X-ray CT apparatus Number:7,522,695 from the United States Patent and Trademark Office (PTO) owispatent A data acquisition device, which has an X-ray detector including a plurality of channel widths at which channels at its central portion are fine and channels at its peripheral portions are coarse or rough, and a plurality of data acquisition ranges including a data acquisition range wide in a channel direction and a data acquisition range narrow in the channel direction, and which is capable of performing switching among the data acquisition ranges for each data acquisition, is used to perform data acquisition on the channels fine at the central portion in the data acquisition range narrow in the channel direction, whereby an X-ray CT apparatus is provided which is capable of performing high-resolution imaging and brings about more satisfactory image quality.<H apparatus, ray, X, ray, CT, appara, 7522695.html, Patent, pto, 7522695

Title: X-ray CT apparatus

Abstract: A data acquisition device, which has an X-ray detector including a plurality of channel widths at which channels at its central portion are fine and channels at its peripheral portions are coarse or rough, and a plurality of data acquisition ranges including a data acquisition range wide in a channel direction and a data acquisition range narrow in the channel direction, and which is capable of performing switching among the data acquisition ranges for each data acquisition, is used to perform data acquisition on the channels fine at the central portion in the data acquisition range narrow in the channel direction, whereby an X-ray CT apparatus is provided which is capable of performing high-resolution imaging and brings about more satisfactory image quality.

Patent Number: 7,522,695 Issued on 04/21/2009 to Nishide, et al.

Inventors: Nishide; Akihiko (Tokyo, JP), Kawachi; Naoyuki (Tokyo, JP)
Assignee: GE Medical Systems Global Technology Company, LLC (Waukesha, WI)

Appl. No.: 11/489,065

Filed: July 19, 2006

Foreign Application Priority Data


Jul 19, 2005 [JP]			2005-208235

Current U.S. Class: 378/4 ; 250/370.09; 378/98.8

Current International Class: G01N 23/083 (20060101); H05G 1/38 (20060101)

Field of Search: 378/4-20,901,62,98.8,114-116,146-152,156,210 250/370.09,370.1,370.11,366,367,362,363.1,363.01,363.02,370.01,370.08-9,370.14

References Cited [Referenced By]

U.S. Patent Documents


3755672	August 1973	Edholm et al.
4200799	April 1980	Saito
4638499	January 1987	Eberhard et al.
5033075	July 1991	DeMone et al.
5054041	October 1991	Hampel
5185775	February 1993	Sirvin
5237599	August 1993	Gunji et al.
5396889	March 1995	Ueda et al.
5974109	October 1999	Hsieh
6023494	February 2000	Senzig et al.
6115448	September 2000	Hoffman
6157696	December 2000	Saito et al.
6198791	March 2001	He et al.
6215843	April 2001	Saito et al.
6243438	June 2001	Nahaliel et al.
6275562	August 2001	He et al.
6292527	September 2001	Guendel
6385278	May 2002	Hsieh
6400793	June 2002	Doubrava et al.
6404841	June 2002	Pforr et al.
6445764	September 2002	Gohno et al.
6895077	May 2005	Karellas et al.
2005/0053191	March 2005	Gohno et al.

Foreign Patent Documents


0950372	Oct., 1999	EP
1502548	Feb., 2002	EP
1216662	Jun., 2002	EP
1498908	Jan., 2005	EP
2000-193750	Jul., 2000	JP
9930616	Jun., 1999	WO
9930616	Jun., 1999	WO

Other References

European Search Report for application for EP 06253787, dated Dec. 17, 2007. cited by other.

Primary Examiner: Midkiff; Anastasia
Attorney, Agent or Firm: Armstrong Teasdale LLP

Claims

The invention claimed is:

1. An X-ray CT apparatus comprising: an X-ray data acquisition device comprising an X-ray generator and an X-ray detector positioned opposite the X-ray generator such that the X-ray generator and the X-ray detector are rotated about a center of rotation placed between the X-ray generator and the X-ray detector, the X-ray data acquisition device configured to acquire projection data of X-rays transmitted through a subject disposed between the X-ray generator and the X-ray detector, the X-ray data acquisition device is further configured such that a second detector channel width, d.sub.2, located at each peripheral portion of the X-ray detector as viewed in a channel direction becomes d.sub.1<d.sub.2 with respect to a first detector channel width, d.sub.1, located at a central portion of the X-ray detector as viewed in the channel direction or such that a plurality of detector widths, (d.sub.1, d.sub.2, . . . d.sub.i, . . . d.sub.n-1, d.sub.n), provided from the central portion of the X-ray detector as viewed in the channel direction to the peripheral portion of the X-ray detector, are sized such that d.sub.1.ltoreq.d.sub.2.ltoreq. . . . .ltoreq.d.sub.i.ltoreq. . . . .ltoreq.d.sub.n-1.ltoreq.d.sub.n,the X-ray data acquisition device further comprising a plurality of data acquisition sampling periods at which data acquisition is performed, the plurality of data acquisition sampling periods differing according to channel positions; an image reconstructing device configured to image-reconstruct the projection data acquired from the X-ray data acquisition device; a display device configured to display an image-reconstructed image generated by the image reconstructing device; and a control device configured to control an X-ray irradiation area in a row direction such that the X-ray irradiation area differs according to positions in the channel direction.

2. An X-ray CT apparatus comprising: an X-ray data acquisition device comprising an X-ray generator and an X-ray detector positioned opposite the X-ray generator such that the X-ray generator and the X-ray detector are rotated about a center of rotation placed between the X-ray generator and the X-ray detector, the X-ray data acquisition device configured to acquire projection data of X-rays transmitted through a subject disposed between the X-ray generator and the X-ray detector, the X-ray data acquisition device comprising a plurality of data acquisition ranges 1.sub.1.gtoreq.1.sub.2.gtoreq. . . . .gtoreq.1.sub.i.gtoreq. . . . .gtoreq.1.sub.n-1.gtoreq.1.sub.n arranged such that a first data acquisition range, 1.sub.1, is wider in a channel direction of the X-ray detector than an n.sup.th data acquisition range, 1.sub.n, the X-ray data acquisition device further configured such that the plurality of data acquisition ranges are switchable every data acquisition, the X-ray data acquisition device further comprising a plurality of data acquisition sampling periods at which data acquisition is performed, the plurality of data acquisition sampling periods differing according to channel positions; an image reconstructing device configured to image-reconstruct the projection data acquired from the X-ray data acquisition device; a display device configured to display an image-reconstructed image generated by the image reconstructing device; and a control device configured to control an X-ray irradiation area in a row direction such that the X-ray irradiation area differs according to positions in the channel direction.

3. The X-ray CT apparatus according to claim 1, wherein the X-ray data acquisition device is further configured to perform data acquisition at the central portion of the X-ray detector as viewed in the channel direction, the central portion having a narrower detector channel width than a detector channel width at the peripheral portion of the X-ray detector.

4. The X-ray CT apparatus according to claim 1, wherein the X-ray data acquisition device comprises a plurality of channels at which data acquisition is performed.

5. The X-ray CT apparatus according to claim 1, wherein the X-ray data acquisition device comprises a plurality of channels at which data acquisition is performed, and a plurality of views.

6. The X-ray CT apparatus according to claim 1, wherein the X-ray data acquisition device comprises a plurality of rows at which data acquisition is performed such that a number of rows differs according to each channel position.

7. The X-ray CT apparatus according to claim 1, wherein the control device is configured to control the X-ray irradiation area in such a manner that X-rays are radiated only into some of the central portion of the X-ray detector as viewed in the channel direction, the central portion of the X-ray detector having a narrower detector channel width than a detector channel width at the peripheral portion of the X-ray detector.

8. The X-ray CT apparatus according to claim 1, wherein the control device comprises a channel-direction collimator.

9. The X-ray CT apparatus according to claim 2, wherein the X-ray data acquisition device is configured to perform data acquisition at the central portion of the X-ray detector as viewed in the channel direction, the central portion having a narrower detector channel width than a detector channel width at the peripheral portion of the X-ray detector.

10. The X-ray CT apparatus according to claim 2, wherein the X-ray data acquisition device comprises a plurality of channels at which data acquisition is performed.

11. The X-ray CT apparatus according to claim 2, wherein the X-ray data acquisition device comprises a plurality of channels at which data acquisition is performed, and a plurality of views.

12. The X-ray CT apparatus according to claim 2, wherein the X-ray data acquisition device comprises a plurality of rows at which data acquisition is performed such that a number of rows differs according to each channel position.

13. The X-ray CT apparatus according to claim 2, wherein said the control device is configured to control an X-ray irradiation area in such a manner that X-rays are radiated only into some of the central portion of the X-ray detector as viewed in the channel direction, the central portion of the X-ray detector having a narrower detector channel width than a detector channel width at the peripheral portion of the X-ray detector.

14. The X-ray CT apparatus according to claim 2, wherein the control device comprises a channel-direction collimator.

15. The X-ray CT apparatus according to claim 1, wherein the control device comprises a beam forming X-ray filter.

16. The X-ray CT apparatus according to claim 2, wherein the control device comprises a beam forming X-ray filter.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Application No. 2005-208235 filed Jul. 19, 2005.

BACKGROUND OF THE INVENTION

An X-ray detector used in an X-ray CT apparatus, like a multi-row X-ray detector or a two-dimensional X-ray area detector of a matrix structure typified by a flat panel ahs heretofore been fabricated at constant intervals (pitches) and with a constant channel width as shown in FIG. 12 (refer to, for example, Japanese Unexamined Patent Publication No. 2000-193750).

An X-ray detector used in an X-ray CT apparatus, like a multi-row X-ray detector or a two-dimensional X-ray area detector of a matrix structure typified by a flat panel has heretofore been fabricated at constant intervals (pitches) and with a constant channel width as shown in FIG. 12 (refer to, for example, a patent document 1).

[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-193750

Therefore, even if the imaging area is made small in an attempt to see it in high resolution, the tomographic image is merely blurred and hence a tomographic image of high resolution was not obtained. Although a slight improvement in contrast and an improvement in resolution are performed by slightly intensifying a high-frequency or RF region of a reconstruction function, an increase in noise and an increase in artifact have been brought about as adverse effects.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide an X-ray CT apparatus capable of imaging or photographing a high-resolution X-ray tomographic image.

Another object of the present invention is to provide an X-ray CT apparatus capable of obtaining high resolution when a small imaging area is taken.

A further object of the present invention is to provide an X-ray CT apparatus which reduces the amount of used X rays and effectively uses the X rays thereby to enable a reduction in exposure of a subject to the X rays.

In a first aspect, the present invention provides an X-ray CT apparatus comprising X-ray data acquisition means which allows an X-ray generator, and a multi-row X-ray detector provided in opposing relationship to the X-ray generator and detecting X rays or a two-dimensional X-ray area detector of a matrix structure to be rotated about a center of rotation placed between the X-ray generator and the detector, thereby acquiring projection data of the X rays transmitted through a subject disposed between the X-ray generator and the detector; image reconstructing means which image-reconstructs the projection data acquired from the X-ray data acquisition means; and display means which displays an image-reconstructed image, wherein the X-ray data acquisition means is configured in such a manner that a detector channel width d2 at each peripheral portion of the detector as viewed in a channel direction becomes d1<d2 with respect to a detector channel width d1 at a central portion thereof as viewed in the channel direction, or a plurality of detector widths (d.sub.1, d.sub.2, . . . d.sub.i, . . . d.sub.n-1, d.sub.n) provided from the central portion of the detector as viewed in the channel direction to the peripheral portion thereof satisfy d.sub.1.ltoreq.d.sub.2.ltoreq. . . . .ltoreq.d.sub.i.ltoreq. . . . .ltoreq.d.sub.n-1.ltoreq.d.sub.n.

In the X-ray CT apparatus according to the first aspect, since X-ray detector channels narrower in channel width concentrate on the central portion, spatially high-resolution X-ray CT imaging can be conducted by performing data acquisition and image reconstruction using the X-ray detector channels narrow in channel width at the central portion.

In a second aspect, the present invention provides an X-ray CT apparatus comprising X-ray data acquisition means which allows an X-ray generator, and a multi-row X-ray detector provided in opposing relationship to the X-ray generator and detecting X rays or a two-dimensional X-ray area detector of a matrix structure to be rotated about a center of rotation placed between the X-ray generator and the detector, thereby acquiring projection data of the X rays transmitted through a subject disposed between the X-ray generator and the detector; image reconstructing means which image-reconstructs the projection data acquired from the X-ray data acquisition means; and display means which displays an image-reconstructed image, wherein the X-ray data acquisition means has a plurality of data acquisition ranges 1.sub.1.gtoreq.1.sub.2.gtoreq. . . . .gtoreq.1.sub.i.gtoreq. . . . .gtoreq.1.sub.n-1.gtoreq.1.sub.n from the data acquisition range 1.sub.1 wide in a channel direction of the detector to the data acquisition range 1.sub.n narrow in the channel direction, and is configured in such a manner that the data acquisition ranges are switchable every data acquisition.

In the X-ray CT apparatus according to the second aspect, since the narrower data acquisition range exists in the central portion, data acquisition is effected on the narrower data acquisition range of the central portion with fine channel widths and intervals and image reconstruction is performed, thereby enabling spatially high-resolution X-ray CT imaging.

In a third aspect, the present invention provides an X-ray CT apparatus wherein the X-ray data acquisition means performs data acquisition at a portion narrow in detector channel width, or the central portion of the detector as viewed in the channel direction when data acquisition is performed in the data acquisition range narrow as viewed in the channel direction of the detector.

In the X-ray CT apparatus according to the third aspect, since the detector channels narrower in channel width concentrate on the central portion and the narrower data acquisition range exists, data acquisition is effected on the narrower data acquisition range of the central portion with fine channel widths and at fine channel intervals and image reconstruction is performed, thereby enabling spatially high-resolution X-ray CT imaging.

In a fourth aspect, the present invention provides an X-ray CT apparatus wherein the X-ray data acquisition means has a plurality of channels at which data acquisition is performed.

In the X-ray CT apparatus according to the fourth aspect, since the number of the detector channels at which the data acquisition is performed, is switched in plural modes, data about a small number of detector channels at the central portion are acquired at high speed in a mode for a small number of channels and at the maximum value of a sampling rate of an A/D converter of the X-ray data acquisition means, and image reconstruction is performed, thereby enabling X-ray CT imaging high in resolution in terms of time.

In a fifth aspect, the present invention provides an X-ray CT apparatus wherein the X-ray data acquisition means has a plurality of channels at which data acquisition is performed, and a plurality of views.

In the X-ray CT apparatus according to the fifth aspect, since the number of the detector channels at which the data acquisition is performed, is switched in plural modes and the number of the views at which the data acquisition is performed, is switched in plural modes, data about a small number of detector channels at the central portion are acquired at high speed in the maximum value of a sampling rate of the A/D converter of the X-ray data acquisition means and in a mode for a small number of views, and image reconstruction is performed, thereby enabling X-ray CT imaging high in resolution in terms of time.

In a sixth aspect, the present invention provides an X-ray CT apparatus wherein the X-ray data acquisition means has a plurality of rows at which data acquisition is performed, and the number of the rows differs according to each channel position.

In the X-ray CT apparatus according to the sixth aspect, detector channels large in the number of rows as viewed in a z direction are concentrated on the central portion and in this condition, data acquisition is effected on the narrow data acquisition range of the central portion with fine channel widths and at fine channel intervals, and image reconstruction is performed, thereby enabling spatially high-resolution X-ray CT imaging.

In a seventh aspect, the present invention provides an X-ray CT apparatus wherein the X-ray data acquisition means has a plurality of data acquisition sampling periods at which data acquisition is performed.

In an eighth aspect, the present invention provides an X-ray CT apparatus wherein the X-ray data acquisition means has a plurality of data acquisition sampling periods at which data acquisition is performed, and the data acquisition sampling periods differ according to channel positions.

In the X-ray CT apparatus according to the seventh and eight aspects, data about a small number of detector channels at the central portion are collected or acquired at high speed in a mode short in data acquisition sampling period, and image reconstruction is carried out, whereby X-ray CT imaging high in resolution in terms of time is enabled.

In a ninth aspect, the present invention provides an X-ray CT apparatus including control means which controls an X-ray irradiation area in such a manner that X rays are radiated only into some of the range narrow in detector channel width, of the central portion of the detector as viewed in the channel direction or its inner range, the data acquisition range narrow in the channel direction of the detector or its inner range, or some of the data acquisition range narrow in the channel direction of the detector, i.e., the range narrow in detector channel width, of the central portion as viewed in the channel direction or its inner range.

In the X-ray CT apparatus according to the ninth aspect, since the irradiated X rays can be optimized narrower and radiated in the channel direction by the control means when data acquisition is done in the narrower data acquisition range of the central portion, subject's tomogram imaging at low exposure to radiation can be carried out.

In a tenth aspect, the present invention provides an X-ray CT apparatus having means which limits an X-ray irradiation area in such a manner that X rays are radiated into some range in the channel direction of the detector, which is fine in channel at the central portion as viewed in the channel direction of the detector or its inner range, the data acquisition range narrow in the channel direction of the detector or its inner range, or the data acquisition range narrow in the channel direction of the detector, i.e., the range for the fine channels of the central portion as viewed in the channel direction or its inner range.

In the X-ray CT apparatus according to the tenth aspect, since the irradiated X rays can be optimized narrower and radiated in the channel direction by the limiting means when data acquisition is performed in the narrower data acquisition range of the central portion, subject's tomogram imaging at low exposure to radiation can be performed.

The present invention can provide an X-ray CT apparatus capable of imaging or photographing a high-resolution X-ray tomographic image. Also, the present invention is capable of providing an X-ray CT apparatus capable of obtaining high resolution when a small imaging area is taken. Further, the present invention can provide an X-ray CT apparatus which reduces the amount of used X rays and effectively use the X rays to thereby enable a reduction in exposure of a subject to the X rays.

Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an X-ray CT apparatus according to one embodiment of the present invention.

FIG. 2 is an explanatory view illustrating an X-ray generator (X-ray tube) and a multi-row X-ray detector.

FIG. 3 is a flow diagram depicting a schematic operation of the X-ray CT apparatus according to the one embodiment of the present invention.

FIG. 4 is a flow diagram showing the details of pre-processing.

FIG. 5 is a flow diagram illustrating the details of a three-dimensional image reconstructing process.

FIGS. 6a and 6b are conceptual diagrams depicting a state of projection of lines on a reconstruction area in an X-ray penetration direction.

FIG. 7 is a conceptual diagram showing lines projected onto an X-ray detector plane.

FIG. 8 is a conceptual diagram illustrating a state of projection of projection data Dr (view, x, y) on a reconstruction area.

FIG. 9 is a conceptual diagram depicting backprojection pixel data D2 of respective pixels on a reconstruction area.

FIG. 10 is an explanatory view showing a state in which backprojection pixel data D2 are added corresponding to pixels over all views to obtain backprojection data D3.

FIGS. 11a and 11b are conceptual diagrams showing a state in which lines on a circular reconstruction area are projected in the X-ray penetration direction.

FIG. 12 is a diagram illustrating a conventional multi-row X-ray detector.

FIGS. 13a and 13b are diagrams showing a multi-row X-ray detector in which a central channel is brought into high resolution.

FIG. 14 is a diagram illustrating a conventional data read mode.

FIG. 15 is a diagram depicting a mode 1 for reading in the number of rows large at an inner central portion.

FIG. 16 is a diagram showing a mode 2 for reading in the number of rows large at the inner central portion.

FIG. 17 is a diagram illustrating a mode 3 for reading in the number of rows large at the inner central portion.

FIG. 18 is a diagram showing a row of modes for reading in row widths large in number and fine at an inner central portion of a multi-row X-ray detector having a plurality of types of channel widths and data acquisition ranges, and in row widths small in number and coarse at its outer peripheral portions.

FIG. 19 is a diagram illustrating the manner in which a subject is large and its area of interest is small.

FIG. 20 is a diagram showing a data transfer rate in a normal mode.

FIG. 21 is a diagram depicting a data transfer rate where data are acquired or collected only at a central portion as viewed in a channel direction.

FIG. 22 is a diagram showing a data transfer rate where data are collected only at the central portion as viewed in the channel direction.

FIG. 23 is a diagram illustrating a multi-row X-ray detector in which a plurality of types of data acquisition sampling periods are provided every data acquisition ranges.

FIG. 24 is a diagram showing an X-ray irradiation range matched with a data acquisition range by a channel-direction collimator.

FIGS. 25a, 25b, and 25c are diagrams illustrating a data acquisition range defined by a beam forming X-ray filter.

FIG. 26(a) is a diagram showing the setting of areas of interest at scout images as viewed in an RL direction (x direction), and FIG. 26(b) is a diagram showing the setting of areas of interest at scout images as viewed in an AP direction (y direction).

FIG. 27 is a diagram illustrating switching between an X-ray detector channel width d and an X-ray detector channel width d/2.

FIG. 28 is a diagram showing switching between a wide data acquisition range and a narrow data acquisition range.

FIGS. 29a and 29b are flow diagrams of the operation of an embodiment 2.

FIG. 30(a) is a diagram showing a channel-direction collimator (rotational-axis eccentric cylindrical system), FIG. 30(b) is a diagram showing a channel-direction collimator (shielding plate system), and FIG. 30(c) is a diagram showing an example of a beam forming X-ray filter.

FIGS. 31(a) and 31(b) are respectively diagrams showing channel-direction collimator control.

FIGS. 32a, 32b, and 32c are diagrams illustrating the manner in which projection data that lack at a channel-direction X-ray collimator are added.

FIG. 33 is a diagram showing feed forward control of the channel-direction collimator.

FIG. 34 is an explanatory view of an imaging area of interest and an irradiation channel range at the time of a view angle=0.degree..

FIG. 35 is an explanatory view of an imaging area of interest, an irradiation minimum channel and an irradiation maximum channel at the time of a view angle=0.degree..

FIG. 36 is an explanatory view of an imaging area of interest, an irradiation minimum channel and an irradiation maximum channel at the time of a view angle=.beta..

FIG. 37 is a diagram showing feedback control of the channel-direction collimator.

FIG. 38(a) is a diagram showing a normal position of a beam forming X-ray filter 32, FIG. 38(b) is a diagram showing position control (part 1) on the beam forming X-ray filter 32, and FIG. 38(c) is a diagram showing position control (part 2) on the beam forming X-ray filter 32.

FIG. 39 is a diagram illustrating image reconstruction functions different every X-ray detector channel intervals.

FIG. 40 is a flow diagram showing convolution of reconstruction functions where a plurality of types of X-ray detector channel widths exist.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will hereinafter be described in further detail by embodiments illustrated in the drawings. Incidentally, the present invention is not limited thereby.

Embodiment 1

FIG. 1 is a configurational block diagram of an X-ray CT apparatus according to one embodiment of the present invention. The X-ray CT apparatus 100 is equipped with an operation console 1, a photographing or imaging table 10, and a scanning gantry 20.

The operation console 1 is equipped with an input device 2 which accepts an operator's input, a central processing unit 3 which executes an image reconstructing process or the like, a data acquisition buffer 5 which acquires or collects projection data acquired by the scanning gantry 20, a monitor 6 which displays a CT image reconstructed from the projection data, and a memory device 7 which stores programs, data and an X-ray CT image therein.

The photographing table 10 is equipped with a cradle 12 which places a subject thereon and which takes it in a cavity section of the scanning gantry 20 and takes it out therefrom. The cradle 12 is moved up and down by a motor built in the photographing table 10 and moved linearly along the photographing table 10. The direction in which the cradle 12 of the photographing table 10 moves is defined as a z direction.

The scanning gantry 20 is equipped with an X-ray tube 21, an X-ray controller 22, a slice-thickness direction collimator 23, a multi-row X-ray detector 24, a DAS (Data Acquisition System) 25, a rotational section controller 26 which controls the X-ray tube 21 or the like being rotated about a body axis of the subject, and a control controller 29 which swaps control signals or the like with the operation console 1 and the photographing table 10. The scanning gantry 20 can be tilted.+-.about 30.degree. or so forward and rearward as viewed in the z direction by a tilt controller 27. In addition to the above, the scanning gantry 20 has a channel-direction collimator 31 and a beam forming X-ray filter 32.

FIG. 2 is an explanatory view of a geometric arrangement of the X-ray tube 21 and the multi-row X-ray detector 24.

The X-ray tube 21 and the multi-row X-ray detector 24 rotate about the center of rotation IC. When the vertical direction is defined as a y direction, the horizontal direction is defined as an x direction, and a table traveling direction orthogonal to these is defined as a z direction, the rotational plane of each of the X-ray tube 21 and the multi-row X-ray detector 24 is expressed as an xy plane. The moving direction of the cradle 12 corresponds to the z direction.

The X-ray tube 21 generates an x-ray beam called "cone beam CB". When the direction of a central axis of the cone beam CB is parallel to the y direction, a view angle is assumed to be 0.degree..

The multi-row X-ray detector 24 has detector rows corresponding to 256 rows, for example. The X-ray detector channels each having a constant channel width have heretofore been arranged in the channel direction, and X-ray detector data for all the channels have always been read upon data acquisition, as shown in FIG. 12. In the present embodiment, each detector row has detector channels corresponding to 1024 channels with respect to a data acquisition range and an angle .theta. in the case of, for example, an X-ray detector channel width d of a data acquiring X-ray detector as shown in FIG. 13. 512 channels equivalent to half of all channels at the central portion of the multi-row X-ray detector 24 are set in such a manner that data can be read even in the case of an X-ray detector channel width d/2 of the data acquiring X-ray detector. In the case of the X-ray detector channel width d/2, each detector row has detector channels corresponding to 1024 channels with respect to a data acquisition range and an angle .theta./2.

That is, in the multi-row X-ray detector 24, a plurality of channels which detect X rays transmitted through the subject to acquire or collect X-ray detector data, are respectively arranged in both directions of a channel direction extending along the direction in which they are rotated by a rotational section 15 and a row direction extending along its rotational axis about which they are rotated by the rotational section 15. As shown in FIG. 13, the multi-row X-ray detector 24 has a first area in which a plurality of channels corresponding to a first channel width d/2 are disposed in a channel direction, and second areas in which a plurality of channels corresponding to second channel widths d larger than the first channel width d/2 are disposed in the channel direction. In the multi-row X-ray detector 24, the first area is formed so as to correspond to the central portion as viewed in the channel direction, and the second areas are formed at its peripheral portions so as to interpose the first area therebetween as viewed in the channel direction.

The multi-row X-ray detector 24 and the DAS 25 in this case have two data acquisition modes shown below.

(1) A mode 1 for collecting or acquiring 1024 channels with channel widths d coarse or rough in a data acquisition range (data acquisition range and angle .theta.) wide as viewed in the channel direction.

(2) A mode 2 for acquiring 1024 channels with a channel width d/2 fine in a data acquisition range (data acquisition angle .theta./2) narrow as viewed in the channel direction.

In this case, the data acquisition system (DAS) 25 effects all-row data acquisition of all 1024 channels on the 1 to 1024 channels shown in FIG. 13(a) with the channel width d upon data acquisition based on the mode 1. Upon data acquisition based on the mode 2, the data acquisition system 25 performs all-row data acquisition of all 1024 channels on the 257 to 768 channels with the channel width d/2.

The data acquisition system (DAS) 25 and the multi-row X-ray detector 24 are electrically connected to each other in such a manner that the data acquisition based on the mode 1 and the data acquisition based on the mode 2 can be conducted. The connection therebetween is switched according to the mode 1 and the mode 2.

In the 257 to 768 channels at this time, as shown in FIG. 27, data of X-ray detector channels corresponding to each channel width d/2 are respectively read in the mode 2. In the mode 1, the data of the X-ray detector channels corresponding to each channel width d/2 are respectively added together, after which the added data is read as X-ray detector channel data corresponding to a channel width d.

Thus, the data acquisition range wide in the channel direction of the mode 1 and the narrow data acquisition range based on the X-ray detector channels high in resolution as viewed in the channel direction of the mode 2 are used by switching, e.g., the data acquisition range wide in the channel direction of the mode 1 is used for a lung examination and the narrow data acquisition range high in resolution as viewed in the channel direction of the mode 2 is used for a cardiac examination, clinically as shown in FIG. 28, thereby making it possible to use the respective modes effectively.

That is, in the present embodiment, the DAS 25 collects X-ray detector data from the multi-row X-ray detector 24 and outputs the X-ray detector data to the central processing unit 3 via the data acquisition buffer 5. As shown in FIG. 13, the DAS 25 performs switching to an area for acquiring or collecting X-ray detector data so as to collect the X-ray detector data from the channels corresponding to any one of the first area (data acquisition range 2) of the multi-row X-ray detector 24, and the first and second areas (data acquisition range 1). Here, the control controller 29 transmits a control signal, based on a command inputted to the input device 2 from an operator. The DAS 25 performs switching to the area for collecting the X-ray detector data. When the area switching is done such that the X-ray detector data are collected in the first area of the multi-row X-ray detector 24, the DAS 25 acquires or collects X-ray detector data from respective channels arranged in the channel and row directions selected in the first area and outputs the same therefrom. On the other hand, when X-ray detector data are acquired in both of the first and second areas of the multi-row X-ray detector 24, the DAS 25 acquires the X-ray detector data from respective channels arranged in the channel and row directions selected in both the first and second areas and outputs the same therefrom. As to the respective X-ray detector data from the channels of the first area, the DAS 25 adds X-ray detector data from a plurality of channels adjacent to one another in the first area so as to correspond to the channel widths d of the channels in the second areas and outputs the result of addition therefrom. That is, the DAS 25 adds up X-ray detector data from two channels adjacent to each other in the first area so as to become identical to the channel width d for the channels in the second area and outputs it therefrom. Respective X-ray detector data from the channels in the second area are outputted without being added up.

The following one is known as another X-ray detector of the present invention. Although the data have heretofore been read over all channels and rows as shown in FIG. 14, the number of reading rows can also be changed depending upon the positions of channels as shown in FIGS. 15, 16, 17 and 18 in one embodiment of the present invention. In this case, data are read in X-ray detector rows small in number at each outer peripheral portion as viewed in the channel direction, and data are read in X-ray detector rows large in number at an inner central portion as viewed in the channel direction. Thus, data acquisition can be performed on the central portion spatially and in high resolution. By collecting some row-direction data at the peripheral portion or non-consecutive data in the row direction or data wide and coarse in row width as viewed in the row direction, a data acquisition number is identical or equivalent to the conventional one, and data acquisition can be effected on the central portion in high resolution as viewed in the channel direction.

X-ray detector data irradiated with X rays and collected are A/D converted by the DAS 25 as viewed from the multi-row X-ray detector 24 and inputted to the data acquisition buffer 5 via a slip ring 30. The data inputted to the data acquisition buffer 5 are processed by the central processing unit 3 in accordance with the program of the memory device 7, after which the data are image-reconstructed as a tomogram or tomographic image, which is displayed on the monitor 6.

FIG. 3 is a flow diagram showing the outline of operation of the X-ray CT apparatus 100 according to the present invention.

In Step S1, the X-ray tube 21 and the multi-row X-ray detector 24 are first rotated about a subject. A helical scan operation is performed while the cradle 12 on the photographing table 10 is being linearly moved. Thus, a table linear movement z-direction position Ztable (view) is added to X-ray detector data D0 (view, j, i) expressed in a view angle view, a detector row number j and a channel number i to collect the X-ray detector data. Upon a conventional scan (axial scan), imaging data are collected with the cradle 12 placed on the photographing table 10 being fixed. In the present embodiment, data acquisition is carried out at fine channel intervals p of (2). Incidentally, the view angle view described above is an angle at which the X-ray tube 21 is rotated and moved about the subject from a predetermined position by the rotational section 15 upon scan's execution. The detector row number j is a number of each detector arranged in the row direction in the multi-row X-ray detector 24. The channel number i is a number of each detector arranged in the channel direction in the multi-row X-ray detector 24. The X-ray detector data D0 (view, j, i) indicate data collected by allowing detectors placed in detector row numbers j and channel numbers i in the multi-row X-ray detector 24 to detect X rays transmitted through the subject when the X-ray tube 21 moved to a predetermined view angle view applies X rays to the subject. The table linear movement Z-direction position Ztable (view) indicates a position where the cradle 12 of the photographing table 10 is moved along the direction of a body axis of the subject upon execution of the scan.

Upon determining the position of the subject, the subject is placed in such a manner that a data acquisition channel interval p at a detector central portion can be used effectively and the subject falls inside a central data acquisition angle .theta./2.

In Step S2, preprocessing is effected on the X-ray detector data D0 (view, j, i) and converted to projection data. As shown in FIG. 4, the preprocessing includes an offset correction of Step S21, logarithmic transformation of Step S22, an X-ray dosage correction of Step S23 and a sensitivity correction of Step S24.

In Step S3, a beam hardening correction is effected on pre-processed projection data D1 (view, j, i). Assuming that projection data subjected to the sensitivity correction S24 of the preprocessing S2 is defined as D1 (view, j, i) and data subsequent to the beam hardening correction S3 is defined as D11 (view, j, i) upon the beam hardening correction S3, the beam hardening correction S3 is expressed in, for example, a polynomial form like the following equation (1): D11(view, j, i)=D1(view,j,i)(Bo(j,i)+B.sub.1(j,i)D1(view,j,i)+B.sub.2(j,i)(D1(view,j,i- ).sup.2 (1)

Since the beam hardening corrections independent every j rows of the detector can be carried out at this time, the difference between detector's X-ray energy characteristics set every rows can be corrected if tube voltages of respective data acquisition systems are different under photographing or imaging conditions.

In Step S4, a z-filter convolution process for exerting a z-direction (row direction) filter on projection data D11 (view, j, i) subjected to the beam hardening correction is carried out.

In Step S4, after preprocessing at respective view angles and respective data acquisition systems, a filter whose row-direction filter size is 5 rows like, for example, (w1(ch), w2(ch), w3(ch), w4(ch), and w5(ch)) is exerted on projection data of multi-row X-ray detectors D11 (ch, row) (where ch=1-CH, row=1-ROW) subjected to the beam hardening correction in the row direction. Incidentally, ch indicates the channel and row indicates the row herein.

However, the above relation is defined as given by an equation (2) as follows:

.times..times..function. ##EQU00001##

The corrected detector data D12 (ch, row) is expressed in an equation (3) shown below:

.times..times..times..times..times..times..times..times..function. ##EQU00002##

Incidentally, when the maximum value of the channel is assumed to be CH and the maximum value of the row is assumed to be ROW, they are shown like the following equations (4) and (5): D11(ch, -1)=D11(ch, 0)=D11(ch, 1) (4) D11(ch, ROW)=D11(ch, ROW+1)=D11(ch, ROW+2) (5)

When a row-direction filter coefficient is changed for each channel, a slice thickness can be controlled according to the distance away from an image reconstruction center. Since the slice thickness becomes thick at a peripheral portion of a tomogram as compared with its reconstruction center in general, row-direction filter coefficients are changed at the central portion and each peripheral portion, the width of each row-direction filter coefficient is widely changed in the neighborhood of a central-portion channel, and the width of each row-direction filter coefficient is narrowly changed in the neighborhood of each peripheral-portion channel. As a result, the slice thicknesses can also be made close evenly even at the peripheral portion and the image reconstruction central portion.

By controlling the row-direction filter coefficients of the central-portion channel and each peripheral-portion channel of the multi-row X-ray detector 24 in this way, the slice thickness can also be controlled at the central portion and the peripheral portion. When the slice thickness is made thick slightly by means of the row-direction filters, both artifacts and noise can be greatly improved. Thus, the degree of an artifact improvement and the degree of a noise improvement can also be controlled. That is, a three-dimensional image reconstructed tomogram, i.e., the quality of an image in an xy plane can be controlled. As another embodiment, a tomogram thin in slice thickness can also be realized by bringing a row-direction (z-direction) filter coefficient to a deconvolution filter.

In Step S5, a reconstruction function convolution process is carried out. That is, data is Fourier-transformed and multiplied by a reconstruction function, followed by being subjected to inverse Fourier-transformation. Assuming that in the reconstruction function convolution process S5, data subsequent to a z filter convolution process is defined as D12, data subsequent to the reconstruction function convolution process is defined as D13, and a reconstruction function to be convoluted is Kernel (j), the reconstruction function convolution process is expressed in an equation (6) shown below: D13(view, j, i)=D12(view, j, i)*Kernel(j) (6)

That is, since the reconstruction function kernel (j) can perform a reconstruction function convolution process independent for each j row of the detector, differences in noise characteristic and resolution characteristic for each row can be corrected.

In Step S6, a three-dimensional backprojection process is effected on projection data D13(view, j, i) subjected to the reconstruction function convolution process to determine backprojection data D3(x, y). While the helical scan is being performed in the present invention, an image-reconstructed image is three-dimensionally image-reconstructed to a plane or xy plane orthogonal to the z axis. The following reconstruction area P is assumed to be parallel to the xy plane. The three-dimensional backprojection process will be described later with reference to FIG. 5.

In Step S7, postprocessing such as image filter convolution, CT-value conversion or the like is effected on the backprojection data D3(x, y, z) to obtain a tomographic image or tomogram D31(x, y).

Assuming that a three-dimensionally backprojected tomogram is D31(x, y, z), data subsequent to the image filter convolution is D32(x, y, z) and an image filter is Filter(z), the image filter convolution process corresponding to the postprocessing can be expressed in an equation (7) as follows: D32(x, y, z)=D31(x, y, z)*Filter(z) (7)

That is, since the image filter convolution process independent for each j row of the detector can be conducted, differences in noise characteristic and resolution characteristic for each row can be corrected.

The thus-obtained tomogram is displayed on the monitor 6.

FIG. 5 is a flow diagram showing the details of the three-dimensional backprojection process (Step S6 of FIG. 4).

In the present embodiment, an image-reconstructed image is three-dimensionally image-reconstructed to the plane or xy plane orthogonal to the z axis. The following reconstruction area P is assumed to be parallel to the xy plane.

In Step S61, attention is paid to one of all views (i.e., views corresponding to 360.degree. or views corresponding to "180.degree.+fan angle") necessary for image reconstruction of a tomogram, and projection data Dr corresponding to each pixel in the reconstruction area P is extracted.

As shown in FIGS. 6(a) and 6(b), the area of a square of 512.times.512 pixels parallel to an xy plane is defined as a reconstruction area P, and a pixel row L0 at y=0, which is parallel to an x axis, a pixel row L63 at y=63, a pixel row L127 at y=127, a pixel row L191 at y=191, a pixel row L255 at y=255, a pixel row L319 at y=319, a pixel row L383 at y=383, a pixel row L447 at y=447, and a pixel row L511 at y=511 are respectively taken as rows. Thus, if projection data on lines T0 through T511 shown in FIG. 7 obtained by projecting these pixel rows L0 through L511 onto the plane of the multi-row X-ray detector 24 as viewed n an X-ray penetration direction are extracted, then they result in projection data Dr(view, x, y) of the pixel rows L0 through L511. However, x and y correspond to each pixel (x, y) of a tomogram.

The X-ray penetration direction is determined depending upon the geometric positions of the X-ray focal point of the X-ray tube 21, the respective pixels and the multi-row X-ray detector 24. Since, however, a z coordinate z(view) of an X-ray detector data D0(view, j, i) is known as a table linear movement z-direction position Ztable(view) concomitantly with X-ray detector data, the X-ray penetration direction can accurately be determined in a data acquisition geometric system of the X-ray focal point and the multi-row X-ray detector even in the case of the X-ray detector data D0(view, j, i) lying in acceleration/deceleration.

Incidentally, when some of each line falls out as viewed in the channel direction of the multi-row X-ray detector 24 as in, for example, the line T0 obtained by projecting the pixel row L0 onto the plane of the multi-row X-ray detector 24 as viewed in the X-ray penetration direction, the corresponding projection data Dr(view, x, y) is assumed to be "0". When it falls out as viewed in the z direction, the corresponding projection data Dr(view, x, y) is determined as extrapolation.

As shown in FIG. 8, projection data Dr(view, x, y) associated with the respective pixels in the reconstruction area P can be extracted in this way.

Referring back to FIG. 5, in Step S62, the projection data Dr(view, x, y) is multiplied by a cone beam reconstruction weight coefficient to create such projection data D2(view, x, y) as shown in FIG. 9.

Here, the cone beam reconstruction weight coefficient w(i, j) is shown as follows. In the case of fan beam image reconstruction, when the angle which a straight line obtained by connecting the focal point of the X-ray tube 21 and each pixel g(x, y) on the reconstruction area P (xy plane) when view=.beta.a forms with a center axis Bc of an X-ray beam, is defined as .gamma. and its opposite beam is defined as view=.beta.b in general, .beta.b results in .beta.b=.beta.a+180.degree.-2.gamma..

Assuming that the angles which the X-ray beam passing through the pixel g(x, y) on the reconstruction area P and its opposite X-ray beam form with the reconstruction plane P are .alpha.a and .alpha.b as indicated by the following equation (8), they are multiplied by cone beam reconstruction weight coefficients .omega.a and .omega.b dependent upon these and added together to determine backprojection pixel data D2(0, x, y). D2(0, x, y)=.omega.aD2(0, x, y).sub.--a+.omega.bD2(0, x, y).sub.--b (8)

However, D2(0, x, y)_a is defined as projection data of the view .beta.a, and D2(0, x, y)_b is defined as projection data of the view .beta.b, respectively.

Incidentally, the sum of the cone beam reconstruction weight coefficients with respect to the beams opposite to each other results in .omega.a+.omega.b=1.

Multiplying the projection data by the cone beam reconstruction weight coefficients .omega.a and .omega.b and adding together makes it possible to reduce cone angle artifacts.

For example, ones determined from the following equations can be used as the cone beam reconstruction weight coefficients .omega.a and .omega.b.

When 1/2 of a fan beam angle is assumed to be .gamma.max, one determined by the equation (14) from the following equation (9) can be used. Incidentally, ga indicates an addition/multiplication coefficient of an X-ray beam in a given direction, and gb indicates an addition/multiplication coefficient of an X-ray beam corresponding to its opposite beam. ga=f(.gamma.max, .alpha.a, .beta.a) (9) gb=f((.gamma.max, .alpha.b, .beta.b) (10) xa=2ga.sup.q/(ga.sup.q+gb.sup.q) (11) xb=2gb.sup.q/(ga.sup.q+gb.sup.q) (12) wa=xa.sup.2(3-2xa) (13) wb=xb.sup.2(3-2xb) (14)

Incidentally, for example, q=1 here.

Assuming that max[ ] are functions which take large values, for example, ones determined from the following equations (15) and (16) can be used as examples of ga and gb. ga=max[0, {(.pi./2+.gamma.max)-|.beta.a|}]|tan(.alpha.a)| (15) gb=max[0, {(.pi./2+.gamma.max)-|.beta.b|}]|tan(.alpha.b)| (16)

In the case of fan beam image reconstruction, each pixel on the reconstruction area P is further multiplied by its corresponding distance coefficient. When the distance from the focal point of the X-ray tube 21 to a detector row j and a channel i of the multi-row X-ray detector 24, corresponding to projection data Dr is r0, and the distance from the focal point of the X-ray tube 21 to each pixel on the reconstruction area P, corresponding to the projection data Dr is r1, the distance coefficient is given as (r1/r0).sup.2.

In the case of parallel beam image reconstruction, each pixel on the reconstruction area P may be multiplied by its corresponding cone beam reconstruction weight coefficient w(i, j) alone.

In Step S63, as shown in FIG. 10, projection data D2(view, x, y) is added to backprojection data D3(x, y) cleared in advance in association with each pixel.

In Step S64, Steps S61 to S63 are repeated over all views (i.e., views corresponding to 360.degree. or views corresponding to "180.degree.+fan angle") necessary for image reconstruction of each tomogram, and thereby backprojection data D3(x, y) is obtained as shown in FIG. 10.

Incidentally, the reconstruction area P may be formed as a circular area as shown in FIGS. 11(a) and 11(b).

The full imaging visual field is normally imaged in the mode of the multi-row X-ray detector 24 with the channel width d as shown in FIG. 12. When, however, a subject small in the imaging field of view is imaged or photographed, data acquisition is performed in the mode in which the central-portion channel is brought to high resolution as shown in FIG. 13, and a tomographic image is created by such image reconstruction as described above.

Since the tomographic image obtained here is image-reconstructed based on projection data acquired at a portion of the multi-row X-ray detector 24 at the time that the fine channel interval is d/2 and the data acquisition angle is .theta./2, a tomographic image corresponding to a small imaging area is obtained in high resolution.

That is, N channels.times.M rows corresponding to a channel width d in a normal mode, and N channels.times.M rows corresponding to a channel width d/2 in a high resolution mode can be switched according to a subject. In this case, the data acquisition system (DAS) 25 corresponds to the N channels.times.M rows and is efficient if N channels corresponding to the channel width d and N channels corresponding to the channel width d/2 are used by switching. If a subject and an area of interest are both small and fall within a range corresponding to an imaging area of N channels.times.channel width d/2 as shown in FIG. 13, then data acquisition, tomogram image reconstruction and a tomogram image display can be conducted in a high resolution mode based on the N channels.times.channel width d/2. In the image reconstruction at this time, image reconstruction functions for the normal mode and the high resolution mode are prepared at the reconstruction function convolution process of Step S5 in the flow of such image reconstruction as shown in FIG. 3. When data acquisition is done in the high resolution mode based on the N channels.times.detector channel width d/2, a Nyquist frequency for sampling at data acquisition increases. Therefore, when the image reconstruction is conducted using the image reconstruction function for the high resolution mode as shown in FIG. 39, a high resolution image whose quality is suitable is obtained.

Incidentally, as the structure of the X-ray detector, the central portion of the X-ray detector as viewed in the channel direction is constituted of scintillators and photodiodes with N channels.times.channel width d/2 as shown in FIG. 13.

The right and left peripheral portions are respectively constituted of scintillators and photodiodes with N/4 channels.times.channel width d. When the central N channels.times.channel width d/2 are read as the high resolution mode, the respective channels corresponding to the channel width d/2 are read independently one by one.

When, however, all channels are read with the N channels.times.channel width d/2 as the normal mode, the X-ray detector having the respective channels corresponding to the channel width d/2 at its central portion is read in the normal mode with two channels unified into one. Thus, an FET switch is known as a switch for reading the outputs of the scintillators and photodiodes of the X-ray detector by switching.

As shown in FIG. 19, however, the area of interest is small and falls within a range for a high resolution mode with N channels.times.channel width d/2. When, however, the size of the subject does not fall within the range of the N channels.times.channel width d/2, the inside of the multi-row X-ray detector 24 as viewed in a channe