Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05G—X-RAY TECHNIQUE
  • H05G1/00—X-ray apparatus involving X-ray tubes; Circuits therefor
  • H05G1/08—Electrical details
  • H05G1/26—Measuring, controlling or protecting
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J35/00—X-ray tubes
  • H01J35/02—Details
  • H01J35/14—Arrangements for concentrating, focusing, or directing the cathode ray
  • H01J35/147—Spot size control
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J35/00—X-ray tubes
  • H01J35/02—Details
  • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
  • H01J35/045—Electrodes for controlling the current of the cathode ray, e.g. control grids

Definitions

• 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.
• 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.
• the power that can be applied is closely proportional to the linear dimension of the focal spot.
• 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.
• 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.
• 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 ⁇ and a “width” B D , as shown in VIEW A-A of Figure 1(e).
• the focal spot is normally elongated (i.e., B ⁇ is greater than B D ) and is tilted at target angle ⁇ (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.
• 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 ⁇ of about 20° or a little more.
• X-ray tube manufacturers quote a "nominal" focal spot height F ⁇ and width F D for an x-ray tube.
• the actual focal spot dimensions F ⁇ ' and F D 'as determined from pinhole images taken on the central axis 12 of the cone beam must lie within the limits
• 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.
• 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.
• the apparent size is F ⁇ ' x F D '.
• the apparent size of the focal spot changes as viewed from off the central axis 12 of the radiation cone.
• the apparent height F ⁇ ' is foreshortened as the viewing angle ⁇ (shown in FIG. 1(b)) approaches tangency with the target itself, and is lengthened as viewing angle ⁇ moves in the opposite direction.
• the apparent width F D ' increases as the viewing angle ⁇ is moved off of the cone axis 12 in the ⁇ (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 ⁇ and an elevation angle ⁇ 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.)
• 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.
• 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 ⁇ and
• (D) is a view of (A) from an angle ⁇ + 90°.
• Angle ⁇ is a measure of rotation around the long axis of view (C), as measured from the normal to projection (C).
• (E) is the projection of (C) at angle ⁇ .
• the effective intensity distribution along the ⁇ 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 .
• 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.
• 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
• the electron lens may include arangements for focussing or blurring the electron flux onto the anode
• the controller may include control elements for varying the control potential or control potentials of the electron lens with respect to the
• 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.
• 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.
• 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 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.
• a method 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:
• the x-ray flux produced by the x-ray tube may be better utilized, in direct proportion to the relative angular field increase.
• a larger field-of-view may be covered in a single scan.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 apparent width is slightly less than the apparent height only near the center of the fan, and is much broader at the edges.
• the apparent height H will be larger than for the conventional usage, but the broadening of width W at large azimuthal angles ⁇ will be significantly less, thereby sharpening the image resolution in the plane of the fan beam.
• 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.
• 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.
• an implementation of an aspect of the invention is by a relatively minor variation on the conventional technology.
• 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.
• 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.
• a hoodless anode can be used in this case as well.
• Figure 7 shows a preferred embodiment of the invention.
• 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).
• 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.
• 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.

Landscapes

• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Toxicology (AREA)
• X-Ray Techniques (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=21776497

Family Applications (1)

Country Status (3)

Families Citing this family (5)

Family Cites Families (6)

• 1997
  • 1997-05-06 EP EP97924779A patent/EP0900449A1/de not_active Withdrawn
  • 1997-05-06 WO PCT/US1997/008515 patent/WO1997042646A1/en not_active Ceased
  • 1997-05-06 AU AU30095/97A patent/AU3009597A/en not_active Abandoned

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events