Patent Number: 055984533
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method for X-ray fluoroscopy or radiography as well as an X-ray apparatus and more particularly, to a technique which is suitable for three-dimensionally measuring a large view field such as human chest in an X-ray computerized tomography (CT) scan method and an X-ray CT apparatus. As a prior art method for measuring an X-ray fluoroscopic or radiographic image from a plurality of directions to observe or record a stereoscopic dynamic image, a rotational digital angiography (DA) or a rotational digital subtraction angiography (DSA) is described in a journal entitled "Toshiba Medical Review", No. 45, pages 2 to 11, 1992. In the journal, a C arm is provided at its one end with an X-ray image intensifier which is positioned opposed to an X-ray tube so that continuous images appear on a monitor while the C arm is rotated, whereby an operator can observe the stereoscopic dynamic images or acquire DSA images taken from a plurality of directions. As one of general methods for obtaining a completer X-ray three-dimensional image, there is known a method in which tomographic images obtained by an X-ray CT apparatus are connected to each other through image processing. This method however has had a problem that the X-ray CT requires a lot of imaging time. For the purpose of reducing the imaging time, there is advantageously used a cone-beam CT apparatus in which a 2-dimensional X-ray detector is used as an X-ray detector and 2-dimensional transmission images of a subject obtained through X-ray beams emitted from an X-ray source in a cone shape are use to reconstruct a 3-dimensional X-ray image of the subject. Disclosed in a journal entitled "Medical Imaging Technology", Vol. 10, pp. 113-118, 1992 is a cone-beam CT apparatus wherein a 2-dimensional X-ray detector is made up of an X-ray image intensifier and a television camera. Also disclosed in a paper entitled "Development of 3D CT with a large area detection system" of a journal "Radiology", Vol. 185(P), p. 271, 1992 is a large-view-field cone-beam CT apparatus wherein a 2-dimensional X-ray detector is made up of a large-area phosphor screen and a television camera. Known a typical algorithm of reconstructing a 3-dimensional image of a subject in a cone-beam CT apparatus is the Feldkamp's method (refer to a paper "practical cone-beam algorithm" of a journal "Optical Society of America," written by L. A. Feldkamp, et al., A/Vol. 1(6), pp. 612-619, 1984). There are also described in IEEE Transactions on Medical Imaging, Vol. 12, No. 3, pp.486-496, 1993 a method in which an imaging unit including an X-ray source and an X-ray detector is rotated around a subject and at the same time the subject is moved in a direction perpendicular to a rotation plane to enlarge a view field with respect to the rotation-axis direction of the subject, as well as a reconstruction algorithm thereof. SUMMARY OF THE INVENTION In a prior art rotational DA or rotational DSA apparatus, a measuring view field is limited by the size of view field of an X-ray image intensifier. Further, in the apparatus described in the above journal "Medical Imaging Technology," it is difficult from the technical viewpoint to obtain an X-ray image intensifier having a large view field as well as high resolution, and thus also difficult to obtain a large-view-field, high-quality three-dimensional image. For this reason, when it is desired to obtain a high quality image of such a subject under inspection requiring a large view field as human chest, imaging is restricted to only a part of the chest. In addition, with the apparatus described in the above radiology journal, since it is technically difficult to obtain a phosphor screen with a high sensitivity as well as a high resolution, and thus also difficult to acquire a high quality of stereoscopic image with a large view field. Even in an X-ray CT apparatus using an ordinary X-ray detector having detection elements one-dimensionally arranged or in a cone-beam CT apparatus using a 2-dimensional X-ray detector, the view field of a transaxial sectional plane has been so far circular. That is, there has not been suggested so far a technology for overcoming such view field limitation by the size of the detector and for enlarging the view field of the transaxial sectional plane. Accordingly, when such an apparatus is applied to medical examinations, there occurs such a problem that a part of a patient to be examined becomes out of the view field. It is accordingly an object of the present invention to provide a technology for enabling the view field of X-ray fluoroscopic, radiographic or CT images to be enlarged. Another object of the present invention is to provide a technology for enabling the view field of a transaxial sectional plane of a high quality three-dimensional image to be enlarged. A further object of the present invention is to provide a technology by which a high quality of three-dimensional image can be obtained at a high speed. Yet another object of the present invention is to provide an X-ray CT apparatus which can improve a diagnostic ability of lung cancer and so on. These and other objects and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. Typical ones of features of the present invention are summarized as follows. (Feature 1): A fluoroscopic or radiographic method or CT scan method in which a pair of an X-ray source and an X-ray detection unit (including an X-ray detector, an optical lens unit and a television camera) opposed to the X-ray source for detecting an X-ray image is rotated on a circular orbit having an identical rotation center and at the same time a subject is moved in a direction parallel to the rotation plane to perform X-ray fluoroscopic or radiographic operation or CT scan. According to such a CT scan method, there can be obtained a 2-dimensional or 3-dimensional tomographic image on the basis of projection data made of X-rays passed through the subject from a plurality of directions. (Feature 2): In the fluoroscopic or radiographic method or CT scan method having the feature 1, the movement of the subject is a periodical reciprocating movement on a straight line parallel to the rotation plane of the pair of X-ray source and X-ray detection unit, and a period of the reciprocating movement coincides with a period of rotation of the pair of X-ray source and X-ray detection unit. (Feature 3): The movement of the subject is a composite movement corresponding to a combination of a periodical reciprocating movement on a straight line parallel to the rotation plane of the pair of X-ray source and X-ray detection unit and a vertical movement with respect to the rotation plane of the pair of X-ray source and X-ray detection unit. (Feature 4): An X-ray apparatus in which an imaging unit including an X-ray source and an X-ray detection unit opposed to the X-ray source is rotated on a circular orbit having an identical rotation center and at the same time, a subject is moved in a direction parallel to the rotation plane to perform fluoroscopic or radiographic operation or CT scan. (Feature 5): An X-ray apparatus in which an imaging unit including an X-ray source and an X-ray detection unit opposed to the X-ray source is rotated on a circular orbit having an identical rotation center and at the same time, a subject is moved in directions parallel and vertical to the rotation plane to perform fluoroscopic or radiographic operation. (Feature 6): An X-ray apparatus having the feature 4, which further comprises a bed board for limiting a part of the subject to be subjected to the X-ray fluoroscopic or radiographic operation or CT scan, means for performing a reciprocating movement of the bed board on a straight line parallel to the rotation plane, and means for causing a period of the reciprocating movement to coincide a rotation period of the imaging unit. (Feature 7): An X-ray apparatus having the feature 6, which further comprises control means, when the X-ray source is located at a point-symmetric position with respect to the rotation center, for controlling a position of the bed board determining the X-ray fluoroscopic or radiographic or CT scan part of the subject to be located at a point-symmetric position with respect to a middle point of the reciprocating movement of the bed board, and when the X-ray source is located at a line-symmetric position with respect to a straight line passing through the rotation center, parallel to the rotation plane and vertical to the reciprocating movement direction of the bed board, for controlling the bed position to be located at a point-symmetric position with respect to the middle point of the reciprocating movement of the bed board. (Feature 8): An X-ray apparatus having the feature 6 or 7, which further comprises control means, at the same time the pair of the X-ray source and X-ray detection unit is rotated by one turn along a circular orbit from a horizontal position as its start point, for controlling the bed board to be horizontally reciprocated with a center position as a reciprocation start point to perform the X-ray fluoroscopic or radiographic operation or CT scan during the rotation and reciprocating movement, and at the same time the pair of the X-ray source and X-ray detection unit is reversely rotated by one turn along the circular orbit after completion of the X-ray fluoroscopic or radiographic operation or CT scan, for controlling the bed board to again perform the same movement as the reciprocating movement to perform the fluoroscopic or radiographic operation or CT scan during the reverse rotation and reciprocating movement. (Feature 9): An X-ray apparatus having any of the features 1 to 8, wherein the X-ray detection unit including a 2-dimensional detector and an X-ray beam emitted from the X-ray source is a conical beam. (Feature 10): An X-ray apparatus having the feature 9, wherein the X-ray fluoroscopic or radiographic image of a subject is displayed with a single imaginary rotational axis parallel to a rotation axis of the imaging unit and fixed to the subject being always fixed on a display screen, thus realizing an image display method which facilitates inspector's easy observation of the image. (Feature 11): An X-ray apparatus having any of the features 1 to 10, wherein transmission data of a subject detected on the rotation plane of the imaging unit is subjected to a filtering operation in a coordinate system fixed to the imaging unit and the data subjected to the filtering operation is subjected to a back projection with respect to any reconstruction points of and X-ray CT image to thereby perform reconstruction of the X-ray CT image of the subject. In the above imaging system, a coordinate system unique to the present imaging system is realized to perform simple reconstruction. (Feature 12): An X-ray apparatus having any of the features 1 to 11, wherein the transmission data of the subject detected at X-ray input plane position other than the rotation plane of the imaging unit are subjected to a filtering operation on a plane which includes both a straight line present on the X-ray input plane of the X-ray detection means and parallel to the rotation plane and an X-ray generation point in a coordinate system fixed to the imaging unit, and the data after subjected to the filtering operation is subjected to a back projection with respect to a given reconstruction point of an X-ray CT image for reconstruction of the X-ray CT image of the subject, whereby the coordinate system realized in the X-ray CT apparatus set forth in the above feature 11 can be expanded to the entire 3-dimensional space. (Feature 13): An X-ray apparatus having the feature 11 or 12, wherein all the transmission X-ray data of the subject collected through a plurality of rotations of the imaging unit around the subject are subjected to a filtering operation, and the data after subjected to the filtering operation are subjected to the back projection with respect to a given reconstruction point, when data to be back-projected in the back projection is missing in a rotational angle in one of the plurality of rotations, back projection is performed using data obtained in another rotation. As a result, there can be realized such a reconstruction method unique to the present imaging system that the X-ray transmission data of the subject which cannot be collected in one rotation of the imaging unit are compensated for by a plurality of rotations, and the need for rearranging all the transmission data thus collected is eliminated. (Feature 14): An X-ray apparatus having the feature 13, wherein, when a plurality of data to be back-projected is present in a rotational angle in the plurality of turn rotations, the overlapped data are averaged and then back-projected. As a result, there can be realized a reconstruction method for obtaining an X-ray 3-dimensional image having a high S/N ratio with effective use of all the projection data collected by the above imaging unit. (Feature 15): An X-ray apparatus having the feature 13 or 14, wherein, in the course of reconstruction of the X-ray image of a subject, intermediate results of the reconstruction are sequentially displayed in the form of an image, whereby, even in the course of the reconstruction, the user can obtain schematic information on the X-ray CT image. In the X-ray fluoroscopic or radiographic methods an X-ray apparatuses in accordance with the features 1 to 8 for obtaining an X-ray fluoroscopic or radiographic image or CX-ray CT image; since the pair of the X-ray source and X-ray detection unit is moved on the circular orbits having an identical rotation center and at the same time, the subject is moved parallelly to the rotation plane to perform X-ray fluoroscopic or radiographic operation or X-ray CT scan, there can be obtained an X-ray image having an area wider than the view field of the X-ray detector. Thus, the view field of transaxial sectional plane of the X-ray fluoroscopic or radiographic image or X-ray CT image can be enlarged to improve a diagnostic ability such as lung cancer. Further, since the pair of the X-ray source and X-ray detection unit is moved on the circular orbits having an identical rotation center and at the same time, the subject is moved parallelly to the circular orbit plane to perform X-ray fluoroscopic or radiographic operation or X-ray CT scan; there can be obtained a transaxial sectional plane of a high quality of stereoscopic image at high speed. In the X-ray CT scan, in particular, the imaging unit including the X-ray source and X-ray detection unit for obtaining the X-ray transmission image is rotated by a plurality of rotation turns around the subject and at the same time, a relative positional relationship between the rotation center of the imaging unit and the subject is changed in a direction parallel to the rotation plane and this changing method is made different for the respective rotations, so that, of all the data necessary for the reconstruction, the data not able to be collected in one rotation in which the view field of the X-ray detector is smaller than the size of the subject can be all collected in the plurality of turns. When the reconstruction area is limited only to within the rotation plane of the imaging unit to obtain the X-ray 2-dimensional tomographic image of the subject, the reconstruction in the cone-beam CT apparatus is equivalent to the reconstruction in an ordinary X-ray CT apparatus. In the ordinary X-ray CT apparatus, the rotation center of the imaging unit is always fixed to the subject, and a coordinate system having the rotation center as its origin and fixed to the imaging unit is used in the calculation of the reconstruction. In the specification, this coordinate system is referred to as the fixed center coordinate system. The reconstruction using the fixed center coordinate system includes a filtering operation procedure of projection data and a back projection procedure of the projection data after subjected to the filtering operation. This is advantageous in that, since the coordinate system for the imaging unit is the same as that for the reconstruction, the calculation can be simplified. Further, in the case of the ordinary cone-beam CT apparatus, of all reconstruction points of an X-ray 3-dimensional image of a subject, reconstruction points contained within the rotation plane are directly subjected to the fixed center coordinate system, whereas reconstruction points not contained within the rotation plane are also subjected the fixed center coordinate system by regarding a plane, which contains the X-ray generation point and the each reconstruction point, and a intersection line between the plane and detection plane of X-ray detector is parallel to the rotation plane, as approximately the rotation plane, whereby the fixed center coordinate system is expanded to an entire 3-dimensional space for the reconstruction. In the imaging system of the present invention, on the other hand, the rotation center of the imaging unit is always moved with respect to the subject. In the present specification, the then coordinate system of the imaging unit will be referred to as the moving center coordinate system. In order to apply the reconstruction method defined in the above fixed center coordinate system to the projection data collected in the moving center coordinate system for the reconstruction, it is necessary prior to the reconstruction to perform the following operations 1 and 2. (Operation 1): Converts all projection data to projection data defined in the fixed center coordinate system. (Operation 2): Rearranges the projection data for enlargement of view field of the X-ray detector. That is, in the operation 1, the projection data obtained in the moving center coordinate system are converted to the projection data in the fixed center coordinate system. In the operation 2, according to the above X-ray CT imaging system, all the projection data necessary for the reconstruction are obtained through a plurality of rotations of the imaging unit. Accordingly, since the then projection data are collected in the fixed center coordinate system separately for the respective rotations, the operation 2 for rearranging such data is required. However, this method involves problems 1 to 4 which follow. (Problem 1): The operation 1 requires much labor. (Problem 2): The operation 2 requires much labor. (Problem 3): It is impossible to perform the rearrangement of the operation 2 in the entire 3-dimensional space. (Problem 4): The operation 2 cannot be carried out until all the projection data are collected. With regard to the problems 1 and 2, the operations 1 and 2 can be executed at the same time, but the simultaneous execution of the operations 1 and 2 requires much labor and much calculation time. Further, the processor must be complicated. With regard to the problem 3, in the cone-beam CT apparatus for reconstructing the X-ray 3-dimensional image of a subject with use of a 2-dimensional X-ray detector, the projection data of the subject based on X rays irradiated from the X-ray source in a conical shape are used for the reconstruction, which results in that it is impossible to rearrange the projection data in a spatially identical plane. This means that this problem is inherent in the cone-beam CT apparatus, the prior art reconstruction method based on the fixed center coordinate system cannot be applied to the present imaging system, and thus the execution of the above reconstruction is impossible in the prior art reconstruction method. With regard to the problem 4, the operation 2 cannot be executed until all the projection data are collected. Accordingly, the same holds true even for the reconstruction. For this reason, in the course of the data collection, the reconstruction calculation cannot be executed at the same time, with the result that a series of works from the imaging to the display of a reconstructed image takes a lot of time. In accordance with the present invention, all the above problems can be solved by employing a new coordinate system unique to the imaging system of the invention and by employing a new calculation method. The feature 11 has a function of performing the reconstruction with use of the moving center coordinate system fixed to the imaging unit in the above imaging system, whereby the coordinate system unique to the present imaging system can be realized for the reconstruction without the need for the execution of the above operation 1. Accordingly, the above problem 1 can be solved. In this case, the reconstruction procedure, as in the case of the use of the fixed center coordinate system, includes a filtering operation of projection data and a back projection of the projection data after subjected to the filtering operation. The feature 12 has a function of expanding the moving center coordinate system set forth in the feature 11 to an entire 3-dimensional space, whereby the projection data collected in the above imaging system can be used to realize a coordinate system for acquisition of an X-ray 3-dimensional CT image for the subject. The feature 13 has a function of rotating the imaging unit by a plurality of turns and also a function of making up for data lacking in each rotation with use of data obtained in another rotation in the apparatus set forth in the features 11 and 12, whereby there can be realized a calculation method unique to the present imaging system for reconstructing the projection data without rearranging them over the view field of the X-ray detector expanded by the above imaging system. This is a calculation method for performing the reconstruction without the above operation 2, and thus the above problems 2 and 3 can be solved. Other features of the present invention will be explained in the following. (Feature 16): This has a function of discarding some of the projection data after subjected to the filtering operation which are present in the peripheral area of view field of the X-ray detector, thereby realizing a reconstruction method for removing the influence of the projection data correcting filter caused by the view field interrupted in the peripheral area of the view field to obtain an accurater X-ray CT image. This method is advantageous in that the operation can be easily executed, though the view field becomes slightly narrow. (Feature 17): This has a function of making projection data outside the view field of the X-ray detector by an extrapolation method using the projection data obtained within the view field of the X-ray detector before subjected to the filtering operation, whereby the influence of the projection data correcting filter caused by the view field interrupted in the peripheral area of the view field can be suppressed, and there can be obtained a X-ray CT image of a subject in a wider range without discarding the data in the peripheral area of the view field, i.e., without decreasing the view field of the X-ray detector, as in the above feature 16. The feature 14 has a function of performing averaging operation over overlapped some of the projection data of a plurality of rotations at the time of the back projection, thus realizing a reconstruction method for obtaining an X-ray CT image having a high S/N ratio by making the most of the projection data of the subject. (Feature 18): This has a function of performing selecting operation over overlapped some of the projection data of a plurality of rotations at the time of the back projection, thus realizing a reconstruction method for using only some of the projection data of the subject suitable for the reconstruction. (Feature 19): This has a function of, in the selection of the projection data in the feature 18, selecting the projection data of an X ray issued from a farmost position from the reconstruction points of the X-ray CT image, so that, when the reconstruction area is expanded approximately to an entire 3-dimensional space, the expansion can be realized with use of the accuratest approximation. (Feature 20): This has a function of performing sequential reconstructing operation over projection data concurrently with the collection of the projection data of the subject in the X-ray apparatus of the present invention, whereby a series of works from the imaging of the subject to the display of the X-ray CT image can be concurrently executed at high speed. Feature 15 has a function of, even in the course of reconstruction of an X-ray image, sequentially displaying intermediate results of the reconstruction in the X-ray apparatus, whereby, even when urgent evaluation of the reconstruction image is required, the evaluation can be realized with use of the image of the intermediate results of the reconstruction without waiting for the full completion of the reconstruction. In accordance with the present invention, in short, there is provided an X-ray fluoroscopic or radiographic method or an X-ray apparatus wherein the pair of the X-ray tube and X-ray detection unit for obtaining an X-ray image is moved on the circular orbits having an identical rotation center and at the same time, the subject is moved in a direction parallel to the rotation plane to obtain an X-ray fluoroscopic or radiographic image or an X-ray CT image. As a result, the view field of the X-ray fluoroscopic or radiographic image or X-ray CT image can be enlarged. The view field of a high quality of stereoscopic image can be enlarged, for example, with respect to such a target requiring a large view field as human chest. In particular, this is effective for enlargement of the view field of a transaxial sectional plane in the X-ray CT, improving the diagnostic ability of lung cancer or the like. Further, a transaxial sectional plane of a high quality of stereoscopic image can be obtained at high speed.