Dynamically Adjustable Focal Spot

Abstract

Methods for maintaining a specified beam profile of an x-ray beam extracted from an x-ray target over a large range of extraction angles relative to the target. A beam of electrons is generated and directed toward a target at an angle of incidence with respect to the target, with the beam of electrons forming a focal spot corresponding to the cross-section of the electron beam. At least one of a size, shape, and orientation of the electron beam cross-section is dynamically varied as the extraction angle is varied, and the extracted x-ray beam is collimated. Dynamically varying the size, shape or orientation of the electron beam cross-section may be performed using focusing and stigmator coils.

Claims

1. A method for maintaining a specified beam profile of an x-ray beam extracted from an x-ray target over a large range of extraction angles relative to said target, the method comprising: generating a beam of electrons; directing the beam of electrons, characterized by a cross-section, toward a target at an angle of incidence with respect to the target wherein the beam of electrons forms a focal spot corresponding to the cross-section of the electron beam; dynamically varying at least one of a size, shape, and orientation of the electron beam cross-section as an extraction angle is varied; and collimating the x-ray beam extracted at the extraction angle.

2. A method in accordance with claim 1, wherein the step of dynamically varying at least one of the size, shape, and orientation of the electron beam cross-section employs focusing and stigmator coils.

3. A method in accordance with claim 1, wherein an apparent focal spot, defined as a projection of the focal spot on a plane transverse to a direction of extraction of an x-ray beam from the target, is kept substantially invariant with increasing extraction angle.

4. A method in accordance with claim 1, wherein an apparent focal spot, defined as a projection of the focal spot on a plane transverse to a direction of extraction of an x-ray beam from the target, is controlled in size, shape, and orientation as a function of the extraction angle.

5. A method in accordance with claim 1, further comprising varying an the angle of incidence of the beam of electrons during generation of x-rays.

6. A method in accordance with claim 1, further comprising varying a centroid of the focal spot on the target as a function of time.

7. A method in accordance with claim 1, wherein focal spot adjustments are functionally related to beam deflection.

8. A method in accordance with claim 1, further comprising using a collimating aperture to collimate the x-ray beam extracted at the extraction angle and controlling an orientation of the electron beam cross section so that the focal spot on the target aligns with the collimating aperture.

9. A method in accordance with claim 1, further comprising adjusting the electron beam cross-section such that a size and shape of an apparent focal spot, defined as a projection of the focal spot on a plane transverse to a direction of extraction of an x-ray beam from the target, does not vary as a function the extraction angle.

10. A method in accordance with claim 1, wherein said dynamically varying at least one of the size, shape, and orientation of the electron beam cross-section yields a dynamic adjustment of the focal spot and wherein said dynamic adjustment of the focal spot maintains alignment of an actual focal spot with an aperture used to collimate the x-ray beam.

11. A method in accordance with claim 10, wherein said dynamically varying at least one of the size, shape, and orientation of the electron beam cross-section and said dynamic adjustment of the focal spot are achieved without any moving parts inside a vacuum tube generating said x-ray beam.

12. In an x-ray system comprising a vacuum tube with a source of a beam of electrons and a target, a method for maintaining a specified beam profile of an x-ray beam extracted from the target over a large range of extraction angles relative to said target, the method comprising: generating a beam of electrons from said source; directing the beam of electrons, characterized by a cross-section, toward a target at an angle of incidence with respect to the target wherein the beam of electrons forms a focal spot corresponding to the cross-section of the electron beam; dynamically adjusting a focal spot by dynamically varying at least one of a size, shape, and orientation of the electron beam cross-section as an extraction angle is varied; and using an aperture to collimate the x-ray beam extracted at the extraction angle.

13. A method in accordance with claim 12, wherein said dynamic adjustment of the focal spot maintains alignment of an actual focal spot with said aperture.

14. A method in accordance with claim 12, wherein said dynamically varying at least one of the size, shape, and orientation of the electron beam cross-section and said dynamically adjusting the focal spot are achieved without any moving parts inside the vacuum tube.

15. A method in accordance with claim 12, wherein the step of dynamically varying at least one of the size, shape, and orientation of the electron beam cross-section employs at least one of focusing and stigmator coils.

16. A method in accordance with claim 12, wherein an apparent focal spot, defined as a projection of the focal spot on a plane transverse to a direction of extraction of an x-ray beam from the target, is kept substantially invariant with increasing extraction angles.

17. A method in accordance with claim 12, wherein an apparent focal spot, defined as a projection of the focal spot on a plane transverse to a direction of extraction of an x-ray beam from the target, is controlled in size, shape, and orientation as a function of the extraction angle.

18. A method in accordance with claim 12, further comprising varying an angle of incidence of the beam of electrons during generation of x-rays.

19. A method in accordance with claim 12, further comprising varying a centroid of the focal spot on the target as a function of time.

20. A method in accordance with claim 12, wherein focal spot adjustments are functionally related to beam deflection.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:

[0039] FIG. 1 depicts a prior art stationary x-ray tube target, with a fixed focal spot.

[0040] FIGS. 2A and 2B show the prior art target of FIG. 1 oriented so that the φ=0 (in FIG. 2A) and φ=60° (in FIG. 2B) direction are normal to the page, illustrating an apparent focal spot 20 in the respective cases.

[0041] FIG. 3 identifies the three angles associated with distortion of an apparent focal spot.

[0042] FIG. 4 shows the distortion of the apparent focal spot at various angles φ for three target angles α.

[0043] FIG. 5 depicts plots of skew angle σ, slant angle δ of the diagonal, and slant angle ε of the ellipse, as a function of extraction angle φ, in case of a 20° target angle α.

[0044] FIGS. 6A and 6B show two views of a target in which the actual focal spot has been adjusted to produce a circular apparent focal spot at the specified extraction angle, in accordance with an embodiment of the present invention.

[0045] FIGS. 7 and 8 show dynamic focal spot adjustment for extraction angles of φ=(−50°, 0, and 35°), for the case of a beam scanned along a path on a target.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0046] Scanning pencil beam systems acquire the image data sequentially, pixel by pixel. That means that, at any given time, x-rays are extracted at one specific angle and not over an entire range. This makes it possible, in accordance with the invention described herein, to correct for the extraction angle dependent distortion of the apparent focal spot.

[0047] In accordance with embodiments of the present invention, of particular advantage in the case of a pencil beam system with a wide field of view, the size and shape of apparent focal spot can be maintained over the entire angular scan range by dynamically adjusting the actual focal spot on the target. This can be achieved by controlling the cross-section of the electron beam by means of focusing and stigmator coils. Most importantly, the orientation of the elongated cross-section of the electron beam can be controlled with stigmator coils so that the actual focal spot on the target always aligns with the collimating aperture. In addition, by controlling the focusing the electron beam cross-section can be adjusted so that size and shape of the apparent focal spot do not vary as a function of the x-ray extraction angle φ.

[0048] In contrast to Safai's concept, described in the Background Section above, dynamical adjustment of the focal spot in accordance with the present invention may advantageously keep the actual focal spot aligned with a moving aperture without requiring any moving parts inside the vacuum tube. It can be implemented purely electronically by controlling the currents through the stigmator coils.

[0049] And, in contradistinction to the teaching of Schardt et al., not only are the actual focal spot shape and size adjusted in accordance with the present invention, but, most importantly, orientation of the focal spot on the target is adjusted as well. Moreover, in accordance with the present invention, focal spot 12 is adjusted in real-time during the image acquisition and not just between images.

[0050] FIGS. 6A and 6B show two views of a target like that depicted in FIG. 1, however the actual focal spot has been adjusted, in accordance with an embodiment of the present invention, to produce a circular apparent focal spot when x-rays are extracted at 45° as shown by the bold arrow in the left view. FIG. 6B is oriented so the x-rays are extracted normal to the page. This is the view through the collimating aperture which shows the apparent focal spot: Dotted ellipse 65 depicts the focal spot seen with the desired circular shape.

[0051] In FIG. 6A, beam-focusing element 15 and stigmation elements 18 are shown for controlling shape, size and orientation of electron beam 16 onto target 10. Focusing and stigmation of beam 16 is governed by controller 20, which applies voltages and/or currents to focusing element 15 and stigmation elements 18 as well-known in the art of charged particle beams. Control of beam focus and orientation is taught in detail in Orloff (ed.), Handbook of Charged Particle Optics (CRC Press, 2008), and in references cited therein.

[0052] The idea of keeping the elongated actual focal spot aligned with the collimating aperture can also be applied to pencil beam systems with stationary aperture and moving x-ray sources like the one described in U.S. Pat. No. 4,045,672 to Watanabe. For these systems keeping the focal spot orientation and the x-ray extraction angle aligned becomes simple as both are controlled electronically. This is illustrated in FIG. 7 where the same controller 20 drives the beam-focusing and stigmation elements 18 as well as the beam steering elements 19. Both, FIGS. 7 and 8 show dynamic focal spot adjustment for 3 representative extraction angles φ(−50°, 0, and35°) which arise as an electron beam is scanned linearly, in this example, along a path 74 on the face of a bremsstrahlung target 75. In particular, the centroid of the focal spot on the target may vary as a function of time. The point 70 where the three arrows meet is the location of the collimating aperture, which is stationary for a pencil beam system with a moving x-ray source. In FIG. 7 focal spot 72 is merely rotated. Again, that the minor axis of the now elliptical apparent focal spot shrinks as φ increases is not detrimental to imaging, rather the opposite.

[0053] In FIG. 8, the focal spot adjustment includes the stretching factor 1/ cos (φ) resulting in a round apparent focal spot for all angles φ.

[0054] Where examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives of beam shaping for inspection with penetrating radiation. Additionally, single device features may fulfill the requirements of separately recited elements of a claim. The embodiments of the invention described herein are intended to be merely exemplary; variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.