Patent Publication Number: US-2022222917-A1

Title: Image processing device, image processing method, and image processing program

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2020/033315 filed on Sep. 2, 2020, which claims priority under 35 U.S.C § 119(a) to Japanese Patent Application No. 2019-184534 filed on Oct. 7, 2019. Each of the above application(s) is hereby expressly incorporated by reference, in its entirety, into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to an image processing device, an image processing method, and a non-transitory computer recording medium storing an image processing program. 
     2. Description of the Related Art 
     In recent years, with the progress of medical devices, such as a computed tomography (CT) device and a magnetic resonance imaging (MRI) device, high-resolution three-dimensional images with higher quality have been used for image diagnosis. 
     Meanwhile, interstitial pneumonia is known as a lung disease. In addition, there has been proposed a method of analyzing a CT image of a patient with interstitial pneumonia to classify lesions, such as honeycomb lung, reticular opacity, and cyst, and tissues, such as normal lungs, blood vessels, and bronchi, which are included in a lung field region of the CT image, as properties and to quantify the properties (see Evaluation of computer-based computer tomography stratification against outcome models in connective tissue disease-related interstitial lung disease: a patient outcome study, Joseph Jacob et al., BMC Medicine (2016) 14:190, DOI 10.1186/s12916-016-0739-7 and Quantitative evaluation of CT images of interstitial pneumonia by computer, Iwasawa Tae, Journal of Japanese Association of Tomography Vol. 41, No. 2, August 2014). In this manner, the CT image is analyzed, and the properties are classified, and the volume, area, number of pixels, and the like of the property are quantified, so that the degree of lung disease can be easily determined. In addition, for each property that is classified and quantified in this manner, different colors are assigned and displayed, so that it is possible to easily diagnose how much a region with a specific property is included in the image. 
     There has also been proposed a method of classifying each pixel of a medical image into a class for each of a plurality of properties using a discriminative model that is constructed by a deep learning method or the like. With such a discriminative model used, it is possible to classify a target pixel of an input medical image into any one of a plurality of types of properties. 
     Meanwhile, in order to diagnose the recovery or progression of the disease, there is a case in which comparative observation over time is performed using past medical images of the same patient. Upon such comparative observation over time, various methods have been proposed to make it easier to visually recognize changes over time. For example, JP2015-167629A has proposed a method of acquiring a plurality of images having different imaging dates, setting a reference point at a part of interest of each image, acquiring a measured value of a measurement item of the part of interest in any direction centered on the reference point, calculating an amount of time-series change of the measured value, and generating and displaying an annotation and a graph representing the amount of change during observation over time. Further, JP3577680B has proposed a method of obtaining a difference image between radiographic images of a chest having different imaging times, labeling a portion corresponding to a time-varying portion in the difference image, and expressing a time-varying region having a predetermined size or larger with a fill pattern or the like, thereby displaying the time-varying portion with an increased emphasis degree. Further, JP5661453B has proposed a method of displaying a motion picture after performing registration between corresponding parts in CT images having different imaging times. 
     SUMMARY OF THE INVENTION 
     However, in the methods described in JP2015-167629A, JP3577680B, and JP5661453B, only the difference between the images is emphasized and displayed. For this reason, in a case in which properties to be classified are a plurality of types as in the above-mentioned interstitial pneumonia, it is not possible to accurately grasp which property the past property has changed to. 
     The present disclosure has been made in view of the above circumstances, and an object thereof is to make it possible to accurately recognize property changes between medical images having different imaging times. 
     There is provided an image processing device according to the present disclosure comprising: a property classification unit that classifies each pixel included in a target region of each of a first medical image and a second medical image of which an imaging time is later than an imaging time of the first medical image for the same subject into any one of a plurality of types of properties; and a quantitative value derivation unit that specifies property pairs each representing a property change between corresponding pixels in at least partial regions within the target regions, and derives a quantitative value in the at least partial regions for each of the property pairs. 
     The “property pair representing a property change between corresponding pixels” means a combination of the property for the first medical image and the property for the second medical image that are derived in the target regions included in the first medical image and the second medical image. For example, in a case in which a certain pixel in the first medical image is a property A and a pixel in the second medical image corresponding to the pixel is a property B, the property of the pixel is changed from the property A to the property B, so that the property pair is (A, B). In addition, the “property pair representing a property change between corresponding pixels” includes a combination of the properties in which there is no change in the property. 
     In the image processing device according to the present disclosure, the quantitative value derivation unit may derive a ratio in the at least partial regions for each of the property pairs as the quantitative value. 
     In addition, the image processing device according to the present disclosure may further comprise a registration unit that performs registration of the target regions between the first medical image and the second medical image. 
     Further, in the image processing device according to the present disclosure, the quantitative value derivation unit may derive an amount of change per unit time in an amount of the property pair between the corresponding pixels in the at least partial regions as the quantitative value. 
     As the “amount of the property pair”, for example, the volume, area, and number of pixels of the property pair, and the ratio of the property pair to the target regions may be used. 
     Further, in the image processing device according to the present disclosure, the quantitative value derivation unit may divide the target region into a plurality of small regions and derive the quantitative value for each of the small regions. 
     In addition, the image processing device according to the present disclosure may further comprise a display control unit that displays the quantitative value for each of the property pairs on a display unit. 
     Further, in the image processing device according to the present disclosure, the display control unit may display the quantitative value such that a property pair in which a symptom is worsened, a property pair in which the symptom is improved, and a property pair in which there is no change in the symptom, out of the property pairs, can be identified. 
     Alternatively, in the image processing device according to the present disclosure, the display control unit may display the quantitative value for each of the property pairs in descending order of the quantitative values. 
     Alternatively, in the image processing device according to the present disclosure, the display control unit may superimpose and display information representing the quantitative value for at least a part of the property pairs, on the target region of the first medical image or the second medical image. 
     Further, in the image processing device according to the present disclosure, the property classification unit may have a discriminative model in which machine learning has been performed so as to classify the plurality of types of properties, and may classify each pixel included in the target region into any one of the plurality of types of properties via the discriminative model. 
     Further, in the image processing device according to the present disclosure, the target region may be a lung field region. 
     There is provided an image processing method according to the present disclosure comprising: classifying each pixel included in a target region of each of a first medical image and a second medical image of which an imaging time is later than an imaging time of the first medical image for the same subject into any one of a plurality of types of properties; and specifying property pairs each representing a property change between corresponding pixels in at least partial regions within the target regions, and deriving a quantitative value in the at least partial regions for each of the property pairs. 
     A non-transitory computer recording medium storing a program causing a computer to execute the image processing method according to the present disclosure may also be provided. 
     There is provided another image processing device according to the present disclosure comprising: a memory that stores a command to be executed by a computer; and a processor configured to execute the stored command, in which the processor executes classifying each pixel included in a target region of each of a first medical image and a second medical image of which an imaging time is later than an imaging time of the first medical image for the same subject into any one of a plurality of types of properties; and specifying property pairs each representing a property change between corresponding pixels in at least partial regions within the target regions, and deriving a quantitative value in the at least partial regions for each of the property pairs. 
     According to the present disclosure, it is possible to accurately recognize property changes between medical images having different imaging times. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a hardware configuration view showing an outline of a diagnostic support system to which an image processing device according to an embodiment of the present disclosure is applied. 
         FIG. 2  is a schematic block diagram showing a configuration of the image processing device according to the present embodiment. 
         FIG. 3  is a view illustrating registration of lung field regions. 
         FIG. 4  is a view showing an example of a multi-layer neural network. 
         FIG. 5  is a view showing evaluation values corresponding to property types for a certain pixel. 
         FIG. 6  is a view showing a cross-section in a mapping image in which colors corresponding to classifications are assigned. 
         FIG. 7  is a view showing a derivation result of the quantitative value. 
         FIG. 8  is a view showing a display screen of the quantitative value for each of property pairs. 
         FIG. 9  is a flowchart showing processing performed in the present embodiment. 
         FIG. 10  is a view showing a display screen of a list in which colors corresponding to the magnitude of the quantitative values are assigned, for each field of the property pairs. 
         FIG. 11  is a view showing a display screen in which information representing the quantitative value is superimposed and displayed on a medical image. 
         FIG. 12  is a view showing a display screen in which the property pairs are ranked and displayed in descending order of the quantitative values. 
         FIG. 13  is a view illustrating the division of the lung field region into an inner region and an outer region. 
         FIG. 14  is a view illustrating the division of the lung field region into an upper region, a middle region, and a lower region. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings.  FIG. 1  is a hardware configuration view showing an outline of a diagnostic support system to which an image processing device according to the embodiment of the present disclosure is applied. As shown in  FIG. 1 , in the diagnostic support system, an image processing device  1 , a three-dimensional image capturing device  2 , and an image storage server  3  according to the present embodiment are connected to communicate with each other through a network  4 . 
     The three-dimensional image capturing device  2  is a device that generates a three-dimensional image showing a part, which is a part to be diagnosed of a subject, by imaging the part. Specifically, the three-dimensional image capturing device  2  is a CT device, an MM device, a positron emission tomography (PET) device, or the like. The three-dimensional image consisting of a plurality of slice images, which is generated by the three-dimensional image capturing device  2 , is transmitted to the image storage server  3  and is stored thereon. In the present embodiment, the diagnostic target part of the patient who is a subject is the lungs, and the three-dimensional image capturing device  2  is a CT device and generates a CT image of the chest including the lungs of the subject as a three-dimensional image. 
     The image storage server  3  is a computer that stores and manages various kinds of data, and comprises a large-capacity external storage device and software for database management. The image storage server  3  communicates with other devices through the wired or wireless network  4  to transmit and receive image data or the like. Specifically, the image storage server  3  acquires various kinds of data including the image data of a medical image generated by the three-dimensional image capturing device  2  through the network, and stores the various kinds of data on a recording medium, such as a large-capacity external storage device, and manages the various kinds of data. The storage format of the image data and the communication between devices through the network  4  are based on a protocol, such as digital imaging and communications in medicine (DICOM). In the present embodiment, the image storage server  3  stores and manages a plurality of medical images having different imaging dates for the same patient. 
     The image processing device  1  of the present embodiment is realized by installing an image processing program of the present embodiment on one computer. The computer may be a workstation or a personal computer that is directly operated by a doctor who performs a diagnosis, or may be a server computer connected to the workstation or to the personal computer through a network. The image processing program is stored in a storage device of a server computer connected to the network or in a network storage so as to be accessible from the outside, and is downloaded and installed on a computer that the doctor uses in response to a request. Alternatively, the image processing program is distributed by being recorded on a recording medium, such as a digital versatile disc (DVD) or a compact disc read only memory (CD-ROM), and is installed onto the computer from the recording medium. 
       FIG. 2  is a diagram showing a schematic configuration of the image processing device that is realized by installing the image processing program on a computer. As shown in  FIG. 2 , the image processing device  1  comprises a central processing unit (CPU)  11 , a memory  12 , and a storage  13 , as a configuration of a standard workstation. Further, a display unit  14 , such as a liquid crystal display, and an input unit  15 , such as a keyboard and a mouse, are connected to the image processing device  1 . 
     The storage  13  includes a hard disk drive or the like, and stores the plurality of medical images of the same subject that are used to perform observation over time, which are acquired from the image storage server  3  through the network  4 , and various information including information required for processing. 
     Further, the image processing program read out from the storage  13  by the CPU or the like is temporarily stored in the memory  12 . The image processing program defines, as processing to be executed by the CPU  11 , image acquisition processing of acquiring a first medical image G 1  and a second medical image G 2  of which an imaging time is later than an imaging time of the first medical image G 1  for the same subject, registration processing of performing registration of target regions between the first medical image G 1  and the second medical image G 2 , property classification processing of classifying each pixel included in the target region of each of the first medical image G 1  and the second medical image G 2  into any one of a plurality of types of properties, quantitative value derivation processing of specifying property pairs each representing a property change between corresponding pixels in at least partial regions within the target regions and deriving a quantitative value in the at least partial regions for each of the property pairs, and display control processing of displaying the quantitative value for each of the property pairs on a display unit  14 . 
     Then, the CPU  11  executes the processing in accordance with the image processing program, so that the computer functions as an image acquisition unit  21 , a registration unit  22 , a property classification unit  23 , a quantitative value derivation unit  24 , and a display control unit  25 . 
     The image acquisition unit  21  acquires the first medical image G 1  and the second medical image G 2  from the image storage server  3  through an interface (not shown) connected to the network. In the present embodiment, it is assumed that the first medical image G 1  and the second medical image G 2  are CT images including the lung field for the same subject. The acquired first and second medical images G 1  and G 2  are stored in the storage  13 . In the present embodiment, it is assumed that the target region is a lung field region. 
     Here, in the first medical image G 1  and the second medical image G 2 , there are differences in size and position of the target region between the images because of changes in the breathing and posture of the patient, which is the subject, and changes in the imaging field of view (FOV). Therefore, the registration unit  22  performs registration of the target regions of the first medical image G 1  and the second medical image G 2 , that is, the lung field regions. Specifically, first, the imaging fields of view of the first medical image G 1  and the second medical image G 2  are matched with each other, and then the pixel densities are matched with each other. Then, the shift direction and shift amount of the corresponding pixel position are derived between the first medical image G 1  and the second medical image G 2 . Then, registration between the lung field regions of the first and second medical images G 1  and G 2  is performed by non-linearly converting at least one of the lung field region of the first medical image G 1  or the lung field region of the second medical image G 2  based on the derived shift direction and shift amount. As the registration method, any method such as template matching, warping, rigid body registration, or non-rigid body registration can be used. 
     By performing registration in this manner, as shown in  FIG. 3 , each pixel of a lung field region H 1  of the first medical image G 1  and each pixel of a lung field region H 2  of the second medical image G 2  (indicated by a black circle in  FIG. 3 ) are associated with each other. Further, with the registration, the number of pixels included in the lung field region H 1  and the number of pixels included in the lung field region H 2  can be regarded as the same. Since registration of all the regions of the first medical image G 1  and the second medical image G 2  is performed so that registration is performed between the lung field region H 1  and the lung field region H 2 , registration of all the regions of the first medical image G 1  and the second medical image G 2  may be performed. 
     The property classification unit  23  classifies each pixel of the lung field regions H 1  and H 2  of the first and second medical images G 1  and G 2  into any one of a plurality of types of properties. Specifically, the property classification unit  23  derives a plurality of evaluation values indicating the possibility of being each of a plurality of types of tissues or lesions (that is, properties) for each pixel of the lung field regions H 1  and H 2 , and classifies each pixel of the lung field regions H 1  and H 2  into any one of the plurality of types of properties based on the plurality of evaluation values. 
     The property classification unit  23  of the present embodiment has a discriminative model  30  consisting of a multi-layer neural network generated by deep learning, which is one of machine learning methods, and classifies each pixel of the lung field regions H 1  and H 2  into any one of the plurality of properties using the discriminative model  30 . A machine learning method is not limited to deep learning, and other methods, such as a support vector machine, can be used. 
     In the multi-layer neural network, in each layer, arithmetic processing is performed using various kernels on data of a plurality of different feature amounts obtained by the previous layer (regarding a first layer, pixel data in the lung field regions H 1  and H 2 ). Then, in the subsequent layers, arithmetic processing is further performed on the data of the feature amounts obtained by the arithmetic processing, so that the recognition rate of the feature amounts can be improved and the input data can be classified into a plurality of classes. 
     In the present embodiment, description will be made using the multi-layer neural network that receives, as an input, the lung field regions H 1  and H 2  as three-dimensional images and that outputs the result of classification into any one of the plurality of types of properties for each pixel of the lung field regions H 1  and H 2 . However, a configuration can be adopted in which the lung field region as a two-dimensional image in each of the plurality of tomographic images constituting the first and second medical images G 1  and G 2  is received as an input and the result of classification into the plurality of types of properties for the lung field region as the two-dimensional image is output. 
       FIG. 4  is a view showing an example of the multi-layer neural network. As shown in  FIG. 4 , a multi-layer neural network  40  consists of a plurality of layers including an input layer  41  and an output layer  42 . In the present embodiment, the multi-layer neural network  40  learns to classify each pixel of the lung field regions H 1  and H 2  into any one of 33 types of properties, such as normal lung, GGO mass nodule opacity, mixed mass nodule opacity, solid mass nodule opacity, ground glass opacity, pale ground glass opacity, centrilobular ground glass opacity, consolidation, low density, centrilobular emphysema, panlobular emphysema, normal pulmonary emphysema tendency, cyst, tree-in-bud (TM), small nodule (non-centrilobular), centrilobular small nodule opacity, interlobular septal thickening, bronchial wall thickening, bronchiectasis, bronchioloectasis, air bronchogram, traction bronchiectasis, cavity consolidation, cavernous tumor, reticular opacity, fine reticular opacity, honeycomb lung, pleural effusion, pleural thickening, chest wall, heart, diaphragm, and blood vessel. The property types are not limited thereto, and more or fewer properties than the above properties may be used. 
     In the present embodiment, the multi-layer neural network  40  learns the properties using a large amount of training data, such as millions of pieces of training data. Upon learning, a region of interest with a predetermined size (for example, 1.5 cm×1.5 cm×1.5 cm) is cut out from a CT image in which the property type is known, and the region of interest is used as the training data. Then, the training data is input to the multi-layer neural network  40 , and the multi-layer neural network  40  outputs the classification result of the property type. Then, the output classification result is compared with the training data, and the weight of connection between the layers of units (indicated by circles in  FIG. 4 ) included in each layer of the multi-layer neural network  40  is corrected from the output side to the input side in response to whether or not the output result is correct or incorrect. The correction of the weight of the connection is repeated using a large amount of training data a predetermined number of times or until the accuracy rate of the output classification result reaches 100%, and the learning ends. As a result, the discriminative model  30  is constructed. 
     Further, in a case in which the input image is a tomographic image, upon the learning by the multi-layer neural network  40 , a two-dimensional region normalized to a predetermined size (for example, 1.5 cm×1.5 cm) is cut out from a tomographic image constituting a three-dimensional image in which the lesion is known, and the image of the cut-out two-dimensional region is used as the training data. 
     The property classification unit  23  extracts the lung field regions H 1  and H 2 , which are target regions, from the first and second medical images G 1  and G 2  in order to classify the properties. As a method of extracting the lung field regions H 1  and H 2 , any method such as a method in which the signal value of each pixel in the first and second medical images G 1  and G 2  is expressed in a histogram and threshold processing is performed to extract the lung or a region growing method based on a seed point showing the lung can be used. The discriminative model  30  in which machine learning has been performed so as to extract the lung field regions H 1  and H 2  may be used. 
     In the property classification unit  23 , upon the property classification processing, a region of interest having the same size as the training data is sequentially cut out from the lung field regions H 1  and H 2 , and the region of interest is input to the discriminative model  30  consisting of the multi-layer neural network  40 . With this, the discriminative model  30  outputs an evaluation value corresponding to each classification of the properties for the central pixel of the cut-out region of interest. The evaluation value corresponding to each classification is an evaluation value indicating the probability of the central pixel belonging to each classification. The larger this evaluation value is, the higher the probability that the central pixel belongs to the classification is. 
       FIG. 5  is a view showing evaluation values corresponding to property types for a certain pixel. In addition, in  FIG. 5 , the evaluation values for a part of the properties are shown for simplicity of illustration. In the present embodiment, the discriminative model  30  classifies the central pixel of the region of interest into a property with the maximum evaluation value out of the plurality of properties. For example, in a case in which the evaluation values shown in  FIG. 5  are acquired, the central pixel of the region of interest has the highest probability of being reticular opacity and has the second highest probability of being ground glass opacity. On the contrary, the probability of the central pixel being normal lung or low density is almost zero. Therefore, in a case in which the evaluation values as shown in  FIG. 5  are acquired, the central pixel of the region of interest is classified into reticular opacity having a maximum evaluation value of 8.5 by the property classification processing. Such property classification processing is performed on all the pixels of the lung field regions H 1  and H 2 , so that all the pixels of the lung field regions H 1  and H 2  are classified into any one of the plurality of types of properties. 
     In the present embodiment, although the property classification unit  23  classifies each pixel of the lung field regions H 1  and H 2  into any one of 33 types of properties as described above, in order to simplify arithmetic operation for generating a mapping image and deriving the quantitative value, which will be described later, in the present embodiment, 33 types of properties are summarized into 10 types of properties, that is, normal opacity, ground glass opacity, reticular opacity, consolidation, honeycomb lung, nodular opacity, hyperlucent lung, others, bronchus, and blood vessel. The types of properties to be summarized are not limited to these 10 types, and may be more than 10 or less than 10. 
     The property classification unit  23  generates a mapping image by assigning a color to the pixel of each classification in at least one of the first medical image G 1  or the second medical image G 2 , based on the result of the property classification processing. Specifically, the property classification unit  23  generates a three-dimensional mapping image by assigning the same color to the pixels classified into the same property, for all the pixels in a three-dimensional space classified into any one of the plurality of types of properties that are summarized into 10 types as described above.  FIG. 6  is a view showing a cross-section in a mapping image in which colors corresponding to a plurality of types of classifications are assigned for each classification. The display control unit  25 , which will be described later, may display the mapping image on the display unit  14 . In a case in which the mapping image is displayed on the display unit  14 , as shown in  FIG. 6 , a cross-sectional image of any cross-section in the three-dimensional mapping image may be displayed. However, the present disclosure is not limited thereto, and the three-dimensional mapping image may be displayed on the display unit  14  by a method such as volume rendering. 
     The quantitative value derivation unit  24  specifies property pairs each representing a property change between the corresponding pixels in at least partial regions of the lung field regions H 1  and H 2  of the first and second medical images G 1  and G 2 , and derives the quantitative value in the at least partial regions for each of the property pairs. In the present embodiment, the quantitative value derivation unit  24  specifies property pairs in all the regions of the lung field regions H 1  and H 2 , and derives the ratio in the lung field regions H 1  and H 2  for each of the property pairs as the quantitative value. 
     In the present embodiment, the property classification unit  23  classifies each pixel of the lung field regions H 1  and H 2  into any one of 10 types of properties. Here, in a case in which the property of a certain pixel in the lung field region H 1  is normal opacity, the property that the pixel may take in the lung field region H 2  of the second medical image G 2  of which the imaging time is later than the imaging time of the first medical image G 1  is 10 types, that is, normal opacity, ground glass opacity, reticular opacity, consolidation, honeycomb lung, nodular opacity, hyperlucent lung, other, bronchus, and blood vessel. Therefore, 10×10=100 combinations of property changes exist between the corresponding pixels of the lung field regions H 1  and H 2 , as the property pair. Accordingly, the quantitative value derivation unit  24  specifies which of the  100  property pairs each pixel of the lung field regions H 1  and H 2  corresponds to. 
     Further, the quantitative value derivation unit  24  counts the number of pixels in the lung field regions H 1  and H 2  for all the property pairs. Then, the quantitative value derivation unit  24  divides the number of pixels for each property pair by the total number of pixels in the lung field regions H 1  and H 2  to derive the ratio of each property pair to all the regions in the lung field regions H 1  and H 2  as the quantitative value.  FIG. 7  is a view showing the derivation result of the quantitative value. The rows in  FIG. 7  represent the properties of the lung field region H 2  of the second medical image G 2 , and the columns represent the properties of the lung field region H 1  of the first medical image G 1 . In addition, in  FIG. 7 , the property pairs of the fields positioned diagonally are each a property pair in which there is no change in the property between the lung field region H 1  and the lung field region H 2 . 
     The display control unit  25  displays the quantitative value for each of the property pairs on the display unit  14 .  FIG. 8  is a view showing a display screen of the quantitative value for each of the property pairs. As shown in  FIG. 8 , a display screen  50  displays a list  51  of the quantitative value for each of the property pairs. Here, in a case in which a property pair representing that the symptom is worsened, out of the property pairs, has a value as the quantitative value (that is, in a case in which the quantitative value is not 0%, the same applies hereinafter), the field of the property pair is displayed in a different form from the other fields. For example, a field having a value as the quantitative value is colored, or the color of a number in the field is changed. In the list  51  shown in  FIG. 8 , the field of the property pair in which the symptom is worsened is colored. In  FIG. 8 , the field to be colored is shown by hatching. Further, the mapping image shown in  FIG. 6 , in addition to the list  51 , may be displayed on the display screen  50 . 
     Although which property change corresponds to the worsening of the symptom differs depending on the disease, in interstitial pneumonia, the symptom is generally worsened in the order of normal opacity, ground glass opacity, reticular opacity, and honeycomb lung. In addition, the bronchus and the blood vessel are tissues that also exist in the normal lung field, but normal opacity may be changed to the bronchus and the blood vessel because the bronchus dilates or the blood vessel dilates in a case in which the symptom is worsened. 
     In the present embodiment, as shown in  FIG. 8 , the property pairs of (normal opacity, ground glass opacity), (ground glass opacity, reticular opacity), (reticular opacity, honeycomb lung), (normal opacity, hyperlucent lung), and (normal opacity, bronchus) have values as the quantitative value. Therefore, for example, a red color is given to the fields of the property pairs so that the worsened symptom can be visually recognized. 
     On the other hand, in a case in which the property other than normal opacity in the lung field region H 1  of the first medical image G 1  is changed to normal opacity in the second medical image G 2 , the property pair represents that the symptom is improved. Therefore, in a case in which the property pairs of (ground glass opacity, normal lung), (reticular opacity, normal lung), (consolidation, normal lung), (honeycomb lung, normal lung), (hyperlucent lung, normal lung), (nodular opacity, normal lung), (other, normal lung), (bronchus, normal lung), and (blood vessel, normal lung), which are the property pairs representing that the symptom is improved, have values as the ratio, for example, a blue color is given to the field so that the improved symptom may be visually recognized. In addition to the above property pairs, examples of the property pair in which the symptom is improved include (reticular opacity, ground glass opacity). In addition, the property pair in which there is no change in the symptom may not be colored or may be given a gray color. 
     The interpretation of the worsening and improvement of the symptom may differ depending on the doctor. Therefore, it is preferable to allow the doctor to set which property pair is the property pair representing property changes of the worsening and improvement. 
     Next, processing performed in the present embodiment will be described.  FIG. 9  is a flowchart showing processing performed in the present embodiment. First, the image acquisition unit  21  acquires the first and second medical images G 1  and G 2  (medical image acquisition; step ST 1 ), and the registration unit  22  performs registration of the lung field region H 1  of the first medical image G 1  and the lung field region H 2  of the second medical image G 2  (step ST 2 ). Further, the property classification unit  23  classifies each pixel included in the lung field region H 1  of the first medical image G 1  and the lung field region H 2  of the second medical image G 2  into any one of the plurality of types of properties (step ST 3 ). The processing of step ST 2  and step ST 3  may be performed in parallel, or the processing of step ST 3  may be performed before the processing of step ST 2 . 
     Next, the quantitative value derivation unit  24  specifies property pairs each representing a property change between the corresponding pixels in the lung field regions H 1  and H 2 , and derives the ratio in at least partial regions for each of the property pairs as the quantitative value (quantitative value derivation; step ST 4 ). Then, the display control unit  25  displays the quantitative value for each of the property pairs on the display unit  14  (step ST 5 ), and the processing ends. 
     As described above, in the present embodiment, in the lung field regions H 1  and H 2  of the first and second medical images G 1  and G 2 , the property pairs each representing a property change between the corresponding pixels are specified, and the ratio in the lung field regions H 1  and H 2  for each of the property pairs is derived as the quantitative value. Therefore, according to the present embodiment, the derived quantitative values are displayed, so that it is possible to recognize the tendency of property changes in the lung field regions H 1  and H 2 . Consequently, according to the present embodiment, it is possible to accurately recognize the property changes between the first and second medical images G 1  and G 2  having different imaging times. 
     In the above embodiment, colors corresponding to the magnitude of the quantitative values may be assigned, for each field of the property pairs of the list  51  of the quantitative values. For example, a ratio of 0% may be assigned a gray color, a ratio of 0% to 5% (greater than 0% and 5% or less, the same applies hereinafter) may be assigned a yellow color, a ratio of 5% to 20% may be assigned an orange color, and a ratio of greater than 20% may be assigned a red color.  FIG. 10  is a view showing a display screen of a list in which colors corresponding to the magnitude of the quantitative values are assigned, for each field of the property pairs. In addition, in  FIG. 10 , the difference between the colors is shown by a difference in hatching. Further, in this case as well, the color may be further changed in accordance with the property pair representing that the lesion is worsened, the property pair representing that the lesion is improved, and the property pair in which there is no change in the lesion. For example, a reddish color may be assigned to the property pair representing that the lesion is worsened, and a bluish color may be assigned to the property pair representing that the lesion is improved. 
     Further, in the above embodiment, information representing the quantitative value for at least a part of the property pairs may be superimposed and displayed on the first medical image G 1  or the second medical image G 2 .  FIG. 11  is a view showing a display screen in which information representing the quantitative value is superimposed and displayed on a medical image. As shown in  FIG. 11 , a newly captured first medical image G 1  is displayed on a display screen  55 , and colors corresponding to the magnitude of the quantitative values are assigned to the regions including pixels having values as the quantitative value, out of the property pairs, for each pixel of the lung field region H 1  of the first medical image G 1 . For example, a ratio of 0% may not be colored, a ratio of 0% to 5% may be assigned a yellow color, a ratio of 5% to 20% may be assigned an orange color, and a ratio of greater than 20% may be assigned a red color. In  FIG. 11 , a difference between the colors is represented by a difference in hatching. 
     Further, in the above embodiment, the quantitative value derivation unit  24  may derive the amount of change per short time in the ratio of the property pair as the quantitative value by dividing the derived ratio by a difference in acquisition time between the first medical image G 1  and the second medical image G 2 . For example, in a case in which the difference in acquisition time between the first medical image G 1  and the second medical image G 2  is 3 months, the quantitative value derivation unit  24  can derive the amount of change in the ratio of the property pair per month by dividing the derived ratio by  3 . 
     Further, in the above embodiment, all the property pairs are included and displayed in the list  51 , but the present disclosure is not limited thereto. Only the property pair representing the property change in which the symptom is worsened may be displayed in the list, only the property pair representing the property change in which the symptom is improved may be displayed in the list, or only the property pair representing that there is no change in the symptom may be displayed in the list. Alternatively, the property pair representing the property change in which the symptom is worsened, the property pair representing the property change in which the symptom is improved, and the property pair representing that there is no change in the symptom may be displayed in a switchable manner. 
     Further, in the above embodiment, the quantitative values may be displayed in descending order of the quantitative values of the property pairs.  FIG. 12  is a view showing a display screen in which the property pairs are ranked and displayed in descending order of the quantitative values. As shown in  FIG. 12 , the property pairs are ranked and displayed from the first place to the fifth place in descending order of the quantitative values on a display screen  58 . With this, it is possible to easily recognize which property pair is more in the first and second medical images G 1  and G 2 . 
     Further, in the above embodiment, property pairs are derived for all the regions of the lung field regions H 1  and H 2  of the first and second medical images G 1  and G 2 , and the quantitative values are displayed, but the present disclosure is not limited thereto. The lung field regions H 1  and H 2  can be divided into a left lung and a right lung. Further, each of the left and right lungs can be divided into an upper lobe, a middle lobe, and a lower lobe. Alternatively, as shown in  FIG. 13 , the lung field region can be divided into inner regions  60 Lin and  60 Rin, and outer regions  60 Lex and  60 Rex for the left lung and the right lung, respectively. Alternatively, as shown in  FIG. 14 , the lung field region can be divided into three regions of upper, middle, and lower regions. That is, the left lung can be divided into an upper region  61 LU, a middle region  61 LM, and a lower region  61 LL, and the right lung can be divided into an upper region  61 RU, a middle region  61 RM, and a lower region  61 RL. Alternatively, the lung field region can be divided into a dorsal region and a ventral region. Alternatively, the lung field region can be divided into a central region near the first branch of the bronchus and a region other than the central region. Therefore, the lung field regions H 1  and H 2  may be divided into small regions as described above, and the quantitative values of the property pairs may be derived and displayed for each small region. The mode of division is not limited to the above modes. The lung field region may be divided in combination with a plurality of division modes. For example, in addition to the division of the inner region and the outer region, the division of three regions of upper, middle, and lower regions may be performed, and in addition thereto or instead thereof, the division into the dorsal region and the ventral region may be performed. 
     Further, in the above embodiment, the quantitative value derivation unit  24  derives the ratio in at least partial regions for each of the property pairs as the quantitative value, but the present disclosure is not limited thereto. The volume (in a case of a three-dimensional image), the area (in a case of a tomographic image), or the number of pixels in at least partial regions for each of the property pairs may be derived as the quantitative value. 
     Further, in the above-described embodiment, for example, the following various processors can be used as a hardware structure of a processing unit that executes various processing, such as the image acquisition unit  21 , the registration unit  22 , the property classification unit  23 , the quantitative value derivation unit  24 , and the display control unit  25 . The various processors include, for example, a programmable logic device (PLD), such as a field programmable gate array (FPGA), which is a processor having a changeable circuit configuration after manufacture, and a dedicated electrical circuit, such as an application specific integrated circuit (ASIC), which is a processor having a dedicated circuit configuration designed to perform specific processing, in addition to the CPU, which is a general-purpose processor that executes software (programs) to function as various processing units as described above. 
     One processing unit may be constituted of one of the various processors or may be constituted of a combination of two or more processors of the same type or different types (for example, a combination of a plurality of FPGAs and a combination of a CPU and an FPGA). Alternatively, the plurality of processing units may be constituted of one processor. 
     A first example of the configuration in which the plurality of processing units are constituted of one processor is an aspect in which one or more CPUs and software are combined to constitute one processor and the processor functions as a plurality of processing units. A representative example of the aspect is a computer such as a client and server. A second example of the configuration is an aspect in which a processor that implements all of the functions of a system including the plurality of processing units with one integrated circuit (IC) chip is used. A representative example of the aspect is a system on chip (SoC). As described above, as the hardware structure of various processing units, one or more of the various processors are used. 
     Furthermore, as the hardware structure of the various processors, more specifically, an electrical circuit (circuitry) in which circuit elements, such as semiconductor elements, are combined can be used. 
     EXPLANATION OF REFERENCES 
     
         
         
           
               1 : image processing device 
               2 : three-dimensional image capturing device 
               3 : image storage server 
               4 : network 
               11 : CPU 
               12 : memory 
               13 : storage 
               14 : display unit 
               15 : input unit 
               21 : image acquisition unit 
               22 : registration unit 
               23 : property classification unit 
               24 : quantitative value derivation unit 
               25 : display control unit 
               30 : discriminative model 
               40 : multi-layer neural network 
               41 : input layer 
               42 : output layer 
               50 ,  55 ,  58 : display screen 
               60 Rin,  60 Lin: inner region 
               60 Rex,  60 Lex: outer region 
               61 RU,  61 LU: upper region 
               61 RM,  61 LM: middle region 
               61 RL,  61 LL: lower region 
             G 1 : first medical image 
             G 2 : second medical image 
             H 1 , H 2 : lung field region