X-RAY TUBES FOR IMAGING SYSTEMS

Abstract: An x-ray tube having novel components and geometry to optimize the resolution of fan beam and flying-spot transmission images by varying the size of the focal region and by compensating the tube current in response to the degree of beam attenuation by the imaged object, and by taking the plane of the fan beam off the x-ray tube at an acute angle to the cathode-anode axis.

Description: X-Ray Tubes for Imaging Systems Field of the Invention The present invention relates to improved x-ray tubes for the generation of x- ray beams, and, more particularly to x-ray tubes wherein the shape and intensity of the emitted beam may be controlled to optimize the resolution of transmitted images of intervening objects. Background Art X-ray tubes are commonly provided with one or two fixed focal spots of different sizes, whereby higher resolution images can be obtained by using the small spot, or lower resolution images of the same object can be obtained in a shorter time by using the larger focus at a higher power. The required time is proportional to the power applied to the focal spot. To the extent that the tube's quoted focal spot sizes, in accordance with industry standards, are nominally square, the power that can be applied is closely proportional to the linear dimension of the focal spot. When tubes of this type are used for flying spot imaging systems, it is usual to use only the large focal spot so as to obtain sufficient flux to penetrate the higher areal density parts of an object without requiring inordinately long scan times. Flying spot systems are described, for example, in U.S. Patent No. 4,342,912, to Bjorkholm, which is herein incorporated by reference. Flying spot systems, in operation, are usually "flux starved," in that the sensitivity is predominantly limited by the number of x-ray photons per resolution element of the target. An unfortunate consequence of the use of large focal spots is that the resolution is adversely affected. This is especially so in flying spot systems, where the resolution is largely determined by a pinhole projection of the focal spot that may be enlarged severalfold. The size of a focal spot in an x-ray tube is established by the size of an electron-emitting filament and a simple electron-optical system that focuses the electrons emitted by the filament onto an anode. The electron lens typically has a fixed focal length that customarily (but not necessarily) produces a demagnified filament image. Dual focal spot tubes simply use filaments of two different sizes. Existing X-Ray Tube Design The basic configuration of the cathode and anode structure of an x-ray tube is shown in Figures l(a)-l(e). (A conventional, stationary anode x-ray tube is illustrated; rotating anode tubes have a very different configuration, but the following description still applies in all important details, and the scope of the invention is also applicable to rotating anode tubes.) The conventional x-ray tube comprises a cathode structure 2 and an anode structure 4, housed in a vacuum envelope (not shown). A high electrical potential is applied between cathode 2 and anode 4 to accelerate electrons emitted by a filament 6 in cathode 2 onto a "target" 8 in anode 4. Electron optics (established by the shapes of the cathode 2 and anode 4, the latter usually having a "hood" to improve the optics and to capture scattered electrons) cause the filament electron source 6 to be imaged onto target 8, usually with a degree of demagnification, as illustrated. X- rays are produced where the energetic electrons strike target 8, either by bremsstrahlung or by ionization of the inner shells of the atoms of target material. In x-ray tubes intended for radiography, a high atomic number, refractory metal, usually tungsten, is used. The area of the target 8 that is struck by electrons is called the "focal spot" of the x-ray tube, having a "height" B τ and a "width" B D , as shown in VIEW A-A of Figure 1(e). (Subscripts "T" and "D" are used to indicate the "transport" and "detector" directions, respectively, in fan-beam geometry imaging systems.) The focal spot is normally elongated (i.e., B τ is greater than B D ) and is tilted at target angle α (shown in FIG. 1(b)) toward an x-ray exit "window" in the tube. X-rays are emitted in all directions from the focal spot. However, the useful beam is usually restricted to a relatively small cone of radiation directed normal to the cathode- anode axis 10. The cathode-anode axis 10 of standard x-ray tubes is concentric with or parallel to the axis of the x-ray tube. This arrangement allows emission of an intense cone beam of x-rays without melting the target material and without absorption of the useful beam in target 8 itself. A typical x-ray tube provides an output field of 40°, so its target is designed with a target angle α of about 20° or a little more. X-ray tube manufacturers quote a "nominal" focal spot height F τ and width F D for an x-ray tube. To be within specification according to industry standards, the actual focal spot dimensions F τ ' and F D 'as determined from pinhole images taken on the central axis 12 of the cone beam, must lie within the limits F τ /0.7 ≤ F τ ' < fF T /0.7 (IA) F D < F D ' < fF D , (IB) where f has a value ranging from 1.3 to 1.5, depending on the nominal focal spot dimension. In practice, the actual focal spots tend to be at the upper limits of the allowable range. For nominal focal spots greater than 1.5 mm, f has the value 1.3. For the remainder of this application the approximation will be used: F τ ' * F τ /0.7, (2A) F D ' - 1.3 F D . (2B) These axially-projected dimensions, together with target angle α, can be used to estimate the electron beam spot dimensions B τ and B D on the tilted anode target 8: B τ = F τ '/sin a * F τ /(0.7 sin α), (3A) B D = F D ' = 1.3 F D . (3B) Focal Spot Projection Geometry and Resolution The resolution of an image produced by an x-ray tube is directly affected by the apparent size of the focal spot as viewed from the image field. On axis, the apparent size is F τ ' x F D '. As is well known, the apparent size of the focal spot changes as viewed from off the central axis 12 of the radiation cone. The apparent height F τ ' is foreshortened as the viewing angle β (shown in FIG. 1(b)) approaches tangency with the target itself, and is lengthened as viewing angle β moves in the opposite direction. Similarly, the apparent width F D ' increases as the viewing angle β is moved off of the cone axis 12 in the θ (azimuthal) direction (shown in FIG. 1(c)). VIEW C-C of Figure 1(d) illustrates the general shape of the focal spot as viewed at an azimuthal angle θ and an elevation angle β off the central axis of the cone. (Note that actual focal spots are neither sharply defined nor uniformly distributed, as is assumed for purposes of this discussion.) As used in this description and in the appended claims, the terms "elevation" and "azimuth" are defined, as stated in the text, with reference to features of the x-ray tube and are not meant to imply orientation with respect to the earth or other externally defined axes. Figure 2 shows the development of the dimensions of the focal spot 20 projected in a direction θ, φ, where θ is the azimuthal angle, as above, and φ is the angle off the focal spot normal in the θ = 0° plane, φ is related to the elevation angle β and target angle α by φ = [90° - (α + β)]. (A) is a larger-scale replication of the focal spot established by the electron beam, which was shown in the cross-section taken along line A-A of Figure 1(e). (B) is an edge-on view of (A). (C) is a view of (A) from an angle φ and (D) is a view of (A) from an angle φ + 90°. Angle θ is a measure of rotation around the long axis of view (C), as measured from the normal to projection (C). Finally, (E) is the projection of (C) at angle θ. The three characteristic dimensions of the parallelogram-shaped distribution, H, w„ and w 2 (shown in FIG. 1(d)), are easily calculated from the projections: H = B τ cos φ (4) W = w, + w 2 = B D cos θ + B τ sin φ sin θ (5) The effective intensity distribution along the θ direction from the parallelogram- shaped focal spot projection can be approximated by a trapezoid whose full-width- half-maximum (FWHM) is the greater of Wj and w 2 . Brief Description of the Drawings The invention will more readily be understood by reference to the following description, taken with the accompanying drawings in which: FIGS. l(a)-l(e) depict the geometrical features of a prior art x-ray tube. FIG. 2 depicts a geometrical derivation of the dimensions of the projected focal spot of an x-ray tube. FIG. 3 shows a side view in cross-section of an improved x-ray tube having a focussing electrode in accordance with an embodiment of the present invention. FIG. 4(a) shows a cut-away side view of a prior art x-ray tube. FIG. 4(b) shows a top view in cross-section of a prior art x-ray tube. FIG. 5(a) is a cut-away side view of an x-ray tube employing an anode disposed at an enhanced target angle in accordance with an embodiment of the present invention. FIG. 5(b) is a top view in cross-section of the x-ray rube of FIG. 5(a). FIG. 6 is a cut-away side view of an x-ray tube with an output window disposed at an acute angle with respect to the cathode-anode axis in accordance with an alternate embodiment of the present invention. FIG. 7 is a top view in cross-section of an x-ray tube having a cathode-anode axis offset with respect to the axis of the vacuum envelope of the x-ray tube, in accordance with an embodiment of the present invention. Brief Summary of the Invention In accordance with one aspect of the invention, in one of its embodiments, there is provided an x-ray tube for generating an x-ray beam for imaging an object. The x-ray tube has a vacuum enclosure, a cathode for emitting an electron flux, an anode structure having a positive voltage potential with respect to the cathode for emitting the x-ray beam, an electron lens having at least one control potential with respect to the cathode for varying the size of a region of the anode impinged upon by the elctron flux, and a controller for varying the at least one control potential so as to vary the size of the region of the anode impinged upon by the elctron flux in a determined relation to the electron flux In accordance with alternate embodiments of the invention, the electron lens may include arangements for focussing or blurring the electron flux onto the anode, and the controller may include control elements for varying the control potential or control potentials of the electron lens with respect to the cathode, as well as the size of the region of the anode impinged upon by the elctron flux and the electron flux in a specified manner in response to a transmission level of the x-ray beam through the object. In accordance with a further aspect of the invention, an improved x-ray tube is provided of the type having a vacuum enclosure, a cathode emitting an electron flux, an anode for emitting an x-ray beam, the cathode and anode defining a cathode-anode axis, and a window formed in the vacuum enclosure for permitting efflux of the x-ray beam. The improvement includes an arrangement for holding the cathode and anode in relation to the vacuum window such that a beam axis, defined by a line segment running between a central portion of the anode and a central portion of the pro j ection of the x-ray beam on an imaged object, is disposed at an acute angle in relation to the cathode-anode axis. This improvement increases the apparent height of the x-ray beamsource while sharpening the apparent width of the x-ray source when it is viewed off the central (beam) axis. The x-ray tube as set forth may also have a beam steering element for reducing the angle of incidence of the electron flux onto the anode. The first aspect of the invention is most suited to flymg-spot scanners wherein detected signals are measured sequentially, point-by-point. The second aspect of the invention is useful for any fan-beam or flying-spot scanner. In accordance with an alternate embodiment of the invention, an x-ray tube is provided for generating an x-ray beam for imaging an object. The x-ray tube has a vacuum enclosure, a cathode for emitting an electron flux, an anode structure for emitting the x-ray beam, a vacuum window formed in the vacuum enclosure for permitting efflux of the x-ray beam, and an arrangement for holding the cathode and anode in relation to the vacuum window such that a beam axis, defined by a line segment running between a central portion of the anode and a central portion of the projection of the beam on the object, is disposed at an acute angle in relation to the cathode-anode axis. In accordance with yet a further aspect of the present invention, a method is provided for scanning an object with x-rays. The method has the steps of providing an x-ray source for emitting an x-ray beam having a variable spot size and a variable x-ray flux, irradiating the object with the x-ray beam, monitoring a level of transmission of the x-ray beam through the object, and varying the x-ray flux and spot size of the x-ray beam in substantially inverse proportion to the fraction of the x-ray beam transmitted through the object such that attenuation of x-rays transmitted through the object is compensated by a substantially corresponding increase in the x-ray flux of the x-ray beam. An advantage of the several aspects of the invention is improved off-axis resolution performance. A further beneficial use may be to enable the use of very broad fan beam angles without an unacceptable loss of resolution. This can be advantageous in several ways: 1. In accordance with embodiments of the invention, the x-ray flux produced by the x-ray tube may be better utilized, in direct proportion to the relative angular field increase. 2. A larger field-of-view may be covered in a single scan. For example, applied to the large-object scanning system described in a co-pending U.S. patent application, Ser. No. 08/799533, which is herein incorporated by reference, it may enable a large vehicle to be scanned in one pass per side, instead of two. 3. As an alternative to increasing the field-of-view, embodiments of the invention may be used to cover a smaller field-of-view by a scanning system having a smaller footprint. Detailed Description of Specific Embodiments Referring now to FIG. 3, one preferred embodiment of the invention has an improved electron lens, designated generally by numeral 30, generally disposed between cathode 2 and anode 4. V A indicates the high voltage potential applied to anode 4 while V c indicates the low reference potential of cathode 2. V F is the low voltage, typically 10 volts, applied across filament 6. Electron lens 30 has one or more focusing elements 32a, 32b, etc., to which control potentials V Ga , V Gb , etc. are applied from controller 7 to cause the size of the focal spot to be varied between a tight focus F and a blurred or less concentrated focus F', in a manner that may be analogous to the function of an optical zoom lens. Electron lens 30 does not affect the total accelerating potential (V A -V C ) of the x-ray tube. Solid lines 34 indicate the envelope of electron paths under conditions of of tight electron focussing. Dashed lines 36 indicate the envelope of electron paths under conditions of less concentration. The shaping and positioning of focussing electrodes 32a, 32b, etc., and the potentials necessary to be applied to them to achieve the desired degree of electron beam concentration, are known to persons skilled in the art of electron optics. Alternatively, rather than focussing the electrons onto anode 4, potentials V Ga , V Gb , etc. can be varied to defocus the spot F into a larger, blurred size F'. A means to quickly adjust the tube current is also employed, as known to persons skilled in the art. The tube current may be simply adjusted by means of a control of filament current, for example, if the thermal response time is fast enough. Alternatively, it may be adjusted by means of a current control grid or electrode in the cathode 2 or in the electron lens 30. The two control elements of controller 7, one for focal length and one for beam current, may be used together in the following manner: At the beginning of a scan (i.e., before the inspected object actually enters the x-ray field) the tube is turned on and the control elements are set to produce a small spot size and the maximum power permissible for that small spot. As the object enters the beam and attenuates the flux, this small focus and (relatively) low current combination continues in effect as long as the attenuation of the object at any point along the flying spot path does not exceed some nominal value established during system design or calibration. When the attenuation exceeds that value, as determined by a drop of signal level below a signal threshold, the amount by which the signal falls below the threshold value is used to generate a feedback signal to provide a proportional increase of tube current. Simultaneously, the electron lens 30 is adjusted to proportionately increase the size of, or to blur, the focal spot so that the thermal limitations of target 8 are not exceeded. The actual, detected (analog) signal thereby remains constant and, thus, effectively compensated, for a range of attenuations greater than the set value, but the signal level is digitally scaled upward in proportion to the beam current. Clearly, the range of the focal spot size and current changes must be within some practical limit, which may typically approach a factor of 10. At a greater level of attenuation the focal spot size and beam current may no longer be increased, and the detected signal resumes its drop in intensity; meanwhile, the digital scaling according to beam current continues at its maximum level. As the flying spot moves back to regions of lesser attenuation, the reverse of the above process occurs. The analog signal first increases until it reaches a level where the current and the focal spot size can be reduced, and then maintains a constant value, to which a continuously declining digital scaling is applied, until the minimum focal spot size is reached, at which time the analog signal is allowed to grow without digital scaling. According to the operation described above, the imaging resolution is higher throughout that range of signal levels from maximum down through the second break point (where the focal spot and current reach their maximum values) than is customarily achieved by use of the "large" focal spot throughout the entire imaging range. This yields a significantly "sharper" image down to attenuation levels where poor photon statistics dominate resolving power. Stated another way, the modulation transfer function is improved at high signal levels where it is limited by the physical characteristics of the equipment, and equals the previously-achieved values for low signal levels where it is dominated by photon statistics. Projected Focal Spots in a Fan Beam Geometry Conventional Usage Fan beam systems (both flying-spot and detector array) use a very narrow range of angles in the elevation direction, but preferably use large angles in the azimuthal direction. (As defined above, "elevation" and "azimuth" are referenced to the x-ray tube.) The fan beam is customarily taken perpendicular to the tube axis (β = 0°, or φ = 90° - a). Consider a conventional fan beam generation system using a tube having parameters α = 22° (φ = 68°), F τ = 4.5 mm, F D = 4.5 mm, to be used over a fan beam angle of ± 45°. Substituting these values into the above equations (3), (4) and (5) yields apparent focal spot dimensions at the limits of the fan beam of: H = 6.43 mm (for all θ), W = 4.14 mm + 11.25 mm = 15.39 mm (for θ = +/- 45°), FWHM = 11.25 mm (for θ = +/- 45°). W = FWHM = B D = F D ' = 1.3 F D = 5.85 mm (for θ = 0°) . For a wide fan beam angle the apparent width is slightly less than the apparent height only near the center of the fan, and is much broader at the edges. Usage for Fan Beam Planes Not Normal to the Tube Axis In embodiments of a second aspect of the current invention, the fan beam is taken off at an angle φ that is significantly smaller than the value φ = 90° - α that corresponds to the plane normal to the tube axis. When viewed under these conditions, the apparent height H will be larger than for the conventional usage, but the broadening of width W at large azimuthal angles θ will be significantly less, thereby sharpening the image resolution in the plane of the fan beam. Consider the same tube used above, but with the only difference being that the fan beam plane is taken off at φ = 30° (instead of φ = 68° ). Again substituting into equations (3), (4) and (5): H = 14.86 mm (for all θ), W = 4.14 mm + 6.07 mm = 10.20 mm (for θ = +/- 45°), FWHM = 6.07 mm (for θ = +/- 45°). W = FWHM = B D = F D ' = 1.3 F D = 5.85 mm (as above, for θ = 0°) . In this case, the FWHM in the plane of the fan beam, which directly affects the system resolution in that direction, is only slightly increased at θ = ±45°, as compared to θ = 0°, whereas it is nearly doubled for the conventional configuration. The beam height projection is more than doubled in this example, but this dimension tends to be less important for some applications (e.g., computed tomography); furthermore, it is possible to postcollimate at the detector to improve resolution in the height direction, albeit at the expense of x-ray flux. Referring now to FIG. 4(a), a cathode 50, anode 52, and vacuum window structure 54 representative of a conventional x-ray tube 70 are shown in a cut-away side view. The target angle 56 between anode 52 and normal 58 to cathode-anode axis 60 is at a relatively shallow angle (22° in the figure), and the cone beam 62 is taken out through an aperture 64 in the hood 66 of the anode 52. FIG. 4(b) shows a cross-section of the representation of FIG. 4(a) along direction A-A. Referring now to FIG. 5(a), an implementation of an aspect of the invention is by a relatively minor variation on the conventional technology. In this case, the target angle 56 is increased (to 60° in the figure) and the hood aperture and vacuum window are opened up to permit a wide fan angle to be emitted. The electron optics are adjusted, as known to persons skilled in the art, to retain the original electron spot size on the target. One way to achieve this is to use a magnetic field to bend the electron beam through an angle of about 60° prior to its impact on the anode, thereby varying the angle of incidence of the electron beam, as known to persons skilled in the art of charged particle optics. Omission of the anode hood is also an option in reconfiguring the electron optics. FIG. 5(b) shows a cross-section of FIG. 5(a) along direction A-A to clearly illustrate hood aperture 64 and vacuum window 54. Figure 6 shows an alternative configuration (more closely related to the geometrical description provided in the earlier sections), wherein the basic x-ray tube structures are unaltered, but the output window 54 is reconfigured. In this case the hood can be made of graphite to provide the necessary electrical, physical, and thermal properties without degrading the electron optics or interfering with the emission of x-rays; graphite anode structures in x-ray tubes are well known. Alternatively, a hoodless anode can be used in this case as well. Figure 7 shows a preferred embodiment of the invention. Here, the cathode - anode axis 10 is offset relative to axis 11 of the vacuum envelope (i.e., the axis of the x-ray tube itself). In this case, the original electron optics are retained, and the fan beam with improved azimuthal resolution is taken off normal to the axis of the tube. A graphite anode may be used. The vacuum envelope may be constructed of a suitable grade of copper, for example, and x-ray window 54 can be a thinned-wall section providing the desired degree of x-ray filtration. As one example, an x-ray tube currently used for conventional scanning typically employs 2.5mm of added copper filtration at 450 kV operating potential. In this way, the invention may be practiced without enlarging the shielded housing of the x-ray tube. The described embodiments of the inventions are intended to be merely exemplary and numerous 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 the appended claims.

Claims: I CLAIM: 1. An x-ray tube for generating an x-ray beam for imaging an object, the x-ray tube comprising: a. a vacuum enclosure; b. a cathode for emitting an electron flux; c. an anode structure having a positive voltage potential with respect to the cathode for emitting the x-ray beam; d. an electron lens having at least one control potential with respect to the cathode for varying the size of a region of the anode impinged upon by the electron flux; and e. a controller for varying the at least one control potential so as to vary the size of the region of the anode impinged upon by the electron flux in a determined relation to the electron flux. 2. An x-ray tube according to claim 1, wherein the electron lens includes an arrangement for focussing the electron flux onto the anode. 3. An x-ray tube according to claim 1, wherein the electron lens includes an arrangement for blurring the electron flux onto the anode. 4. An x-ray tube according to claim 1, wherein the controller includes a control element for varying the at least one control potential of the electron lens with respect to the cathode. 5. An x-ray tube according to claim 1, wherein the controller further includes a control element for varying the focal region and the electron flux in a specified manner in response to a transmission level of the x-ray beam through the object. 6. An improved x-ray tube of the type having a vacuum enclosure, a cathode emitting an electron flux, an anode for emitting an x-ray beam, the cathode and anode defining a cathode-anode axis, and a window formed in the vacuum enclosure for permitting efflux of the x-ray beam, wherein the improvement comprises an arrangement for holding the cathode and anode in relation to the vacuum window such that a beam axis, defined by a line segment running between a central portion of the anode and a central portion of the projection of the x-ray beam on an imaged object, is disposed at an acute angle in relation to the cathode- anode axis. 7. An x-ray tube accroding to claim 4, further comprising a beam steering element for reducing the angle of incidence of the electron flux onto the anode. 8. An x-ray tube for generating an x-ray beam for imaging an object, comprising: a. a vacuum enclosure; b. a cathode for emitting an electron flux; c. an anode structure for emitting the x-ray beam; d. a vacuum window formed in the vacuum enclosure for permitting efflux of the x-ray beam; and e. an arrangement for holding the cathode and anode in relation to the vacuum window such that a beam axis, defined by a line segment running between a central portion of the anode and a central portion of the projection of the beam on the object, is disposed at an acute angle in relation to the cathode-anode axis. 9. A method for scanning an object with x-rays, comprising: a. providing an x-ray source for emitting an x-ray beam having a variable spot size and a variable x-ray flux; b. irradiating the object with the x-ray beam; c. monitoring a level of transmission of the x-ray beam through the object; and d. varying the x-ray flux and spot size of the x-ray beam in substantially inverse proportion to the fraction of the x-ray beam transmitted through the object such that attenuation of x-rays transmitted through the object is compensated by a substantially corresponding increase in the x-ray flux of the x-ray beam.

Inventors: SWIFT RODERICK, SWIFT, RODERICK
Applicants: AMERICAN SCIENCE & ENG INC, AMERICAN SCIENCE AND ENGINEERING, INC.
Application year: 1997
Main classification: H01J35
IPC codes: H01J35, H05G1