Patent Number: 
Section: description

The present application claims priority from U.S. Provisional Patent Application Ser. No. 61/638,555, filed Apr. 26, 2012, and incorporated herein by reference. The present invention relates to sources of X-ray radiation, and, more particularly, to an X-ray tube with a rotating anode. X-ray backscatter imaging relies on scanning an object with a well-collimated beam, typically referred to as “pencil beam”. Several approaches for forming the collimated scanning beam have been suggested. Commonly, beam formation and steering relies on an aperture moving in front of a stationary X-ray tube. In most cases the radiation from an X-ray tube is first collimated into a fan beam by a stationary collimator. Then, a moving part with an opening forms a scanning beam. This moving part can be, for example, a rotating disk with radial slits, or a wheel with openings at the perimeter. The rotating disk covers the fan beam and the scanning beam is formed by the radiation emitted through the slits traversing the length of the fan beam opening. This approach is illustrated, e.g., in the U.S. Pat. No. 3,780,291 (to Stein and Swift). In the case of a rotating wheel, a wheel with radial bores spins around the X-ray source. If the source is placed at the center of the wheel (or hub), the scanning beam is emitted in radial direction with the angular speed of the wheel. Alternatively, the source may be placed off-center with respect to the rotating wheel, which changes the beam geometry. In most X-ray tubes, an electron beam impinges upon a stationary target, which, in turn, gives off X-ray radiation produced by stopping the fast electrons, i.e., Bremsstrahlung. Most of the kinetic energy of the electron beam is converted into heat and only a small fraction is given off as X-rays. For imaging purposes, a small electron beam focal spot is desirable, however anode heating limits the acceptable current for a given focal spot size. To allow smaller focal spots, X-ray tubes 100 have been designed to have rotating anodes, as depicted in FIG. 1. X-ray tube 100 represents a typical design, as produced, for example, by Varian Medical Systems. Rotating anode 102 distributes the heat over a larger area and allows a considerably smaller focal spot 104 of electrons 106 emanating from cathode block 107 than would be possible using a stationary anode. Rotating anode 102 is rotated by rigid coupling to rotor 108 which moves relative to stator 110. X-rays 112 are emitted through exit window 114, and they are subsequently collimated by some external collimating structure. In accordance with various embodiments of the present invention, an X-ray tube is provided that both generates and collimates an X-ray beam. The X-ray tube has a vacuum enclosure, a cathode disposed within the vacuum enclosure for emitting a beam of electrons, and an anode adapted for rotation with respect to the vacuum enclosure about an axis of rotation. The X-ray tube also has at least one collimator opening adapted for co-rotation with respect to the anode within the vacuum enclosure. In accordance with other embodiments of the present invention, the collimator opening or openings may be disposed within the anode itself Each collimator opening may be contiguous with a wedge opening in the anode. In accordance with further embodiments of the present invention, the X-ray tube may have an external collimator opening disposed outside the vacuum enclosure. The collimator openings (or opening) may be disposed above a plane transverse to the axis of rotation containing a locus of focal spots of the beam of electrons. In accordance with embodiments of the present invention, described now with reference to FIGS. 2-6, an X-ray tube 200 is provided that uses a rotating anode, not only to distribute the heat, but also to act as a rotating collimator to create a scanning beam. To that end, referring first to FIG. 2, anode 202 is preferably concave, with an electron beam 204 impinging upon focal spot 205 on an inner surface 206 in such a manner that the X-rays 208 are emitted towards the center 210 of anode 202. In the embodiment depicted in FIG. 2, X-rays 208 are emitted perpendicularly to axis of rotation 212 about which anode 202 rotates. The elevated rim 216 of anode 202 may also be referred to herein as an anode “ring” 216. To form a scanning collimated pencil beam 214, anode ring 216 has openings 218 which allow X-rays 208 to be emitted out of the X-ray tube 200. In the depicted embodiment, anode ring 216 has three openings 120° apart creating a scanning beam coverage of approximately 50°. FIG. 3 is a top cross-sectional view of anode 202 of FIG. 2. The circular focal spot path 220 comprises the locus of regions serving as focal spot 205 as anode 202 rotates. Partially collimated pencil beam 214 emerges from wedge opening 230. An external collimator slit 232 may be situated outside glass envelope 234 of the X-ray tube 200. In FIG. 4, rotating anode 202 has been rotated relative to the cathode block 107 in order to illustrate a near-extremal position of the beam span, where the focal spot 205 will fall into the wedge opening 230 just as collimated pencil beam 214 is about to be vignetted by an edge of wedge opening 230. More generally, within the scope of the present invention, opening 218 is to be considered an instance of a collimator aperture which co-rotates with anode 202, whether or not the aperture is integral with the anode. In the embodiment of rotating anode X-ray tube 500, depicted in FIG. 5, X-rays 502 are emitted at a slight angle to clear the height of the slanted anode 504. This eliminates the need to cut openings into the slanted anode area and thus allows for continuous X-ray generation not interrupted by gaps in the anode area. X-rays 502 are emitted, instead, through an aperture 506 above the plane transverse to rotation axis 212 containing the intersection of focal spot 205 with the surface of rotating anode 504. A further advantage of this design is the greater flexibility in choosing the number of apertures. FIG. 6 is a top view of the anode of FIG. 5. The largest possible angular span of the scanning beam depends on the number of apertures in the ring as well as on the ratio of the ring diameter 2R to the distance r between the focal spot and the center of rotation, see FIG. 6. A single aperture 506 theoretically allows for a 360° angular beam span. For two opposite apertures 506, the theoretical beam span is twice the arc tangent of the ratio R/r, where, as shown in FIG. 6, R is the radius of the an anode rim ring wall 602, and r is the radial distance from the axis of rotation 212 to focal spot 205. Using three equally spaced apertures limits the theoretical beam span to twice the arc tangent of the ratio                     3        ⁢        R                            2        ⁢        r            +      R        .These formulas are exact for a dimensionless focal spot 205 and an infinitesimally thin anode ring wall 602. Assuming the ring wall radius R is 4/3 of the focal spot distance r, two opposite apertures 506 create a span of about 106°; three equally spaced apertures 506 create a span of just over 69°. In preferred embodiments of the present invention, the aperture 506 in the anode ring wall 602 are vertical cuts (parallel to the axis of rotation) and the collimation in the vertical direction is accomplished by an external collimator slit 232 positioned outside the x-ray tube 500. In order for the scanning beam to span a plane without curvature, the external collimator slit 232 should be coplanar with the focal spot 205. X-ray tubes with anodes rotating at up to 10,000 rpm are commercially available. With three openings and 150 rotations per second, X-ray tube 500, in accordance with embodiments of the present invention, creates a scan rate of 450 lines per second, a rate compatible, for example, with typical applications like whole body scanners. 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 objective of x-ray scanning 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.