Patent Publication Number: US-7218700-B2

Title: System for forming x-rays and method for using same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/576,147, filed May 28, 2004, which is incorporated in its entirety herein by reference. 
    
    
     BACKGROUND 
     The invention relates generally to a system for forming x-rays, and more particularly to a system configured to direct electron beams at a plurality of discrete spots on a target to form x-rays. 
     X-ray scanning has been used in medical diagnostics, industrial imaging, and security related applications. Commercially available x-ray sources typically utilize conventional thermionic emitters, which are helical coils made of tungsten wire and operated at high temperatures. Each thermionic emitter is configured to emit a beam of electrons to a single focal spot on a target. To obtain a total current of 10 to 20 mA with an electron beam size of 10 mm 2 , helical coils formed of a metallic wire having a work function of 4.5 eV must be heated to about 2600 K. Due to its robust nature, tungsten wire has been the electron emitter of choice. 
     There are disadvantages to the use of conventional thermionic filament emitters. Such filament emitters lack a uniform emission profile necessary for proper beam steering and focusing. Further, a higher electron beam current will cause a reduction in the lifetime of such filament emitters. Additionally, such filament emitters require high quiescent power consumption, which leads to the need for larger, more complex cooling architectures, a larger system envelope, and greater cost. 
     SUMMARY 
     An exemplary embodiment of the invention provides a system for forming x-rays that includes a target and at least one electron emission subsystem for generating a plurality of spots on the target. The at least one electron emission subsystem includes a plurality of electron sources and each of the plurality of electron sources generates at least one of the plurality of spots on the target. The system also includes a beam focusing subsystem for focusing electron beam emissions from the plurality of electron sources prior to the electron beam emissions striking the target. 
     Another exemplary embodiment of the invention provides a system for forming x-rays that includes a target, an electron emission subsystem for generating a plurality of spots on the target, and a transient beam protection subsystem for protecting the electron emission subsystem from transient beam currents, material emissions from the target, and electric field transients. The electron emission subsystem includes a plurality of electron sources. 
     Another exemplary embodiment of the invention provides a system for forming x-rays that includes a target and an electron emission subsystem including a plurality of electron sources. The electron emission subsystem is configured to generate a plurality of discrete spots on the target from which x-rays are emitted. The target is enclosed within a first vacuum chamber and the electron emission subsystem is enclosed within a second vacuum chamber. 
     Another exemplary embodiment of the invention provides a method for x-ray scanning an object that includes emitting a beam of electrons from an electron source to strike a discrete or swept focal spot on a target for creating x-rays from the discrete or swept focal spot. The method further includes focusing the beam of electrons from the electron source prior to the electron beam emissions striking the target and detecting the x-rays created from the discrete or swept focal spots. 
     These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an x-ray system constructed in accordance with an exemplary embodiment of the invention. 
         FIG. 2  is a schematic view of an exemplary embodiment of an x-ray generation subsystem for use in the x-ray system of  FIG. 1 . 
         FIG. 3  is a schematic view of an exemplary embodiment of an electron source array for use in the x-ray system of  FIG. 1 . 
         FIG. 4  is a side view of an electron source for use in the x-ray system of  FIG. 1 . 
         FIG. 5  is a schematic view, of multiple steerable electron emission subsystems within the x-ray system of  FIG. 1 . 
         FIG. 6  is a schematic representation of the source and target vacuums of  FIG. 5 . 
         FIG. 7  is an expanded view of the beam dump mechanism within circle VII of  FIG. 2 . 
         FIG. 8   a  is a perspective view of an alternative source for use in the x-ray system of  FIG. 1 . 
         FIG. 8   b  is a cross-sectional view of the electron source of  FIG. 8   a  taken along line VIIIa—VIIIa. 
         FIG. 9  is a perspective view of a target constructed in accordance with another exemplary embodiment of the invention. 
         FIG. 10  is a side view of a portion of the target of  FIG. 9 . 
         FIG. 11  illustrates process steps for obtaining x-rays of a subject in accordance with another exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIGS. 1 and 2 , first will be described an x-ray system  10 . The x-ray system  10  includes an x-ray generation subsystem  15  including a target  46  ( FIG. 2 ), a detector  60 , and an electronic computing subsystem  80 . A portion of the x-ray generation subsystem  15 , which may include a steerable electron emission subsystem  20 , may be encompassed in a first vacuum vessel  25 , while the target  46  may be encompassed within a second vacuum vessel or target chamber  47  ( FIG. 6 ). The x-ray system  10  may be configured to accommodate a high throughput of articles, for example, screening of upwards of one thousand individual pieces of luggage within a one hour time period, with a high detection rate and a tolerable number of false positives. Conversely, the x-ray system  10  may be configured to accommodate the scanning of organic subjects, such as humans, for medical diagnostic purposes. Alternatively, the x-ray system  10  may be configured to perform industrial non-destructive testing. The electron emission subsystem  20  and the target  46  may be stationary relative to the detector  60 , which may be stationary or rotating, or the electron emission subsystem  20  and the target  46  may rotate relative to the detector  60 , which may be stationary or rotating. 
     With specific reference to  FIGS. 2 and 4 , next will be described an exemplary embodiment of the x-ray generation subsystem  15  including the electron emission subsystem  20 . It should be appreciated that multiple electron emission subsystems  20  may be arranged around the target  46 . The electron emission subsystem  20  includes an electron source  26 . Each electron beam generated within the electron emission subsystem  20  is steerable. The electron source  26  is positioned within the electron emission subsystem  20  such that the electron emission subsystem  20  serves as a transient beam protection subsystem protecting the electron source  26  from transient voltages and/or currents. In addition, the electron emission subsystem  20  protects the electron source  26  from sputter damage gasses in the target chamber  47  ( FIG. 6 ). Specifically, a channel  33  extends between the target  46  and the electron source  26  to alleviate the deleterious effects of transient beam currents and material emissions striking at or near the electron source  26 . The transient beam protection subsystem functions more efficiently if the differential between the voltage potential of the target  46  is significantly higher than the voltage potential of the electron source  26  and its surrounding environs. Such a transient beam protection subsystem serves to sink current from one or more electron sources if the potential of the anode or target  46  drops and to provide protection for one or more electron sources during transient beam emissions. 
     It should be appreciated that a different architecture may be utilized to effect the emission of electron beams to more than one focal spot on the target  46 . Instead of utilizing a steerable electron emission subsystem as described with reference to the x-ray generation subsystem  15 , a dedicated emitter design architecture may be used. For example, and with specific reference to  FIG. 3 , an x-ray generation subsystem  115  may be used, which includes an electron emission subsystem  120  having an emitter array  122 . The emitter array  122  includes a plurality of electron sources  26 , each positioned within an alcove  29  and each being configured to emit a beam  44  of electrons to a discrete focal spot  48  on the target  46 . The transient beam protection subsystem for the  FIG. 3  embodiment may include the combination of the channel  33 , and the alcoves  29 . The transient beam protection subsystem may also include guard electrodes (not shown) as a further protection mechanism. Furthermore, such a transient beam protection subsystem serves to (a) sink current from one or more electron sources if the potential of the target  46  drops and (b) provide protection for one or more electron sources during transient beam emissions. 
     It also should be appreciated that several types of electron sources, or emitters, may be utilized. Examples of suitable electron emitters include tungsten filament, tungsten plate, field emitter, thermal field emitter, dispenser cathode, thermionic cathode, photo-emitter, and ferroelectric cathode, provided the electron emitters are configured to emit an electron beam at multiple discrete focal spots on a target. 
     The x-ray generation subsystem  15  includes a beam focusing subsystem  40 , a beam deflection subsystem  42 , and a pinching electrode for selectively inhibiting or permitting electron beams from the electron source  26  to be emitted toward the target  46 . One such mechanism is a pinch-off plate or beam grid, which is configured to pinch off electron beams  44  when activated. Another such mechanism is a conducting gate  32  ( FIG. 4 ), which is configured to facilitate electron beam  44  generation when activated. Yet another mechanism is a beam dump  105  ( FIGS. 2 ,  7 ). The beam dump  105 , when activated, diverts the electron beams  44  away from an undeflected path  27  toward the target  46  ( FIGS. 2 ,  6 ,  7 ) to a deflected path  27   c  into the container. 
     The beam focusing subsystem  40  serves to form and focus a beam  44  of electrons into a pathway  27  ( FIG. 5 ) toward the target  46 . The beam focusing subsystem  40  may include an electrostatic focusing component, such as, for example, a plurality of focusing plates each biased at a different potential, or a magnetic focusing component, such as, for example, a suitable combination of focusing solenoids, deflecting dipoles and beam-shaping quadrupole electromagnets. Electromagnets that produce higher order moments (6-pole, 8-pole, etc.) can be used to improve beam quality or to counter effects of edge-focusing that may occur due to a particular choice or design of elements in subsystem  40 . 
     The beam deflection subsystem  42  serves to steer or deflect the electrons from the pathway  27  onto deflected pathways  27   a ,  27   b  ( FIG. 5 ) toward numerous discrete focal spots  48  on the target  46  ( FIG. 10 ). The ability to steer electron beams to more than one focal spot  48  on the target  46  is significant in that it facilitates the use of a reduced number of electron emitters relative to the required number of x-ray focal spots. The electron source  26  may be a low current-density electron source. Optics, such as the beam focusing subsystem  40 , is used to form high current-density beams  44  at the target  46  from a low current-density electron source. Each discrete electron beam  44  strikes the focal spots  48  on the target  46 , creating x-ray beams  50  ( FIG. 3 ) which will be used to scan a subject, be it inorganic or organic. It should be appreciated that a beam deflection subsystem  42  may be unnecessary for an arrangement of electron sources such as the x-ray generation subsystem  115  having an emitter array  122  illustrated in  FIG. 3 , although a beam focusing subsystem  40  may still be employed. Since a plurality of electron sources  26  would be located adjacent to one another, steering the electron beams  44  from each electron source  26  likely would not be needed to produce electron beam strikes at a plurality of focal spots  48  on the target  46 . 
     The beam deflection subsystem  42  may be electrostatically-based, magnetically-based, or a combination of the two. For example, the beam deflection subsystem  42  may include an electrostatic steering mechanism that has one or more free standing electrically conducting plates that may be positioned within the channel  33 . As beam currents  44  of electrons are emitted from the electron source  26 , the plates can be charged to a fairly high negative potential with respect to ground. The plates may be formed of an electrically conductive material, or be formed of an insulating material and coated with an electrically conductive coating. The beam deflection subsystem  42  may include a magnetic steering mechanism with a magnetic core for correcting magnetic fields that have other higher-moment fields, such as, for example, hexapoles, so that the focal spot  48  ( FIGS. 3 ,  10 ) shape is maintained over a wide set of deflection angles. Alternatively, the magnetic steering mechanism may have no magnetic core. Examples of suitable magnetic steering mechanisms include one or more coils, a coil-shaped electromagnet, and a fast switching magnetic-field-producing magnet, each of which being capable of producing magnetic fields with substantial quadrupole moments as well as dipole moments. 
     As described above, each electron emission subsystem  20  may be encompassed in a first vacuum vessel  25 , while the target  46  may be encompassed within a second vacuum vessel  47  ( FIGS. 5 ,  6 ). Each of the first vacuum vessels  25  is separated from the second vacuum vessel  47  via a channel  33 . The differential pressures of each of the vacuum vessels  25 ,  47  are maintainable through the use of differential pumping through a narrow diameter pipe. As an exemplary embodiment, two gate valves  70 ,  72  connect each first vacuum vessel  25  with the second vacuum vessel  47  through the channels  33 . Through this arrangement, if replacement of any single electron source  26  is required, the gate valve  70  may be kept in a closed state while the gate valve  72  is opened to allow removal of the electron source  26  from the vacuum vessel  25 . Alternatively, a single gate valve may be used to separate the two vacuum vessels  25 ,  47 . 
     Referring now to  FIG. 4 , next will be described an exemplary embodiment of the electron source  26  of  FIGS. 2 and 3 . The electron source  26  illustrated in  FIG. 4  includes a base or substrate  28  and carbon nanotubes  36 . The carbon nanotubes  36  are positioned on a catalyst pad  34 , which is itself located on a surface of the substrate  28 . The substrate  28  may be formed of silicon or another like material. A dielectric spacer  30  is positioned over the substrate  28 . A well  35  is etched in the dielectric spacer  30 , and the catalyst pad  34  is positioned therein. A conducting gate  32 , positioned over the spacer  30 , serves to generate high electric fields in the vicinity of the tips of the carbon nanotubes  36 , which promotes electron emissions within electron source  26 . The carbon nanotubes  36  may be grown selectively on the catalyst pad  34  through the use of chemical vapor deposition. The inherently high aspect ratio makes them particularly well suited for field emission. 
     Alternatively, and with specific reference to  FIGS. 8   a ,  8   b , a dispenser cathode  126  may be utilized as an electron source. The dispenser cathode  126  may include a container  128  with a porous tungsten plug  129 . A coil  130 , preferably formed of tungsten, is positioned within the container  128  and surrounded by an oxide-based solution, such as, for example, barium oxide, calcium oxide, or tin oxide. A gridding mechanism  140  ( FIG. 8   b ) may be placed between the dispenser cathode  126  and the target  46  ( FIGS. 2 ,  5 ,  6 ) to permit or inhibit electron emissions from the dispenser cathode  126  from striking the target  46 . The oxide materials coat the tungsten plug  129 , thereby lowering the work function for the dispenser cathode  126 . One advantage of using a dispenser cathode  126  is that the lowered work function requires that the tungsten coil  130  only needs to be heated up to 1300° C., instead of the 2500° C. required for uncoated tungsten thermionic emitters. A further advantage is the low cost of off-the-shelf dispenser cathodes  126 . When the oxide materials have evaporated away, the dispenser cathode  126  can be discarded and replaced with another. 
     Next will be described the x-ray system  10  as illustrated in  FIG. 5 . A plurality of electron emission subsystems  20  is arrayed around a target  46 . Each of the electron emission subsystems  20  is within a first vacuum vessel  25 , while the target  46  is within a second vacuum vessel  47 . Each of the vacuum vessels  25 ,  47  are pumped so as to obtain a differential pressure between each of the first vacuum vessels  25  and the second vacuum vessel  47 . Each of the first vacuum vessels  25  is connectable with the second vacuum vessel  47  through a channel  33 . The differential pressure between the first vacuum vessels  25  and the second vacuum vessel  47  is maintained through the use of differential pumping. While six discrete electron emission subsystems  20  are illustrated each within a separate first vacuum vessel  25 , it should be appreciated that any number of electron emission subsystems  20  may be utilized. The beam deflection subsystem  42  steers the electron beams  44  ( FIGS. 2 ,  3 ) from the pathway  27  to a deflected pathway  27   a ,  27   b  to strike the target  46  at an alternative discrete focal spot  48  ( FIG. 3 ). 
     With specific reference to  FIGS. 9 ,  10 , next will be described an exemplary embodiment of the target  46 . The target  46 , as illustrated in  FIGS. 9 and 10  includes target planes  49 ,  49   a , and  49   b . Target planes  49   a  and  49   b  are at an angle to target plane  49 . An undeflected electron beam  44  is intended to follow pathway,  27  to strike the target  46  at a focal spot  48  along target plane  49 . Alternatively, a deflected electron beam  44  is intended to follow the deflected pathway  27   a  or  27   b  to strike the target  46  at a focal spot  48  along target plane  49   a  or  49   b . The target planes  49 ,  49   a ,  49   b  may be curved surfaces or they may be flat surfaces at an angle relative to one another. The angle of incidence of target planes  49   a  and  49   b  is chosen such that the deflected electron beams  44  strike the focal spots  48  along the target planes  49   a ,  49   b  at the same angle as the undeflected electron beam  44  strikes the focal spot  48  along the target plane  49 . In this manner, the beam deflection subsystem  42  ( FIGS. 2 ,  5 ) can deflect electron beams  44  to strike a plurality of focal spots  48  along the target  46  such that the similar x-ray energy spectrum is exhibited from strikes along all the target planes  49 ,  49   a ,  49   b  and such that each strike produces a similar angle of emission of x-ray beams  50  ( FIGS. 2 ,  3 ). 
     Next, with reference to  FIG. 1 , will be described the detector  60  and the electronic computing subsystem  80 . The detector  60  may include a detector ring positioned adjacent to the x-ray generation subsystem  15 . The detector ring may be offset from the x-ray generation subsystem  15 . It should be appreciated, however, that “adjacent to” should be interpreted in this context to mean the detector ring is offset from, contiguous with, concentric with, coupled with, abutting, or otherwise in approximation with the x-ray generation subsystem  15 . The detector ring may include a plurality of discrete detector modules that may be in linear, multi-slice, or area detector arrangements. Moreover, energy-integration, photon-counting, or energy-discriminating detectors may be utilized, comprising scintillation or direct conversion devices. An exemplary embodiment of the detector module includes a detector cell having a pitch of, for example, two millimeters by two millimeters, providing an isotropic resolution on the order of one millimeter in each spatial dimension. Another exemplary embodiment of the detector module includes a detector cell having a pitch of one millimeter by one millimeter. 
     The electronic computing subsystem  80  is linked to the detector  60 . The electronic computing subsystem  80  functions to reconstruct the data received from the detector  60 , segment the data, and perform automated detection and/or classification. One embodiment of the electronic computing subsystem  80  is described in U.S. patent application Ser. No. 10/743,195, filed Dec. 22, 2003, which is incorporated in its entirety by reference herein. 
     There are several advantages to the aforementioned arrangement of features in the x-ray system  10 . By utilizing steerable electron sources, such as the electron sources in the x-ray generation subsystem  15 , and the target planes  49 ,  49   a ,  49   b , the range of electron beams  44  ( FIG. 2 ) from each electron source  26  is expanded with a minimal loss of resolution. The expanded range of electron beams  44  may translate into some redundancy, wherein some of the electron beams  44  from one electron source  26  may overlap others of the electron beams  44  from adjacent electron sources  26 . Further, the expanded range of electron beams  44  may translate into a longer working life of the x-ray system  10  between maintenance since the increased redundancy may allow the x-ray system  10  to be used with a larger number of inoperable electron emission subsystems  20 . 
     Another advantage of the x-ray system  10  is that the arrangement of the transient beam protection subsystem inhibits transient vacuum arcs, vacuum discharges, or spits from the target  46  striking at or near the electron sources  26 . The channel  33  provides a narrow pathway through which a spit will unlikely be able to traverse all the way back to the electron sources  26 . Further, the alcoves  29  can minimize any sputter damage to the electron sources  26 . Additionally, the transient beam protection subsystem can sink current from the electron source  26  if the electric field within the x-ray generation subsystem  15  collapses due to discharges. 
     Furthermore, using the architecture of the x-ray system  10  reduces the concern about the power dissipation of the electron sources  26 , since the amount of power that is used is considerably less than in a comparable x-ray system utilizing thermionic electron emitters. In a conventional x-ray system, the focal spot positions are positioned adjacent to one another, providing little space in which to place focusing mechanisms. In a dedicated emitter design ( FIG. 3 ) of x-ray generation subsystem  15 , an electron source is required for each x-ray spot  48 . The emitters are positioned so close to each other that incorporating beam optics to deflect the beam would be difficult to achieve. Thus, to generate, for example, one-thousand x-ray spots  48 , one-thousand electron emitters would be necessary. As thermionic emitters typically require approximately 10 watts of power to emit electrons, the overall power requirement is difficult to accommodate. The use of the beam focusing subsystem  40  allows for lower-density electron sources to be used, and the use of a beam deflection subsystem  42  permits multiple x-ray spots from a single electron source, and the use of alternative electron emitters (dispenser cathodes, field emission devices, for example) reduces quiescent power consumption, all of which reduce the overall power consumption. 
     With specific reference to  FIG. 11 , next will be described a method for x-ray scanning an object. At Step  200 , a plurality of electron emission subsystems is provided adjacent to a target. At Step  205 , a transient beam protection subsystem is positioned in the vicinity of each electron emission subsystem arranged about the target. For example, each electron emission subsystem  20 ,  120  may be segregated from the target  46  through the use of the transient beam protection subsystem, including one or more of channel  33 , the alcove  29 , or guard electrodes (not shown). The transient beam protection subsystem is designed to provide protection to the electron sources  26  against transient beam currents/voltages, material emissions from the target  46 , and collapse of the electric field. 
     At Step  210 , a first electron beam current is emitted from an electron emission subsystem to a first focal spot  48  on the target  46 . At Step  215 , a second electron beam current is emitted from an electron emission subsystem to a second focal spot  48  on the target. For electron emission subsystems  20 , a single electron source  26  transmits both of the electron beam currents and one of the electron beam currents is subjected to deflection. For electron emission subsystems  120 , which each incorporate an array of electron sources  26 , no deflection of the electron beam currents is necessary, since each electron source is offset from the others. It should be appreciated that there may be numerous times that a current is emitted to a focal spot  48  on the target  46 , and that there may be a loop executed N number of times, depending on the number of focal spots  48  desired. 
     Finally, at Step  220 , a detector, such as the detector  60 , is provided to measure the x-rays emitted from the focal spots on the target. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while field emitters and dispenser cathodes have been generally described, it should be appreciated that various embodiments of the invention may incorporate field emitters and/or dispenser cathodes that are anode grounded, cathode grounded, or multi-polar. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.