JP2009213534A - X-ray ct apparatus and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus which diagnoses a fat degree more easily. <P>SOLUTION: The X-ray CT apparatus generate tomographic images of the periphery of the liver, the periphery of the spleen (muscle), the periphery of the intestine of a subject by irradiating at least the peripheries with X rays without changing conditions for irradiation. A parameter computing section 72 computes the CT value of the liver portion, the CT value of the muscle portion, and the CT value of the fat portion on the basis of the obtained tomographic images. Subsequently, the system computes the fat content of the kidney in accordance with formula äfat content=(CT value of the muscle portion-CT value of the liver portion)/(CT value of the muscle portion-CT value of the fat portion)×100%}. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus and a program for generating a tomographic image of a subject based on projection data obtained when the subject is irradiated with X-rays.

  Conventionally, an X-ray CT apparatus that generates a tomographic image of a subject based on projection data obtained by irradiating the subject with an X-ray beam is known. A tomographic image generated by an X-ray CT apparatus is generally called a CT image, and the pixel value of this CT image is called a CT value. The CT value is a value reflecting the X-ray absorption rate of each substance. Usually, the CT value of water is 0, and the CT value of air is about -1000.

  In recent years, it has been proposed to use this X-ray CT apparatus for the evaluation of the fat content of a target part. For example, a parameter called an average CT value or LS ratio of the liver part is calculated from a CT image obtained by photographing the liver with an X-ray CT apparatus, and the degree of progression of fatty liver is diagnosed based on the value of this parameter (For example, Patent Document 1 below). Here, the LS ratio is a parameter indicating the degree of fat content in the liver, and is a value obtained by dividing the average CT value of the liver portion by the average CT value of the muscle portion (for example, spleen). The diagnostician determines the fat content degree of the target part based on the LS ratio and the CT value.

JP 2006-312030 A

  However, since the CT value and LS ratio are not parameters that directly indicate the fat content, it is extremely difficult to intuitively determine the fat content from these values. Further, since the CT value and the LS ratio both vary according to the drive voltage supplied to the X-ray generator, it is difficult to say that the parameters are reproducible.

  Therefore, an object of the present invention is to provide an X-ray CT apparatus and a program that can more easily diagnose the degree of fat content.

  An X-ray CT apparatus according to the present invention is an X-ray CT apparatus that generates a tomographic image of a subject based on projection data obtained when the subject is irradiated with X-rays. Target part when acquiring CT value of muscle part, CT value of fat part, CT value of target part, and CT value of fat part and CT value of muscle part obtained upon irradiation And a calculating means for calculating the relative magnitude of the CT value as the fat content of the target part.

  In a preferred aspect, the acquisition means actually calculates the CT value of the muscle portion, the CT value of the fat portion based on the result of X-ray irradiation around the muscle portion, fat portion, and target region around the subject, The CT value of the target part is calculated. Specifically, the acquisition means includes means for acquiring projection data by irradiating X-rays around the muscle portion, fat portion, and target region of the subject without changing the irradiation condition, and the obtained projection data Based on the above, a means for generating a tomographic image around the muscle part, the fat part, and the target part, and the generated tomographic image, the muscle ROI indicating the muscle part in the tomographic image, and the fat indicating the fat part Means for accepting the setting of each ROI and the target ROI indicating the target region, and calculating the average value of the CT values in each set ROI as the CT value of the muscle portion, the CT value of the fat portion, and the CT value of the target portion And means.

  Another program according to the present invention is obtained when a computer is irradiated with X-rays under the same irradiation conditions based on a tomographic image of a subject generated from projection data obtained when the subject is irradiated with X-rays. CT value calculation means for calculating the CT value of the muscle part, the CT value of the fat part, and the CT value of the target part, and the CT value of the target part when the CT value of the fat part and the CT value of the muscle part are used as a reference It is made to function as a calculation means which calculates the relative magnitude | size of as a fat content rate of an object part.

  According to the present invention, the relative size of the CT value of the target part when the CT value of the fat part and the CT value of the muscle part are used as a reference is calculated as the fat content of the target part. Therefore, the fat content degree of a target part can be recognized intuitively.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of an X-ray CT apparatus according to an embodiment of the present invention. FIG. 2 is a perspective view of the measurement unit of the X-ray CT apparatus.

  As is well known, an X-ray CT apparatus is an apparatus that forms a tomographic image (CT image) of a subject based on projection data obtained by irradiating the subject with an X-ray beam. In addition to this CT image forming function, the X-ray CT apparatus of the present embodiment further calculates a fat content rate as a parameter that quantitatively indicates the fat content level in a part designated by the user, for example, the liver. It also has. That is, in order to quantitatively determine the degree of progression of fatty liver, it is necessary to quantitatively evaluate the degree of fat content in the liver. The X-ray CT apparatus of the present embodiment calculates the fat content as an index parameter effective for quantitative evaluation of the fat content. Hereinafter, the X-ray CT apparatus will be described in detail.

  The X-ray CT apparatus of this embodiment has a configuration suitable for CT measurement of small animals such as mice, rats, guinea pigs, and hamsters used in animal experiments. However, as a matter of course, the present invention may be applied to CT measurement of a person or the like by changing the configuration of the gantry and the container 24 described later.

  As shown in FIG. 1, the X-ray CT apparatus includes a measurement unit 10 that acquires projection data, and a calculation control unit 12 that controls driving of the measurement unit 10 and executes various calculations based on the obtained projection data. It is roughly divided into

  As shown in FIG. 2, the measurement unit 10 is provided with a main body having a gantry 18. An opening is formed in the upper surface 16A of the main body 16, and an arm 26 projects upward from the opening. The arm 26 forms a part of the slide mechanism 68. The arm 26 is connected to the container 24, and slides (moves and scans) the container 24 in the direction of the rotation center axis.

  On the other hand, a measurement unit (X-ray generator, X-ray detector), which will be described later, is housed in the gantry 18 and rotates around the rotation center axis. A hollow portion 18A is formed in the central portion of the gantry 18 in the direction of the rotation center axis. The hollow portion 18A is a non-penetrating type, but may be a penetrating type.

  The container 24 is a capsule for storing a subject (a small animal or a tissue extracted therefrom), and the shape thereof is a hollow, substantially cylindrical shape in the present embodiment. The container 24 is arranged in a state where the container center axis coincides with the rotation center axis. Specifically, the base end portion of the container 24 is detachably attached to the upper end portion of the arm 26 described above. In this case, examples of the attachment / detachment mechanism include various engagement mechanisms or screwing mechanisms. As described above, the container 24 has a hollow cylindrical shape, and one or a plurality of small animals are disposed in the container 24 in the present embodiment. With such a configuration, the hair of a small animal can be prevented from coming into direct contact with the gantry 18. In addition, it is possible to prevent the problem that small animal excrement or detached hair is released to the outside. Furthermore, since it becomes possible to restrain the small animal in the container 24 by a fixing tool, problems such as image blurring when reconstructing a CT image can be prevented. It is desirable to prepare a plurality of types of containers having different sizes and shapes and selectively use the containers.

  After the container 24 is attached to the arm 26, the arm 26 is driven forward along the direction of the rotation center axis, whereby the container 24 is inserted into the cavity 18A of the gantry 18. At this time, the container 24 is positioned so that the X-ray beam is set at the measurement position in the specimen. Further, such a measurement position is changed continuously or stepwise. As a result, a large number of CT sections spatially aligned at a predetermined pitch are formed.

  An operation panel 20 is provided on the upper surface 16A of the main body 16, and the operation panel 20 includes a plurality of switches, indicators, and the like. Using this operation panel 20, the user can operate the operation of the apparatus at the measurement site. A plurality of casters 22 are provided below the main body 16.

  In the measurement unit 10, an X-ray generator 52 is provided on one side and the X-ray detector 60 is provided on the other side with the rotation center axis O therebetween (see FIG. 1). A collimator 54 is provided on the irradiation side of the X-ray generator 52. The X-ray generator 52 emits an X-ray beam 56 having an intensity corresponding to the supplied drive voltage. As shown in FIG. 1, the X-ray beam has a divergent or fan shape (that is, a fan beam shape). On the other hand, the X-ray detector 60 is configured as a plurality of (for example, 100) X-ray sensors arranged in a line, and an X-ray receiving opening is set according to the opening angle of the X-ray beam 56. Incidentally, the arrangement of the plurality of X-ray sensors may be linear or arcuate. In the present embodiment, a highly sensitive X-ray sensor is used. The detection value detected by the X-ray detector 60 is output to the processor 30 as projection data. In FIG. 1, the voltage source connected to the X-ray generator 52 and the signal processing circuit connected to the X-ray detector 60 are not shown.

  In FIG. 1, reference numeral 58 denotes an effective field of view. This is a circular area in which a CT image can be formed when the X-ray beam 56 is rotated and scanned. Incidentally, the effective visual field 58 is determined according to the positional relationship between the rotation center axis, the X-ray generator 52, and the X-ray detector 60. In this embodiment, since the displacement mechanism 62 is provided, it is possible to change the positional relationship between them and mechanically vary the magnification of the CT image.

  That is, the X-ray generator 52 and the X-ray detector 60 are connected to the displacement mechanism 62, and the displacement mechanism 62 maintains the distance between the X-ray generator 52 and the X-ray detector 60. They (that is, the measurement unit) are displaced in the beam axis direction of the X-ray beam 56. In this case, the rotation center axis O is not changed, that is, the magnification can be changed by moving the measurement unit without moving the container described above. The displacement mechanism 62 includes a motor 62A for generating a displacement force.

  The gantry rotation mechanism 66 is a mechanism that rotates the entire base including the displacement mechanism mounted thereon by rotating the rotation base. Since the measurement unit is mounted on the displacement mechanism 62, the measurement unit positioned at a desired position by the displacement mechanism 62 is rotationally driven while maintaining the position. The gantry rotating mechanism 66 has a motor 66A for generating the driving force.

  The slide mechanism 68 is a moving mechanism that slides the arm 26 shown in FIG. 2, and the driving force is generated by the motor 68A. The operation panel 20 is provided on the upper surface of the main body as described above. The operation panel 20 may be connected to a local controller (not shown) provided on the measurement unit 10 side so that the local controller and the arithmetic control unit 12 communicate with each other.

  Incidentally, although various mechanisms 62, 66, 68 and the like are shown in FIG. 1, it is desirable to provide a sensor to detect a position or a position change by these mechanisms. And it is desirable for the arithmetic control part 12 to perform feedback control based on the output signal of those sensors. The magnification change by the displacement mechanism 62 may be performed by user input. For example, the subject size or the container size is automatically detected, and the magnification is automatically set based on the detected data. Also good. Furthermore, when the container type or the like is registered in advance, the magnification may be set using the registered information. Further, in the example shown in FIG. 1, the slide mechanism 68 has the motor 68 </ b> A as a drive source, but the slide force may be generated artificially.

  Next, the arithmetic control unit 12 will be described. Connected to the processor 30 are a display 32, a storage device 34, a keyboard 36, a mouse 38, a printer 40, and the like. In addition, a communication unit 42 for communicating with an external device via a network is connected.

  The processor 30 includes a CPU and various programs. FIG. 1 shows typical functions thereof, and the processor 30 includes an operation control unit 44, an image forming unit 46, a parameter calculation unit 72, and the like.

  The operation control unit 44 controls the overall operation of the measurement unit 10. The image forming unit 46 calculates a pixel value called a CT value based on projection data obtained by rotational scanning of the X-ray beam, and generates a CT image (tomographic image) based on the obtained CT value. In addition, since the well-known well-known technique can be utilized about the specific production | generation method of this CT image, detailed description here is abbreviate | omitted.

  The parameter calculation unit 72 calculates various diagnostic parameters using the CT value calculated by the image forming unit 46. One of the diagnostic parameters calculated here is the fat content in the target region designated by the user. The specific calculation procedure and the like will be described in detail later.

  The display 32 displays a CT image generated by the image forming unit 46, various diagnostic parameters calculated by the parameter calculating unit 72, and the like. The user diagnoses the state of the subject based on the contents displayed on the display 32. If necessary, the user sets ROI (region of interest) necessary for calculation of various diagnostic parameters while referring to the CT image displayed on the display device 32.

  Next, the fat content calculated by the parameter calculation unit 72 will be described by taking the case of the liver as an example. As is well known, in the diagnosis of the degree of progression of fatty liver, the degree of fat content in the liver is very important. Conventionally, the degree of fat content has been determined based on a visual impression on the CT image of the liver portion. That is, in the CT image, a substance having a higher X-ray absorption rate is displayed brighter, and conversely, a substance having a lower X-ray absorption rate is displayed darker. The liver can be regarded as a mixed substance of muscle and fat, but fat has a lower X-ray absorption rate than muscle, so that the higher the fat content, the darker the liver is displayed. Become. In other words, the degree of fat content in the liver can be known to some extent by looking at the brightness in the CT image.

  However, the diagnosis based on such a visual impression has a problem that it is easily influenced by subjectivity and experience and lacks accuracy. Thus, in some cases, it has been proposed to use the average CT value of the liver portion or the LS ratio as a parameter that quantitatively represents the fat content.

  The CT value and LS ratio will be briefly described with reference to FIG. The CT value corresponds to a pixel value in the CT image, and is generally set so that the CT value of water is 0 and the CT value of air is −1000. In general, the CT value of the muscle portion is around +50, and the CT value of the fat portion is around −250. The CT value of the liver portion, which is a mixture of muscle and fat, approaches the CT value of the fat portion (about −250) as the fat content increases, and the CT value of the muscle portion (about +50 as the fat content decreases). The value is close to). The LS ratio is a value obtained by dividing the CT value of the liver portion by the CT value of the muscle portion. Therefore, the closer the LS ratio is to 1, the lower the fat content in the liver.

  However, neither the CT value nor the LS ratio is a parameter that directly represents the fat content. For this reason, it has been difficult for general users to accurately recognize the fat content from the CT value and LS ratio. The CT value depends on the drive voltage supplied to the X-ray generator 52. When the drive voltage is different, the obtained CT value and LS ratio are different even when the same liver is imaged. . As a result, it has become more difficult to properly diagnose the degree of fat content.

  Therefore, in the present embodiment, the fat content is calculated as a parameter that can more easily and more accurately diagnose the degree of fat content. The fat content rate is a parameter indicating the ratio (percentage) in which fat is included in the target site as the name suggests. In this embodiment, this fat content is calculated by the following procedure.

  When calculating the fat content rate in the liver, the processor 30 drives and controls the measurement unit 10 to acquire CT images of a plurality of locations of the subject. 4 to 6 are schematic diagrams of CT images obtained when the subject is a mouse. More specifically, FIG. 4 is a CT image around the liver, FIG. 5 is a CT image around the spleen, and FIG. 6 is a CT image around the intestine. This CT imaging is performed while keeping the driving voltage supplied to the X-ray generator 52 constant, in other words, keeping the X-ray irradiation conditions constant. In other words, during this CT imaging, the operation control unit 44 controls the drive voltage of the X-ray generator 52 so that the same type of substance has the same CT value.

  If a plurality of CT images can be acquired, the processor 30 subsequently displays the obtained CT images on a display. In addition, the user is prompted to set the liver ROI, muscle ROI, and fat ROI by outputting a message. Here, the liver ROI is an ROI indicating a liver portion, that is, a region where the fat content is to be calculated. This liver ROI is set by operating a mouse or the like, and is, for example, an area indicated by reference numeral E1 in FIG.

  The muscle ROI is an ROI indicating a muscle portion, and is usually set in the spleen as indicated by a symbol E2 in FIG. Here, the reason why the muscle ROI is set in the spleen is that the spleen is an organ in which fat is not accumulated, and the spleen portion can be regarded as almost 100% muscle tissue. The fat ROI is an ROI indicating a fat portion, and is set to a subcutaneous fat portion existing around the intestine, for example, as indicated by a symbol E3 in FIG. The ROI setting may be automatically performed on the processor 30 side. That is, the processor 30 may perform image analysis processing on the obtained CT image to automatically extract the liver portion, the muscle portion, and the fat portion and set the ROI.

  If the liver ROI, muscle ROI, and fat ROI are set, then the parameter calculation unit 72 calculates the average CT value in each ROI as the liver average CT value, muscle average CT value, and fat average CT value. Then, the three types of average CT values are applied to the following equation 1 to calculate the fat content of the liver. In addition, FIG. 7 is a graph representation of the formula 1. In FIG. 7, the horizontal axis indicates the CT value, and the vertical axis indicates the fat content.

    Fat content = (Muscle average CT value−Liver average CT value) / (Muscle average CT value−Fat average CT value) × 100% Formula 1

  As is apparent from Equation 1 and FIG. 7, in this embodiment, both the muscle CT value and the fat CT value are used as reference values, and the relative magnitude of the liver CT value with respect to these two types of reference values is determined as the fat content rate. It is calculated as According to such a fat content rate, the degree of fat content in the liver can be intuitively recognized. The calculation formula shown in Formula 1 is an example, and other calculation formulas may be used as long as they represent the relative magnitude of the liver CT value with respect to the muscle CT value and the fat CT value. For example, in Equation 1, (Muscle Average CT Value−Liver Average CT Value) is a numerator, but (Liver Average CT Value−Fat Average CT Value) may be a numerator.

  Further, the fat content obtained by this procedure is hardly affected by the drive voltage, unlike the average CT value and LS ratio that have been frequently used conventionally. Therefore, the fat content degree can always be diagnosed quantitatively and accurately. As a result, for example, even when it is difficult to keep the driving voltage constant, such as when the degree of progression of fatty liver is observed over several months, the degree of progression of fatty liver can be accurately determined. The calculated fat content is displayed on the display 32 together with the CT image, liver CT value, LS ratio, and the like.

  FIG. 8 is a table showing a comparison between liver CT values and LS ratios that have been frequently used in the past and the fat content calculated in the present embodiment. As shown in the upper side of FIG. 8, it is assumed that CT values obtained under a certain drive voltage are liver CT value = −100, fat CT value = −250, and muscle CT value = + 50. In this case, the LS ratio is -2.0, and the fat content is 50%. Here, the LS ratio reflects the muscle CT value but does not reflect the fat CT value, as is apparent from the calculation formula of (liver CT value) / (muscle CT value). Therefore, it is difficult to intuitively recognize the degree of fat content in the liver even if only the LS ratio value of “2.0” is seen. If the user does not refer to the fat CT value or the like, “2. The meaning of the value “0” cannot be accurately evaluated. On the other hand, the fat content calculated in the present embodiment is a value reflecting both the muscle CT value and the fat CT value. Therefore, if only the fat content rate of “50%” is observed, it can be intuitively recognized how much fat is contained in the liver. As a result, it is possible to grasp the degree of progression of fatty liver more accurately and easily.

  Further, it is assumed that the CT value has moved 10 to the negative side as the drive voltage changes. In this case, as shown in the lower side of FIG. 8, liver CT value = −110, fat CT value = −260, and muscle CT value = + 40. In this case, the LS ratio is −2.75. In other words, the LS ratio fluctuates despite the same liver measurement results as shown in the upper part of FIG. Such fluctuation of the LS ratio is a major factor that makes it difficult to appropriately diagnose the degree of fat content in the liver. On the other hand, the fat content calculated in the present embodiment remains 50% as in the case illustrated in the upper side of FIG. In other words, since the fat content does not depend on the drive voltage, it can always be used as a suitable determination index.

  As is apparent from the above description, in this embodiment, the fat content is calculated based on the fat CT value and the muscle CT value. As a result, the fat content degree can be quantitatively evaluated more accurately. In the present embodiment, the liver is taken as an example of the site to be diagnosed, but naturally, the fat content of other sites may be calculated.

  In the present embodiment, muscle CT values and fat CT values are calculated from CT images actually obtained by X-ray irradiation. However, these values may be measured and stored in advance. For example, the results of measuring muscle CT values and fat CT values by changing the drive voltage are stored in advance in the storage device 34 as a table or a function as shown in FIG. Then, when calculating the fat content of the liver, the muscle CT value and fat CT value that would be obtained with the drive voltage when CT imaging of the liver is calculated from the stored data. Also good.

  Furthermore, you may make it change the luminance value and color in CT image according to fat content rate. For example, the liver is divided into a plurality of blocks, and the fat content is calculated for each block. If the brightness and color are changed according to the obtained fat content, the distribution of fat in the liver can be easily grasped.

  The calculation of the fat content described above is not necessarily performed by the X-ray CT apparatus, and may be performed on a computer separate from the X-ray CT apparatus. That is, the tomographic image generated by the X-ray CT apparatus may be input to a computer in which a fat content rate calculation program is installed, and the fat content rate may be calculated on the computer.

It is a block diagram of the X-ray CT apparatus which is embodiment of this invention. It is a perspective view of a measurement part among X-ray CT apparatuses. It is a figure which shows the relationship between a substance kind and CT value. It is the schematic of the CT image around the liver. It is the schematic of the CT image around a spleen. It is the schematic of a CT image around the intestine. It is a graph which shows fat content rate. It is a figure which shows a comparison of CT value, LS ratio, and fat content rate. It is a figure which shows an example of the data memorize | stored beforehand.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Measurement part, 12 Operation control part, 16 Main body, 18 Gantry, 24 Container, 26 Arm, 30 Processor, 32 Display, 34 Storage device, 42 Communication part, 44 Operation control part, 46 Image formation part, 52 X-ray generation , 54 collimator, 56 X-ray beam, 58 effective field of view, 60 X-ray detector, 72 parameter calculation unit.

Claims (4)

An X-ray CT apparatus that generates a tomographic image of a subject based on projection data obtained when the subject is irradiated with X-rays,
An acquisition means for acquiring a CT value of a muscle part, a CT value of a fat part, and a CT value of a target part obtained when X-ray irradiation is performed under the same irradiation conditions;
A calculation means for calculating the relative magnitude of the CT value of the target part as a fat content of the target part when the CT value of the fat part and the CT value of the muscle part are used as a reference;
An X-ray CT apparatus comprising: The X-ray CT apparatus according to claim 1,
The acquisition means actually calculates the CT value of the muscle part, the CT value of the fat part, and the CT value of the target part based on the result of X-ray irradiation around the muscle part, fat part, and target part of the subject. An X-ray CT apparatus characterized by calculating The X-ray CT apparatus according to claim 2,
The acquisition means includes
Means for obtaining projection data by irradiating X-rays around the muscle portion, fat portion, and target region around the subject without changing the irradiation conditions;
Based on the obtained projection data, means for generating a tomographic image around the muscle part, around the fat part, around the target part;
Means for displaying the generated tomographic image and receiving settings of a muscle ROI indicating a muscle part, a fat ROI indicating a fat part, and a target ROI indicating a target part in the tomographic image;
Means for calculating an average value of CT values in each set ROI as a CT value of a muscle part, a CT value of a fat part, and a CT value of a target part;
An X-ray CT apparatus comprising: Computer
Based on the tomographic image of the subject generated from the projection data obtained when the subject is irradiated with X-rays, the CT value of the muscle portion and the CT value of the fat portion obtained when X-ray irradiation is performed under the same irradiation conditions CT value calculating means for calculating the CT value of the target site;
A calculation means for calculating the relative magnitude of the CT value of the target part as a fat content of the target part when the CT value of the fat part and the CT value of the muscle part are used as a reference;
A program characterized by making it function.

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