Medical image processing device

A medical image processing device according to an embodiment includes a first reconstruction unit, a second reconstruction unit, and an image combining unit. The first reconstruction unit generates a first reconstructed image on the basis of an X-ray collection image using a first reconstruction filter. The second reconstruction unit generates a second reconstructed image on the basis of the X-ray collection image, using a second reconstruction filter having a high-frequency emphasis effect more than that of the first reconstruction filter. The image combining unit combines the first reconstructed image with the second reconstructed image.

FIELD

An embodiment of the invention relates to a medical image processing device.

BACKGROUND

As one of intervention treatments (intravascular treatments), there is a coiling treatment for an aneurysm. The coiling treatment is performed only when the size of a neck at the boundary between the aneurysm and the parent vessel is less than that of the aneurysm. In the case in which the size of the neck is substantially equal to that of the aneurysm, when a coil is inserted into the aneurysm, the coil is taken off from the aneurysm and embolism occurs in a peripheral vessel.

However, in recent years, a stent for a coiling treatment has been developed. In a new treatment method using the stent, as illustrated inFIGS. 19 and 20, the stent is placed into the parent vessel and the coil is inserted through a mesh of the stent. According to the new treatment method, since the mesh of the stent prevents the coil from being taken off, it is possible to select the coiling treatment, regardless of the size of the neck.FIGS. 19 and 20are diagrams illustrating the coiling treatment using the stent.

In the coiling treatment using the stent, it is indispensable that, after the stent is placed into the parent vessel, it comes into close contact with the parent vessel and a neck portion of the aneurysm. However, in the stent for the coiling treatment, since the diameter of a metal material forming the mesh in a cross-sectional view is very small, it is very difficult to view the metal material on a radiographic image, such as a fluoroscopic image or a captured image, and it is difficult to evaluate the degree of adhesion. Therefore, in general, a doctor checks a three-dimensional image which is reconstructed in a high spatial resolution mode from projection data (hereinafter, referred to as an X-ray collection image) and checks that the stent comes into close contact with the parent vessel and the neck portion of the aneurysm. Then, the doctor performs the coiling treatment.

However, in the related art, it is difficult to clearly observe both the stent and the vessel. When the X-ray collection image is reconstructed with high resolution, the stent is clearly drawn up to a strut, but is mixed with concave and convex portions of the surface of the vessel. As a result, there is a concern that the boundary therebetween will be unclear. On the other hand, when the X-ray collection image is reconstructed with standard resolution, stent markers are drawn in the image, but the strut is not generally drawn. For example, when the number of projection directions increases in order to solve the problems, an exposure dose increases or the injection amount of contrast medium increases.

DETAILED DESCRIPTION

According to an embodiment, a medical image processing device includes a first reconstruction unit, a second reconstruction unit, and an image combining unit. The first reconstruction unit generates a first reconstructed image on the basis of an X-ray collection image using a first reconstruction filter. The second reconstruction unit generates a second reconstructed image on the basis of the X-ray collection image, using a second reconstruction filter having a high-frequency emphasis effect more than that of the first reconstruction filter. The image combining unit combines the first reconstructed image with the second reconstructed image.

Hereinafter, medical image processing devices according to exemplary embodiments will be described. First, an example in which a medical image processing device, which is a medical image processing device according to a first embodiment, is incorporated into an X-ray diagnosis apparatus will be described.

[Structure of X-Ray Diagnosis Apparatus According to First Embodiment]

FIG. 1is a block diagram illustrating the structure of an X-ray diagnosis apparatus1according to the first embodiment. As illustrated inFIG. 1, the X-ray diagnosis apparatus1according to the first embodiment includes a radiographic mechanism10and an image processing device100. The radiographic mechanism10includes an X-ray tube11, a detector (flat panel detector (FPD))12, a C-shaped arm13(hereinafter, referred to as C-arm13), and a bed14. The C-arm13supports the X-ray tube11and the detector12and is rotated around a subject P at a high speed by a motor which is provided in the base (not illustrated), like a propeller.

The image processing device100includes a control unit20, an analog/digital (A/D) conversion unit21, a two-dimensional image storage unit22, a subtraction unit23, a three-dimensional reconstruction unit24, a three-dimensional image storage unit25, a three-dimensional image combining unit26, a three-dimensional image display unit27, and a monitor28.

The control unit20controls the overall operation of the X-ray diagnosis apparatus1. Specifically, the control unit20controls, for example, the collection of an X-ray collection image, the reconstruction of a three-dimensional image, and the display of the three-dimensional image. The A/D conversion unit21is connected to the detector12, converts an analog signal input from the detector12into a digital signal, and stores the converted digital signal as the X-ray collection image in the two-dimensional image storage unit22. The two-dimensional image storage unit22stores the X-ray collection images.

The subtraction unit23performs subtraction between the X-ray collection image and a non-uniformity correcting image to acquire a subtraction image. The three-dimensional reconstruction unit24reconstructs a three-dimensional image from the subtraction image. As illustrated inFIG. 1, the three-dimensional reconstruction unit24includes a first reconstruction unit24a, a second reconstruction unit24b, a specifying unit24c, and an identification unit24d, which will be described in detail below. The three-dimensional image storage unit25stores three-dimensional images. The three-dimensional image combining unit26combines the reconstructed three-dimensional images. The three-dimensional image display unit27displays the three-dimensional image as a volume rendering image or a multi planar reconstruction (MPR) image on the monitor28.

[Process Performed by X-Ray Diagnosis Apparatus According to First Embodiment]

Next, a process performed by the X-ray diagnosis apparatus1according to the first embodiment will be described in detail.FIG. 2is a flowchart illustrating the flow of the entire process of the X-ray diagnosis apparatus1according to the first embodiment.

As illustrated inFIG. 2, first, imaging is prepared (Step S1). Specifically, when an intervention treatment starts, the doctor inserts a catheter into a vessel. When the catheter reaches the vicinity of a desired aneurysm, the doctor places a stent so as to cover a neck at the boundary between the aneurysm and a parent vessel. The doctor performs 3D imaging in order to check whether the stent completely covers the neck, the stent comes into close contact with a vessel wall, or the stent is damaged.

In the 3D imaging, the C-arm13is rotated around the subject P (for example, at an angle of 180 degrees or more around the subject P) by the motor provided in the base at a high speed, like a propeller. The doctor adjusts any one of the position of the bed14, the height of the bed14, and the position of the C-arm13or a combination thereof such that the main vessel, which is a target, is within the field of view. Then, the doctor rotates the C-arm13to check whether there is an injury in the subject P. In this way, preparations for imaging are completed.

Then, the doctor sets a contrast medium injector to the X-ray diagnosis apparatus1. A contrast medium with a concentration that is five to eight times lower than that of a general contrast medium used for angiography is set. The reason is that, when the contrast medium with high concentration is used, a contrast medium component negates the information of the stent.

Then, the injection of the contrast medium starts (Step S2). After a predetermined period of time (for example, 1 to 2 seconds) has elapsed from the injection of the contrast medium, the C-arm13is rotated and an imaging operation starts (Step S3). Then, the X-ray collection image is captured (Step S4).

For example, the C-arm13is rotated at an angle of 25 degrees for each second and the X-ray diagnosis apparatus1captures about 250 frames of X-ray collection images at an angular interval of about 0.8 degrees. The A/D conversion unit21converts the collected 250 frames of X-ray collection images into digital signals and stores the digital signals corresponding to 250 frames of X-ray collection images with a size of 1,024 pixels×1,024 pixels in the two-dimensional image storage unit22.

When the X-ray collection images are stored in the two-dimensional image storage unit22, the control unit20transmits the X-ray collection images stored in the two-dimensional image storage unit22and a non-uniformity correcting image which is collected in advance to the subtraction unit23, and the subtraction unit23performs subtraction (Step S5). Specifically, the subtraction unit23subtracts the X-ray collection image from the corresponding non-uniformity correcting image.

Next, the non-uniformity correcting image will be described.FIGS. 3A and 3Bare diagrams illustrating non-uniformity correction according to the first embodiment. The non-uniformity correcting image is an image which is captured with only air between the X-ray tube11and the detector12, as illustrated inFIG. 3A, and is periodically collected once every few months in a calibration process. In addition, the non-uniformity correcting image is collected for SID (Source Image Distance), FOV (Field Of View), and a radiation quality adjusting filter.

It is assumed that the image captured in the imaging environment illustrated inFIG. 3Ais “F(x,y)”. In addition, it is assumed that the image captured in a state in which a uniform material is interposed between the X-ray tube11and the detector12as illustrated inFIG. 3Bis “P(x,y)”. When the transmission distance of X-rays in the material is “1” and the X-ray absorption coefficient of the material is “μ”, the following Expression 1 is established. The subtraction unit23obtains a subtraction image Qθ(x,y) using the X-ray collection image, the non-uniformity correcting image, and the following Expression 2:

(where θ indicates an imaging angle, Pθ(x,y) is the X-ray collection image of the subject at the imaging angle θ, F(x,y) is a non-uniformity correcting image, and Qθ(x,y) is a subtraction image at the imaging angle θ).

When the subtraction unit23performs subtraction, the control unit20transmits the subtraction image Qθ(x,y) to the three-dimensional reconstruction unit24and the three-dimensional reconstruction unit24performs reconstruction (Step S6). In the first embodiment, for convenience of explanation, the “subtraction image Qθ(x,y)” is referred to as an “X-ray projection image”.

The three-dimensional reconstruction unit24and the three-dimensional image combining unit26according to the first embodiment perform two kinds of reconstructions, combine two kinds of reconstructed images obtained by two kinds of reconstructions, and output a combined image. Specifically, first, the first reconstruction unit24aof the three-dimensional reconstruction unit24generates a first reconstructed image on the basis of the X-ray collection image using a first reconstruction filter. The second reconstruction unit24bof the three-dimensional reconstruction unit24generates a second reconstructed image on the basis of the X-ray collection image using a second reconstruction filter. The second reconstruction filter has a high-frequency emphasis effect more than that of the first reconstruction filter. The three-dimensional image combining unit26combines the first reconstructed image with the second reconstructed image. In the first embodiment (except for the following modifications 1 and 2), the processes of the specifying unit24cand the identification unit24dare not performed. Therefore, the three-dimensional reconstruction unit24may not include each of the units.

FIG. 4is a diagram illustrating a reconstruction and combination process according to the first embodiment. As illustrated inFIG. 4, the three-dimensional reconstruction unit24performs a first reconstruction using a standard reconstruction filter (hereinafter, appropriately referred to as a “standard reconstruction filter”) and a second reconstruction using a reconstruction filter (hereinafter, appropriately referred to as a “high-frequency emphasis reconstruction filter”) with a great high-frequency emphasis effect on the X-ray projection image transmitted from the subtraction unit23. Each reconstruction filter may be a combination of two kinds of reconstruction filters such that one of, for example, a Ramachandran reconstruction filter, a Smoothed Shepp & Logan reconstruction filter, a Shepp & Logan reconstruction filter has a high-frequency emphasis effect more than that of another reconstruction filter, which will be described in detail below with reference toFIG. 16.

When the first reconstruction is performed using the standard reconstruction filter, as illustrated inFIG. 4, the first reconstruction unit24aobtains a reconstructed image (“vessel image” illustrated inFIG. 4) from which the vessels are extracted. Since the reconstructed image is reconstructed by the standard reconstruction filter, most of the stent is not drawn in the reconstructed image. The reconstructed image is formed such that concave and convex portions of the surface of the vessels are not excessively emphasized and it is easy to observe the overall structure of the vessels.

When the second reconstruction is performed using the high-frequency emphasis reconstruction filter, the second reconstruction unit24bobtains a reconstructed image (“stent image” illustrated inFIG. 4) from which the stent is clearly extracted up to a strut, as illustrated inFIG. 4.FIG. 4illustrates a stent image from which only a stent portion is extracted. However, in practice, an image from which both the stent and the vessels are extracted is used. The stent is made of metal and has a large computed tomography (CT) value. The second reconstruction unit24badjusts a window width or a window level for the reconstructed image to increase the density of the image of the stent and decrease the density of the image of the vessel. For example, the window width is the range of a density value allocated to the range from the maximum brightness to the minimum brightness and the window level is, for example, an intermediate density value of the window width.

FIG. 4illustrates an example in which two kinds of reconstructions are performed in parallel to each other, but the embodiment is not limited thereto. For example, after the reconstruction using the high-frequency emphasis reconstruction filter is performed, the reconstruction using the standard reconstruction filter may be performed. Alternatively, after the reconstruction using the standard reconstruction filter is performed, the reconstruction using the high-frequency emphasis reconstruction filter may be performed.

Returning toFIG. 2, when two kinds of reconstructed images are stored in the three-dimensional image storage unit25, the control unit20transmits the two kinds of reconstructed images stored in the three-dimensional image storage unit25to the three-dimensional image combining unit26. Then, the three-dimensional image combining unit26combines the two kinds of reconstructed images. Specifically, the three-dimensional image combining unit26combines the reconstructed image obtained by the first reconstruction with the reconstructed image obtained by the second reconstruction and outputs the combined image. In the combined image, the image of the stent and the image of the vessel overlap each other. The reconstructed image in which the stent is not drawn and the overall structure of the vessels is drawn so as to be easily observed and the reconstructed image in which only the stent is clearly drawn and the vessel is drawn with low density overlap each other in the combined image. As a result, as illustrated inFIG. 4, the overall structure of the vessels can be observed from the combined image and the stent is clearly drawn in the combined image.

When two kinds of reconstructed images are combined with each other, the control unit20transmits the combined three-dimensional image to the three-dimensional image display unit27, and the three-dimensional image display unit27displays the combined three-dimensional image as a volume rendering image or an MPR image on the monitor28.

In this embodiment, it is premised that various kinds of correction are not performed in the reconstruction process. However, the embodiment is not limited thereto. For example, scattered ray correction, beam hardening correction, and ring correction may be performed.

In this way, according to the first embodiment, both the stent and the vessels can be clearly observed. That is, according to the first embodiment, two kinds of reconstructions are performed on the X-ray collection image and the images are combined with each other. In this way, it is possible to obtain an image from which the overall structure of the vessels can be observed and in which the stent is clearly drawn. In particular, it is possible to clearly observe both the stent and the vessels without increasing the number of projection directions.

A first modification of the first embodiment will be described below.FIG. 5is a diagram illustrating a reconstruction and combination process (first modification) according to the first embodiment. The difference from the reconstruction and combination process described with reference toFIG. 4will be mainly described. As illustrated inFIG. 5, the three-dimensional reconstruction unit24according to the first modification extracts the area of interest using the reconstructed image which is obtained by the first reconstruction using the standard reconstruction filter and performs the second reconstruction using the high-frequency emphasis reconstruction filter on only the extracted area of interest.

The area of interest may be determined by instructions from the operator. However, a method of automatically extracting the area of interest from the reconstructed image will be described. Specifically, the three-dimensional reconstruction unit24according to the first modification performs the processes of the specifying unit24cand the identification unit24d. The specifying unit24cperforms threshold processing on the reconstructed image obtained by the first reconstruction to specify stent markers. The identification unit24didentifies the area of interest surrounded by the stent markers in the reconstructed image obtained by the first reconstruction. The second reconstruction unit24bapplies the high-frequency emphasis reconstruction filter only to the area of interest identified by the identification unit24din the X-ray collection image, and generates a reconstructed image only for the area of interest.

FIG. 6is a diagram illustrating the stent markers according to the first embodiment. For example, the specifying unit24caccording to the first modification performs image processing on the reconstructed image obtained by the first reconstruction to specify the stent markers. As illustrated inFIG. 6, the stent markers are positioned at both ends of the stent (for example, four stent markers are provided at each end). The stent marker has high absorptance and a CT value that is as large as a portion of the stent marker. Therefore, the specifying unit24ccan perform, for example, threshold processing to specify the stent markers. The identification unit24dfurther extracts an area which is surrounded by the stent markers specified by the specifying unit24c, thereby identifying the area of interest, that is, a stent area. Similarly, a metal prosthetic tooth has a large CT value, but the volume of the metal prosthetic tooth is significantly larger than that of the stent marker. Therefore, when the volume is measured, the prosthetic tooth can be excluded.

Since the second reconstruction unit24bperforms the second reconstruction on only the stent area using the high-frequency emphasis reconstruction filter, only the stent portion is exactly extracted and a stent image in which the stent is clearly drawn is obtained, as illustrated inFIG. 5. As such, according to the first modification, it is easy to adjust the window width or the window level. In addition, since a second reconstruction area is small, the time required for reconstruction is reduced. Then, the three-dimensional image combining unit26combines the reconstructed image obtained by the first reconstruction with the reconstructed image obtained by the second reconstruction, similarly to the first embodiment described with reference toFIG. 4.

A second modification of the first embodiment will be described below.FIG. 7is a diagram illustrating a reconstruction and combination process (second modification) according to the first embodiment. The difference from the reconstruction and combination process described with reference toFIG. 5will be mainly described. In the second modification, the extraction of the area of interest is performed as a post-process of the second reconstruction using the high-frequency emphasis reconstruction filter.

That is, the second reconstruction unit24baccording to the second modification performs the second reconstruction using the high-frequency emphasis reconstruction filter on the entire X-ray projection image, similarly to the first embodiment described with reference toFIG. 4. The specifying unit24cperforms threshold processing on the reconstructed image obtained by the first reconstruction to specify the stent markers, and the identification unit24didentifies the area of interest surrounded by the stent markers in the reconstructed image obtained by the second reconstruction. That is, the identification unit24dcuts out the stent area from the reconstructed image obtained by the second reconstruction, using the information of the area of interest which is obtained from the reconstructed image obtained by the first reconstruction. Then, the three-dimensional image combining unit26combines the reconstructed image obtained by the first reconstruction with the reconstructed image which is obtained by the second reconstruction and from only the stent area is cut out. As such, in the second modification, it is also easy to adjust the window width or the window level.

Next, a second embodiment will be described. It is assumed that the second embodiment is implemented by the same X-ray diagnosis apparatus1as the X-ray diagnosis apparatus1according to the first embodiment. The difference between the second embodiment and the first embodiment is as follows. In the first embodiment, two kinds of reconstructions are performed on the image after the contrast medium is injected. However, in the second embodiment, an image is collected before and after the contrast medium is injected and two kinds of reconstructions are performed using the subtraction image and the image before the contrast medium is injected. Next, the difference from the first embodiment will be mainly described.

FIG. 8is a diagram illustrating a reconstruction and combination process according to the second embodiment. In the second embodiment, a subtraction unit23performs subtraction between an image (hereinafter, appropriately referred to as a “mask image”) before a contrast medium is injected and a non-uniformity correcting image and also performs subtraction between the mask image and an image (hereinafter, appropriately referred to as a “contrast image”) after the contrast medium is injected. Hereinafter, the image obtained by the former subtraction is appropriately referred to as an X-ray projection image and the image obtained by the latter subtraction is appropriately referred to as a digital subtraction angiography (DSA) image. Then, the subtraction unit23according to the second embodiment transmits the X-ray projection image and the DSA image to a three-dimensional reconstruction unit24.

The three-dimensional reconstruction unit24according to the second embodiment performs a first reconstruction using a standard reconstruction filter on the DSA image transmitted from the subtraction unit23, and performs a second reconstruction using a high-frequency emphasis reconstruction filter on the X-ray projection image. That is, a first reconstruction unit24aperforms the first reconstruction on the DSA image using the standard reconstruction filter and a second reconstruction unit24bperforms the second reconstruction on the X-ray projection image using the high-frequency emphasis reconstruction filter.

In this case, since the X-ray projection image is obtained before the contrast medium is injected, the vessel is not drawn in the X-ray projection image. In the reconstructed image obtained by the second reconstruction, the vessel is not drawn and only the stent is clearly drawn. Therefore, according to the second embodiment, it is easy to adjust the window width or the window level. Since the DSA image is a subtraction image before and after the contrast medium is injected, only the vessel is drawn in the DSA image. Then, similarly to the first embodiment, a three-dimensional image combining unit26combines the reconstructed image obtained by the first reconstruction with the reconstructed image obtained by the second reconstruction.

Therefore, according to the second embodiment, it is possible to clearly observe both the stent and the vessels. That is, according to the second embodiment, two kinds of reconstructions are performed on the image from which only the stent is extracted and the image from which only the vessel is extracted and the images are combined with each other. Therefore, unnecessary overlap, such as overlap between one of the two images and the image of the stent or the vessels which are unclearly drawn, does not occur and it is possible to clearly observe both the stent and the vessels.

A first modification of the second embodiment will be described.FIG. 9is a diagram illustrating a reconstruction and combination process (first modification) according to the second embodiment. The difference from the reconstruction and combination process described with reference toFIG. 8will be mainly described below. As illustrated inFIG. 9, the three-dimensional reconstruction unit24according to the first modification performs the first reconstruction using the standard reconstruction filter on the X-ray projection image in addition to the DSA image. That is, as described above, no vessel is drawn in the X-ray projection image. Therefore, the specifying unit24caccording to the first modification performs the first reconstruction using the standard reconstruction filter on the X-ray projection image and performs image processing on the reconstructed image, thereby specifying the stent markers.

The other processes are the same as those in the first modification of the first embodiment. The second reconstruction unit24bperforms the second reconstruction on only the stent area using the high-frequency emphasis reconstruction filter. Therefore, as illustrated inFIG. 9, only a stent portion is exactly extracted and a stent image in which the stent is clearly drawn is obtained. As such, according to the first modification of the second embodiment, it is unnecessary to adjust the window width or the window level. In addition, since the second reconstruction area is small, the time required for reconstruction is reduced.

Similarly to the first modification of the first embodiment, the area of interest may be determined by instructions from the operator.

A second modification of the second embodiment will be described.FIG. 10is a diagram illustrating a reconstruction and combination process (second modification) according to the second embodiment. The difference from the reconstruction and combination process described with reference toFIG. 9will be mainly described below. In the second modification, the extraction of the area of interest is performed as a post-process of the second reconstruction using the high-frequency emphasis reconstruction filter.

That is, the second reconstruction unit24baccording to the second modification performs the second reconstruction using the high-frequency emphasis reconstruction filter on the entire X-ray projection image, similarly to the second embodiment described with reference toFIG. 7. The specifying unit24cperforms the first reconstruction on the X-ray projection image using the standard reconstruction filter, and performs threshold processing on the reconstructed image to specify the stent markers. The identification unit24didentifies the area of interest surrounded by the stent markers in the reconstructed image obtained by the second reconstruction. That is, the identification unit24dcuts out the stent area from the reconstructed image obtained by the second reconstruction, using the information of the area of interest which is obtained from the reconstructed image obtained by the first reconstruction. Then, the three-dimensional image combining unit26combines the reconstructed image obtained by the first reconstruction with the reconstructed image which is obtained by the second reconstruction and from only the stent area is cut out. As such, in the second modification, it is also easy to adjust the window width or the window level.

Next, a third embodiment will be described. Before the third embodiment is described in detail, a reconstruction filter will be described. In the first embodiment and the second embodiment, two kinds of reconstruction filters may be appropriately combined with each other such that, among the Ramachandran reconstruction filter, the Smoothed Shepp & Logan reconstruction filter, and the Shepp & Logan reconstruction filter, one reconstruction filter has a high-frequency emphasis effect more than that of another reconstruction filter. In the following description, when a reconstruction filter has a relatively great high-frequency emphasis effect, it is referred to as a “sharp convolution filter” and, when a reconstruction filter has a less high-frequency emphasis, it is referred to as a “smooth convolution filter”.

In the first embodiment and the second embodiment, two kinds of reconstructions are performed by the “sharp convolution filter” and the “smooth convolution filter”. However, in the third embodiment, a method of performing a reducing process or a low-pass filtering process as a pre-process will be described.

That is, when the reducing process is performed as the pre-process and the “sharp convolution filter” is used as the reconstruction filter, the same effect as that when the “smooth convolution filter” is used is obtained as the entire result including the result of the reducing process. Similarly, when the low-pass filtering process is performed as the pre-process and the “sharp convolution filter” is used as the reconstruction filter, the same effect as that when the “smooth convolution filter” is used is obtained as the entire result including the result of the low-pass filtering process. Therefore, in the third embodiment, a first reconstruction unit24aof a three-dimensional reconstruction unit24performs the reducing process or the low-pass filtering process in advance.

Next, a reconstruction and combination process according to the third embodiment will be described in detail. The three-dimensional reconstruction unit24according to the third embodiment performs reconstruction in two stages, similarly to the first embodiment and the second embodiment.FIG. 11is a flowchart illustrating the flow of a reconstruction process according to the third embodiment. As illustrated inFIG. 11, the three-dimensional reconstruction unit24performs a “first-stage reconstruction” and a “second-stage reconstruction”. InFIG. 11, the three-dimensional reconstruction unit24performs the “first-stage reconstruction” and the “second-stage reconstruction” in parallel, but the embodiment is not limited thereto. For example, after the “first-stage reconstruction” is performed, the “second-stage reconstruction” may be performed. Alternatively, after the “second-stage reconstruction” is performed, the “first-stage reconstruction” may be performed.

Next, each of the “first-stage reconstruction” and the “second-stage reconstruction” will be described with reference toFIGS. 11 to 18.FIG. 12is a diagram illustrating the reconstruction process according to the third embodiment.

The “first-stage reconstruction” will be described. First, the three-dimensional reconstruction unit24(first reconstruction unit24a) reduces an X-ray projection image (Step S101). For example, as illustrated inFIG. 12, the three-dimensional reconstruction unit24(first reconstruction unit24a) reduces the X-ray projection image with a size of 1,024 pixels×1,024 pixels such that the size of a reduced pixel corresponds to that of four pixels before reduction. Then, the reduced X-ray projection image corresponds to 250 frames of X-ray projection images with a size of 512 pixels×512 pixels.

Then, the three-dimensional reconstruction unit24(first reconstruction unit24a) performs reconstruction on 250 frames of the reduced X-ray projection images with a size of 512 pixels×512 pixels using the smooth convolution filter, such as a Smoothed Shepp & Logan filter (Step S102).

Then, the three-dimensional reconstruction unit24(first reconstruction unit24a) performs a back-projection process to generate a first three-dimensional image and stores the generated first three-dimensional image in a three-dimensional image storage unit25(Step S103).

Next, the “second-stage reconstruction” will be described. First, the three-dimensional reconstruction unit (second reconstruction unit24b) extracts a portion of the X-ray projection image (Step S201). For example, as illustrated inFIG. 12, the three-dimensional reconstruction unit24(second reconstruction unit24b) extracts an X-ray projection image with a size of 512 pixels×1,024 pixels from a subtraction image with a size of 1,024 pixels×1,024 pixels. As a result, the three-dimensional reconstruction unit24(second reconstruction unit24b) acquires 250 frames of X-ray projection images with a size of 512 pixels×1,024 pixels.

Then, the three-dimensional reconstruction unit24(second reconstruction unit24b) performs a reconstruction process on the extracted X-ray projection image. In this embodiment, a filtered back-projection method proposed by Feldkamp, et al. is used as an example of the reconstruction method. The three-dimensional reconstruction unit24(second reconstruction unit24b) performs reconstruction on 250 frames of the extracted X-ray projection images with a size of 512 pixels×1,024 pixels, using the sharp convolution filter, such as the Shepp & Logan filter or the Ramachandran filter (Step S202).

Then, the three-dimensional reconstruction unit24(second reconstruction unit24b) performs the back-projection process to generate a second three-dimensional image and stores the generated second three-dimensional image in the three-dimensional image storage unit25(Step S203).

Next, a reconstruction area reconstructed by the three-dimensional reconstruction unit24will be described.FIG. 13is a diagram illustrating the reconstruction area according to the third embodiment.FIGS. 14 and 15are diagrams illustrating a discrete interval according to the third embodiment.

First, the second reconstruction area is defined as a cylinder which is inscribed in the overlap between the X-ray tube11and a central area (FOV/2×FOV/2) of the FOV of the detector12when a quadrangular pyramid including the X-ray tube11and the central area of the FOV of the detector12is defined in all directions, as illustrated inFIG. 13. In addition, as illustrated inFIGS. 14 and 15, for example, the inside of the cylinder is three-dimensionally discretized with a length “d” at the center of a reconstruction area which is projected with the width “D” of one detection element of the detector12. When the distance between the X-ray tube11and the detector12is “L” and the distance between the X-ray tube11and the center of the reconstruction area is “l”, the length “d” is represented by the following Expression 3:

As such, the three-dimensional reconstruction unit24generates, as the second three-dimensional image, the data of a discrete point with a size of 512×512×512 voxels which is three-dimensionally discretized with the length “d”. Since the second three-dimensional image is reconstructed from the X-ray projection image with a size of 512 pixels×1,024 pixels, the three-dimensional reconstruction unit24can generate the data of the inside of a cube with a size of 512×512×512 voxels, which is the data of the outside of the cylinder, in addition to the data of the inside of the cylinder illustrated inFIG. 15. In this embodiment, an example of the discrete interval is given, but the discrete interval varies depending on devices or manufacturers. Therefore, basically, a discrete interval which is defined by a device may be used.

The first reconstruction area is defined as a cylinder which is inscribed in the overlap between the X-ray tube11and the area (FOV/2×FOV/2) of the FOV of the detector12when the quadrangular pyramid including the X-ray tube11and the area of the FOV of the detector12is defined in all directions, as illustrated inFIG. 13. In addition, the inside of the cylinder is three-dimensionally discretized with a length “2d”. As such, the three-dimensional reconstruction unit24generates, as the first three-dimensional image, the data of a discrete point with a size of 512×512×512 voxels which is three-dimensionally discretized with the length “2d”.

Next, the convolution filter will be described.FIG. 16is a diagram illustrating the convolution filter according to the third embodiment. In general, examples of the convolution filter include a “smooth” convolution filter used in the “first-stage reconstruction” and a “sharp” convolution filter used in the “second-stage reconstruction”. The high-frequency emphasis effect of the “sharp” convolution filter is more than that of the “smooth” convolution filter.

For example,FIG. 16illustrates three kinds of convolution filters. The three kinds of convolution filters are the Ramachandran convolution filter, which is the “sharpest” convolution filter, the Smoothed Shepp & Logan convolution filter, which is the “smoothest” convolution filter, and the Shepp & Logan convolution filter, which is an intermediate convolution filter.

In general, when the “sharp” convolution filter is used for reconstruction, a high frequency is emphasized. As a result, it is easy to obtain a clear image. That is, the X-ray diagnosis apparatus1according to the third embodiment performs different processes, such as the “first-stage reconstruction” and the “second-stage reconstruction”, extracts a clear image in the area of interest, without changing the convolution filters applied to the first-stage reconstruction and the second-stage reconstruction, and prevents the deterioration of the image of a peripheral portion.

In this way, as illustrated inFIG. 12, the first three-dimensional image and the second three-dimensional image are generated. As can be seen from the comparison between the first three-dimensional image generated by the “first-stage reconstruction” and the second three-dimensional image generated by the “second-stage reconstruction”, the size of a voxel of the second three-dimensional image is less than that of a voxel of the first three-dimensional image and the spatial resolution of the second three-dimensional image is higher than that of the first three-dimensional image. That is, since the second three-dimensional image is generated from the subtraction image which is extracted without being reduced, the size of a voxel of the second three-dimensional image maintains high spatial resolution when the X-ray collection image is collected. In addition, since the first three-dimensional image is generated from the reduced subtraction image, the size of a voxel of the first three-dimensional image is less than the spatial space resolution when the X-ray collection image is collected.

The three-dimensional image combining unit26according to the third embodiment combines the three-dimensional images, similarly to the first and second embodiments. For example, as illustrated inFIG. 12, the three-dimensional image combining unit26combines the first three-dimensional image with the second three-dimensional image. The three-dimensional image combining unit26according to the third embodiment preferentially uses the second three-dimensional image in an overlap area between the first three-dimensional image and the second three-dimensional image.

In this embodiment, the Feldkamp method is given as an example of the reconstruction method, but the embodiment is not limited thereto. For example, an iterative reconstruction method, such as an algebraic reconstruction technique (ART), may be used. In this embodiment, it is premised that various kinds of correction are not performed in the reconstruction process, but the embodiment is not limited thereto. For example, correction, such as scattered ray correction, beam hardening correction, and ring correction, may be performed.

In this embodiment, the second reconstruction area is at the center of the first reconstruction area. However, the embodiment is not limited thereto, and the second reconstruction area may deviate from the center. However, since a phenomenon, such as the deterioration of image quality, is likely to occur in the periphery of the first reconstruction area, it is not preferable that the second reconstruction area greatly deviate from the center.

FIG. 17is a diagram illustrating the position of the second reconstruction area according to the third embodiment. For example, as illustrated inFIG. 17, it is preferable that the second reconstruction area be in an area included in a cylinder with a radius that is two-thirds of the radius “R” of the first reconstruction area and a height that is two-thirds of the height “h” of the first reconstruction area.

In addition, for example, there is a method of designating the position of the second reconstruction area on the X-ray collection image which is collected at an angle of about 90 degrees.FIG. 18is a diagram illustrating the designation of the position of the second reconstruction area according to the first embodiment. InFIG. 18, a central circle indicates the cylinder of the first reconstruction area or the second reconstruction area, as viewed from the upper side. As illustrated inFIG. 18, for example, when the position of the aneurysm on the X-ray collection image at two angles (for example, at an angular interval of about 90 degrees) relative to the cylinder is designated, the position of the aneurysm in a three-dimensional space is determined. The three-dimensional reconstruction unit24may extract the X-ray projection image in the second reconstruction stage such that the aneurysm determined in the three-dimensional space is disposed at the center.

In this way, according to the third embodiment, it is possible to achieve both the extraction of a wide area (for example, the entire area in the field of view) and the extraction of a clear area of interest at the same time. For example, it is possible to satisfy clinical needs for minutely observing the area of interest including the stent while checking the overall structure of the vessels and for significantly reducing the amount of exposed dose to the patient.

The three-dimensional reconstruction unit24according to the third embodiment performs a convolution filtering process when the first three-dimensional image and the second three-dimensional image are generated. The high-frequency emphasis effect of the convolution filtering process performed when the second three-dimensional image is generated is more than that of the convolution filtering process performed when the first three-dimensional image is generated.

In this way, according to the third embodiment, it is possible to extract a clear image of the area of interest.

In the third embodiment, the three-dimensional image combining unit26simply combines the first three-dimensional image with the second three-dimensional image, but the embodiment is not limited thereto. In some cases, the pixel levels (density values) of the first three-dimensional image and the second three-dimensional image are different from each other due to some factors, such as various correction effects. Therefore, it is preferable that the three-dimensional image combining unit26compare the pixel level of a portion of the first three-dimensional image corresponding to the second reconstruction area with the pixel level of the second three-dimensional image and perform the correction. For example, the three-dimensional image combining unit26calculates the difference (AV1−AV2) between the average pixel level AV2 of the second three-dimensional image and the average pixel level AV1 of a portion of the first three-dimensional image corresponding to the second reconstruction area and adds the difference (AV1−AV2) to the pixels of the second three-dimensional image. This is global correction.

The three-dimensional image combining unit26may perform local correction after the global correction. For example, the three-dimensional image combining unit26compares the average pixel level of a small area (for example, a size of 32 pixels×32 pixels) of the second three-dimensional image with the average pixel level of a small area of the first three-dimensional image corresponding to the small area of the second three-dimensional image and performs correction such that there is no large difference between the two pixel levels. When the correction is performed in this way, the pixel levels of the first three-dimensional image and the second three-dimensional image are adjusted and the boundary between the two images is smoothly extracted from the combined three-dimensional image. In addition, the correction described in the first modification may be similarly applied to the first and second embodiments.

In the third embodiment, the three-dimensional reconstruction unit24(first reconstruction unit24a) performs the reducing process as the pre-process of the first-stage reconstruction. However, the three-dimensional reconstruction unit24may perform a low-pass filtering process, not the reducing process, on the X-ray collection image and may reconstruct the filtered image subjected to the low-pass filtering process. For example, when the low-pass filter is of a type in which a value is reduced to one-fourth in an area with a size of 2 pixels×2 pixels, the same effect as that when the reducing process is performed is obtained.

In addition, the three-dimensional reconstruction unit (first reconstruction unit24a) may combine a given low-pass filter with a convolution filter as the first-stage reconstruction and may directly perform a reconstruction process on the X-ray collection image using the combined filter. In this modification, the discrete interval of the first-stage reconstruction is not necessarily the length “2d”. For example, the three-dimensional reconstruction unit24(first reconstruction unit24a) may perform reconstruction at a discrete interval of a length “1.5d” using a low-pass filter which is slightly sharper than the low-pass filter equivalent to the reducing process.

In the third embodiment, the three-dimensional reconstruction unit24(second reconstruction unit24b) extracts only a central portion with a size of 512 pixels×1,024 pixels and performs reconstruction. However, the second-stage reconstruction attaches importance to spatial resolution rather than to density resolution. Therefore, the three-dimensional reconstruction unit24(second reconstruction unit24b) may extract a central portion with a size of 512 pixels×512 pixels, instead of extracting the portion with a size of 512 pixels×1,024 pixels and process the extracted portion. In this case, the density resolution of the reconstructed image is reduced a little, but the filtering process ends in a short time.

In this case, the second three-dimensional image generated by the three-dimensional reconstruction unit24(second reconstruction unit24b) is reconstructed from the X-ray projection image with a size of 512 pixels×512 pixels, not the X-ray projection image with a size of 512 pixels×1,024 pixels. Therefore, the three-dimensional reconstruction unit24(second reconstruction unit24b) can generate the data of only the inside of the cylinder illustrated inFIG. 15and cannot generate the data of the outside of the cylinder, which is also the data of the inside of a cube with a size of 512×512×512 voxels (a predetermined value is appropriately given).

As the medical image processing device according to the embodiment, the X-ray diagnosis apparatuses according to some embodiments have been described above, but the embodiment is not limited thereto.

For example, the medical image processing device according to each of the embodiments may be incorporated into an X-ray CT apparatus. In addition, for example, the medical image processing device according to each of the embodiments may be a single medical image processing device. In this case, the medical image processing device acquires the X-ray collection image collected by the medical image diagnosis apparatus and generates a three-dimensional image on the basis of the acquired X-ray collection image. For example, the medical image processing device may be connected to the medical image diagnosis apparatus, such as an X-ray diagnosis apparatus or an X-ray CT apparatus, through a network and receive the X-ray collection image from the medical image diagnosis apparatus, thereby acquiring the X-ray collection image.

When a single medical image processing device is used, the medical image processing device includes the three-dimensional reconstruction unit24and the three-dimensional image combining unit26according to the above-described embodiments. The A/D conversion unit21and the subtraction unit23may be appropriately provided according to the format of data, for example, whether data received from the medical image diagnosis apparatus is an analog signal, an X-ray collection image converted into a digital signal, or an X-ray projection image subjected to the subtraction process. The two-dimensional image storage unit22and the three-dimensional image storage unit25may be appropriately provided according to an operation form, for example, whether the storage device is provided inside or outside the medical image processing device. The monitor28may be appropriately provided according to an operation form, for example, whether the display device is provided inside or outside the medical image processing device.

In the third embodiment, the “first-stage reconstruction” is performed on the basis of the data subjected to the pre-process and the “second-stage reconstruction” is performed on the basis of the data which has not been subjected to the pre-process. Since the pre-process is similar to a process equivalent to the process to which the “smooth” convolution filter is applied, the convolution filter applied to each of the “first-stage reconstruction” and the “second-stage reconstruction” is not necessarily changed. For example, the same convolution filter may be used.

[Area of Interest Other than Stent]

In the above-described embodiments, an example of extracting the clear image of the stent in the intervention treatment is given, but the case in which the medical image processing device is applied is not limited thereto. When the user wants to extract a wide area and the clear image of the area of interest at the same time, the medical image processing device may be similarly applied. For example, the medical image processing device may be similarly applied when the user wants to extract the clear image of a part with a complicated structure, such as the bone of the ear, as the area of interest.

Various numerical values (for example, “1,024” and “512”) are illustrative, and may be arbitrarily changed depending on, for example, an operation form.

According to the medical image processing device according to at least one of the above-described embodiments, it is possible to clearly observe both the stent and the vessels.

Some embodiments of the invention have been described above. However, these embodiments are illustrative, but are not intended to limit the scope of the invention. These embodiments can be performed in other various ways, and various omissions, substitutions, and changes can be made without departing from the scope and spirit of the invention. These embodiments or modifications thereof are included in the scope and spirit of the invention and are also included in the scope of equivalents to the invention disclosed in the claims.