Image processing device, image processing method, image processing system and program

Abstract

To generate a difference image high in visibility.SOLUTION: An image processing device for generating a difference image from a first medical image and a second medical image, which are three-dimensional images composed of a plurality of slices obtained by imaging a subject includes: an acquisition unit for acquiring the first medical image and the second medical image; a determination unit for determining, as a slice interval of the difference image, an interval that a slice corresponding to the slice of any one reference medical image of the first medical image and the second medical image is included in the difference image in a range of the difference image in a slice direction of the subject, which is an interval equal to or smaller than a slice interval of any one reference medical image of the first medical image and the second medical image; and a generation unit for generating the difference image with the slice interval determined by the determination unit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image processing device, an image processing method, an image processing system, and a program. [Background technology]

[0002] In the medical field, attempts have been made to visualize changes over time in lesions and other conditions by presenting users with a subtraction image generated from two images taken at different times using various modalities.

[0003] Non-Patent Document 1 discloses a technique for displaying a difference image generated from two three-dimensional images obtained by imaging using a CT device as a two-dimensional tomographic image, and also discloses a technique for generating and displaying a two-dimensional projection image by projecting pixel values ​​of the difference image in a direction parallel to the slice plane (generally a direction perpendicular to the body axis of the subject in CT). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] R. Sakamoto, et.al, Temporal subtraction system for detecting bone metastasis using LDDMM : preliminary study, CARS2014. Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the technology of Non-Patent Document 1, when a difference image is generated from two 3D images (original images) with a large slice interval, the slice interval of the difference image also becomes large. Therefore, when a projection image in which pixel values ​​of the difference image are projected in a direction parallel to the slice plane is displayed on a display unit, a problem occurs in that visibility is reduced.

[0006] In view of the above-mentioned problems, an object of the present invention is to provide an image processing technique that can generate a difference image with high visibility. [Means for solving the problem]

[0007] In order to achieve the object of the present invention, an image processing device according to one aspect of the present invention displays on a display unit a first medical image and a second medical image, which are three-dimensional images formed by a plurality of slices obtained by imaging an object. Display the projected image do It is a 3D image for An image processing device for generating a difference image, an acquisition means for acquiring the first medical image and the second medical image; a conversion means for converting the resolution of at least one of the first medical image and the second medical image to a predetermined resolution so that differences in details between the first medical image and the second medical image can be calculated; a generating means for generating the subtraction image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; the generating means of the difference image generated by The predetermined A determining means for determining a slice interval, the determining means determining a slice interval of one of the first medical image and the second medical image as a reference. The slice corresponding to The slice interval of the slices in the difference image is smaller than the slice interval of the slices in the one medical image. So that , The aforementioned Predetermined resolution to Based on the neighboring values ​​of the set reference resolution The predetermined slice interval is Decision making means and 、 of It is characterized by being equipped with: In order to achieve the object of the present invention, an image processing method according to another aspect of the present invention includes: displaying on a display unit a first medical image and a second medical image, which are three-dimensional images configured from a plurality of slices obtained by imaging an object; Display the projected image do It is a three-dimensional image for An image processing method for an image processing device that generates a difference image, comprising: an acquisition step of acquiring the first medical image and the second medical image; a conversion step of converting the resolution of at least one of the first medical image and the second medical image to a predetermined resolution so that differences in details between the first medical image and the second medical image can be calculated; generating the difference image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; The generating step of the difference image generated by The predetermined A slice interval determination step includes determining a slice interval of either one of the first medical image and the second medical image as a reference. The slice corresponding to The slice interval of the slices in the difference image is smaller than the slice interval of the slices in the one medical image. So that , The aforementioned Predetermined resolution to Based on the neighboring values ​​of the set reference resolution The predetermined slice interval is a decision-making process for making a decision; 、 of It is characterized by having: In order to achieve the object of the present invention, an image processing apparatus according to another aspect of the present invention displays on a display unit a first medical image and a second medical image, which are three-dimensional images configured by a plurality of slices obtained by imaging an object. Display the projected image do It is a three-dimensional image for An image processing device for generating a difference image, an acquisition means for acquiring the first medical image and the second medical image; the first medical image and of the second medical image At least one of them The resolution can be calculated as the difference in image detail between the first medical image and the second medical image. As such, A conversion means for converting the resolution to a predetermined resolution; a generating means for generating the subtraction image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; the generating means of the difference image generated by The predetermined A determining means for determining a slice interval, the determining means determining a slice interval of one of the first medical image and the second medical image as a reference. The slice corresponding to The slice interval of the slices in the difference image is smaller than the slice interval of the slices in the one medical image. in the difference image so that The aforementioned predetermined The slice interval is set to the predetermined resolution. Set to a determination means for determining the reference resolution based on values ​​in the vicinity of the reference resolution; 、 of Preparation 、 The determining means compares a first value obtained based on a difference between a value obtained by dividing a slice interval between slices of the one medical image by a first natural number and the reference resolution with a second value obtained based on a difference between a value obtained by dividing a slice interval between slices of the one medical image by a second natural number and the reference resolution, and sets one of the values ​​obtained by dividing the slice interval between slices of the one medical image by the first natural number or the second natural number, which corresponds to a smaller absolute value of the first value and the second value, as the neighboring value, wherein the first natural number is a natural number such that the value obtained by dividing the slice interval between slices of the one medical image by the first natural number gives a maximum value equal to or smaller than the reference resolution, and the second natural number is a natural number such that the value obtained by dividing the slice interval between slices of the one medical image by the second natural number gives a minimum value equal to or larger than the reference resolution.It is characterized by the following. In order to achieve the object of the present invention, an image processing method according to another aspect of the present invention includes: displaying on a display unit a first medical image and a second medical image, which are three-dimensional images configured from a plurality of slices obtained by imaging an object; Display the projected image do It is a three-dimensional image for An image processing method for an image processing device that generates a difference image, comprising: an acquisition step of acquiring the first medical image and the second medical image; the first medical image and of the second medical image At least one of them The resolution can be calculated as the difference in image detail between the first medical image and the second medical image. As such, a conversion step of converting the resolution to a predetermined resolution; a generating step of generating a difference image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; The generating step of the difference image generated by The predetermined A slice interval determination step includes determining a slice interval of either one of the first medical image and the second medical image as a reference. The slice corresponding to The slice interval of the slices in the difference image is smaller than the slice interval of the slices in the one medical image. in the difference image so that The aforementioned predetermined The slice interval is set to the predetermined resolution. Set to a determining step of determining the reference resolution based on values ​​in the vicinity of the reference resolution; 、 of Yes death, The determining step compares a first value obtained based on a difference between a value obtained by dividing an interval between slices of the one medical image by a first natural number and the reference resolution with a second value obtained based on a difference between a value obtained by dividing an interval between slices of the one medical image by a second natural number and the reference resolution, and sets one of the values ​​obtained by dividing the interval between slices of the one medical image by the first natural number or the second natural number, which corresponds to a smaller absolute value of the first value and the second value, as the neighboring value, wherein the first natural number is a natural number such that the value obtained by dividing the interval between slices of the one medical image by the first natural number gives a maximum value equal to or smaller than the reference resolution, and the second natural number is a natural number such that the value obtained by dividing the interval between slices of the one medical image by the second natural number gives a minimum value equal to or larger than the reference resolution. It is characterized by the following. [Effects of the Invention]

[0008] According to the present invention, it is possible to generate a difference image with high visibility. That is, the difference image can be displayed as a cross section in the same slice as the original image, and even when the pixel values ​​of the difference image are projected parallel to the slice plane, a projection image with high visibility can be displayed. [Brief explanation of the drawings]

[0009] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. [Figure 1] FIG. 1 is a diagram showing the functional configuration of an image processing system according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an overall processing procedure in the embodiment. [Figure 3] FIG. 10 is a diagram illustrating slice intervals of a difference image. [Figure 4] 10A and 10B are diagrams illustrating axial cross sections of an original image and a differential image. [Figure 5] 1A and 1B are diagrams illustrating coronal sections of an original image and a differential projection image. [Figure 6] FIG. 4 is a diagram illustrating an example of setting of coordinate axes in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the present invention, and not all of the combinations of features described in the embodiments are necessarily essential to the solution of the present invention.

[0011] [First embodiment] (Image processing system configuration) 1 is a diagram showing the configuration of an image processing system 10 according to an embodiment. The image processing system 10 has an image processing device 100, which is connected to a data server 130 via a network 120. The image processing device 100 according to an embodiment is a device that can align two images captured at different times and generate a difference image from two images (a first medical image and a second medical image) that are three-dimensional images obtained by capturing images of a subject.

[0012] The data server 130 stores a plurality of medical images. The data server 130 represents, for example, a medical image management system (PACS: Picture Archiving and Communication Systems) that receives medical image data captured by a modality and stores and manages it via a network. In the following description, it is assumed that the data server 130 stores a plurality of 3D tomographic images obtained by capturing images of a subject in advance under different conditions (different modalities, imaging modes, dates and times, body positions, etc.) as first and second medical images. In this embodiment, it is assumed that the first and second medical images are 3D tomographic images (3D medical images) captured by an X-ray CT scanner, as examples of medical images.

[0013] FIG. 6 is a diagram illustrating the setting of coordinate axes in an embodiment. As shown in FIG. 6, the axis representing the direction from the right hand to the left hand of the subject 600 is defined as the X-axis, the axis representing the direction from the back to the front of the subject 600 is defined as the Y-axis, and the axis representing the direction from the head to the feet of the subject is defined as the Z-axis. The X-Y cross section is defined as the axial plane, the Y-Z cross section as the sagittal plane, and the Z-X cross section as the coronal plane. That is, the X-axis direction is a direction perpendicular to the sagittal plane (hereinafter referred to as the "sagittal direction"). The Y-axis direction is a direction perpendicular to the coronal plane (hereinafter referred to as the "coronal direction"). The Z-axis direction is a direction perpendicular to the axial plane (hereinafter referred to as the "axial direction"). In this case, in the case of a CT image, which is a three-dimensional image configured as a collection of two-dimensional tomographic images (slices), the slice plane of the image represents the axial plane, and the direction perpendicular to the slice plane (hereinafter referred to as the slice direction) represents the axial direction. The coordinate system shown in FIG. 6 is an example, and it is possible to set the coordinate system using other definitions.

[0014] The modality for capturing the 3D tomographic images may be, for example, an MRI apparatus, an X-ray CT apparatus, a 3D ultrasound apparatus, a photoacoustic tomography apparatus, a PET / SPECT apparatus, or an OCT apparatus. Furthermore, the first medical image and the second medical image may be any 3D tomographic images from which a subtraction image is generated. For example, they may be images captured simultaneously using different modalities or different imaging modes. Furthermore, they may be images captured of the same patient using the same modality in the same position on different dates and times for follow-up observation. The first medical image and the second medical image are 3D medical images (3D tomographic images) configured as a collection of 2D tomographic images. The positions and orientations of the 2D tomographic images are converted into a reference coordinate system (a coordinate system in space based on the subject) and stored in the data server 130. At this time, the first medical image and the second medical image expressed in the reference coordinate system are input to the image processing device 100 in response to an instruction from a user operating the instruction unit 140.

[0015] The image processing device 100 is a device that receives a processing request from a user via the instruction unit 140, performs image processing, and outputs the results of the image processing to the display unit 150, and functions as a terminal device for interpretation operated by a user such as a doctor. Specifically, based on an instruction from the user via the instruction unit 140, the image processing device 100 acquires a first medical image and a second medical image to be subjected to image processing from the data server 130 as a pair of images (image pair) to be subjected to image processing. The image processing device 100 then performs registration processing on the acquired first medical image and second medical image, generates a difference image, etc., and outputs it to the display unit 150.

[0016] The image processing device 100 is composed of the components described below. The functions of the following components are realized, for example, by one or more central processing units (CPUs) that function as control units of the image processing device 100 executing programs. The components of the image processing device 100 may be composed of integrated circuits or the like, as long as they perform similar functions.

[0017] The acquisition unit 101 functions as an interface that realizes communication between an external device (e.g., a data server 130, etc.) and the image processing device 100 via the network 120, and acquires information on a first medical image and a second medical image input to the image processing device 100 via the network 120. When a user instruction is input from the instruction unit 140, the acquisition unit 101 acquires, as an image pair, the first medical image and the second medical image to be subjected to image processing from the data server 130.

[0018] The conversion unit 102 converts the resolution of each of the first and second medical images to a predetermined resolution (hereinafter referred to as "processing resolution"). The alignment unit 103 performs alignment processing between the resolution-converted first and second medical images (hereinafter referred to as "first converted image" and "second converted image"), and obtains a displacement field that associates the positions of the images. Note that, here, the image resolution represents the pixel size in each of the X, Y, and Z axis directions.

[0019] The determining unit 104 determines the resolution in each of the X, Y, and Z axis directions of the difference image to be output (hereinafter referred to as "output resolution"). The determining unit 104 determines, as the slice interval of the difference image (i.e., the output resolution in the Z direction of the difference image), an interval such that slices corresponding to slices of either one of the first medical image and the second medical image serving as a reference are included in the difference image within the range of the difference image in the slice direction of the three-dimensional medical image, and the interval is equal to or less than the slice interval of either one of the first medical image and the second medical image serving as a reference.

[0020] The difference image generating unit 105 generates a difference image at the output resolution determined by the determining unit 104. Specifically, the difference image generating unit 105 generates a difference image between the first medical image and an image (hereinafter referred to as a "second deformed image") obtained by deforming the second medical image based on the displacement field acquired by the alignment unit 103 so that it matches the first medical image, based on the output resolution. Note that the image deformed based on the displacement field acquired by the alignment unit 103 is not limited to the second medical image, and the first medical image may also be deformed. For example, the difference image generating unit 105 can also generate a difference image between the second medical image and an image (first deformed image) obtained by deforming the first medical image based on the displacement field acquired by the alignment unit 103 so that it matches the second medical image, based on the output resolution.

[0021] The projection image generation unit 106 generates a projection image (hereinafter referred to as a "difference projection image") by two-dimensionally projecting pixel values ​​of the difference image. The display control unit 107 controls the output of cross-sectional images of the first medical image and the second medical image, the cross-sectional image of the generated difference image, the difference projection image, etc. to the display unit 150.

[0022] The display unit 150 is configured with any device such as an LCD or a CRT, and displays medical images and the like for a doctor to interpret. Specifically, it displays cross-sectional images of the first medical image and the second medical image acquired from the image processing device 100. It also displays cross-sectional images of the difference image generated by the image processing device 100 and projection images (difference projection images). Here, the display unit 150 is provided with a GUI for receiving instructions from a user such as a doctor. The user can freely switch the displayed cross-sectional images of the first medical image and the second medical image to cross-sectional images of the difference image or difference projection images using the GUI. Furthermore, based on instructions from the GUI, the display control unit 107 can also control the display unit 150 to combine and display each image.

[0023] FIG. 2 is a flowchart showing the overall processing procedure performed by the image processing device 100.

[0024] (S201: Get image) In step S201, the acquisition unit 101 acquires a first medical image and a second medical image specified by the user via the instruction unit 140 from the data server 130. The acquisition unit 101 then acquires the first medical image and the second medical image as a pair of images to be subjected to image processing and outputs them to the conversion unit 102, the determination unit 104, the subtraction image generation unit 105, and the display control unit 107. The first medical image and the second medical image automatically acquired based on a predetermined rule may be reviewed by the user and finally acquired. An example of the process of automatically acquiring images based on a predetermined rule is to automatically acquire the first medical image as the most recent examination image of the subject under examination, and the second medical image as the second most recent examination image of the same subject. The automatic acquisition method is not limited to this, and the second medical image may be automatically acquired as the oldest examination image of the same subject, for example. Alternatively, the configuration may be such that the user specifies only the first medical image, and the second medical image is automatically acquired based on a predetermined rule in the same manner as above.

[0025] (S202: Convert image resolution) In step S202, the conversion unit 102 converts the resolution of the first medical image and the second medical image acquired from the acquisition unit 101 to a predetermined processing resolution, thereby acquiring a first converted image and a second converted image. For example, when the resolution (pixel size) of the first medical image and the second medical image, which are the original images of the difference image, is anisotropic, the conversion unit 102 acquires the first converted image and the second converted image in which the resolution is isotropic so that the resolution in each axial direction is equal, in order to perform high-precision image-to-image registration in subsequent processing. More specifically, for example, when the first medical image and the second medical image are CT images, the resolution in the slice plane (e.g., the sagittal direction and the coronal direction) is higher than the resolution in the slice direction (axial direction). Therefore, the conversion unit 102 performs a process of upsampling pixels in the slice direction to match the resolution in the slice plane. Note that a known image processing method can be used to interpolate density values ​​during resolution conversion.

[0026] In this embodiment, the processing resolution is set to, for example, 1 mm in each three-dimensional axis direction so that differences in details between the images can be calculated, and first and second converted images with isotropic resolution are acquired by resolution conversion. The processing resolution does not have to be 1 mm as long as it is sufficient to calculate differences in details between the images. Furthermore, isotropy is not necessary; it is sufficient to convert the resolution to a level that allows for highly accurate alignment between images in subsequent processing. Furthermore, resolution conversion is not necessarily required when the resolutions of the first and second medical images acquired in step S201 are already the processing resolution, or when resolution conversion processing is not required. The conversion unit 102 then outputs the generated first and second converted images to the alignment unit 103.

[0027] (S203: Align two images) In step S203, the registration unit 103 performs registration so that pixels representing the same part between the first converted image and the second converted image substantially coincide with each other, and obtains a displacement field that associates the positions between the images. Then, information on the displacement field that is the result of the registration is output to the difference image generation unit 105.

[0028] In this embodiment, image registration refers to the process of calculating a displacement field for displacing each pixel position in one image to the corresponding pixel position in the other image. For example, when two images are captured at different times, the displacement field from one reference image to the other can be calculated by estimating the corresponding pixel position in the other image for each pixel position in the reference image. In this embodiment, when a first medical image is used as the reference, the pixel positions in the second converted image corresponding to each pixel position in the first converted image are determined to obtain displacement vectors at each pixel position. In other words, when the first medical image is used as the reference, the generated displacement field is an image that stores displacement vectors from each pixel position in the first converted image to the corresponding position in the second converted image, and has the same resolution and number of pixels as the first converted image used as the reference. Therefore, if the processing resolution is, for example, 1 mm isotropic, the displacement field will also be a displacement vector field isotropized at 1 mm. This allows the displacement field to retain enough information to calculate detailed differences between images.

[0029] In this embodiment, the registration unit 103 can perform registration using a known image processing method. For example, registration can be performed by calculating a displacement field by deforming one image so as to increase the image similarity between the images. As the image similarity, known methods such as the commonly used Sum of Squared Difference (SSD), mutual information, and cross-correlation coefficient can be used. Furthermore, as an image deformation model, known deformation models such as a deformation model based on a radial basis function such as Thin Plate Spline (TPS), Free Form Deformation (FFD), and Large Deformation Diffeomorphic Metric Mapping (LDDMM) can be used.

[0030] (S204: Determine output resolution) In step S204, the determination unit 104 determines the output resolution based on the resolution of the first medical image and the processing resolution used in step S202. Then, the value of the determined output resolution is output to the difference image generation unit 105.

[0031] Here, the process of determining the output resolution by the determination unit 104 will be described in detail. A common method for generating a difference image between two images (a first medical image and a second medical image) is to generate a difference image in such a way that the positions of a medical image (the first medical image in this embodiment) that serves as a reference for alignment and the difference image correspond to each other for each pixel. In this case, the difference image is generated at the same resolution as the medical image (the first medical image in this embodiment) that serves as a reference for alignment. Furthermore, within the range in which the difference image is generated, the difference image is generated with the same number of pixels as the image (the first medical image) that serves as a reference for alignment. This allows, for example, when it is desired to confirm the positions of a two-dimensional tomographic image included in the first medical image and a two-dimensional tomographic image included in the difference image in association with each other, the two-dimensional tomographic image can be easily displayed in association with each other.

[0032] However, when generating and displaying a difference projection image or volume rendering image obtained by two-dimensionally projecting a difference image in a direction parallel to the slice plane, or a cross-sectional image obtained by cutting the difference image at an arbitrary cross section, including a sagittal or coronal plane, different from the slice plane (axial plane), (hereinafter, referred to as a difference projection image, etc.), the following problem may arise. That is, if the slice spacing of the reference first medical image is coarse, the slice spacing of the difference image also becomes coarse. As a result, although the cross-sectional image of the slice plane of the difference image displayed on a display unit has sufficient resolution, when the difference projection image, etc., is displayed on a display unit, the resolution is displayed at a coarse resolution in the slice direction. This problem arises because, when three-dimensional tomographic images are acquired using various modalities, the resolution is often not consistent in all axial directions. For example, in the case of an X-ray CT scanner, the resolution in the slice direction (axial direction) (slice spacing) is often coarser than the resolution in the slice plane direction (e.g., the sagittal direction or coronal direction) (intra-slice resolution) in order to reduce radiation exposure and data volume.

[0033] Therefore, in this embodiment, the resolution in the slice plane (in the XY plane) of the output resolution is matched to the resolution in the slice plane of the first medical image. Then, the slice interval (value in the Z direction) of the output resolution is determined so that slices corresponding to the slices of the first medical image are included in the difference image and are not coarser than the set predetermined resolution. That is, the determination unit 104 corresponds to an example of a determination means for determining, as the slice interval of the difference image, an interval that allows slices corresponding to slices of either one of the first medical image and the second medical image serving as a reference to be included in the difference image and is equal to or less than the slice interval of either one of the first medical image and the second medical image serving as a reference. In this case, the predetermined resolution is a resolution at which differences in the details between images can be calculated, and is set as the upper limit of the Z direction value (slice interval) of the output resolution (hereinafter referred to as the "upper limit resolution"). A specific setting method is described below.

[0034] In this embodiment, the determination unit 104 sets the value of the output resolution in the Z direction to a value that satisfies the following two conditions.

[0035] <Condition 1> The slice interval of the first medical image must be divided by a natural number. <Condition 2> The maximum value below the upper limit resolution Here, the Z-axis value of the output resolution is s z , the slice interval of the first medical image is s1, a natural number is n, and the upper limit resolution is s MAX Then, the Z-axis value of the output resolution is s Z Regarding the above, the following formula (1A) holds. According to formula (1A), the determining unit 104 obtains the slice interval (s1) of the reference medical image (in this embodiment, the first medical image) divided by a natural number (n) as the slice interval of the difference image. In addition, the determining unit 104 obtains the slice interval (s2) of the difference image based on the set resolution (upper limit resolution s MAX ) is set as the upper limit value to determine the slice interval of the difference image.

[0036] s Z = s1 / n ≦ s MAX (1A) Furthermore, from equation (1A), the following equation (1B) holds.

[0037] n ≧ s1 / s MAX (1B) Here, the Z-axis value of the output resolution s Z To maximize the value that satisfies the formula (1A), the natural number n must be the minimum value that satisfies the formula (1B). Therefore, the slice interval s1 and the upper limit resolution s of the first medical image are MAX If we know the natural number n, we can calculate it. For example, s1=3.5mm, s MAX = 1.0 mm (= processing resolution). Here, the upper limit resolution s MAX The reason for setting the processing resolution as s will be explained later. In this case, equation (1B) becomes n ≥ 3.5, and the smallest natural number n = 4 that satisfies equation (1B) can be found. Therefore, from equation (1A), the value of the output resolution in the Z direction is s Z =3.5 / 4=0.875mm.

[0038] FIG. 3 is a diagram for explaining the Z-direction value of the output resolution (slice interval of the difference image). In FIG. 3, a three-dimensional tomographic image 300 represents a difference image. The interval 301 between the thick lines in the Z direction represents the slice interval of the first medical image, and the interval 302 between the dotted lines represents the Z-direction value s of the output resolution. Z As described above, when the interval 301 between the thick lines is s1=3.5 mm, the interval 302 between the dotted lines is 0.875 mm, which is the value obtained by dividing it by n=4. In this case, in the slice direction (Z-axis direction in FIG. 3) of the difference image (3D tomographic image 300), the slice interval at the position of the interval 301 between the thick lines is the same as that of the first medical image, that is, the slice corresponding to the slice of the first medical image is included in the difference image. At the positions of the interval 302 between the dotted lines other than the interval 301 between the thick lines, the upper limit resolution s MAX Compared to the above, the slice interval is finer (the Z-direction value of the output resolution s Z ) is set.

[0039] In this way, since the slices corresponding to the slices of the reference medical image (the first medical image in this embodiment) are included in the subtraction image, when displaying the cross section, it becomes possible to easily associate the first medical image and the subtraction image in slice units and observe them. Furthermore, the slice interval of the subtraction image is limited to the upper limit value (upper limit resolution s MAX ), so even if the slice interval of the original image is coarse, the resolution in the slice direction is fine (the Z-direction value of the output resolution s Z ) can be generated, resulting in a highly visible image (such as a differential projection image).

[0040] Here, the value of the processing resolution can be used as the upper limit resolution. In other words, the upper limit of the Z-direction value of the output resolution is set as the processing resolution. By matching the upper limit of the Z-direction value of the output resolution (upper limit resolution) with the processing resolution, as described in step S203, the upper limit resolution also matches the resolution of the displacement field obtained by the registration process. As described in step S203, the displacement field is generated at a resolution (e.g., 1 mm) that allows calculation of the fine differences between images. Therefore, the upper limit of the Z-direction value of the output resolution is not simply set so that the resolution in the body axis direction of the difference projection image does not become coarse, but is also set as a value that allows the amount of information to represent the fine differences between images.

[0041] The value used as the upper limit resolution is merely an example and is not limited to the above example. That is, the upper limit resolution does not have to be the same as the processing resolution, and the upper limit resolution may be a value obtained by multiplying the processing resolution by a predetermined constant. For example, the predetermined constant may be 1.5. Here, the predetermined constant can be empirically set to a value that displays a differential projection image, etc., with sufficient resolution. Setting the predetermined constant to 1 or greater can speed up the generation of differential images. Alternatively, a predetermined constant may be used as the upper limit resolution. Preferably, the upper limit resolution may be set to a predetermined resolution in the range of 0.5 mm or more and 2.0 mm or less, which is a resolution that is not unnecessarily fine but can depict sufficiently fine differential values.

[0042] The Z-axis value of the output resolution s Z In the calculation of <Condition 1>, a predetermined resolution is set as an upper limit value and the resolution is always set to a value equal to or less than that value, but this setting is not necessary. That is, the predetermined resolution may be set as a reference value (hereinafter referred to as "reference resolution"), and a value that satisfies a predetermined condition for the reference resolution may be set as the Z-direction resolution. That is, the above upper limit resolution is an example of the reference resolution, and <Condition 2> is an example of a predetermined condition. For example, the value closest to the reference resolution may be set as the Z-direction resolution. That is, as a condition to be combined with <Condition 1>, it is also possible to set a value that satisfies the following <Condition 2'> instead of the above <Condition 2>.

[0043] <Condition 2'> The value must be closest to the reference resolution. In this case, the reference resolution is s B Then, the following equations (2) to (5) hold for <Condition 2'>. According to the equations (2) to (5), the determination unit 104 determines the set resolution as the reference resolution (s B ) and the value near the reference resolution (d MIN The determining unit 104 determines the slice interval of the difference image based on the first natural number (n - ) divided by (s1 / n - ) and reference resolution (s B The first value (d - ) and the interval between slices of the reference medical image is the second natural number (n + ) divided by (s1 / n + ) and reference resolution (s B ) and the second value (d + ) and the smaller absolute value is selected as the neighboring value (d MIN ), where the first natural number (n - ) is the first natural number (n - ) is the natural number that gives the maximum value below the reference resolution. Also, the second natural number (n + ) is the interval between slices of the reference medical image, expressed as the second natural number (n + ) is a natural number that gives the smallest value above the reference resolution.

[0044] s Z =s B +d MIN ···(2) d MIN ={d - (if |d - |≦|d + |) d + (if |d - |>|d + |)}···(3) d - =s1 / n- -s B ···(4) d + =s1 / n + -s B ···(5) Here, in equation (4), n - is the natural number (first natural number) that gives the maximum value below the reference resolution when s1 is divided by itself, and in equation (5), n + is the natural number (the second natural number) that gives the smallest value above the reference resolution when s1 is divided by itself. And, d - is the difference between the maximum value below the reference resolution and the reference resolution, d + represents the difference between the minimum value equal to or greater than the reference resolution and the reference resolution. MIN is d - Absolute value of and d + The smaller absolute value is set by comparing with the absolute value of

[0045] For example, s1=2.25mm, s B When = 1.0 mm, from equations (2) to (5), n - =3, n + =2, d - =0.25, d + =0.125. In this case, d in equation (3) MIN The value is d - Absolute value of and d + The smaller absolute value of d MIN =0.125. The standard resolution is s B = 1.0 mm, the Z-axis value of the output resolution (s Z ) is calculated from equation (2) as s Z =s B +d MIN =1.0+0.125=1.125.

[0046] On the other hand, if the parameters are set to satisfy the above-mentioned <Condition 1> and <Condition 2>, then in this case, equation (1B) becomes n≧2.25, and the smallest natural number n=3 that satisfies equation (1B) is found. Therefore, from equation (1A), the value of the output resolution in the Z direction is s Z= 2.25 / 3 = 0.75 mm. The Z-direction value of the output resolution (s Z ) is s Z = 0.75. In this case, the reference resolution s B The difference with (=1.0) is 0.25, and the standard resolution s B When using <Condition 2>, the difference between the standard resolution s B The more distant value is set.

[0047] In this way, by treating the specified resolution as a reference value and setting a value closest to the reference value, it is possible to set a value closer to the specified resolution, compared to when the specified resolution is treated as the upper limit of the resolution in the Z direction.

[0048] This is useful in the following cases, for example. As mentioned above, the amount of information contained in the displacement field due to the alignment is, for example, the amount of information at the processing resolution of 1 mm isotropic resolution. Therefore, when <Condition 2> is used, the Z-direction value of the output resolution (s Z ) does not exceed the processing resolution, so even if the slice interval of the original image is coarse, it will not be set to a coarse value. However, in some cases, as in the case described above, it may be set to a fine value (0.75 mm) far from 1 mm, which is finer than the amount of information contained in the displacement field. In such cases, by using <Condition 2'> to determine the slice interval of the difference image to a value close to the set resolution (reference resolution), that is, by setting the value closest to the processing resolution as the slice interval (value of output resolution) of the difference image, the output resolution value (s) will be set to a value (1.125 mm) closer to the processing resolution, although it is coarser than the amount of information contained in the displacement field. Z ) can be set.

[0049] (S205: Generate a difference image) In step S205, the difference image generation unit 105 performs difference processing at the output resolution determined in step S204 between the first medical image and the second medical image to generate a difference image at the output resolution. The generated difference image is then output to the projection image generation unit 106 and the display control unit 107. A specific method for generating a difference image will be described below.

[0050] Generally, the generation of a difference image based on the subtraction process between a first medical image and a second medical image is performed by the following method. That is, based on the displacement field acquired in step S203, the difference image generation unit 105 calculates, for each pixel on the first medical image, the position (coordinate) of the pixel on the second medical image that corresponds to the position (coordinate) of the pixel. Then, for each pixel on the first medical image, the difference value between the pixel value and the position (coordinate) of the corresponding pixel on the second medical image is calculated to generate a difference image. As a result, the position (coordinate) of each pixel in the difference image corresponds to that of the first medical image, so the difference image can be acquired at the same resolution as the first medical image.

[0051] In the subtraction processing of this embodiment, since the output resolution of the subtraction image does not necessarily match the resolution of the first medical image, the subtraction image generating unit 105 first converts the resolution of the first medical image to the output resolution, and then performs the same subtraction processing as above to generate a subtraction image. After converting the resolution of the first medical image described in the subtraction processing method above to the output resolution, the subtraction image generating unit 105 performs processing to generate a subtraction image based on the first medical image after resolution conversion.

[0052] That is, for each pixel on the first medical image after resolution conversion, the difference image generating unit 105 calculates the position (coordinates) of the pixel on the second medical image that corresponds to the position (coordinates) of that pixel. Then, for each pixel on the first medical image after resolution conversion, the difference image is generated by calculating the difference in pixel value between the position (coordinates) of the corresponding pixel on the second medical image. As a result, the position (coordinates) of each pixel in the difference image corresponds to the first medical image after conversion to the output resolution, so the difference image can be acquired at the output resolution.

[0053] The method for generating a difference image in this embodiment is not limited to this. For example, a method may be used in which the first medical image is not first converted to the output resolution. Specifically, an empty area for a difference image having the output resolution is first prepared. Next, for each pixel position (coordinate) in that area, a difference value of pixel values ​​may be obtained between the position (coordinate) on the first medical image to which the position is originally associated and the position (coordinate) on the second medical image to which the position is associated using a displacement field, thereby generating a difference image.

[0054] As described in Fig. 3, the difference image generated at the output resolution includes slices corresponding to the slices of the first medical image at the same slice intervals as the first medical image, represented by the intervals 301 between the thick lines. Therefore, by extracting slices corresponding to the slices of the first medical image from the difference image, it is possible to display the slices of the first medical image and the slices of the difference image in correspondence with each other when displaying in step S208, which will be described later. This can be achieved, for example, by sequentially acquiring slices of the difference image at the same slice intervals as the first medical image.

[0055] It is also possible to display the slices in correspondence with each other by associating the first medical image and the subtraction image with each other in a coordinate system of physical space, rather than in units of slices.

[0056] Furthermore, the method of associating slices of the first medical image with slices of the difference image is not limited to the above method. For example, it is possible to display slices of both images in a corresponding manner by adding, to the difference image, correspondence information such as "which slice in the first medical image does the slice in the difference image correspond to?" for the slice of the difference image corresponding to the slice of the first medical image. That is, the difference image generating unit 105 adds, to the difference image, correspondence information that associates the slice of the first medical image with the slice of the difference image. Note that, for example, when the slice spacing of the difference image is shorter than the slice spacing of the first medical image, there may be no slice of the first medical image that corresponds to a slice of the difference image. In such a case, correspondence information need not be added to a difference image for which no corresponding slice exists, or the closest slice of the first medical image may be added as correspondence information.

[0057] Fig. 4 is a diagram illustrating axial cross sections of original images (first medical image, second medical image) and the difference image. Fig. 4 shows an axial cross section 400 of the first medical image, an axial cross section 401 of the second medical image, and an axial cross section 402 of the difference image. Furthermore, an area 403 on the axial cross section 400 of the first medical image represents an abnormal area, and an area 404 on the axial cross section 402 of the difference image represents a difference area corresponding to the area 403 (abnormal area).

[0058] The region 403 (abnormal region) is present in the first medical image, but the region of the second medical image corresponding to the region 403 (abnormal region) does not contain an abnormal region. The difference region corresponding to the region 403 (abnormal region) is depicted as a difference in pixel values ​​between corresponding regions in the first and second medical images. The difference image and the first medical image are associated on a slice-by-slice basis. Therefore, by specifying the original region 403 (abnormal region) on the axial cross section 400 of the first medical image, the position of the corresponding region 404 (difference region) can be identified. This facilitates displaying corresponding positions between images, such as by displaying the axial cross section 402 of the difference image including the identified region 404 (difference region).

[0059] Here, the difference image is generated only in the area common between the first and second medical images as a result of aligning the two, so in reality, there are areas of the first medical image that are outside the range of the difference image. In other words, strictly speaking, the difference image and the first medical image correspond pixel by pixel only within the range of the difference image. Therefore, it goes without saying that the fusion of the above-mentioned images and the display of corresponding positions between the images can be easily performed within the range of the difference image.

[0060] In this embodiment, the generated difference image is stored in a storage unit (not shown). As a result, when it is desired to acquire the difference image again after the processing of the image processing device 100 is completed, the stored difference image can be read from the storage unit to easily acquire the difference image. However, it is not necessary to store the generated difference image in a storage unit (not shown).

[0061] (S206: Generate a differential projection image) In step S206, the projection image generation unit 106 generates a difference projection image by two-dimensionally projecting the pixel values ​​of the difference image generated in step S205, and outputs the generated difference projection image to the display control unit 107.

[0062] More specifically, the projection image generating unit 106 generates a differential projection image by projecting the second differential image, which is a three-dimensional image, parallel to a slice plane (for example, the xy plane in FIG. 6). When the original image of the differential image is an X-ray CT image, the slice direction coincides with the body axis direction (Z axis), and therefore the differential projection image is an image projected in a direction (for example, the y direction in FIG. 6) perpendicular to the body axis direction (Z axis) of the subject 600 (FIG. 6). For example, a differential projection image projected in the front direction (coronal direction) of the subject, which is the direction perpendicular to the body axis direction (Z axis), is generated. This makes it possible to easily grasp the information of the differential image of the entire imaging region of the subject.

[0063] The projection image generating unit 106 can generate a difference projection image by, for example, calculating the average of the maximum and minimum pixel values ​​in the projection direction. Hereinafter, this method is referred to as an MIP / MinIP (maximum intensity projection / minimum intensity projection) image. This allows values ​​that take into account both positive and negative difference values ​​in the difference image to be reflected in the projection image. The projection method for generating the difference projection image is not limited to this method, and other methods such as maximum intensity projection (MIP) and minimum intensity projection (MinIP) can also be used. In this case, the resolution of the difference projection image in the slice direction is the output resolution determined in step S204.

[0064] 5 is a diagram illustrating an example of a coronal cross-sectional image and a differential projection image of an original image. Fig. 5 shows a coronal cross-sectional image 500 of the original image (first medical image) and a differential projection image 501. Furthermore, an area 502 on the coronal cross-sectional image 500 of the first medical image represents an abnormal area, and an area 503 on the differential projection image 501 represents a differential projection area onto which the differential area corresponding to the area 502 (abnormal area) is projected.

[0065] Region 503 (difference projection region) corresponds to the region obtained by projecting the three-dimensional difference region including region 404 (difference region) on the slice (axial section 402 of the difference image) in Figure 4 in the coronal direction (e.g., the y direction in Figure 6).

[0066] In FIG. 5, the slice interval of the original image (first medical image) is coarse (for example, 5 mm), so the resolution of the coronal cross-sectional image 500 in the slice direction (Z-axis direction in FIG. 5) is coarse.

[0067] However, the difference projection image 501 does not become coarse in the slice direction because its resolution in the slice direction is the upper limit resolution (= processing resolution, for example, 1 mm) and it is generated as an image that retains the amount of differential information in the details of the three-dimensional image. That is, although the display of the region 502 (abnormal region) on the coronal cross-sectional image 500 becomes coarse because the slice interval of the original image (first medical image) is coarse, the region 503 (difference projection region) on the difference projection image 501 retains the amount of differential information in the details of the three-dimensional image and is therefore depicted in detail.

[0068] In the above example, a differential projection image is generated by projecting the image in the direction perpendicular to the body axis (Z axis) toward the front of the subject (e.g., the y direction in FIG. 6). However, the present invention is not limited to this example. It is also possible to generate multiple projection images by uniformly changing the projection direction so as to rotate around the body axis (e.g., the z axis in FIG. 6), including this direction. It is also possible to generate (reconstruct) data by combining a set of these projection images. For simplicity, the data of the set of individual projection images will also be referred to as a differential projection image. For example, the projection image generation unit 106 generates, as differential projection images, a set of data consisting of a total of 36 projection images divided around the body axis at 10° intervals. The method for generating a differential projection image is not limited to this method, and a process of projecting pixel values ​​of the differential image from any direction may also be used.

[0069] When the difference projection image is to be observed again after the processing of the image processing device 100 is completed, the projection image generation unit 106 reads the saved difference image from the storage unit and then performs the processing of this step alone to generate the difference projection image, thereby making it easy to observe the difference projection image. Therefore, in this embodiment, it is not necessary to save the generated difference projection image in a storage unit (not shown). Since it is sufficient that at least the difference image is stored in the storage unit, further saving the difference projection image in the storage unit can prevent an increase in the storage capacity of the storage unit. In this case, it is sufficient to store the difference images collectively in the storage unit, which allows for efficient use of the storage capacity of the storage unit and simplifies data management. The generated difference projection image may also be saved in a storage unit (not shown). In this case, the processing time required to generate the difference projection image can be reduced when the difference projection image is to be observed again.

[0070] (S207: Show image) In step S207, the display control unit 107 controls the display unit 150 to display the cross-sectional image of the difference image acquired from the difference image generation unit 105 and the difference projection image acquired from the projection image generation unit 106. The display control unit 107 also controls the display unit 150 to display the cross-sectional images of the first medical image and the second medical image acquired from the acquisition unit 101. In this way, the processing of the image processing device 100 is performed.

[0071] According to this embodiment, the difference image can be displayed as a cross section in the same slice as the original image, and when generating a projection image by projecting pixel values ​​of the difference image parallel to the slice plane, it is possible to generate a projection image with fine resolution in the slice direction even if the slice interval of the original image is coarse. As a result, when generating a projection image by projecting pixel values ​​of the difference image parallel to the slice plane as in Non-Patent Document 1, it is possible to generate a projection image with fine resolution in the slice direction even if the slice interval of the original image is coarse.

[0072] According to this embodiment, it is possible to generate a difference image with high visibility, and the difference image can be displayed cross-sectionally in the same slice as the original image, and even when the pixel values ​​of the difference image are projected parallel to the slice plane, a projection image with high visibility can be displayed.

[0073] (Variation 1) In the first embodiment, when a subtraction image is displayed, a slice corresponding to the first medical image is extracted from the subtraction image and displayed. However, it is not necessary to associate the slices of the first medical image with the slices of the subtraction image during display. For example, before displaying the subtraction image in step S207, a subtraction image (hereinafter referred to as a "derived subtraction image") may be generated by extracting only the slices corresponding to the first medical image. In this case, since all slices of the first medical image and the slices of the derived subtraction image correspond to each other within the range of the subtraction image, it is not necessary to extract slices from the subtraction image every time the subtraction image is displayed in step S207. Furthermore, since all slices of the first medical image and the derived subtraction image correspond to each other within the range of the subtraction image, that is, the images correspond to each other on a pixel-by-pixel basis, fusion of the images can be easily performed. In this way, by generating a derived subtraction image, an image more suitable for cross-sectional display can be obtained.

[0074] In this modification, when a derived difference image for cross-sectional display is acquired after the image processing device 100 completes processing, the display control unit 107 reads the difference image saved in step S205 from the storage unit and then performs the processing of this step alone to generate a derived difference image, thereby easily acquiring the derived difference image for cross-sectional display. Therefore, storing the derived difference image generated in this modification in a storage unit (not shown) is not a necessary process. Since it is sufficient for the storage unit to store at least the difference image generated in step S205, additionally storing the derived difference image for cross-sectional display in the storage unit can prevent an increase in the storage capacity of the storage unit. In this case, it is sufficient to store the difference images collectively in the storage unit, which allows for efficient use of the storage capacity of the storage unit and simplifies data management. The generated derived difference image may also be stored in a storage unit (not shown), which reduces the processing time required to generate the derived difference image.

[0075] (Variation 2) In the first embodiment and Modification 1, only the difference image generated at the output resolution is stored in a storage unit (not shown), and images derived from the difference image processed for display, such as the derived difference image or the difference projection image, are not stored. However, the storage method is not necessarily limited to this. For example, only images processed for display, such as the derived difference image or the difference projection image, may be stored, and the difference image generated at the output resolution may not be stored. As a result, when a derived difference image or a difference projection image is desired for cross-sectional display or projection display after the processing of the image processing device 100 is completed, the required image can be obtained simply by reading the stored derived difference image or the difference projection image from the storage unit without performing the processing of Modification 1 or the processing of step S206. Furthermore, although the stored data is divided into two, if the original first medical image is coarse, the derived difference image to be stored will also be coarse. Therefore, the storage capacity of the storage unit can be reduced compared to when the difference image generated at the output resolution, which is a high resolution, is stored. Furthermore, the difference image generated at the output resolution and the derived difference image and the difference projection image may be stored in different destinations. The image processing device 100 further includes a storage unit that stores the difference image and the derived difference image in different storage units. The storage unit may store the difference image in a storage unit included in the image processing device 100, and may store the derived difference image in a storage unit included in a data server connected to the image processing device 100 via a network. More specifically, the difference image generated at the output resolution may be stored in a storage unit (not shown) in the image processing device 100, and the derived difference image and the difference projection image may be stored in a storage unit (not shown) in a data server 130 such as a PACS.

[0076] As a result, the image processing device 100 stores the difference images that are the basis for the derived difference images and the difference projection images. Therefore, based on the difference images, it is possible to, for example, check slices that are not included in the generated derived difference images, or generate and view a new difference projection image using parameters different from those of the generated difference projection image. Meanwhile, a data server 130 such as a PACS typically stores a large number of images and has limited storage capacity. Therefore, by storing only the derived difference images and the difference projection images, it is possible to reduce storage capacity while retaining the minimum information required to view the difference images.

[0077] (Variation 3) In the first embodiment, the upper limit (upper limit resolution) when determining the value of the output resolution in the slice direction is equal to the processing resolution. However, the upper limit of the output resolution does not necessarily have to be equal to the processing resolution. For example, the processing resolution may be a value close to the upper limit of the output resolution. In this modification, the close value can be defined as a value within ±0.5 mm of the target resolution. In other words, it is a value based on and including the upper limit resolution. For example, in the output resolution determination process in step S204, the upper limit of the output resolution is set to 1 mm, as described in step S202, at which the difference in details between images can be calculated. Meanwhile, in step S202, the processing resolution for resolution conversion is set to 1.5 mm, a value close to 1 mm.

[0078] Then, in the registration process between the two images in step S203, the first converted image with resolution converted to 1.5 mm and the second converted image are registered. By performing registration at a resolution close to that when the resolution is converted to 1 mm, registration can be performed quickly without reducing the registration accuracy as much as possible (without reducing the amount of information of the displacement field as much as possible).

[0079] Then, in the subtraction image generation process in step S205, a displacement field with a resolution of 1.5 mm is used to generate a subtraction image with an upper limit of output resolution set to 1 mm, and in step S206, a subtraction projection image is generated based on that subtraction image. Since the displacement field used to generate the subtraction image does not lose much information compared to the case of 1 mm, the subtraction projection image can be generated without losing much information in the slice direction. In this way, as long as the processing resolution is coarser than the upper limit of the output resolution but is a nearby value, the subtraction projection image can be output at high speed with as little loss in quality as possible compared to the subtraction projection image generated in the first embodiment.

[0080] (Other embodiments) The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0081] The present invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0082] 101 Acquisition Department 102 Conversion unit 103 Alignment section 104 Decision Section 105 Differential image generation unit 106 Projection image generation unit 107 Display control unit

Claims

1. 1. An image processing device that generates a difference image, which is a three-dimensional image for displaying a projection image on a display unit, from a first medical image, which is a three-dimensional image configured from a plurality of slices obtained by imaging a subject, and a second medical image, an acquisition means for acquiring the first medical image and the second medical image; a conversion means for converting the resolution of at least one of the first medical image and the second medical image to a predetermined resolution so that differences in details between the first medical image and the second medical image can be calculated; a generating means for generating the subtraction image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; a determination means for determining the predetermined slice interval of the difference image generated by the generation means, the determination means determining the predetermined slice interval based on a value close to a reference resolution set to the predetermined resolution so that slices corresponding to slices of one of the first and second medical images serving as a reference are included in the difference image and the slice interval of the slices in the difference image is finer than the slice interval of the one of the medical images; An image processing device comprising:

2. The determining means a first value obtained based on a difference between a value obtained by dividing a slice interval between slices of the one medical image by a first natural number and the reference resolution; The image processing device according to claim 1, characterized in that by comparing a value obtained by dividing the slice spacing of the slices of the one medical image by a second natural number with a second value obtained based on the difference from the reference resolution, either the value obtained by dividing the slice spacing of the slices of the one medical image by the first natural number or the value obtained by dividing the slice spacing by the second natural number, which corresponds to the smaller absolute value of the first value or the second value, is set as the neighboring value.

3. the first natural number is a natural number that gives a maximum value when a slice interval between slices of the one medical image is divided by the first natural number and is equal to or less than the reference resolution, 3. The image processing device according to claim 2, wherein the second natural number is a natural number that gives the smallest value when the slice spacing of the one medical image is divided by the second natural number and is equal to or greater than the reference resolution.

4. 4. The image processing device according to claim 1, wherein the generation means converts the reference medical image to a resolution based on the slice interval, and performs processing to generate the difference image based on the reference medical image after resolution conversion.

5. 5. The image processing apparatus according to claim 1, wherein the generating means adds, to the difference image, correspondence information that associates a slice of the reference medical image with a slice of the difference image.

6. 2. The image processing apparatus according to claim 1, wherein said generating means generates at least one slice corresponding to a slice of said one medical image from said difference image as a derived difference image.

7. a registration unit that performs a registration process between the first medical image and the second medical image and obtains a displacement field that associates positions between the images; 7. The image processing device according to claim 1, wherein the generating means generates a difference image between an image obtained by deforming the second medical image based on the displacement field so as to match the first medical image and the first medical image.

8. a registration unit that performs a registration process between the first medical image and the second medical image and obtains a displacement field that associates positions between the images; 2. The image processing apparatus according to claim 1, wherein the alignment means aligns the images based on the resolution-converted first medical image and the resolution-converted second medical image.

9. 7. The image processing apparatus according to claim 6, further comprising a projection image generating means for generating a projection image by two-dimensionally projecting pixel values ​​of the difference image in a direction parallel to the slice plane.

10. 10. The image processing apparatus according to claim 9, further comprising a display control unit that causes at least one of the difference image, the derived difference image, and the projected image to be displayed on a display unit.

11. 7. The image processing apparatus according to claim 6, further comprising a storage unit for storing the difference image and the derived difference image in different storage units.

12. The image processing device according to claim 11, characterized in that the storage means stores the difference image in a storage means provided in the image processing device, and stores the derived difference image in a storage means provided in a data server connected to the image processing device via a network.

13. 10. An image processing system comprising: the image processing device according to claim 1; an instruction unit that receives input of instructions from an operator operating the image processing device and inputs the instructions to the image processing device; and a display unit that displays the generated difference image.

14. 1. An image processing method of an image processing device for generating a difference image, which is a three-dimensional image for displaying a projection image on a display unit, from a first medical image, which is a three-dimensional image configured from a plurality of slices obtained by imaging a subject, and a second medical image, the method comprising: an acquisition step of acquiring the first medical image and the second medical image; a conversion step of converting the resolution of at least one of the first medical image and the second medical image to a predetermined resolution so that differences in details between the first medical image and the second medical image can be calculated; generating the difference image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; a determining step of determining the predetermined slice interval of the difference image generated by the generating step, the determining step determining the predetermined slice interval based on a value close to a reference resolution set to the predetermined resolution so that slices corresponding to slices of one of the first and second medical images serving as a reference are included in the difference image, and the slice interval of the slices in the difference image is finer than the slice interval of the one of the medical images; An image processing method comprising:

15. 1. An image processing device that generates a difference image, which is a three-dimensional image for displaying a projection image on a display unit, from a first medical image, which is a three-dimensional image configured from a plurality of slices obtained by imaging a subject, and a second medical image, an acquisition means for acquiring the first medical image and the second medical image; a conversion means for converting the resolution of at least one of the first medical image and the second medical image to a predetermined resolution so that differences in details between the first medical image and the second medical image can be calculated; a generating means for generating the subtraction image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; a determination means for determining the predetermined slice interval of the difference image generated by the generation means, the determination means determining the predetermined slice interval in the difference image based on a value close to a reference resolution set to the predetermined resolution so that slices corresponding to slices of one of the first and second medical images serving as a reference are included in the difference image and the slice interval of the slices in the difference image is finer than the slice interval of the one of the medical images; Equipped with the determining means compares a first value obtained based on the difference between the value obtained by dividing the slice spacing of the slices of the one medical image by a first natural number and the standard resolution with a second value obtained based on the difference between the value obtained by dividing the slice spacing of the slices of the one medical image by a second natural number and the standard resolution, and sets either the value obtained by dividing the first value by the first natural number or the value obtained by dividing the second value by the smaller of the absolute values ​​of the first value and the second value as the nearby value, wherein the first natural number is a natural number such that the value obtained by dividing the slice spacing of the slices of the one medical image by the first natural number gives the largest value at or below the standard resolution, and the second natural number is a natural number such that the value obtained by dividing the slice spacing of the slices of the one medical image by the second natural number gives the smallest value at or above the standard resolution.

16. 1. An image processing method of an image processing device for generating a difference image, which is a three-dimensional image for displaying a projection image on a display unit, from a first medical image, which is a three-dimensional image configured from a plurality of slices obtained by imaging a subject, and a second medical image, the method comprising: an acquisition step of acquiring the first medical image and the second medical image; a conversion step of converting the resolution of at least one of the first medical image and the second medical image to a predetermined resolution so that differences in details between the first medical image and the second medical image can be calculated; a generating step of generating a difference image including a plurality of slices having a predetermined slice interval in a slice direction of the subject; a determining step of determining the predetermined slice interval of the difference image generated by the generating step, the determining step determining the predetermined slice interval in the difference image based on a value close to a reference resolution set to the predetermined resolution so that slices corresponding to slices of one of the first and second medical images serving as a reference are included in the difference image and the slice interval of the slices in the difference image is finer than the slice interval of the one of the medical images; and the determining step compares a first value obtained based on the difference between the reference resolution and a value obtained by dividing the slice spacing of the slices of the one medical image by a first natural number with a second value obtained based on the difference between the reference resolution and a value obtained by dividing the slice spacing of the slices of the one medical image by a second natural number, and sets one of the values ​​obtained by dividing the first value by the first natural number or the second natural number, which corresponds to the smaller of the absolute values ​​of the first value and the second value, as the neighboring value, wherein the first natural number is a natural number such that the value obtained by dividing the slice spacing of the slices of the one medical image by the first natural number gives the largest value at or below the reference resolution, and the second natural number is a natural number such that the value obtained by dividing the slice spacing of the slices of the one medical image by the second natural number gives the smallest value at or above the reference resolution.

17. A program for causing a computer to execute each step of the image processing method according to claim 14 or 16.

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