voltage projection data PHV and PLV on which the reconstruction function is superimposed. Alternatively, the first substance density that strongly expresses the density distribution of the first substance by performing high-order weighted addition processing on the first and second X-ray tube voltage projection data PHV and PLV before superimposing the reconstruction function Projection data and second substance density projection data strongly representing the density distribution of the second substance are obtained.

  The projection data of the object or the tomographic image reconstructed based on the projection data is a high-order weighted addition process or weighted addition of projection data representing the density distribution of two different substances or projection data representing the X-ray absorption coefficient thereof. Required by processing. For this reason, the component of the first substance is suppressed by performing the first high-order weighted subtraction process on the first X-ray tube voltage projection data PHV and the second X-ray tube voltage projection data PLV. Second substance density projection data P strongly representing the density distribution of the second substance is obtained. Similarly, by performing the second high-order weighted subtraction process on the first X-ray tube voltage projection data PHV and the second X-ray tube voltage projection data PLV, the second substance is suppressed, First material density projection data strongly representing one material is obtained.

  Here, as an example, the first substance is water, the second substance is iodine, and water density projection data Pw representing the water density distribution is obtained as the first substance density projection data, and the second substance density projection is obtained. As data, iodine density projection data Pio representing the density distribution of iodine is obtained.

  Next, an image reconstruction function is superimposed on the water density projection data Pw and the iodine density projection data Pio as necessary, and an image reconstruction process is performed, respectively, to obtain a water density tomogram Gw and an iodine density tomogram Gio. And get.

  The coefficient of the first higher-order weighted subtraction process is determined so that the iodine density is substantially 0 (zero) in the water density tomographic image Gw. The coefficient of the second higher-order weighted subtraction process is determined so that the density of water is substantially 0 (zero) in the iodine density tomographic image Gio.

  Next, as shown in FIG. 4, a weighted addition process is performed on the water density tomogram Gw and the iodine density tomogram Gio using a weighted addition coefficient considering the X-ray absorption values of water and iodine at each effective X-ray energy. To obtain a monochromatic tomographic image Gm corresponding to a predetermined effective X-ray energy and an X-ray tube voltage NV.

  The monochromatic tomographic image Gm can be obtained, for example, by performing a weighted addition process according to the following formula.

  Here, keV1 is an effective X-ray energy corresponding to the X-ray tube voltage NV, μw (keV1) is an X-ray absorption coefficient of water with respect to the X-ray of the effective X-ray energy keV1, and μio (keV1) is an effective X-ray energy keV1. X-ray absorption coefficient of iodine with respect to X-ray, kc is a change coefficient for changing the pixel value of the tomographic image corresponding to the X-ray tube voltage NV to 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 the weighted addition process for obtaining the monochromatic tomographic image Gm, the weighting coefficient to be multiplied to the water density tomographic image Gw is kc · μw (keV1), and the weighting factor to be multiplied to the iodine density tomographic image Gio is kc. .Mu.io (keV1). These μw and μio vary depending on the effective X-ray energy keV1.

  Incidentally, in the above method, the first and second X-ray tube voltage projection data are subjected to different first and second weighted subtraction processes to obtain two types of substance density projection data, and these two types The first and second material density tomograms are obtained by performing image reconstruction processing on the respective material density projection data. However, each of the first and second X-ray tube voltage projection data is subjected to image reconstruction processing to obtain two types of tomographic images, and after performing beam hardening processing on these two types of tomographic images, they are different from each other. The first and second material density tomograms can also be obtained by performing the first and second weighted subtraction processes.

  Further, the weighted subtraction process and the weighted addition process are not limited to the linear linear weighted subtraction process and the linear linear weighted addition process, and a high-order linear weighted subtraction process, a high-order linear weighted addition process, and the like can also be used.

  For details of the processing for generating the monochromatic tomographic image Gm, refer to, for example, US Patent Document US2009 / 0052612A1.

  Incidentally, the quality of X-rays radiated from the X-ray tube becomes non-uniform in the z direction due to the heel effect. That is, the X-ray quality of the X-ray by the first X-ray tube voltage and the X-ray by the second X-ray tube voltage are different between the anode side and the cathode side of the X-ray focal point. Therefore, even if the effective X-ray energy keV1 in this process is fixed in the above-described generation process of the monochromatic tomographic image Gm, if the position of the conventional scan with respect to the scan region R in the z direction changes, it is generated for that position. The true effective X-ray energy of the monochromatic tomographic image Gm also slightly changes between the anode side and the cathode side. That is, even in the case of monochromatic tomographic images Gm in the same scan region R, pixel values corresponding to the same material are slightly shifted between tomographic images.

  Therefore, when generating a monochromatic tomographic image Gm at a predetermined position in the z direction, a process for generating the monochromatic tomographic image Gm according to this position with respect to the scan region R when the slice at the predetermined position is subjected to conventional scanning. The processing conditions are changed to reduce pixel value shift due to the heel effect.

  Here, an example of the specific method will be described. However, the effective X-ray energy (hereinafter referred to as target effective X-ray energy) keVa to be targeted as the effective X-ray energy of the generated monochromatic tomographic image Gm is 65 [keV], and the number of tomographic images in the scan region R is , 256 sheets at positions A128 to A1 and B1 to B128 in the z direction.

  First, a phantom 42 as shown in FIG. 5 is prepared. The phantom 42 is a thin cylindrical container in which an aqueous iodine solution having an X-ray tube voltage of 120 [kVp] and a CT value of about 100 [HU] to about 500 [HU] is contained in an acrylic cylindrical container containing water. Several phantoms are arranged. The size of the phantom 42 is such that the length of the acrylic cylindrical container exceeds the scan region R, for example, about 30 [cm], and the diameter thereof is, for example, about 20 [cm]. The inner diameter of the cylindrical container is, for example, about 1.5 [cm].

  Next, as shown in FIG. 6, the phantom 42 is placed in the imaging space so that its cylindrical axis coincides with the rotation axis IC of the scanning gantry 20, and dual energy imaging is performed by conventional scanning or cine scanning.

  A monochromatic tomographic image Gm at each of the positions A128 to A1 and B1 to B128 in the z direction is generated with an effective X-ray energy keV1 for processing being 65 [keV]. When the pixel value (CT value) of the iodine aqueous solution portion in the phantom 42 at each coordinate position in the z direction is obtained based on these monochromatic tomographic images Gm, the pixel value gi is obtained in the z direction as shown in FIG. Change. In other words, since the X-ray on the cathode side is softer and the X-ray on the anode side is harder, the X-ray quality is different in the z direction, and the iodine aqueous solution portion in the monochromatic tomographic image Gm by dual energy imaging is constant. The pixel value is lost. Therefore, for example, the iodine aqueous solution portion of the tomographic images A1 and B1 is 210 [HU], and it is considered that the pixel value of the iodine aqueous solution portion of the tomographic image at each coordinate position in the z direction is matched with this. For example, assuming that the pixel value of the iodine aqueous solution portion of the B64 tomographic image is 220 [HU], keV1 is 65 in the change curve of the pixel value gi of the monochrome tomographic image Gm having the z-direction coordinate position B64 as shown in FIG. The keV1 of the monochrome tomographic image Gm of B64 is set to 67 [keV] so that the value of 220 [HU] in [keV] is matched with the pixel value 210 [HU] of the iodine aqueous solution portion of the A1 and B1 tomographic images. Thus, the pixel value of the iodine aqueous solution portion is set to 210 [HU].

  If such adjustment is performed on the tomographic image at each coordinate position in the z direction, the pixel values for iodine in the z direction can be made uniform, and the pixel values can be made substantially uniform for other substances.

  The target effective X-ray energy setting unit 54 sets the target effective X-ray energy keVt based on the input by the operator. In other words, the target effective X-ray energy keVt is effective X having a correlation with the X-ray tube voltage to be set when it is assumed that the subject 40 is imaged by conventional X-ray CT imaging. Line energy.

  When the monochrome tomographic image generation unit 53 generates the monochrome tomographic image Gm in accordance with the target effective X-ray energy keVt, the processing condition storage unit 55 sets the processing conditions to be used for the generation processing to a plurality of effective X-rays. I remember the line energy. However, as described above, the processing conditions need to be changed according to the position in the z direction, which is the generation target of the monochromatic tomographic image Gm. Therefore, the processing condition storage unit 55 actually sets the target effective X-ray energy keVt for the position in the z direction in the scan region R by one conventional scan and the tomographic image generated by the monochromatic tomographic image generation unit 53. When the monochrome tomographic image Gm is generated in accordance with the above, a relationship with processing conditions to be used for the generation processing (hereinafter referred to as processing condition relationship) is stored. This processing condition relationship is such that when the monochrome tomographic image generation unit 53 generates a tomographic image at each position in the scan region R, a pixel value (excluding air) corresponding to a predetermined substance (excluding air) in these tomographic images. (CT value) is determined to be substantially constant. As the predetermined substance, for example, at least one of water, fat, iodine, and calcium can be considered.

  As the processing conditions, for example, weighting of weighted subtraction processing or weighted addition processing in the processing for generating the above-described monochromatic tomographic image Gm can be considered. Specifically, the effective X-ray energy keV1 in processing is set to a value. Generally, the effective X-ray energy keV1 for processing is set to a desired value by the operator. Therefore, in the implementation, the effective X-ray energy keV1 in processing is considered to be one of the parameters that originally fluctuate. Therefore, this is a processing condition for changing the generated tomographic image according to the position in the z direction. Good.

  Further, the processing condition relationship can be determined based on a pixel value of a tomographic image obtained by photographing a phantom containing the predetermined substance by a conventional scan. For example, a dual energy image of a phantom 42 as shown in FIG. 5 is taken by conventional scanning or cine scanning. A monochromatic tomographic image Gm is generated at a position A1 or B1 in the z direction, which is near the center of the scan region R, with the processing effective X-ray energy keV1 set to a predetermined value keVa, and corresponds to the aqueous iodine solution in this tomographic image. The pixel value gi0 to be obtained is obtained. Then, monochrome tomographic images Gm of the positions AN to A2 and B2 to BN in the scan region R are generated while changing the effective X-ray energy keV1 for processing, and pixel values corresponding to the iodine aqueous solution in these tomographic images. However, the effective X-ray energy keV1 in processing that matches the pixel value gi0 is obtained for each position of each tomographic image. Thereby, as a processing condition relationship to be adopted when the target effective X-ray energy keVt is keVa, the position in the z direction in the scan region R and the processing to be used when generating the monochromatic tomographic image Gm at this position. The effective X-ray energy keV1 can be obtained. Thereafter, the same process is performed while changing the value keVa, and processing condition relationships corresponding to a plurality of values that can be set as the target effective X-ray energy keVt are obtained.

  FIG. 9 shows an example of the processing condition relationship. This example is a processing condition relationship corresponding to the target effective X-ray energy keVt = 65 [keV]. When the position SL in the z direction is A1 and B1 which are substantially the centers of the scan region R, the effective X-ray energy keV1 for processing is keV1 = 65 [keV]. On the other hand, when the position SL in the z direction approaches the anode side, the effective X-ray energy keV1 for processing gradually decreases, and when it reaches the AN on the most anode side in the scan region R, keV1 = 60 [keV]. Further, when the position SL in the z direction approaches the cathode side, the effective X-ray energy keV1 in processing gradually increases, and when the BN on the most cathode side in the scan region R is reached, keV1 = 70 [keV]. Such an effective X-ray energy value can be adjusted in the z direction.

  Note that the monochrome tomographic image generation unit 53 refers to a relationship corresponding to the target effective X-ray energy keVt among a plurality of relationships stored in the processing condition storage unit 55, and generates a monochrome tomographic image Gm. The effective X-ray energy keV1 in the generation process is determined based on the position of the slice in the scan area when a certain slice is scanned.

  The display control unit 56 displays the monochromatic tomographic image Gm generated by the monochromatic tomographic image generation unit 53 or the tomographic image reconstructed based on the monochromatic tomographic image Gm on the screen of the monitor 6. For example, a tomographic image along a predetermined direction different from the z direction, such as an MPR image or an oblique image, is reconstructed based on the monochrome tomographic image Gm generated for each position in the z direction of the scan range SR. indicate.

  Hereafter, the flow of processing in the X-ray CT apparatus of this embodiment will be described.

  FIG. 10 is a flowchart showing the flow of processing in the X-ray CT apparatus of this embodiment.

  In step T1, a scan range SR in the z direction of the subject 40 is set based on an input by the operator. An example is shown in FIG. In this example, with the z direction of the subject 40 on the cradle 12 as the Z coordinate, the scan range is between the scan start position Zs set on the neck of the subject 40 and the scan end position Zf set on the waist. SR. In this example, the width of the scan range SR in the z direction is 4d, which is four times as long as the width in the z direction of the scan region R for one conventional scan is d.

  In step T2, dual energy imaging of the scan range SR of the subject 40 is performed by conventional scanning. In the example of FIG. 11, since the width of the scan range SR is 4d, conventional scans S1 to S4 for four times in which the scan region R is spatially adjacent are performed. In this example, as shown in FIG. 11, the scan areas R1 to R4 correspond to the conventional scans S1 to S4, respectively.

  In step T3, the target effective X-ray energy keVt is set based on the input by the operator.

  In step T4, a processing condition relationship suitable for the set target effective X-ray energy keVt is specified from the plurality of stored processing condition relationships. If there is a processing condition relationship corresponding to the same effective X-ray energy as the target effective X-ray energy keVt, the processing condition relationship is specified. If there is no processing condition relationship corresponding to the same effective X-ray energy, the processing condition relationship corresponding to the effective X-ray energy closest to the target effective X-ray energy keVt is specified. Alternatively, a plurality of processing condition relationships corresponding to effective X-ray energy close to the target effective X-ray energy keVt are designated, and processing condition relationships corresponding to the target effective X-ray energy keVt are generated and specified by interpolation.

  In step T5, a monochrome tomographic image is generated.

  FIG. 12 is a flowchart showing a process for generating a monochrome tomographic image.

  In step T51, variables i and k are initialized so that i = 1 and t = 1.

  In step T52, the position SLt in the z direction in the scan region Ri is selected as a processing target.

  In step T53, the processing condition relationship specified in step T4 is referred to, and based on the position of the position SLt in the z direction, the processing condition used for the monochromatic tomographic image generation process, that is, the effective X-ray energy keV1 on the process. Is identified.

  In step T54, using the processing conditions specified in step T53, a monochrome tomographic image Gm at the position SLt in the z direction in the scan region Ri is generated.

  In step T55, it is determined whether or not the selected position SLt in the z direction is the last position (t = m), and if it is the last, the process proceeds to step T57. If it is not the last, the process proceeds to step T56, t is incremented by 1, and the process returns to step T52.

  In step T57, it is determined whether or not the selected scan area Ri is the last scan area (i = n), and if it is the last, the process ends. If it is not the last, the process proceeds to step T58, i is incremented by 1, and the process returns to step T52.

  In step T6, an MPR image, an oblique image, etc. are reconstructed and displayed based on the generated monochrome tomographic image Gm.

  FIG. 13 shows an example of an MPR image on the xz plane displayed according to this embodiment. FIG. 14 shows an example of an MPR image on the xz plane displayed by a conventional method. In these images G0 and G1 shown in FIGS. 13 and 14, conventional scanning is continuously performed four times from the scan start position Zs to the scan end position Zf as shown in the example of FIG. It is an example of the MPR image corresponding to the space adjacent to the z direction. When a tomographic image is generated by performing a plurality of conventional scans adjacent in the z direction, a tomographic image scanned with hard X-rays and a tomographic image scanned with soft X-rays are adjacent to each other. Therefore, according to the conventional method, in the MPR image such as the xz plane or the yz plane, the pixel value of the substance is not uniform in the z direction, and as shown in FIG. appear. For example, the CT value of a blood vessel containing a contrast medium containing iodine is shifted and appears as a banding artifact. On the other hand, according to this embodiment, even in an MPR image having a cross section of the xz plane, the yz plane, or the like, pixel values corresponding to a substance are substantially uniform in the z direction, and discontinuous lines and banding artifacts are generated as shown in FIG. Does not occur.

  According to the present embodiment, the tomographic image of the scan range is generated by the dual energy imaging using the process of generating the monochrome tomographic image, and further, the correspondence relationship between the substance and the pixel value is determined. When each tomographic image in the scan range is generated by the above processing, the tomographic image of the tomographic image is set so that the pixel values corresponding to a predetermined substance in these tomographic images are substantially uniform in the z direction. It can be changed according to the position. Thereby, the non-uniformity of the pixel value corresponding to the substance in each tomographic image in the scan range due to the X-ray heel effect can be reduced.

  Further, according to the present embodiment, the non-uniformity in the z direction of the X-ray quality due to the heel effect is not corrected by a physical X-ray filter (filter) formed of metal or the like, but dual energy. Since correction is performed by the image reconstruction algorithm, attenuation of the X-ray intensity by the X-ray filter can be prevented, and a signal can be obtained with a high S / N ratio in the X-ray detector.

  Incidentally, the focal point of the X-ray tube in the present embodiment is generally small in the z-direction of the focal point as viewed from the anode-side X-ray detector, and the focal point as viewed from the cathode-side X-ray detector. The size in the z direction increases. Therefore, in the conventional scan and the cine scan, the slice thickness of the tomographic image is thin on the anode side and thick on the cathode side. That is, the spatial resolution in the z direction of the tomographic image is not uniform. The slice thickness in the conventional scan and cine scan is obtained by obtaining each tomographic image using a phantom including a tungsten wire inclined by an angle θ with respect to the xy plane, and the tungsten wire reflected in the tomographic image is obtained. From the length L, it can be obtained as slice thickness D = L · tan θ. If the slice thickness D is made uniform in the z direction, for example, a column of each tomographic image is obtained by using a z direction filter as shown in the equations [0125] to [0131] of Japanese Patent Application Laid-Open No. 2007-325853. Adjust the direction filter coefficient. Thereby, the non-uniformity of the slice thickness in the z direction of the tomographic image can be removed, and the spatial resolution in the z direction can be made more uniform.

  As mentioned above, although embodiment of invention was described, embodiment of invention is not limited to said embodiment, In the range which does not deviate from the meaning of invention, various embodiment can be considered.

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Memory | storage device 10 Imaging table 12 Cradle 15 Rotation part 16 Support part 20 Scanning gantry 21 X-ray tube 21a Anode 21c Cathode 21f Focus 22 X-ray tube controller 23 Collimator 24 X-ray detector 25 Data acquisition device (DAS)
DESCRIPTION OF SYMBOLS 26 Rotation part controller 29 Control controller 30 Slip ring 40 Subject 42 Phantom 51 Scan condition setting part 52 Scan control part 53 Monochromatic tomographic image generation part 54 Target effective X-ray energy setting part 55 Processing condition storage part 56 Display control part 81 X X-ray CT system

Claims (9)

An X-ray tube that emits X-rays with different quality in the body axis direction of the subject;
A multi-row detector;
Dual energy imaging is performed by conventional scanning on the scan range of the subject, and the first projection data by the X-ray of the first X-ray tube voltage is different from the first X-ray tube voltage. Data collecting means for collecting second projection data by X-rays of an X-ray tube voltage;
Based on the first and second projection data, a process for obtaining an image obtained when photographing with X-rays having a desired effective X-ray energy is performed, and images at a plurality of positions in the body axis direction are obtained. Generating means for generating,
The generating unit is an X-ray CT apparatus that changes the processing condition in which a substantial effective X-ray energy of a generated image can be adjusted according to a position of the generated image in the body axis direction. A relationship between each position in the region in the body axis direction scanned by one conventional scan and the condition to be used when generating an image of the position, and processing the image at each position in the region Storage means for storing the relationship determined so that the pixel value of the predetermined substance in these images is constant when generated by
The X-ray CT apparatus according to claim 1, wherein the generation unit determines the condition with reference to the stored relationship. A setting means for setting the desired effective X-ray energy based on an input by an operator;
The storage means stores, for a plurality of effective X-ray energies, the relationship to be referred to when the effective X-ray energy is the desired effective X-ray energy,
The generating means uses a relationship corresponding to an effective X-ray energy that is the same as the desired effective X-ray energy among the plurality of stored relationships or a relationship corresponding to an effective X-ray energy in the vicinity thereof, or uses the relationship. The X-ray CT apparatus according to claim 2, wherein a relationship obtained by interpolation is referred to. In the processing, for each of the plurality of positions, different first and second weighted subtraction processes are performed on the first and second projection data corresponding to the slice at the position, and two types of projections are performed. A process of obtaining data, performing image reconstruction processing on the two types of projection data to obtain first and second images, and performing weighted addition processing on the first and second images,
The X-ray CT apparatus according to any one of claims 1 to 3, wherein the condition is a weight used for the first and second weighted subtraction processes and / or the weighted addition process. In the processing, for each of the plurality of positions, the first and second projection data corresponding to the slice at the position are subjected to image reconstruction processing to obtain two types of images, and the two types of images are obtained. Performing first and second weighted subtraction processes different from each other to obtain first and second images, and performing a weighted addition process on the first and second images,
The X-ray CT apparatus according to any one of claims 1 to 3, wherein the condition is a weight used for the first and second weighted subtraction processes and / or the weighted addition process.   The X-ray CT apparatus according to any one of claims 1 to 5, wherein the predetermined substance is at least one of water, fat, iodine, and calcium.   The X-ray CT according to any one of claims 1 to 6, wherein the relationship is determined based on a pixel value of an image obtained by conventional scanning of a phantom containing the predetermined substance. apparatus. The scan range is wider than the region in the body axis direction scanned by one conventional scan,
The X-ray CT apparatus according to any one of claims 1 to 7, wherein the data collection unit performs a plurality of conventional scans in which the region is adjacent to or partially overlapping in the body axis direction.   The X-ray CT apparatus according to claim 8, further comprising display means for reconstructing and displaying an image along a predetermined direction different from the body axis direction based on the generated image.

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