Patent Publication Number: US-11037672-B2

Title: Medical image processing apparatus, medical image processing method, and system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2019-013563 filed on Jan. 29, 2019, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a medical image processing apparatus, a medical image processing method, and a system. 
     BACKGROUND 
     In organ resection and excision, a tubular tissue containing blood vessels is ligated and separated. It is preferable to perform pre-operative planning before surgery the tubular tissue to be ligated and separated. An image display method for visualizing tubular tissue is known in the related art. The image display method displays an image with respect to a tubular tissue through three-dimensional image processing of a region obtained by performing cutting at a cut surface along a path representing a center line of the tubular tissue and through two-dimensional image processing with respect to the cut surface. The three-dimensional image processing is image processing performed through a ray-casting method. The two-dimensional image processing is image processing performed through an MPR method. 
     It is possible to visualize a cut surface of a tissue with technique of U.S. Patent Application Publication No. 2008/0297509. However, in a case where an image is generated by volume rendering through a ray-casting method, in some cases, it is difficult to grasp the position of a tubular tissue (for example, a blood vessel) in the image. It is more difficult to grasp where the cut surface exists in the tubular tissue in the image. Accordingly, it is difficult to visually recognize a tissue to be ligated in the image. 
     The present disclosure has been made in consideration of the above-described circumstances and provides a medical image processing apparatus which can easily visually recognize a tissue to be ligated, a medical image processing method, and a system. 
     SUMMARY 
     One aspect of the present disclosure provides a medical image processing apparatus for visualizing a tissue that includes: a memory; and a processor configured to execute a process. The process includes: acquiring volume data including the tissue; setting a cut surface for cutting the tissue in the volume data; and first performing processing relating to visualization of the tissue. The first performing includes: second performing rendering that causes ray attenuation on the volume data to generate a rendering image including the tissue cut along the cut surface; and displaying on a display unit display information including the rendering image in which a contour line of the tissue on the cut surface is highlighted. 
     According to the present disclosure, it is possible to easily visually recognize a tissue to be ligated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a hardware configuration example of a medical image processing apparatus according to a first embodiment; 
         FIG. 2  is a block diagram illustrating a functional configuration example of the medical image processing apparatus; 
         FIG. 3A  is a diagram illustrating a ligation and separation point in a raycast image in the related art; 
         FIG. 3B  is a diagram illustrating a ligation and separation point in a raycast image in the related art; 
         FIG. 4A  is a diagram representing a first example of a cross section in the vicinity of a blood vessel in the related art; 
         FIG. 4B  is a diagram representing a second example of a cross section in the vicinity of a blood vessel in the related art; 
         FIG. 5  is a diagram illustrating highlighted ligation and separation points; 
         FIG. 6A  is a diagram illustrating an example of a raycast image including the hepatic veins and the portal vein before excision; 
         FIG. 6B  is a diagram illustrating an example of a raycast image displaying non-highlighted cut surfaces of the hepatic veins and the portal vein after excision; 
         FIG. 6C  is a diagram illustrating an example of a raycast image representing highlighted cut surfaces of the hepatic vein and the portal vein after excision; 
         FIG. 7A  is a diagram illustrating a first example of display of a highlighted cut surface of a blood vessel and each voxel value in a rendering image; 
         FIG. 7B  is a diagram illustrating a second example of display of a highlighted cut surface of a blood vessel and each voxel value in a rendering image; 
         FIG. 8A  is a diagram illustrating a third example of display of a highlighted cut surface of a blood vessel and each voxel value in a rendering image; 
         FIG. 8B  is a diagram illustrating an example of display of a highlighted cut surface in a case where the cut surface is not perpendicular to a running direction of a blood vessel; 
         FIG. 9  is a diagram illustrating performing display of a highlighted cut surface while offsetting the display; 
         FIG. 10  is a diagram illustrating an example of display of highlighted cut surfaces in a surface rendering image; 
         FIG. 11  is a diagram illustrating an example of a surface rendering image represented by a polygon mesh; 
         FIG. 12  is a diagram illustrating a display example of a surface rendering image shown in  FIG. 11 ; and 
         FIG. 13  is a diagram illustrating an example of display of highlighted cut surfaces of an organ to be cut. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. 
     A medical image processing apparatus for visualizing a tissue, which relates to the embodiment, includes: a memory; and a processor configured to execute a process. The process includes: acquiring volume data including the tissue; setting a cut surface for cutting the tissue in the volume data; and first performing processing relating to visualization of the tissue. The first performing includes: second performing rendering that causes ray attenuation on the volume data to generate a rendering image including the tissue cut along the cut surface; and displaying on a display unit display information including the rendering image in which a contour line of the tissue on the cut surface is highlighted. 
     Accordingly, in the medical image processing apparatus, it is possible to easily identify where the cut surface in the tissue is by checking highlighted rendering image. Namely, it is possible to easily visually recognize a tissue to be ligated. 
     Circumstances on How Aspect of Present Disclosure is Obtained 
       FIGS. 3A and 3B  are diagrams illustrating a ligation and separation point in a raycast image in the related art. In ligation and separation, for example, an organ is cut except for blood vessels. The blood vessels are ligated and cut in the state of being ligated.  FIG. 3A  is a diagram illustrating a cut surface F 1 X obtained by partially excising an organ Z 1 X.  FIG. 3B  is a diagram illustrating a raycast image of the organ Z 1 X which has been generated based on each voxel positioned in an arrow αX direction than the cut surface F 1 X. Accordingly, the interior of the organ Z 1 X is shown in  FIG. 3B . However, in many cases, a shading of the raycast image becomes unclear, and in  FIG. 3B , the boundary between the surface of the organ Z 1 X and the cut surface F 1 X is unclear. For example, it is difficult to distinguish whether a blood vessel K 1 X included in the raycast image of  FIG. 3B  is a blood vessel to be ligated and separated or a blood vessel that does not require ligation and separation since the blood vessel is folded back to the back side of the organ Z 1 X. Voxel values in a case where there is noise or calcification in the image become relatively large. Therefore, it is difficult to distinguish noise or calcification from blood vessels in the image. Blood vessels may be ligated after an organ is partially cut away while leaving the blood vessels intact. 
       FIG. 4A  represents a first example of a cross section in the vicinity of a blood vessel in the related art. In  FIG. 4A , a blood vessel K 2 X is planarly represented, but is actually represented by voxels in a three-dimensional space (the same applies to  FIG. 4B ). The region positioned on the right side in  FIG. 4A  is a mask region MR 1 X. The region positioned on the left side in  FIG. 4A  is a non-mask region MR 2 X. The numerical values (for example, “0” or “100”) illustrated in  FIG. 4A  represent voxel values. In  FIG. 4A , the voxel value of a blood vessel portion is 100 and the voxel value of an organ portion that is not a blood vessel is 0. The non-mask region MR 2 X represents an excision region and the mask region MR 1 X represents a residual region. 
     In a case where voxels having a voxel value greater than or equal to  50  are volume rendered (for example, ray-casted) without using a mask while setting the visual line direction (for example, a direction from the front left side of the paper of  FIG. 4A  toward the back right side) as a projection direction of a virtual ray, a contour surface F 3 X is visualized and the blood vessel K 2 X is drawn. That is, the entire blood vessel K 2 X shown in  FIG. 4A  is visualized. In this case, it is unclear which part of the blood vessel is to be excised. 
       FIG. 4B  represents a second example of a cross section in the vicinity of a blood vessel in the related art. In a case where voxels having a voxel value greater than or equal to 50 are volume rendered (for example, ray-casted) using a mask while setting the visual line direction (for example, a direction from the front left side of the paper of  FIG. 4B  toward the back right side) as a projection direction of a virtual ray, the blood vessel K 2 X within the mask region MR 1 X is drawn. There is a distal end K 2 X 1  of the blood vessel, to be excised, at the boundary between the mask region MR 1 X and the non-mask region MR 2 X. Accordingly, the residual portion of the excised blood vessel K 2 X is visualized in  FIG. 4B . The distal end K 2 X 1  of the blood vessel is visualized as a part of the blood vessel K 2 X. For this reason, it is difficult to determine whether the distal end K 2 X 1  of the blood vessel K 2 X is obtained by visualizing the boundary (mask boundary) between the mask region MR 1 X and the non-mask region MR 2 X or is obtained by visualizing the contour surface F 3 X of the voxel values. It is also difficult to determine whether the actual point to be subjected to ligation and separation is a position RX or the blood vessel K 2 X. 
     A medical image processing apparatus which can easily visually recognize a tissue to be ligated, a medical image processing method, and a medical image processing program will be described in the following embodiment. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating a configuration example of a medical image processing apparatus  100  according to a first embodiment. The medical image processing apparatus  100  includes a port  110 , a UI  120 , a display  130 , a processor  140 , and a memory  150 . 
     A CT (Computed Tomography) scanner  200  is connected to the medical image processing apparatus  100 . The medical image processing apparatus  100  obtains volume data from the CT scanner  200  and processes the acquired volume data. The medical image processing apparatus  100  may be constituted by a PC and software mounted on the PC. 
     The CT scanner  200  irradiates a subject with X-rays to capture an image (CT image) using a difference in absorption of X-rays due to tissues in the body. The subject may include a living body, a human body, an animal, and the like. The CT scanner  200  generates volume data including information on any portion inside the subject. The CT scanner  200  transmits the volume data as a CT image to the medical image processing apparatus  100  via a wire circuit or a wireless circuit. When capturing a CT image, imaging conditions relating to CT imaging or contrast conditions relating to administration of a contrast medium may be considered. The imaging may be performed on arteries or veins of an organ. The imaging may be performed a plurality of times at different timings depending on the characteristics of the organ. 
     The port  110  in the medical image processing apparatus  100  includes a communication port, an external device connection port, or a connection port to an embedded device and acquires volume data obtained from the CT image. The acquired volume data may be immediately sent to the processor  140  for various kinds of processing, or may be sent to the processor  140  as necessary after being stored in the memory  150 . The volume data may be acquired via a recording medium or a recording media. The volume data may be acquired in the form of intermediate data, compressed data, or sinogram. The volume data may be acquired from information from a sensor device attached to the medical image processing apparatus  100 . The port  110  functions as an acquisition unit that acquires various data such as volume data. 
     The UI  120  may include a touch panel, a pointing device, a keyboard, or a microphone. The UI  120  receives any input operation from a user of the medical image processing apparatus  100 . The user may include a medical doctor, a radiology technician, a student, or other paramedic staffs. 
     The UI  120  receives various operations. For example, the UI receives operations such as designation of a region of interest (ROI) or setting of luminance conditions in volume data or an image based on the volume data (for example, a three-dimensional image or a two-dimensional image to be described below). The region of interest may include regions of various tissues (for example, blood vessels, the bronchi, organs, bones, and the brain). The tissues may include lesion tissue, normal tissue, tumor tissue, and the like. 
     The display  130  may include, for example, an LCD, and displays various pieces of information. Various pieces of information may include a three-dimensional image or a two-dimensional image obtained from volume data. The three-dimensional image may include a volume rendering image, a surface rendering image, a virtual endoscopic image, a virtual ultrasound image, a CPR image, and the like. The volume rendering image may include a ray-sum image, an MIP image, a MinIP image, an average value image, or a raycast image. The two-dimensional image may include an axial image, a sagittal image, a coronal image, an MPR image, and the like. 
     The memory  150  includes various primary storage devices such as ROM or RAM. The memory  150  may include a secondary storage device such as an HDD or an SSD. The memory  150  may include a tertiary storage device such as a USB memory or an SD card. The memory  150  stores various pieces of information and programs. The various pieces of information may include volume data acquired by the port  110 , an image generated by the processor  140 , setting information set by the processor  140 , and various programs. The memory  150  is an example of a non-transitory recording medium in which programs are recorded. 
     The processor  140  may include a CPU, a DSP, or a GPU. The processor  140  functions as a processing unit  160  performing various kinds of processing and controls by executing a medical image processing program stored in the memory  150 . 
       FIG. 2  is a block diagram illustrating a functional configuration example of the processing unit  160 . 
     The processing unit  160  includes a cut surface setting unit  166  that sets a cut surface for cutting a tissue in volume data and a visualization processing unit  167  that performs processing relating to visualization of the tissue. The cut surface setting unit  166  includes a region processing unit  161 . The visualization processing unit  167  includes an image generator  162 , a highlight information generator  163 , and a display controller  164 . The processing unit  160  controls each portion of the medical image processing apparatus  100 . The processing unit  160  performs processing relating to visualization of the tissue. Each portion included in the processing unit  160  may be realized as different functions using one hardware device or may be realized as different functions using a plurality of hardware devices. Each portion included in the processing unit  160  may be realized using exclusive hardware components. 
     The region processing unit  161  acquires volume data of a subject via the port  110 , for example. The region processing unit  161  extracts any region contained in the volume data. The region processing unit  161  may extract a region of interest by automatically designating the region of interest based on voxel values of the volume data, for example. The region processing unit  161  may extract a region of interest by manually designating the region of interest via, for example, the UI  120 . The region of interest may contain regions of the lung, the liver, the bronchi, the pulmonary arteries, the pulmonary veins, the portal vein, and the hepatic veins. The region of interest may be at least a part of an organ to be excised from a subject. 
     The region processing unit  161  may segment an organ of a subject. The segments may be roughly coincident with at least anatomical segments. The organ may contain the lung, the liver, and other organs. The segments may be at least some regions out of a combination of a plurality of segments. The segments may include sub-segments, sub-sub-segments, and the like which are units in a finer range than that of the segments. 
     The region processing unit  161  may set a cut surface for cutting a tissue. In this case, a cut surface may be manually set via the UI  120 , or may be automatically set based on operation results. For example, when segmenting the lung, the region processing unit  161  may extract a plurality of segments of the lung to set boundary surfaces between the plurality of segments as cut surfaces. The cut surfaces may be flat or curved. The cut surfaces may be coincident with ligation and separation points (ligation and separation surfaces). 
     The image generator  162  generates various images. The image generator  162  generates a three-dimensional image or a two-dimensional image based on at least a part of acquired volume data (for example, volume data of extracted regions or segments). The image generator  162  may generate an image by performing rendering (for example, ray-casting or surface rendering) that causes ray attenuation. The image generator  162  may generate an image using a mask. If a mask is used, drawing is performed within an image while being limited to voxels of a mask region, and drawing is not performed within an image with voxels of a non-mask region. A plurality of masks can be used for each region. The generation of an image using a mask is disclosed, for example, in Japanese Patent No. 4188900 which is incorporated herein by reference. 
     The highlight information generator  163  generates highlight information for highlighting a contour line of a tissue on a cut surface or a cut surface itself of a tissue (the inside surface of the contour line). The highlight information includes at least contour line highlight information in which a contour line of a tissue on a cut surface is highlighted. 
     The contour line highlight information may be a ring formed substantially along a contour line of a tubular tissue on a cut surface. For example, the contour line highlight information may include information in which voxel values of voxels of a contour line of a tissue on a cut surface are greater than acquired actual values. The contour line highlight information may include information in which a contour line of a tissue on a cut surface is made thick. The contour line highlight information may include information in which voxels of a contour line have a color different from other voxels adjacent to the contour line. 
     The highlight information may include surface highlight information in which the inside surface of the contour line of a cut surface is highlighted. The surface highlight information may be a shape, a pattern, a color, plotting out, and the like of the inside of a ring indicating the contour line of the cut surface. For example, the surface highlight information may include information in which voxel values of voxels on a cut surface are greater than acquired actual values. The surface highlight information may include information in which the colors of voxels of a cut surface are different from those of other voxels adjacent to the cut surface. 
     A tissue having a cut surface may be a tissue to be ligated and separated. The tissue may be, for example, tubular tissue. The tubular tissue may include blood vessels, lymphatic vessels, the bronchi, the bile duct, and the like. The ligation and separation may be carried out along with removal of a tumor from an organ, segmentectomy of an organ, wedge-shaped excision of an organ, and the like. The tubular tissue may be a tissue contained in an organ (for example, the lung or the liver). The highlight information may be information in which the direction of the tubular tissue is visualized. The highlight information may be generated based on a path of the tubular tissue. The highlight information may be displayed while being offset from a cut surface. 
     The display controller  164  displays various data, information, and images on the display  130 . The images include images generated in the image generator  162 . The display controller  164  displays highlight information so that the highlight information is superimposed on a rendering image. 
       FIG. 5  is a diagram illustrating highlighted ligation and separation points. A raycast image GZ 1  including a bronchus  11 , a pulmonary artery  12 , and a pulmonary vein  13  is shown in  FIG. 5 . In  FIG. 5 , the bronchus  11 , the pulmonary artery  12 , and the pulmonary vein  13  have a cut surface F 1 . The bronchus  11 , the pulmonary artery  12 , and the pulmonary vein  13  may be visualized through volume rendering (for example, ray-casting). The cut surface F 1  may be visualized through surface rendering. The cut surface F 1  may be substantially coincide with a ligation and separation point. 
     The cut surface F 1  is highlighted according to highlight information. The highlight information includes at least one of contour line highlight information M 1  or surface highlight information M 2 . The display controller  164  highlights boundaries between voxels, voxels visualized on the cut surface F 1  and non-visualized voxels adjacent to the cut surface F 1 , based on the highlight information. A user easily visually recognizes the boundaries between the voxels pertaining to the cut surface F 1  according to the display of a highlighted cut surface F 1 . The display controller  164  may perform display while distinguishing the voxels visualized on the cut surface F 1  from the non-visualized voxels adjacent to the cut surface F 1  using different display modes (for example, different colors). 
       FIGS. 6A to 6C  are diagrams illustrating examples of rendering images after excision.  FIG. 6A  illustrates a raycast image GZ 21  including a hepatic vein  21  and a portal vein  22  before excision.  FIG. 6B  illustrates a raycast image GZ 22  displaying non-highlighted cut surfaces F 1  of the hepatic vein  21  and the portal vein  22  after excision.  FIG. 6C  illustrates a raycast image GZ 23  displaying the highlighted cut surfaces F 1  of the hepatic vein  21  and the portal vein  22  after excision using highlight information. When comparing  FIG. 6B  with  FIG. 6C , it is possible to understand that blood vessels simply ended (here, the hepatic vein  21  and the portal vein  22 ) are easily distinguished from blood vessels cut at the cut surfaces F 1  using the contour line highlight information M 1  and the surface highlight information M 2  as highlight information. Accordingly, even in a case of segmenting a portion where the complicatedly intertwined hepatic vein  21  and portal vein  22  exist, it is possible to clearly recognize points to be cut. For example, it is possible to easily visually recognize points at which fine blood vessels are to be ligated during surgery for performing segmentectomy by making cut surfaces coincide with segmentation surfaces of segmentation. 
       FIG. 7A  is a diagram illustrating a first example of display of the highlighted cut surface F 1  of the blood vessel K 1  and each voxel value in a rendering image. In  FIG. 7A , highlighted display is performed using the contour line highlight information M 1 . In  FIG. 7A , the blood vessel K 1  is planarly represented, but is actually represented by voxels in a three-dimensional space (the same applies to  FIGS. 7B and 8A ). The contour line highlight information M 1  is represented by a closed-ring curve in the three-dimensional space. The region positioned on the right side in  FIG. 7A  is a mask region MR 1 . The region positioned on the left side in  FIG. 7A  is a non-mask region MR 2 . The numerical values (for example, “0” and “100”) described in  FIG. 7A  represent voxel values. In  FIG. 7A , a voxel value of a blood vessel portion is 100 and a voxel value of an organ portion which is not a blood vessel is 0. 
     The highlight information generator  163  calculates an intersection point C 1  between a boundary (mask boundary surface F 1 A) between the mask region MR 1  and the non-mask region MR 2  and a contour surface F 1 B indicating boundaries between the voxel values 0 and 100. The intersection points C 1  coincide with two points obtained at which the contour line of a tissue (here, the blood vessel K 1 ) on the cut surface F 1  is projected onto the plane of  FIG. 7A . The contour lines of the tissue are represented by the two intersection points C 1  in the plane of  FIG. 7A , but are represented by intersection lines between the mask boundary surface F 1 A and the contour surface F 1 B on three-dimensional volume data. The intersection lines are cyclic and coincide with contour lines of tissues on set cut surfaces F 1 . Although the cut surface F 1  is set corresponding to the mask boundary surface F 1 A, the cut surface F 1  itself is not visualized in  FIG. 7A . 
       FIG. 7B  is a diagram illustrating a second example of display of a highlighted cut surface F 1  of the blood vessel K 1  and each voxel value in a rendering image. In  FIG. 7B , highlighted display is performed using the contour line highlight information M 1  and the surface highlight information M 2 . The mask region MR 1 , the non-mask region MR 2 , and voxel values of voxels are shown in  FIG. 7B  in the same manner as in  FIG. 7A . The surface formed by the intersection points C 1  within the closed-ring curve represents the cut surface F 1  of the blood vessel K 1 . The contour line highlight information M 1  is represented by a closed-ring curve in the three-dimensional space. The surface highlight information M 2  may be flat or curved. The display controller  164  may highlight the surface highlight information M 2  along the cut surface F 1  in a display mode (for example, different colors, patterns, and line types) different from that of the contour surface F 1 B. 
       FIG. 8A  is a diagram illustrating a third example of display of the highlighted cut surface F 1  of the blood vessel K 1  and each voxel value in a rendering image. The mask region MR 1 , the non-mask region MR 2 , and voxel values of voxels are shown in  FIG. 8A  in the same manner as in  FIGS. 7A and 7B . In  FIG. 8A , a central path ps 1  of the blood vessel K 1  is added thereto and highlighted. The region processing unit  161  may extract the region of the blood vessel K 1  and calculate the central path ps 1  of the blood vessel K 1 . The highlight information generator  163  generates a ring RG indicating a contour line of the cut surface F 1  around an intersection point C 2  between the central path ps 1  and the mask boundary surface F 1 A. The display controller  164  superimposes the contour line highlight information M 1  (for example, the ring RG) or the surface highlight information M 2  on a rendering image for display. The ring RG is an example of the contour line highlight information M 1 . 
     The direction of the ring RG can be adjusted. For example, the highlight information generator  163  may determine the direction of the ring RG based on the direction of the central path ps 1  of the blood vessel K 1 . In this case, the direction of the ring RG may be determined based on voxel values of 64 voxels (4×4×4) around the intersection point C 2 . By adjusting the direction of the ring RG, it is possible to perform highlighted display using the contour line highlight information M 1  or the surface highlight information M 2  perpendicular to the running direction (direction in which the central path ps 1  extends) of the blood vessel K 1  using the ring RG in a state in which the cut surface F 1  is not perpendicular to the running direction of the blood vessel K 1  (refer to  FIG. 8B ). The surface highlight information M 2  may perform highlighted display by displaying the inner surface of the ring RG in a display mode (for example, different colors, patterns, and line types) different from that of the contour surface F 1 B. 
       FIG. 9  is a diagram illustrating performing display of the highlighted cut surface F 1  while offsetting the display. In  FIG. 9 , the blood vessel K 1  is branched into a branch E 1  and a branch E 2 . The cut surface F 1  passes through both the branches E 1  and E 2  and is set along the branch E 2 . In this case, the display controller  164  may not display the ring RG in the branch E 2  extending along the cut surface F 1 , but may display the ring RG in the branch E 1  that does not extend along the cut surface F 1 . In this case, the display controller  164  may set an offset surface F 2  which is offset from the cut surface F 1 . The offset surface F 2  may or may not be parallel to the cut surface F 1 . The ring RG may be displayed on the contour line of the branch E 1  of the blood vessel K 1  which has passed through both the cut surface F 1  and the offset surface F 2 . The position at which the ring RG is drawn may be a position passing through the offset surface F 2  or a position different from that of the offset surface F 2 . The direction of the ring RG may or may not be parallel to the cut surface F 1 . 
     The offset between the cut surface F 1  and the position where the ring RG is to be displayed can also be applied to the cases of  FIGS. 7A, 7B, and 8A . For example, in  FIGS. 7A, 7B, and 8A , it is assumed that the mask boundary surface F 1 A moves (offsets) along the blood vessel K 1  in the horizontal direction of the drawing. In this case, the display controller  164  may display the ring RG at any position (for example, a position of the offset mask boundary surface) offset from the cut surface F 1  regarding a blood vessel or a branch of a blood vessel which passes through both a non-offset mask boundary surface (the mask boundary surface corresponding to the cut surface F 1 ) and the offset mask boundary surface. 
     The display controller  164  may analyze main components of a figure generated in the cross section at the cut surface F 1  of the blood vessel K 1 . As a result of the analysis of the main components, the ring RG may be displayed in a case where the projected figure has a shape closer to a circle than that of a predetermined standard. The ring RG may not be displayed in a case where the projected figure has a flat shape rather than a shape closer to a circle than that of a predetermined standard. The highlighted display may be offset in the case where the projected figure has a flat shape rather than a shape closer to a circle than that of a predetermined standard. 
       FIG. 10  is a diagram illustrating an example of display of the highlighted cut surfaces F 1  in a surface rendering image. A surface rendering image GZ 3  including a bronchus  31 , a pulmonary artery  32 , and a pulmonary vein  33  is shown in  FIG. 10 . In  FIG. 10 , the cut surfaces F 1  are set at the same position as ligation and separation points. The cut surfaces F 1  are present in the pulmonary artery  32  and the pulmonary vein  33 , and the contour line highlight information M 1  and the surface highlight information M 2  are displayed corresponding to the cut surfaces F 1 . An ended part M 3  of the pulmonary vein  33  regardless of the cut surfaces F 1  is reflected in  FIG. 10 . A user can grasp that the ended part M 3  of the pulmonary vein  33  is so imaged (the pulmonary vein  33  is ended) because the ended part is not highlighted. 
       FIG. 11  is a diagram illustrating an example of a surface rendering image GZ 4  represented by a polygon mesh. A blood vessel K 2  represented by a polygon mesh is included in the surface rendering image GZ 4 . The blood vessel K 2  has two branches which respectively have cut surfaces F 1 . In this case, the image generator  162  segments polygons along the cut surfaces F 1  to form new polygons PG. Various kinds of image processing are performed using the surface rendering image GZ 4  in consideration of the polygon mesh. 
       FIG. 12  is a diagram illustrating a display example of a surface rendering image GZ 5  shown in  FIG. 11 . The wire frame of the polygon mesh is not usually drawn in the display of the surface rendering image GZ 5 . On the other hand, the display controller  164  only draws and displays the wire frames of the cut surfaces F 1 . Here, the wire frames of the cut surfaces F 1  are set as the contour line highlight information M 1 . Accordingly, the contour line of the blood vessel K 2  on the cut surfaces F 1  is highlighted. The display controller  164  may display all the wire frames, and the contour lines of the wire frames of the cut surfaces F 1  may be highlighted by changing the color or the thickness of the wire frames. 
       FIG. 13  is a diagram illustrating an example of display of highlighted cut surfaces F 3  of an organ Z 1  to be cut. The highlight information generator  163  may generate highlight information for highlighting the organ Z 1  itself on the cut surfaces F 3 . The highlight information includes at least contour line highlight information M 11  in which a contour line of the organ Z 1  on the cut surfaces F 3  is highlighted. The highlight information may include surface highlight information M 12  in which the inside surface of the contour lines of the cut surfaces F 3  is highlighted. In  FIG. 13 , the contour line of the organ Z 1  on the cut surfaces F 3  and contour lines of one or more blood vessels K 3  existing in the organ Z 1  are highlighted using the contour line highlight information M 1  and M 11 . In the raycast image, in some cases, it is difficult to grasp which portion of the organ Z 1  is cut. In contrast, it is easy for a user to simultaneously grasp which portion of the organ Z 1  is cut and which portion of a blood vessel in the organ Z 1  is cut by highlighting the blood vessels K 3  and the organ Z 1  according to the cut surfaces F 3 . The contour line highlight information M 11  may be displayed for the organ Z 1  and the contour line highlight information M 1  and the surface highlight information M 2  may be displayed for the blood vessels K 3 . Accordingly, it is possible for a user to simultaneously grasp cut surfaces of an organ and points to be ligated and separated on the cut surfaces of an organ. In  FIG. 13 , the cut surfaces F 1  according to the blood vessels K 3  are included in the cut surfaces F 3  according to the organ Z 1 . 
     In this manner, the medical image processing apparatus  100  can display highlighted points to be ligated and separated regarding blood vessels crossing the cut surfaces F 1 . Accordingly, a user easily grasps blood vessels to be ligated and separated while planning surgery. A user easily grasps blood vessels inside an organ which are usually difficult to grasp. 
     Even if a tissue is actually cut at the cut surface F 1  using a mask, there is volume data of the tissue which has not been cut since the tissue has not yet cut at a point in time when the volume data is acquired. The display controller  164  can display the tissue so as to be cut by not displaying the non-mask region MR 2 . If masking is performed, it is difficult to determine whether there is the cut surface F 1  on a mask boundary surface or a tissue is merely so imaged from the beginning. However, if the cut surface F 1  is highlighted using highlight information such as a ring, it becomes clear that the position of the cut surface F 1  is a ligation and separation point. 
     Up to here, although various embodiments have been described with reference to the accompanying drawings, it is needless to say that the present disclosure is not limited to such examples. It would be apparent for those skilled in the art that various modification examples or corrected examples are conceivable within the scope recited in the claims, and it would be understood that these fall within the technical scope of the present disclosure. 
     For example, a threshold value th 2  (for example, a voxel value  50  for calculating the contour surface F 1 B) of voxel values for calculating a contour line of a blood vessel may not be coincident with a threshold value th 3  (for example, a threshold value for making pixels used in ray-casting opaque or a threshold value for generating a surface through a marching cubes method or the like) for volume rendering. 
     The region processing unit  161  may not calculate a contour line of a blood vessel using the threshold value th 2  by comparison with surrounding voxel values, but may simulate and calculate ray attenuation due to progression of a virtual ray and calculate a contour line of a blood vessel based on the ray attenuation. For example, the position at which initial ray attenuation is greater than or equal to a threshold value th 4  when seen from a viewpoint side may be set to a surface (corresponding to a contour line) of a blood vessel. Each threshold value may be a fixed value or an adaptive value. 
     The cut surface F 1  may be represented by a method other than masking using the region processing unit  161 . For example, the cut surface setting unit  166  may set the cut surface F 1  in a relative coordinate system with respect to volume data, limit voxels to those further on a back side than the cut surface F 1  when seen from a viewpoint side, and generate volume rendering using voxel values of the voxels. In this case, if there are opaque voxels on the cut surface F 1 , these voxels are visualized. Accordingly, it is possible to visually recognize the cut surface F 1  without using a mask. 
     The region processing unit  161  may generate masks representing the cut surfaces F 1  based on the mask representing the shape of the organ Z 1  and the masks representing the shapes of the blood vessels K 1  to K 3 . The region processing unit  161  may generate highlight information based on an intersection of the surface of the mask representing the cut surfaces F 1  and the surfaces of the mask representing the shape of the organ Z 1  and the masks representing the shapes of the blood vessels K 1  to K 3 . Accordingly, it is possible to flexibly visualize ligation and separation points according to how each mask region is generated in comparison with the case of highlighting cut surfaces based on an organ or blood vessels simply region-extracted as a single mask, for example. Respective offset distances may differ in contour lines generated for the organ Z 1  and the blood vessels K 1  to K 3 . As a result, it is possible to simulate a situation in which deviation occurs between the cut surfaces F 1  of the organ and points of the blood vessels K 1  to K 3 , included in the organ, to be ligated and separated. The blood vessels K 1  to K 3  are at least one of the blood vessels K 1  to K 3 . 
     The cut surfaces F 1  may not be cross sections for completely cutting an organ, but may be cross sections for cutting a part of an organ. Accordingly, it is possible to express a state in the middle of surgery by highlighting the cut surfaces F 1 . Even in a case where a cut is made while anticipating proliferation of hepatocytes, it is possible to apply display of the highlighted cut surfaces F 1 . In this case, the processing unit  160  may perform a deformation simulation by deforming the liver by pulling a piece of the liver after cutting a part of the liver, and highlight deformed cut surfaces F 1 . 
     The display of the highlighted cut surfaces F 1  may be performed by being offset from the cut surfaces F 1 . The display controller  164  may highlight, for example, blood vessel paths or cut surfaces F 1  using the ring RG or the like in front of a visual line direction. This is because actual ligation and separation points are positioned in front of a visual line direction of cut surfaces F 1  or highlighted display is easily seen together with a rendering image during the highlighted display since the organ is incised in surgery and blood vessels are ligated and separated at an appropriate position on the blood vessels when the blood vessels are exposed. 
     The rendering image may include both an image portion obtained through volume rendering and an image portion obtained through surface rendering. For example, a contour line of an organ itself may be visualized through surface rendering and blood vessels may be visualized through volume rendering. For example, in a case of performing a simulation of segmentectomy of an organ, segments of the organ may be visualized through surface rendering and blood vessels may be visualized through volume rendering. 
     A subtype of a ray-casting method or a rendering method other than the ray-casting method may be included in the volume rendering. The volume rendering may be limited to rendering that causes ray attenuation (for example ray-casting method, ray-tracing method). The rendering may include stochastic raycast, cinematic volume rendering technique, ray tracing with respect to volume data, and the like. In a case of rendering in which a ray-casting method is combined with an MIP method, a tissue in which the cut surfaces F 1  are highlighted may be visualized through the ray-casting method and other organs may be visualized through the MIP method. Accordingly, highlight information may be superimposed on the organ visualized through the MIP method. The rendering may include surface rendering. In the surface rendering, a surface to be rendered on the cut surface F 1  may or may not be stretched. 
     The display controller  164  may not add highlight information to a rendering image after rendering, but may add highlight information thereto at a stage of rendering. For example, in a case of surface rendering, the image generator  162  may be set so as to perform rendering (contour line highlight rendering) for highlighting a contour line at a stage of rendering, to perform contour line highlight rendering. The highlighting of a contour line may include highlighting of a contour line of a tissue on a cut surface. This is not limited to the surface rendering. The same may apply to volume rendering. 
     The contour line may be at least a solid line, a dotted line, or a double line. A color may be set for the contour line. The contour line may be set according to the type of target organ. For example, arteries may be a red double line and veins may be a blue dotted line. 
     The medical image processing apparatus  100  may include at least the processor  140  and the memory  150 . The port  110 , the UI  120 , and the display  130  may be externally attached to the medical image processing apparatus  100 . 
     Volume data as a captured CT image which is transmitted from the CT scanner  200  to the medical image processing apparatus  100  is exemplified. Alternatively, the volume data may be stored by being transmitted to a server or the like (for example, image data server (PACS) (not shown)) on a network so as to be temporarily accumulated. In this case, the port  110  of the medical image processing apparatus  100  may acquire volume data from the server or the like when necessary via a wire circuit or a wireless circuit or may acquire volume data via any storage medium (not shown). 
     Volume data as a captured CT image which is transmitted from the CT scanner  200  to the medical image processing apparatus  100  via the port  110  is exemplified. It is assumed that this also includes a case where the CT scanner  200  and the medical image processing apparatus  100  are substantially combined as one product. This also includes a case where the medical image processing apparatus  100  is treated as a console of the CT scanner  200 . 
     It has been exemplified that an image is captured by the CT scanner  200  to generate volume data including information on the inside of a subject. However, an image may be captured by other devices to generate volume data. Other devices include a magnetic resonance imaging (MRI) apparatus, a positron emission tomography (PET) device, an angiography device, or other modality devices. The PET device may be used in combination with other modality devices. 
     It can be expressed as a medical image processing method in which an operation of the medical image processing apparatus  100  is defined. It can be expressed as a program for causing a computer to execute each step of the medical image processing method. 
     Outline of Above Embodiment 
     One aspect of the above-described embodiment is a medical image processing apparatus  100  for visualizing a tissue and may include: an acquisition unit (for example, a port  110 ) having a function of acquiring volume data including the tissue; a cut surface setting unit  166  that sets a cut surface for cutting the tissue in the volume data; and a visualization processing unit  167  having a function of performing processing relating to visualization of the tissue. The visualization processing unit  167  may have a function of performing rendering that causes ray attenuation on the volume data to generate a rendering image including the tissue cut along the cut surface. The visualization processing unit  167  has a function of displaying display information including the rendering image, in which a contour line of the tubular tissue on the cut surface is highlighted, on a display unit (for example, a display  130 ). 
     Accordingly, in the medical image processing apparatus  100 , it is possible to easily identify where the cut surface in the entire tissue is by checking highlighted display. In the highlighted display, for example, a branch of a blood vessel extends beyond a cut surface. However, it is possible to add a mark (an example of highlight information) which clearly indicates that a more distal side than the cut surface is excised. Even in a case where no mask is used, it is possible for a user to identify which position of a tissue is resected or excised by checking highlighted display. Even in a case where a mask is used, it is possible for a user to easily determine whether a more root side (non-terminal side) than the cut surface in a tissue is visualized or a contour surface of voxel values is visualized using the presence or absence of the highlighted display of a contour line on the cut surface. Accordingly, it is possible for a user to appropriately grasp points to be resected or excised, ligated, and separated in preoperative planning and intraoperative navigation. 
     The cut surface setting unit  166  may have a function of setting the cut surface F 1  based on a mask boundary surface F 1 A which is a boundary between a mask region MR 1  including voxels to be rendered among a plurality of voxels included in the volume data and a non-mask region MR 2  including voxels not to be rendered. The visualization processing unit  167  may have a function of performing display by generating the rendering image based on voxel values of each of the voxels included in the mask region MR 1  excluding voxel values of each of the voxels of the non-mask region MR 2 . 
     Accordingly, a tissue portion to be resected or excised on a more distal side than the cut surface is not displayed. Accordingly, a user can intuitively understand that the tissue is cut along the cut surface. 
     The tissue may be a tubular tissue (for example, a blood vessel K 1 ). Accordingly, a user easily visually recognizes a resection point or an excision point of the tubular tissue that is often smaller than organs. 
     The tubular tissue may be included in an organ Z 1 . Accordingly, a user can visually recognize cut a point of the tubular tissue inside an organ which cannot be checked from the appearance. 
     The processing unit  160  may display a rendering image by indicating a direction of the tubular tissue. Accordingly, even in a case where, for example, the tubular tissue is cut obliquely against a path, a user can check resection points or excision points by adding the direction of the tubular tissue to easily resect or excise the actual tissue. 
     The visualization processing unit  167  may display the rendering image by offsetting a contour line of the tubular tissue on the cut surface F 1  from the cut surface F 1  for highlighting. Accordingly, in a case where the cut surface F 1  set in the medical image processing apparatus  100  is different from a point that will actually be cut in surgery, the medical image processing apparatus  100  can clearly indicate highlight information in the point of a tissue corresponding to a resection point or an excision point, and it is possible to promptly set the cut surface F 1  in the medical image processing apparatus  100 . It is possible to simulate a situation in which deviation occurs between a cut surface F 3  of an organ and a point of the tubular tissue, which is included in the organ, to be ligated and separated. 
     The tissue may be an organ. Accordingly, even in a case where it is difficult to grasp an organ in a rendering image, a contour line of the organ is clearly indicated. Therefore, a user can easily visually recognize the organ. 
     The visualization processing unit  167  may display the rendering image by highlighting a cut surface of an organ including the tubular tissue in addition to a cut surface of the tubular tissue. Accordingly, it is possible for a user to simultaneously grasp the cut surface F 3  of the organ and a point to be ligated and separated on the cut surface F 3  of the organ. 
     The visualization processing unit  167  may highlight the inside surface of the contour line on the cut surface and display the rendering image. Accordingly, a user easily appropriately grasps the cut surface of the tissue. 
     The rendering image may be a volume rendering image. In the volume rendering image, it is difficult to grasp a specific position in a three-dimensional space since the internal state of a tissue in the three-dimensional space is visualized on a two-dimensional plane. However, it is possible to easily visually recognize a resection point or an excision point through the above-described highlighted display. 
     One aspect of the above-described embodiment is a medical image processing method for visualizing a tissue, the method including steps of: acquiring volume data including the tissue; setting the cut surface F 1  for cutting the tissue in the volume data; performing rendering that causes ray attenuation on the volume data to generate a rendering image including the tissue cut along the cut surface; and displaying display information including the rendering image, in which a contour line of the tissue on the cut surface is highlighted, on a display unit. 
     One aspect of the present embodiment may be a medical image processing program for causing a computer to execute the above-described medical image processing method. 
     The present disclosure is useful for a medical image processing apparatus which can easily visually recognize a tissue to be ligated, a medical image processing method, a medical image processing program, and the like.