WO2007020318A2

WO2007020318A2 - X-ray imaging apparatus and x-ray imaging method for eccentric ct scanning

Info

Publication number: WO2007020318A2
Application number: PCT/FI2005/050293
Authority: WO
Inventors: Juuso SIRÉN
Original assignee: Palodex Group Oy
Current assignee: KaVo Kerr Group Finland
Priority date: 2005-08-17
Filing date: 2005-08-17
Publication date: 2007-02-22
Anticipated expiration: 2008-02-17
Also published as: WO2007020318A3

Abstract

The invention relates to a method and device for performing CT type X-ray imaging. The device comprises a frame part (25), an X-ray source (22; 41) for producing X- rays (42), radiation receiving means (21; 44) for detecting radiation transmitted through an object, support means (26) for supporting the X-ray source and the receiving means in positions on opposite sides of an object. The support means are connected to the frame part so as to rotate around the axis of rotation. The device further comprises actuator means for rotating the support means around the axis of rotation and collimation means arranged in conjunction with the X-ray source for causing a conical or pyramid-shaped beam of rays to be directed through the object, whereupon the field of view becomes essentially a circular cylinder. In the method, the rotation centre (45) between the X-ray source (41) and the radiation receiving means (44) is arranged to move along a predetermined route as the support means rotate around the axis of rotation (40).

Description

X-ray imaging apparatus and X-ray imaging method for eccentric ct scanning

The present invention relates to a method and device for performing CT type (X-ray Computed Tomography) X-ray imaging.

Background art

US 6,434,214 describes an X-ray CT imaging device and X-ray imaging method. Figure 1 shows the general construction of a conventional cone-beam X-ray CT device. The conventional cone-beam X-ray CT device is divided into an imaging unit 1 for carrying out the imaging and an image processing unit 2 for processing the image data detected. The control unit 3 performs the overall control of the imaging unit 1 and the image processing unit 2. In the imaging unit 1, an X-ray source 5 and a two-dimensional radiation receiving means 6 are arranged in such a way as to be opposite each other through an object. The X-ray source 5 and the two-dimensional radiation receiving means 6 are both arranged in a scanner which rotates around the object 7 with the central axis 9 of rotation as the rotation centre.

The scanner 4 is rotated to each predetermined angle and the receiving means 6 measures the intensity of the X-rays 8 transmitted from the X-ray source, the said X-rays having been transmitted through the object 7 at each predetermined angle. The X-ray image imaged by means of the receiving means is converted into digital image data to be transmitted further to the image processing unit 2.

In the image processing unit 2, pre-processing with means 10 is first carried out, for example, gamma correction, distortion correction, logarithmic transformation and non-uniformity correction of the receiving means 6. The reconstruction means 11 then reconstruct, on the basis of the X-ray images (all projected images) transmitted, after the completion of the pre-processing, a three-dimensional reconstructed image, the image being the three-dimensional X-ray absorption coefficient distribution of the part of the object 7 imaged.

As concerns this reconstruction arithmetical operation method, Feldkamp's cone- beam reconstruction arithmetical operation method or the like is known as described in an article by L. A. Feldkamp et al.: PRACTICAL CONE-BEAM ALGORITHM, Journal of Optical Society of America, A.Vol. 1, No. 6, pp. 612 to 619 (1984) (article 1).

Finally, the imaging means 12 subject the three-dimensional reconstructed image to image processing, such as volume-rendering or maximum-intensity-projection processing for displaying the resultant image as a two-dimensional image on a display 13. In that case, the imaging means 12 execute image processing on the basis of the angle of view, the region to be observed, and similar parameters entered through instruction means, such as a keyboard, a mouse and a track ball.

In conventional cone-beam X-ray CT imaging, a scanner 4 provided with an imaging system comprising an X-ray source 5 and a two-dimensional radiation receiving means 6 is rotated around the object 7. The transmitted radiation obtained is imaged and the reconstruction means 11 form a three-dimensional X-radiation absorption coefficient distribution of the object 7 placed on a stationary coordinate system fixed to the frame of the device. The stationary coordinate system is defined by means of the imaging system, i.e., the z-axis as the rotation centre 9 of the scanner, and the rectangular x- and y-axes on the plane on which the rotation orbit of an x-ray focus 14 of the x-ray source 5 lies.

Summary of the invention

The aim of the present invention is to provide an improved CT-type X-ray imaging method, by means of which a larger field of view (= FOV) can be obtained with the same exposure parameters and receiving means, or correspondingly, a smaller amount of radiation can be used. A further aim of the invention is to provide a device by means of which the method can be implemented. To implement the method relating to the invention, a device is used which comprises a frame part, an X-ray source for producing X-rays, radiation receiving means for detecting radiation transmitted through an object, support means for supporting the X-ray source and the receiving means in positions on opposite sides of the object, the said support means being connected to the frame part so as to rotate around the axis of rotation, and actuator means for rotating the said support means around the said axis of rotation, where in conjunction with the X-ray source are arranged collimation means for causing a conical or pyramid-shaped cone of rays to be directed at the object, whereupon the field of view becomes essentially a circular cylinder. In the method, the rotation centre between the X-ray source and the radiation receiving means is arranged to move along a predetermined route as the support means rotate around the axis of rotation, whereby the field of view can be enlarged with the same exposure parameters, or alternatively, the radiation dose may be reduced while the field of view remains the same in comparison to a case where the said rotation centre is connected to the axis of rotation. In the method, the centreline between the X-ray source and the radiation receiving means is offset with respect to the axis of rotation between the support means and the frame structure, whereupon when the support means rotate around the said axis of rotation, the rotation centre between the X-ray source and the radiation receiving means follows a circular path, the radius of which corresponds to the extent of the offset of the centreline between X-ray source and the radiation receiving means.

The offset is preferably within a range from 5 mm to 20 cm, in which case an offset closer to the lower limit is particularly useful when it is desirable to reduce the radiation dose required by a small field of view. When the field of view is to be maximised, the offset is preferably within the range from 4 to 6 cm.

The device according to the invention comprises a frame part, an X-ray source for producing X-rays, radiation receiving means for detecting radiation transmitted through an object, support means for supporting the X-ray source and receiving means in positions on opposite sides of the object, the said support means being connected to the frame part so as to rotate around the axis of rotation, actuator means for rotating the support means around the axis of rotation, collimation means arranged in conjunction with the X-ray source for producing a conical or pyramid-shaped cone of rays to be directed at the object, whereby the field of view becomes essentially a circular cylinder, in which device the rotation centre between the X-ray source and the radiation receiving means is arranged to move along a predetermined route when the support means rotate around the axis of rotation.

Brief description of the drawings

Other aims, characteristics and advantages of the invention appear from the following, more detailed description, with reference to the drawings, in which: Figure 1 shows diagrammatically the structure of a conventional cone-beam X- ray CT device,

Figure 2 shows diagrammatically an X-ray device according to the invention which is suitable for implementing the method according to the invention,

Figure 3 shows diagrammatically the field of view produced by a conventional X-ray CT imaging method, each point of which area is exposed from a

360° angle area when the X-ray source and the radiation receiving means rotate a full cycle around the object,

Figure 4 shows an extended field of view provided by the invention, where the most of the area is exposed within the 360° angle area, while the edge areas are exposed within an angle area well over 180°,

Figure 5 shows a field of view extended further, where only a very small area is exposed within the 360° angle area, while most of the area is exposed within an angle area of 180°, and

Figure 6 shows a field of view extended even further, where an area is formed in the centre which is not exposed to radiation at all when the X-ray source and the radiation receiving means rotate a full circle while the rest of the area is exposed within an angle area of about 180°.

Description of the preferred embodiments

Figure 2 illustrates a device 20 for implementing the method according to the invention comprising a frame part 25, on which is supported a rotating part 23 with a support part 26, at one end of which is an X-ray source 22 and on the opposite side of it, at the other end, radiation receiving means 21, where the object to be imaged is position between the X-ray source 22 and the receiving means 21. The rotating part 23 is arranged to rotate around the axis of rotation with respect to the frame 25, the said axis of rotation passing through the field of view of the object being imaged. The device further comprises a seat 30 for receiving the person being imaged and adjustable support means 27 as well as positioning means 28 and 29 for positioning the object correctly with respect to the X-ray source 22 and the radiation receiving means 21.

In accordance with Figure 3, in conventional CT imaging, the rotation centre between the X-ray source 41 and the radiation receiving means 44 remains in place at the axis of rotation 40, when the X-ray source 41 and the radiation receiving means 44 turn 360° around the object being imaged, whereby the size of the field of view 43 is determined on the basis of the size of the collimated conical cone of rays 42. Each point in the field of view 43 is exposed from the 360° angle area. As regards the actual imaging of the transmitted X-radiation, continuous analogical imaging is. not carried out, but discrete digital imaging. The number of exposure directions may be as few as 4 to 16, but it may also be several thousand..

Figure 4 shows the basic idea of the method according to the invention, where the central ray 42a of the cone-beam 42 between the X-ray source 41 and the radiation receiving means 44 is moved to travel in an offset manner with respect to the axis of rotation 40. This offset causes the rotation centre 45 between the X-ray source 41 and the radiation receiving means 44 to travel along a circular path, the radius of which corresponds to the magnitude of the offset. In this case, the field of view 43 can be extended by the edge area 43b, which edge area is exposed in an angle area exceeding 180°. This edge area produces sufficient image quality, since the aim is to position the actual objects of interest in the centre of the field of view, at the axis of rotation, while the edge area will provide other necessary additional information. This basic idea of the invention may also be utilised in such a way that the amount of radiation exposure is reduced, whereby the field of view will remain constant. The offset may be fixed or adjustable.

Figure 5 shows diagrammatically a situation where the offset is greater than in the case of Figure 4, the rotation centre 45 travelling along a circle with a larger radius, whereupon the field of view is extended further, but only a very small area is exposed within the 360° angle area, whereas most of the area is exposed within an angle area of 180°. Figure 6 shows an offset extended even further, whereupon an area 43 is formed in the centre of the field of view which is not subjected to radiation at all when the X- ray source 40 and the radiation receiving means 44 rotate a full circle, the rest of the area being exposed within an angle area of about 180°, which is the minimum requirement for producing sufficient image information.

The arrangement of the central ray of the cone-beam between the X-ray source and the radiation receiving means to travel in an offset manner with respect to the axis of rotation may, in the device solution according to Figure 2, be realised, for example, so that between the rotating part 23 and the support part 26 are arranged slide means with actuator means which make possible mutual linear movement between them, the said slide means allowing the support part 26 to be moved sideways with respect to the rotating part 23 in such a way that the centreline between the X-ray source and the radiation receiving means is offset with respect to the centreline running via the axis of rotation between the rotating part 23 and the frame structure 25. The extent of this offset is preferably adjustable. A conceivable implementation is also one where the support part is placed in a fixed position with respect to the rotating part in such a way that the said centreline runs in an offset manner with respect to the axis of rotation. The offset may also be realised so that only one of the X-ray sources and radiation receiving means is moved, so that the centreline between them will settle in an angle position with respect to the centreline running via the axis of rotation. This angular transfer is preferably also arranged to be adjustable.

Once the centreline offset has been carried out, the support part 26 and the rotating part 23 remain in a mutually fixed position during the rotating motion around the object, whereby the rotation centre between the X-ray source and radiation receiving means will follow a circular path.

It is also possible to realise the offset in such a way that the support part 26 is moved in directions X and Y during the rotating motion around the object so that the rotation centre between the X-ray source and the radiation receiving means will follow a path deviating from the circular path, for example, an elliptic path.

Claims

1. A method for performing CT type X-ray imaging by means of a device comprising a frame part (25), an X-ray source (22; 41) for producing X-rays (42), radiation receiving means (21; 44) for detecting radiation transmitted through an object, support means (26) for supporting the X-ray source (22; 41) and the receiving means (21; 44) in positions on opposite sides of the object, the said support means (26) being connected to the frame part (25) so as to rotate around the axis of rotation, and actuator means for rotating the said support means around the said axis of rotation, where in conjunction with the X-ray source are arranged collimation means for causing a conical or pyramid-shaped beam of rays to be directed through the object, whereupon the field of view becomes essentially a circular cylinder, characterised in that in the method, the rotation centre (45) between the X-ray source (41) and the radiation receiving means (44) is arranged to move along a predetermined route as the support means rotate around the axis of rotation (40).

2. A method as claimed in claim 1, wherein the centreline (42a) between the X-ray source (41) and the radiation receiving means (44) is offset with respect to the axis of rotation (40) between the support means and the frame structure, whereupon when the support means rotate around the said axis of rotation, the rotation centre (45) between the X-ray source (41) and the radiation receiving means (44) follows a circular path, the radius of which corresponds to the extent of the offset of the centreline between X-ray source and the radiation receiving means.

3. A method as claimed in claim 2, wherein the offset is within the range from 5 mm to 20 cm.

4. A method as claimed in claim 2, wherein the offset is within the range from 4 cm to 6 cm.

5. A device for performing CT type X-ray imaging, the device comprising a frame part (25), an X-ray source (22; 41) for producing X-rays (42a), radiation receiving means (21, 44) for detecting radiation transmitted through an object, support means (26) for supporting the X-ray source and the receiving means in positions on opposite sides of the object, the said support means being connected to the frame part so as to rotate around the axis of rotation, and actuator means for rotating the said support means around the axis of rotation, collimation means arranged in conjunction with the X-ray source for causing a conical or pyramid-shaped beam of rays to be directed through the object, whereupon the field of view becomes essentially a circular cylinder, characterised in that in the device, the rotation centre (45) between the X-ray source (41) and the radiation receiving means (44) is arranged to move along a predetermined route as the support means rotate around the axis of rotation (40).

6. A device as claimed in claim 5, wherein the centreline (42a) between the X-ray source (41) and the radiation receiving means (44) is offset with respect to the axis of rotation (40) between the support means and the frame structure, whereupon when the support means rotate around the said axis of rotation, the rotation centre (45) between the X-ray source (41) and the radiation receiving means (44) follows a circular path, the radius of which corresponds to the extent of the offset of the centreline between X-ray source and the radiation receiving means.

7. A device as claimed in claim 6, wherein the offset is set permanently.

8. A device as claimed in claim 6, wherein the offset is adjustable.