Abstract:
An X-ray imaging apparatus includes a substantially C-arm, a support mechanism which rotatably supports substantially the C-arm, a rotation driving unit which drives rotation of substantially the C-arm, an X-ray tube mounted on substantially the C-arm, an X-ray detector mounted on substantially the C-arm, and a control unit which controls at least one of the rotation driving unit and an imaging control unit to make intervals between a plurality of contrast-enhanced images shorter than intervals between a plurality of mask images by changing a rotational speed of substantially the C-arm before and after injection of a contrast medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2007-187426, filed Jul. 18, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an X-ray imaging apparatus which generates a three-dimensional image of a blood vessel from images before and after the injection of a contrast medium. 
     2. Description of the Related Art 
     There is an examination technique called CTHA (CT during hepatic arteriography) for diagnosing a liver tumor. This is a method of performing CT (Computed Tomography) of a hepatic artery while injecting a contrast medium into it. In general, however, this technique can be implemented by only an apparatus (called an IVR-CT) comprising both a CT apparatus and an angiography apparatus. An IVR-CT is very expensive, and hence only some large hospitals can purchase it. 
     Recently, there has been proposed a method (to be referred to as soft tissue imaging hereinafter) of improving the visibility of a soft tissue by acquiring many projection images using an X-ray imaging apparatus and reconstructing an image from the many projection images. 
     It is expected that if soft tissue imaging can do the same thing as CTHA, even hospitals which do not own IVR-CTs can perform the same examination as that described above. On the other hand, even if soft tissue imaging can perform the same examination as that by CTHA, conventional 3D-DSA is an indispensable examination for identifying a nutrition blood vessel and an approach route. 
     CTHA as soft tissue imaging is, however, inferior to CT in density resolution, and hence requires a larger amount of contrast medium. It is therefore feared that an increasing amount of contrast medium will increase the burden on a patient. See Jpn. Pat. Appln. KOKAI Publication No. 2007-130244. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an X-ray imaging apparatus which can generate two types of three-dimensional images by one rotation radiography. 
     According to an aspect of the present invention, there is provided an X-ray imaging apparatus comprising a substantially C-arm, a support mechanism which rotatably supports substantially the C-arm, a rotation driving unit which drives rotation of substantially the C-arm, an X-ray tube mounted on substantially the C-arm, an X-ray detector mounted on substantially the C-arm, and a control unit which controls at least one of the rotation driving unit and an imaging control unit to make intervals between a plurality of contrast-enhanced images shorter than intervals between a plurality of mask images by changing a rotational speed of substantially the C-arm before and after injection of a contrast medium. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out herein after. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a block diagram showing the arrangement of an X-ray imaging apparatus according to an embodiment; 
         FIG. 2  is a perspective view showing the outer appearance of an X-ray imaging mechanism in  FIG. 1 ; 
         FIG. 3  is a view showing a radiography sequence in this embodiment; and 
         FIG. 4  is a view showing an image processing sequence in this embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An X-ray imaging apparatus according to a preferred embodiment of the present invention will be described below with reference to the views of the accompanying drawing. 
     As shown in  FIG. 1 , the X-ray imaging apparatus includes an X-ray imaging mechanism  10 . As shown in  FIG. 2 , the X-ray imaging mechanism  10  includes an X-ray tube  12  and an X-ray detector  14 . The X-ray detector  14  comprises an image intensifier  15  and a TV camera  16 . Alternatively, the X-ray detector  14  comprises a flat panel detector (FPD) having semiconductor detection elements arrayed in a matrix form. The X-ray tube  12  and the X-ray detector  14  are mounted on a C-arm  60  so as to face each other. A subject P on a top  50  of a bed is placed between the X-ray tube  12  and the X-ray detector  14 . The C-arm  60  is supported by a column  64  suspended from a ceiling base  63  or by a floor-type stand. The C-arm  60  is rotatable with respect to three orthogonal axes A, B, and C. A rotation driving unit  22  is housed in the column  64 . The rotation driving unit  22  includes two power sources for separately rotating the C-arm  60  in the directions indicated by arrows A and B. The rotation driving unit  22  can rotate the C-arm  60  at a high speed like a propeller. 
     In addition to the X-ray imaging mechanism  10 , the X-ray imaging apparatus includes a system controller  20 , a camera controller  21 , a rotation controller  23 , an image memory  25 , a sensitivity correcting unit  26 , a corresponding image selecting unit  19 , a subtraction unit  27 , a body thickness identifying unit  28 , a scattered radiation correcting unit  29 , a beam hardening correction unit  30 , a filtering unit  31  which performs harmonic enhancement filtering or the like, an affine transformation unit  32  which performs image enlarge/movement and the like, a three-dimensional reconstruction unit  34 , a three-dimensional image processing unit  35 , a D/A conversion unit  36 , and a display unit  37 . 
     While rotating the C-arm  60  at a high speed like a propeller using the rotation driving unit  22 , as described above, and changing the projection angle, the apparatus repeats radiography at intervals of, for example, 1° and acquires obtained X-ray intensity distributions (X-ray images) of 200 patterns corresponding to a rotation angle, for example, 200°. After a contrast medium is injected, the apparatus repeats radiography at intervals of, for example, 0.5° while rotating the C-arm  60  and changing the projection angle in the same manner as described above, and acquires obtained X-ray intensity distributions (X-ray images) of 400 patterns corresponding to a rotation angle, for example, 200°. The analog/digital converter (A/D converter) in the camera controller  21  converts the projected X-ray images into digital signal. Note that the X-ray images generated before the contrast medium is injected or before the contrast medium flows into a radiography region are called mask images, and the X-ray images generated after the contrast medium is injected or after the contrast medium flows into the radiography region are called contrast-enhanced images. 
     The image memory  25  is provided to store data associated with a plurality of mask images obtained by radiography before the injection of a contrast medium and data associated with a plurality of contrast-enhanced images obtained by radiography after the injection of the contrast medium. The corresponding image selecting unit  19  selects contrast-enhanced images after the injection of the contrast medium which match, in radiographic angle, the plurality of mask images obtained by radiography before the injection of the contrast medium. That is, the corresponding image selecting unit  19  identifies a plurality of contrast-enhanced images after the injection of the contrast medium which are obtained at radiographic angles equal to or closest to those of the plurality of mask images obtained by radiography before the injection of the contrast medium. The subtraction unit  27  generates a plurality of difference images (DSA (Digital Subtraction Angiography) images) which differ in radiographic angle by subtracting the plurality of mask images and the plurality of contrast-enhanced images selected by the corresponding image selecting unit  19 , which are equal or closest to each other in terms of radiographic angle. 
     The three-dimensional reconstruction unit  34  reconstructs a three-dimensional image (first three-dimensional image) on the basis of a plurality of difference images. 
     That is, the three-dimensional reconstruction unit  34  reconstructs a first three-dimensional image on the basis of the difference images based on a plurality of mask images and some of a plurality of contrast-enhanced images. 
     The three-dimensional image processing unit  35  converts the reconstructed first three-dimensional image into a three-dimensional image by, for example, volume rendering or the like. This image is a three-dimensional blood vessel image (3D-DSA image) having only information about blood vessels. The filtered backprojection method proposed by Feldkamp et al. will be described as an example of the reconstruction methods. A proper convolution filter like a Shepp &amp; Logan filter or a Ramachandran filter is applied to the DSA images of 200 frames. This method then obtains reconstruction data by performing backprojection computation. In this case, a reconstruction region is defined as a cylinder inscribed in a bundle of X-rays in all direction of the X-ray tube  12 . For example, the interior of this cylinder must be three-dimensionally discretized with a length d of the central portion of the reconstruction region projected by the width of one detection element of the X-ray detector  14 , and a reconstructed image of data of the discrete points must be obtained. In this case, the discretization interval is an example. Since various techniques are available, the discretization interval defined by the apparatus may be basically used. 
     The three-dimensional reconstruction unit  34  reconstructs a three-dimensional image (second three-dimensional image) on the basis of all the generated contrast-enhanced images. This three-dimensional image corresponds to a so-called CTHA image (soft tissue image) which improves the visibility of the soft tissue. The details of this image will be described later. 
     The greatest advantage of this embodiment is that the operation of generating contrast-enhanced images can be made common to radiographing operation for 3D-DSA and radiographing operation for CT-like imaging. That is, this embodiment generates mask images and contrast-enhanced images in radiographing operation for 3D-DSA, and executes CT-like imaging using the contrast-enhanced images. In other words, the embodiment generates contrast-enhanced images in radiographing operation for CT-like imaging, but executes 3D-DSA using several images of the contrast-enhanced images and the mask images generated in 3D-DSA radiographing operation. Note that CT-like imaging requires a larger number of contrast-enhanced images than 3D-DSA. In addition, the intervals between the images are shorter. 
     As shown in  FIG. 3 , the acquisition of projection data is performed twice before and after the injection of a contrast medium. In radiography before the injection of a contrast medium, the apparatus repeats radiographing operation at a predetermined frame rate, typically 30 fps, while rotating the C-arm  60  at, for example, a rate of 30°/sec. With this operation, mask images of 200 frames are acquired at intervals of 1°. The analog/digital converter in the camera controller  21  converts the data of the acquired mask images of 200 frames into digital signals, and stores the signals in the image memory  25  in correspondence with the respective radiographic angle data. Thereafter, the C-arm  60  is returned to the initial rotation start position. A contrast medium is then injected with a contrast medium injector, and the apparatus repeats radiography at the same frame rate (30 fps) while rotating the C-arm  60  at a rate of 15°/sec, which is ½ the rate in radiography before the injection of the contrast medium. With this operation, contrast-enhanced images are acquired at intervals of 0.5°, which is ½ the intervals in radiography before the injection of the contrast medium. The data of the acquired contrast-enhanced images of 400 frames are stored in the image memory  25 . 
     Note that if the read rate (frame rate) of the X-ray detector  14  can be increased, it suffices to adjust the rotational speed of the C-arm  60  to 30°/sec and the image read rate of the detector to 60 frames/sec. The data of the contrast-enhanced images of 400 frames are stored in the image memory  25  in correspondence with the respective radiographic angle data. 
     The number of contrast-enhanced images is almost two times, three times, or four times that of mask images. 
     As shown in  FIG. 4 , after the radiography, for the  200  mask images (IM N ), the corresponding image selecting unit  19  selects 200 contrast-enhanced images (IC n ), from the 400 contrast-enhanced images, which are equal in radiographic angle to the mask images. The 200 contrast-enhanced images (IC N ) and the 200 mask images (IM n ) which are equal in radiographic angle are subtracted from each other. The three-dimensional reconstruction unit  34  reconstructs a first three-dimensional image (3D-DSA image) on the basis of the 200 difference images. This three-dimensional image mainly represents a blood vessel form as a contrast-enhanced region, with a non-blood vessel region such as a bone and soft tissue which are not contrast-enhanced being mainly removed. The reconstructed image is transferred to the three-dimensional image processing unit  35 . The three-dimensional image processing unit  35  then converts the image into a three-dimensional image by volume rendering or the like, and displays it on the display unit  37  via the D/A conversion unit  36 . 
     This apparatus generates a CTHA image (second three-dimensional image) concurrently with or before or after the generation of this three-dimensional image and display processing. A CTHA image is generated by using all the acquired 400 contrast-enhanced images (IC n ). First of all, the sensitivity correcting unit  26  subtracts the contrast-enhanced images and images for detector sensitivity correction. The images for detector sensitivity correction are data representing the sensitivity of the detector and X-ray intensity differences. The sensitivity correcting unit  26  subtracts the contrast-enhanced images after the injection of the contrast medium from the images for detector sensitivity correction. The three-dimensional reconstruction unit  34  reconstructs a three-dimensional image from a plurality of sensitivity-corrected contrast-enhanced images. The body thickness identifying unit  28  performs threshold processing for this three-dimensional image to separate the image into a born portion, a soft tissue portion, and a background region. The apparatus then generates projection images of 400 frames by projecting this three-dimensional image in the same directions as those in the radiography. The apparatus calculates thicknesses B(θ, i, j) and T(θ, i, j) of a bone and soft tissue on an X-ray path for each pixel of each projection image. 
     The thickness data B(θ, i, j) and T(θ, i, j) and projection data P(θ, i, j) are sent to the scattered radiation correcting unit  29 . Scattered radiation correction is performed by using the thicknesses of the bone and soft tissue and referring to a two-dimensional correction table. The beam hardening correction unit  30  then corrects the values of the projection images by also referring to the two-dimensional correction table on the basis of the thickness data. Note that the correction table is empirically obtained. 
     The projection images of 400 frames having undergone the scattered radiation correction and beam hardening correction are sent to the three-dimensional reconstruction unit  34  to be used for the reconstruction of a third three-dimensional image. This three-dimensional image is an image approximate to a CTHA image. Obtaining a CTHA image as an image obtained by soft tissue imaging and a 3D-DSA image by one radiography in this manner makes it possible to reduce the dose to the patient and the amount of contrast medium used and shorten the examination time, thereby reducing the burden on the patient. 
     This embodiment has exemplified the method of changing an angle sampling pitch by changing a rotational speed while fixing a frame rate. In the embodiment, the rotational speed in radiography for mask images is different from that in radiography for contrast-enhanced images, and hence the vibrations of the C-arm and the like vary. In the strict sense, therefore, correction data for vibrations and the like must be measured separately in advance. If, however, the frame rate can be increased, it suffices to change the angle sampling pitch by changing the frame rate while fixing the rotational speed. In the embodiment, correction data can be commonly used for the acquisition of mask images and for the acquisition of contrast-enhanced images. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.