USRE41219E1 - Apparatus and method for X-ray computer tomography - Google Patents

Abstract

An X-ray computed tomographic apparatus of the invention includes a display portion to display, on a screen, a scanogram related to a subject together with a quadrilateral frame line specifying a reconstruction range, an input portion to input a command to transform the frame line specifying the reconstruction range to a parallelogram or rotate the frame line specifying the reconstruction range, a gantry to perform scanning in a scan range corresponding to the reconstruction range, and a reconstruction portion to reconstruct image data related to plural slices, parallel to one another and included in the reconstruction range, slice-by-slice on the basis of projection data acquired by the scanning.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2003-091970, filed Mar. 28, 2003, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and method for the X-ray computed tomography.

2. Description of the Related Art

The X-ray computed tomographic apparatus (also referred to as the CT scanner) provides information of the subject in the form of images on the basis of the intensity of X-rays having passed through the subject, and plays an important role in many medical practices including diagnosis of illness, treatment and operation planning, etc. The advent of helical scan has made it possible to achieve wide-range data acquisition in a short time.

The patient throughput has become one of critical issues associated with such achievement. Due to ultrafast scans as well as weight saving of X-ray tubes, widespread use of helical scan, increasing number of detector arrays, and enhancement of detection sensitivity in recent years, the patient throughput is influenced more by a time needed for pre-scan setting of the subject than the scan time. The subject lies on his back on the tabletop of the diagnostic table and fine adjusts the body position according to radiologist's instructions. However, only a limited time is allowed for fine adjustment of the body position. Hence, as is shown inFIG. 1A, scans are often performed while the body axis of the subject is tilted with respect to the center line (Z-axis, the rotational axis of the X-ray tube) of the scan range. This results in an event that, as is shown in FIG.1B andFIG. 1C, the center of the subject is offset from the center of the image and a degree of offset differs from image to image, which makes observations quite difficult.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is therefore to address an event such that scans are performed while the body axis of the subject is tilted with respect to the center line (Z-axis, the rotational axis of the X-ray tube) of the scan range.

An X-ray computed tomographic apparatus of the invention includes: a display portion to display, on a screen, a scanogram related to a subject together with a quadrilateral frame line specifying a reconstruction range; an input portion to input a command to transform the frame line specifying the reconstruction range to a parallelogram or rotate the frame line specifying the reconstruction range; a gantry to perform scanning in a scan range corresponding to the reconstruction range; and a reconstruction portion to reconstruct image data related to plural slices, parallel to one another and included in the reconstruction range, slice-by-slice on the basis of projection data acquired by the scanning.

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 hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, server to explain the principles of the invention.

FIG. 1A,FIG. 1B, andFIG. 1C are views used to explain problems in the related art;

FIG. 2 is a view showing the configuration of an X-ray computed tomographic apparatus according to an embodiment of the invention;

FIG.3A andFIG. 3B are perspective views of an X-ray detector ofFIG. 2;

FIG. 4 is a view showing helical path of an X-ray tube ofFIG. 2;

FIG. 5 is a view showing an example of a scan procedure screen constructed by a scan procedure system ofFIG. 2;

FIG. 6 is a view showing a frame line specifying a reconstruction range transformed with a click on a “transformation icon” ofFIG. 5;

FIG. 7 is a view showing a frame line specifying a reconstruction range rotated with a click on a “rotation icon” ofFIG. 5;

FIG. 8A,FIG. 8B, andFIG. 8C are views used to explain reconstruction processing corresponding to the transformed reconstruction range ofFIG. 6;

FIG. 9A,FIG. 9B, andFIG. 9C are views used to explain reconstruction processing corresponding to the rotated reconstruction range ofFIG. 7; and

FIG.10A andFIG. 10B are views showing two types of scan range corresponding to the rotated reconstruction range of FIG.7.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of an X-ray computed tomographic apparatus of the invention will now be described with reference to the accompanying drawings. The X-ray computed tomographic apparatus includes various types, such as a rotate/rotate type in which a unit comprising the X-ray tube and the radiation detector rotates about the subject, and a stationary/rotate type in which a number of detection elements are aligned in a ring-shaped array and the X-ray tube alone rotates about the subject, and the invention is applicable to any type. Herein, the currently most popular rotate/rotate type will be described. Also, in order to reconstruct tomographic data for one slice, it is necessary to obtain projection data of about 360° for a full circle of the subject, and projection data of 180° plus a view angle is needed even in the half scan method. The invention is applicable to either reconstruction method. Herein, the former method will be described by way of example. Also, popular mechanisms to convert incident X-rays to charges are: an indirect conversion scheme,-by which X-rays are converted first into light by a fluorescent material, such as a scintillator, and the light is then converted to charges by a photoelectric converting element, such as a photodiode; and a direct conversion scheme, by which generation of electron-hole pairs in the semiconductor by X-rays and their movement to the electrodes, that is, the photoelectric phenomenon, are exploited. The X-ray detection elements adopting either scheme can be used, and herein, those adopting the former indirect conversion scheme will be described. In addition, a so-called multi-tube type X-ray computed tomographic apparatus, in which plural pairs of an X-ray tube and an X-ray detector are mounted to a rotational ring, has become commercially available recently, and the peripheral techniques are also under development. The invention is applicable to either a conventional single-tube type X-ray computed tomographic apparatus or a multi-tube type X-ray computed tomographic apparatus. Herein, a single-tube type X-ray computed tomographic apparatus will be described.

FIG. 2 shows the configuration of the X-ray computed tomographic apparatus according to this embodiment. The X-ray computed tomographic apparatus includes agantry1 configured to acquire projection data related to the subject. Thegantry1 includes anX-ray tube10 and anX-ray detector23. Both theX-ray tube10 and theX-ray detector23 are mounted to a ring-shapedrotational frame12, which is driven to rotate about the Z-axis by agantry driving device25. Therotational frame12 is provided with an aperture at the center thereof, and the subject P laid on thetabletop2a of the diagnostic table2 is inserted into the aperture. A slit22 used to vary the irradiation width of X-rays depending on the slice thickness is placed between theX-ray tube10 and the aperture.

A tube voltage from a highvoltage transformer assembly21 is applied between the cathode and the anode of theX-ray tube10, while a filament current from the highvoltage transformer assembly21 is supplied to the filament of theX-ray tube10. X-rays are generated by the application of the tube voltage and the supply of the filament current.

As is shown in FIG.3A andFIG. 3B, theX-ray detector23 includes pluralX-ray detection elements100 each having, for example, a 0.5 mm×0.5 mm tetragonal light-reception surface. In the case ofFIG. 3A, for example, 916X-ray detection elements100 are aligned in an array along the channel direction. In the case ofFIG. 3B, arrays ofFIG. 3A are provided, for example, in 40 rows in parallel along the slice direction. The detector ofFIG. 3A is referred to as the single-slice type, and the detector ofFIG. 3B is referred to as the multi-slice type. TheX-ray detector23 can be of either type.

Adata acquisition device24, generally referred to as a DAS (data acquisition system), converts a signal in each channel outputted from thedetector23 to a voltage signal, amplifies the voltage signal, and converts the amplified voltage signal to a digital signal. Data (raw data) thus obtained is supplied to acomputer unit3 installed at the outside of the gantry. Apre-processing unit34 of thecomputer unit3 performs compensation processing, such as sensitivity compensation, on the raw data outputted from thedata acquisition device24, and outputs projection data. The projection data is then sent to and stored in adata storage device35 of thecomputer system3.

Thecomputer system3 comprises asystem controller29, aninput device39 provided with a keyboard, a mouse, etc., adisplay38, ascan controller30, areconstruction unit36, and ascan procedure system42 in addition to theaforementioned pre-processing unit34 and thestorage device35. Thereconstruction unit36 is able to selectively perform the reconstruction processing according to either of the followings: the typical fan-beam reconstruction method (also referred to as the fan-beam convolution back projection method); and a reconstruction method in a case where projection rays cross with the reconstruction plane like a cone beam, other than the helical interpolation that can be used together with the fan-beam reconstruction method in finding projection data on the reconstruction plane through interpolation from projection data of, for example, two rotations, the method including the Feldkamp method, known as an approximate image reconstruction method, by which convolution is performed by deeming the beam as a fan projection beam on the assumption that the cone angle is small and back projection is performed along rays at the time of scanning, and the cone-beam reconstruction method, known as a method capable of suppressing cone-angle induced errors compared with the Feldkamp method, by which projection data is compensated in response to the angle of rays with respect to the reconstruction plane.

Thescan procedure system42 is provided to assist the operator in a work of determining the scan procedure, and constructs a scan procedure screen used to set scan conditions, such as a helical pitch (HP) indicating a distance the tabletop moves while theX-ray tube10 rotates once as shown inFIG. 4, and a scan speed (SS) indicating a time needed for theX-ray tube10 to rotate once.

FIG. 5 shows an example of the scan procedure screen. The scan procedure screen includes patient information, gantry information, and detailed information of the scan conditions at the bottom of the screen as well as ascanogram image99. Thescanogram image99 is displayed in an orientation such that the Z-axis (the center of rotation) thereof is parallel to the vertical direction (possibly, the horizontal direction in some cases) of the screen. Thus, when scanogram imaging is performed while the body axis of the subject is tilted with respect to the Z-axis, thescanogram image99 is displayed on the screen as being tilted with respect to the vertical direction of the screen as well.

The scan conditions include the activation (distinction between the manual trigger and the automatic trigger to start the scan), scan start time (start time), start position of helical scan, a pause between scans, end position of helical scan, scan mode (distinction among single-slice/multi-slice/helical), start position of scan, end position of scan, tube voltage kV, tube current mA, scan speed (time in parentheses indicates a time needed for the entire scans), the number of slices (the number of arrays used), helical pitch, reconstruction mode, and FOV (width of reconstruction range).

Aquadrilateral frame line101 specifying the reconstruction range is displayed on thescanogram image99. A frame line, generally in a dotted line, specifying the scan range corresponding to the reconstruction range is displayed together with theframe line101 specifying the reconstruction range in some cases. Thequadrilateral frame line101 specifying the reconstruction range is initially provided as an oblong with itscenter line109 being parallel to the Z-axis (central axis of rotation).

Also,rhombic icons102 for scaling up/down the range vertically andrhombic icons103 for scaling up/down the range horizontally are displayed at the four corners of theframe line101 specifying the reconstruction range, so that the operator is able to scale up/down the reconstruction range as needed by moving thepointer104 to any of theicons102 and103 and dragging thepointer104 with the use of, for example, the mouse of theinput device39. Also, the operator is able to move the reconstruction range in parallel vertically and/or horizontally by moving thepointer104 on theframe line101 and dragging thepointer104 with the use of, for example, the mouse of theinput device39.

Further, on thescanogram image99 are superposedtransformation icons105 and106 androtation icons107 and108. A transformation command is inputted with a click on thetransformation icon105. Upon input of the transformation command, as is shown inFIG. 6, theframe line101 specifying the reconstruction range is transformed to a parallelogram. A degree of transformation, that is, a tilt of thecenter line109 with respect to the vertical direction of the screen, is determined, for example, by the number of clicks. For instance, a tilt of 2.5° is given with one click. With a click on theother transformation icon106, theframe line101 specifying the reconstruction range is transformed in an opposite direction to the direction ofFIG. 6. A degree of transformation is also determined by the number of clicks.

A rotation command is inputted with a click on therotation icon107. Upon input of the rotation command, as is shown inFIG. 7, theframe line101 specifying the reconstruction range is rotated about its center. A degree of rotation, that is, a tilt of thecenter line109 with respect to the vertical direction of the screen, is determined, for example, by the number of clicks. For instance, a rotation by 2.5° is given with one click. With a click on theother rotation icon108, theframe line101 specifying the reconstruction range is rotated in a direction opposite to the direction ofFIG. 7. A degree of rotation is also determined by the number of clicks. Basically, transformation and rotation are performed alternatively.

As has been described, thescanogram image99 is displayed in an orientation such that the Z-axis (center of rotation) thereof is parallel to the vertical direction of the screen. Thus, when scanogram imaging is performed while the subject is tilted with respect to the Z-axis, the tilt is reflected on thescanogram image99 on the screen as are shown in FIG.6 and FIG.7.

The operator thus drags and moves theframe line101 specifying the reconstruction range in parallel and clicks either thetransformation icon105 in the forward direction or thetransformation icon106 in the backward direction as many times as necessary, so that thecenter line109 of theframe line101 specifying the reconstruction range becomes parallel to and agrees as much as possible with the body axis of the subject on the tiltedscanogram image99. Also, the operator drags and moves theframe line101 specifying the reconstruction range in parallel and clicks either therotation icon107 in the forward direction or therotation icon108 in the backward direction as many times as necessary, so that thecenter line109 of theframe line101 specifying the reconstruction range becomes parallel to and agrees as much as possible with the body axis of the subject assumed on the tiltedscanogram image99. Alternatively, it may be possible to transform and rotate theframe line101 specifying the reconstruction range with the use of thetransformation icon105 or106 together with therotation icon107 or108.

There is a slight difference between the examples of FIG.6 andFIG. 7 in terms of image reconstruction. When theframe line101 specifying the reconstruction range is transformed as shown inFIG. 6, thescan procedure system42 determines, as is shown inFIG. 8A, thereconstruction range111 to correspond to theframe line101, and determines thescan range112 to correspond to thereconstruction range111. Thescan range112 is set to the shape of a cylindrical column whose longitudinal cross section is an oblong having the Z-axis (axis of rotation) at the center and covering thereconstruction range111.

As is shown inFIG. 8B, thereconstruction unit36 extracts projection data corresponding to respective slices from projection data acquired by scans, and reconstructs image data on the basis of the projection data thus extracted. Widths of the respective slices are set according to the horizontal width of theframe line101 specifying the reconstruction range, and the centers of the respective slices are set on thecenter line109 of theframe line101 specifying the reconstruction range. Because thecenter line109 of the reconstruction range is set with a tilt with respect to the center line of the scan range, the horizontal positions of the respective slices, that is, a distance from the center line of the scan range to the center of each slice, vary from slice to slice. However, the centers of the respective slices are all placed on the body axis of the subject.

Because the center of the slice can be set for each slice in response to the tilted body position of the subject in the manner descried above, even when scans are performed while the body axis of the subject is tilted with respect to the Z-axis, the body axis of the subject can be positioned on almost the center of the image as shown in FIG.8C. This eliminates offset between the position on the image and the position on the subject, which makes observation quite easy. Also, by converting a horizontal distance of the subject to an actual distance on the basis of the tilt of theframe line101, it is possible to reduce errors in measurement of a distance or a volume.

Then, in a case where theframe line101 specifying the reconstruction range is rotated as shown inFIG. 7, thescan procedure system42 determines, as is shown inFIG. 9A, thereconstruction range111 corresponding to theframe line101 together with thescan range112. In this case, thescan range112 is set to the shape of a cylindrical column whose longitudinal cross section is an oblong having the Z-axis (axis of rotation) at the center and covering thereconstruction range111.

Thescan range112 can be selected from the range shown in FIG.10A and the range shown inFIG. 10B wider than the one shown in FIG.10A. In the helical reconstruction, as is known, projection data at the slice position is generated through interpolation from projection data of two, preceding and following rotations. In other words, the helical reconstruction needs projection data that covers awider range113, which is wider than the outermost slice within thereconstruction range111 to the outside by at least one rotation.FIG. 10B shows an example when thescan range112 is set to acquire the entire projection data of thiswider range113. In contrast,FIG. 10A shows an example of anarrow scan range112, in which part of projection data shaded by diagonal lines is replenished through extrapolative interpolation. The operator may make a selection between these two types, or the selection may be made automatically depending on the various conditions of the region to be imaged.

As is shown inFIG. 9B, thereconstruction unit36 reconstructs image data by either the cone-beam reconstruction method or the tilt reconstruction method for each of plural slices (reconstruction planes) orthogonal to thecenter line109 of the reconstruction range rotated in response to a tilt of the body axis of the subject, on the basis of the projection data acquired by scans. In other words, the pixel value of each pixel within the slice tilted with respect to the central axis of the scan range is computed as a filter integral value of projection data of plural X-ray paths that cross with the each pixel diagonally.

The width of each slice is set according to the width of theframe line101 specifying the reconstruction range, and the center of each slice is set on thecenter line109 of theframe line101 specifying the reconstruction range. Because thecenter line109 of the reconstruction range is set with a tilt with respect to the center line of the scan range, the horizontal positions of the respective slices, that is, a distance from the center line of the scan range to the center of each slice, vary from slice to slice.

Because the center of the slice can be set for each slice in response to the tilted body position of the subject in the manner described above, even when scans are performed while the body axis of the subject is tilted with respect to the Z-axis, the body axis of the subject can be positioned on almost the center of the image as is shown in FIG.9C. Moreover, in this example, an image of a plane orthogonal to the body axis of the subject can be obtained, which reduces errors in horizontal distance associated with a tilt of the body axis with respect to the Z-axis. In short, it is possible to substantially eliminate a state where the body axis of the subject is tilted with respect to the Z-axis. Because errors in distance can be eliminated not only vertically but also horizontally, errors can be reduced in measurement of a distance or a volume; moreover, MPR (multi-planar reconstruction) processing or 3-D processing can be performed directly without the need for special compensation processing.

The invention can be applied to, for example, PET (Positron Emission computed Tomography), as an image diagnosis apparatus of a type that reconstructs a planar image on the basis of the subject's data acquired in many directions, as with the X-ray computed tomographic apparatus.

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.

Claims (26)

1. An X-ray computed tomographic apparatus, comprising:

a display portion to display, on a screen, a scanogram related to a subject together with a quadrilateral frame line specifying a reconstruction range;

an input portion to input a command to transform the frame line specifying said reconstruction range to a parallelogram or rotate the frame line specifying said reconstruction range;

a scan portion to perform scanning in a scan range corresponding to said reconstruction range; and

a reconstruction portion to reconstruct image data related to plural slices, parallel to one another and included in said reconstruction range, slice-by-slice on the basis of projection data acquired by said scanning.

2. The X-ray computed tomographic apparatus according toclaim 1, wherein:

a center line of said transformed or rotated reconstruction range is tilted with respect to a center line of said scan range.

3. The X-ray computed tomographic apparatus according toclaim 2, wherein:

a distance from the center line of said scan range to a center of each of said plural slices varies from slice to slice.

4. The X-ray computed tomographic apparatus according toclaim 2, wherein:

said slices are tilted with respect to the center line of said transformed reconstruction range.

5. The X-ray computed tomographic apparatus according toclaim 4, wherein:

said slices are orthogonal to the center line of said scan range.

6. The X-ray computed tomographic apparatus according toclaim 2, wherein:

said slices are orthogonal to the center line of said rotated reconstruction range.

7. The X-ray computed tomographic apparatus according toclaim 6, wherein:

said slices are tilted with respect to the center line of said scan range.

8. The X-ray computed tomographic apparatus according toclaim 1, wherein:

an icon corresponding to transformation or rotation is displayed in the vicinity of the frame line specifying said reconstruction range.

9. The X-ray computed tomographic apparatus according toclaim 1, wherein:

said scan range is set to a shape of a cylindrical column whose longitudinal cross section is an oblong having an axis of rotation at a center and covering said transformed or rotated reconstruction range.

10. The X-ray computed tomographic apparatus according toclaim 1, wherein:

said scan range covers said transformed or rotated reconstruction range.

11. The X-ray computed tomographic apparatus according toclaim 10, wherein:

said scan range has a length longer than said transformed or rotated reconstruction range with respect to a direction parallel to a central axis thereof.

12. The X-ray computed tomographic apparatus according toclaim 10, wherein:

said scan range has a width almost equal to said transformed or rotated reconstruction range with respect to a direction orthogonal to a central axis thereof.

13. A method for X-ray computed tomography, comprising:

displaying, on a screen, a scanogram related to a subject together with a quadrilateral frame line specifying a reconstruction range;

inputting a command to transform the frame line specifying said reconstruction range to a parallelogram or rotate the frame line specifying said reconstruction range;

performing scanning in a scan range corresponding to said reconstruction range; and

reconstructing image data related to plural slices, parallel to one another and included in said reconstruction range, slice-by-slice on the basis of projection data acquired by said scanning.

14. The method for X-ray computed tomography according toclaim 13, wherein:

a center line of said transformed or rotated reconstruction range is tilted with respect to a center line of said scan range.

15. The method for X-ray computed tomography according toclaim 14, wherein:

a distance from the center line of said scan range to a center of each of said plural slices varies from slice to slice.

16. The method for X-ray computed tomography according toclaim 14, wherein:

said slices are tilted with respect to the center line of said transformed reconstruction range.

17. The method for X-ray computed tomography according toclaim 16, wherein:

said slices are orthogonal to the center line of said scan range.

18. The method for X-ray computed tomography according toclaim 14, wherein:

said slices are orthogonal to the center line of said rotated reconstruction range.

19. The method for X-ray computed tomography according toclaim 18, wherein:

said slices are tilted with respect to the center line of said scan range.

20. The method for X-ray computed tomography according toclaim 13, wherein:

an icon corresponding to transformation or rotation is displayed in the vicinity of the frame line specifying said reconstruction range.

21. The method for X-ray computed tomography according toclaim 13, wherein:

said scan range is set to a shape of a cylindrical column whose longitudinal cross section is an oblong having an axis of rotation at a center and covering said transformed or rotated reconstruction range.

22. The method for X-ray computed tomography according toclaim 13, wherein:

said scan range covers said transformed or rotated reconstruction range.

23. The method for X-ray computed tomography according toclaim 22, wherein:

said scan range has a length longer than said transformed or rotated reconstruction range with respect to a direction parallel to a central axis thereof.

24. The method for X-ray computed tomography according toclaim 22, wherein:

said scan range has a width almost equal to said transformed or rotated reconstruction range with respect to a direction orthogonal to a central axis thereof.

25. An X-ray computed tomographic apparatus, comprising:

a display unit which displays, on a screen, an image specifying a reconstruction range together with a scanogram related to a subject,

an input unit which inputs a command to change a tilt angle of a center axis of the reconstruction range;

a scan unit which performs scanning; and

a reconstruction unit which reconstructs image data related to plural slices slice-by-slice based on projection data acquired by the scanning, almost a center of the image data being placed on the center axis, said reconstruction unit changing a tilt angle of a central axis shown in the scanogram without a shift in an axis of rotation of the scan unit.

26. A method for X-ray computed tomography, comprising:

displaying, on a screen, a scanogram related to a subject together with a center axis of a reconstruction range,

inputting a command to change a tilt angle of the center axis,

performing scanning by irradiating the subject with an X-ray; and

reconstructing image data related to plural slices slice-by-slice on the basis of projection data acquired by the scanning, almost a center of the image data being placed on the center axis, said reconstruction image data changing a tilt angle of a central axis shown in the scanogram without a shift in an axis of rotation during performing scanning.

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