Abstract:
Embodiments of the disclosure relate to electron emitters for use in conjunction with X-ray devices. In one embodiment, the emitter features a round emission area capable of emitting electrons when heated, wherein the round emission area comprises at least one of a gap, a channel, or a combination thereof that separates a first portion of the round emission area from a second portion of the round emission area and permits thermal expansion of the first portion and the second portion within the at least one gap or channel without permitting the first portion and the second portion to touch one another. The two electrically conductive legs coupled to the surface at respective locations outside the round emission area and that are capable of supplying current to the round emission area.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to electron emission surfaces, such as for use in an electron gun. 
         [0002]    In non-invasive imaging systems, X-ray tubes are used in various X-ray systems and computed tomography (CT) systems as a source of X-ray radiation. The radiation is emitted in response to control signals during an examination or imaging sequence. Typically, the X-ray tube includes a cathode and an anode. An emitter within the cathode may emit a stream of electrons in response to heat resulting from an applied electrical current, and/or an electric field resulting from an applied voltage to a properly shaped metallic plate in front of the emitter. The anode may include a target that is impacted by the stream of electrons. The target may, as a result of impact by the electron beam, produce X-ray radiation to be emitted toward an imaged volume. 
         [0003]    In such imaging systems, the radiation passes through a subject of interest, such as a patient, baggage, or an article of manufacture, and a portion of the radiation impacts a digital detector or a photographic plate where the image data is collected. In digital X-ray systems a photodetector produces signals representative of the amount or intensity of radiation impacting discrete elements of a detector surface. The signals may then be processed to generate an image that may be displayed for review. In CT systems a detector array, including a series of detector elements, produces similar signals through various positions as a gantry is rotated about a patient. 
         [0004]    In other systems, such as systems for oncological radiation treatment, a source of X-rays may be used to direct ionizing radiation toward a target tissue. In some radiation treatment configurations, the source may also include an X-ray tube. X-ray tubes used for radiation treatment purposes may also include a thermionic emitter and a target anode that generates X-rays, such as described above. Such X-ray tubes or sources may also include one or more collimation features for focusing or limiting emitted X-rays into a beam of a desired size or shape. 
       BRIEF DESCRIPTION 
       [0005]    In one embodiment, an X-ray emitter is provided. The X-ray emitter includes a round emission area capable of emitting electrons when heated. The round emission area includes a surface comprising a round emission area capable of emitting electrons when heated, wherein the round emission area comprises at least one of a gap, a channel, or a combination thereof that separates a first portion of the round emission area from a second portion of the round emission area and permits thermal expansion of the first portion and the second portion within the at least one gap or channel without permitting the first portion and the second portion to touch one another. The round emission area also includes two electrically conductive legs coupled to a surface of the emitter at respective locations outside the round emission area and that are capable of supplying current to the round emission area. 
         [0006]    In another embodiment, an X-ray emitter is provided. The X-ray emitter includes a disc-shaped emission area capable of emitting electrons when heated with a driving current of 10 A or less. The disc-shaped emission area includes two electrically conductive legs coupled to the surface at respective locations outside the disc-shaped emission area and that are capable of supplying current to the disc-shaped emission area such that, when current is applied to the disc-shaped emission area, the disc-shaped emission area heats to a temperature of at least 2000 degrees Celsius with a temperature variation across the emission surface of less than 6% of a maximum temperature achieved. 
         [0007]    In a further embodiment, an X-ray tube is provided. The X-ray tube includes an electron beam source. The electron beam source includes an electron emitter configured to emit an electron beam. The electron emitter includes a disc-shaped emission area capable of emitting electrons when heated, comprising a serpentine radial electrical path wherein the serpentine electrical path extends from an outer diameter of the disc-shaped emission area to a center of the disc-shaped emission area and back. The electron emitter also includes a plurality of electrically conductive legs coupled to the electron emitter at respective locations outside the disc-shaped emission area and that are capable of supplying current to the disc-shaped emission area. The X-ray tube also includes an anode assembly configured to receive the electron beam and to emit X-rays when impacted by the electron beam and a housing in which the electron beam source and the anode assembly are disposed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0009]      FIG. 1  is a pictorial view of a CT imaging system incorporating an embodiment of the present disclosure; 
           [0010]      FIG. 2  is a block schematic diagram of the system illustrated in  FIG. 1 ; 
           [0011]      FIG. 3  is a schematic view of an X-ray source in accordance with an embodiment of the present disclosure; 
           [0012]      FIG. 4  is a top view of an emitter in accordance with an embodiment of the present disclosure; 
           [0013]      FIG. 5  is a perspective view of the emitter of  FIG. 5  in accordance with an embodiment of the present disclosure; 
           [0014]      FIG. 6  is a top view of the emitter of  FIG. 5  in which an emission surface is marked in accordance with an embodiment of the present disclosure; and 
           [0015]      FIG. 7  is a top view of an alternate embodiment of an emission surface with an axial current path. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Provided herein are electron emitters for use in conjunction with a cathode assembly of an X-ray tube. The electron emitters incorporate structural features that result in an electron emission surface with a relatively larger diameter (e.g., in one embodiment having a diameter of about 7 mm to about 11 mm) as compared to existing electron emitters suitable for use in electron gun configurations. Larger emitters such as those disclosed herein result in higher electron emissions at the desired drive current. Drive current refers to the current passing through the emitter to heat it. In one example, the emission is greater than 1250 mA. Further, the electron emitters are capable of maintaining a relatively uniform temperature across the entire electron emission surface, which results in a robust focal spot for imaging purposes. In addition, a lack of hot spots on the emission surface, which is a benefit of relatively uniform temperatures maintained during electron emission, may result in a longer usable life for the emitter, which in turn is cost-effective for maintenance of the X-ray device. Accordingly, the emitters provided may be larger diameter emitters that provide high emission and long usage lives. 
         [0017]    To that end, the electron emitters disclosed herein may be used in conjunction with any suitable X-ray device. The operating environment of the disclosure is described with respect to a sixty-four-slice computed tomography (CT) system. While described with respect to an embodiment of a CT scanner, the present techniques are equally applicable to other X-ray based systems, including fluoroscopy, mammography, angiography, and standard radiographic imaging systems as well as radiation therapy treatment systems. Additionally, it will be appreciated by those skilled in the art that the disclosed embodiments are suitable for use with other applications in which an electron gun and/or electron emitter is implemented, whether for x-ray emission or otherwise. 
         [0018]    Referring to  FIG. 1 , a computed tomography (CT) imaging system  10  is shown as including a gantry  12 . The gantry  12  has an X-ray source  14  that projects a beam of X-rays  16  toward a detector assembly on the opposite side of the gantry  12 . A detector assembly  18  is formed by a collimator  18 , a plurality of detectors  20  and data acquisition system  32 . The plurality of detectors  20  (see  FIG. 2 ) sense the projected X-rays that pass through a medical patient  22 , and the data acquisition system  32  converts the data to digital signals for subsequent processing. Each detector  20  produces an electrical signal that represents the intensity of an impinging X-ray beam and hence the attenuated beam as it passes through the patient  22 . During a scan to acquire X-ray projection data, the gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
         [0019]    Referring to  FIG. 2 , rotation of the gantry  12  and the operation of X-ray source  14  are governed by a control mechanism  26  of CT system  10 . The control mechanism  26  includes an X-ray controller  28  that provides power and timing signals to an X-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . An image reconstructor  34  receives sampled and digitized X-ray data from the data acquisition system  32  and performs high speed reconstruction. The reconstructed image is applied as an input to a computer  36  that stores the image in a mass storage device  38 . 
         [0020]    The computer  36  also receives commands and scanning parameters from an operator via a console  40  that has some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated display  42  allows the operator to observe the reconstructed image and other data from the computer  36 . The operator-supplied commands and parameters are used by the computer  36  to provide control signals and information to the data acquisition system  32 , the X-ray controller  28 , and the gantry motor controller  30 . In addition, the computer  36  operates a table motor controller  44  that controls a motorized table  46  to position the patient  22  and the gantry  12 . In particular, the table  46  moves the patient  22  through a gantry opening  48  of  FIG. 1 , either in whole or in part. 
         [0021]    Referring now to  FIG. 3 , the X-ray source  14  included in CT system  10  is shown in detail. The X-ray source  14  includes an X-ray generating tube  14 , which includes a electron gun  50 , which may be configured as a Pierce electron gun, and an anode assembly  52  encased in a housing  54 . The anode assembly  52  includes a rotor  56  configured to turn a rotating anode disc  58  (i.e., target. When struck by an electron current  60  from the electron gun  50 , the anode  58  emits an X-ray beam  62 . 
         [0022]    The X-ray tube  50  is supported by the anode and cathode assemblies within a housing  54  defining an area of relatively low pressure (e.g., a vacuum). For example, the housing  54  may include glass, ceramics, or stainless steel, or other suitable materials. The anode  58  may be manufactured of any metal or composite, such as tungsten, molybdenum, copper, or any material that contributes to Bremsstrahlung (i.e., deceleration radiation) when bombarded with electrons. The anode&#39;s surface material is typically selected to have a relatively thermal diffusivity to withstand the heat generated by electrons impacting the anode  58 . The space between the cathode assembly  66  and the anode  58  may be evacuated to minimize electron collisions with other atoms and to increase high voltage stability. Moreover, such evacuation may advantageously allow a magnetic flux to quickly interact with (i.e., steer or focus) the electron beam  62 . In some X-ray tubes, electrostatic potential differences in excess of 20 kV are created between the cathode assembly  66  and the anode  58 , causing electrons emitted by the cathode assembly  66  to accelerate towards the anode  58 .  FIG. 4  is a top view of an emitter  100  that may be incorporated as part of the cathode assembly  66  for emission of electrons. The emitter  100  includes a top surface  106  on which an electron emission surface  110  is formed and that emits electrons when heated. Joule heating increases the temperature of the emitter  100  when voltage is applied across the emission surface  110  causing current to flow through the serpentine radial path of each of the four quadrants. By providing an emitter  100  with multiple separate areas (e.g., four quadrants), a larger emitter may be formed. For example, in the depicted four-quadrant pattern, the additional turns may be kept at a desired width and nonetheless maintain the driving current. 
         [0023]    The electrical path is shown via arrow  114 . The path is radial in that the arrow  114  enters the circle at the outer diameter and follows a pathway to the center of the circle before entering another quadrant of the circle and following a path to the outer diameter again. The top surface  106  includes slots  116  that separate ligaments or segments  120  from one another, thus defining a single serpentine radial electrical path. The slots are sized to define the electrical path and to allow for thermal expansion in the radial direction without shorting between neighboring ligaments or segments  120 . In one embodiment, the slots are about 60 μm wide and the segments  120  are about 320 μm wide. The size and number of the segments  120  may be selected to influence the characteristics of the emission surface  110 . For example, the segments  120  provide a radial path that changes direction at each turn  122 , which is defined by the slots  116  and any other physical separation from the adjacent segments  120 . The electrical path winds around the emission surface  110  along the segments  120 , changing direction at the turns  122 . An electrical path with more turns  122  (and more segments  120 ) may result in improved temperature uniformity and smaller driving current. However, an emitter  100  with more turns  122  may be more complex to manufacture. Further, the width of the turns  122  may be adjusted to compensate for any hot spots, thereby improving the temperature uniformity of the emission surface when in operation. 
         [0024]    The flow of electricity across the top surface  106  and within the emission surface  110  results in the heating of the emission surface  110  and eventual electron emission when the emitter  100  reaches sufficiently high temperatures. In certain embodiments, the emitter  100  may include any suitable materials to facilitate electron emission, including tungsten, hafnium carbide (HfC), or other materials. Further, although the emitter  100  is depicted as featuring a flat top surface  106  (and emission surface  110 ) it should be understood that the emitter  100 , in certain embodiments, may be curved or otherwise nonplanar. 
         [0025]    The emitter  100  may also include additional features that define the electrical path, including passageways  124  that electrically separate the terminal  112   a  from other terminals (e.g. terminal  112   b ). A channel  130  separates a top half  132  of the emission surface  110  from a bottom half  134 , further preventing the segments  120  from having multiple paths within the emission surface. As illustrated, the channel  130  bisects the emission surface  110 . The channel  130  may separate the emission surface into substantially equal portions, depending on the shape of the emission surface. The channel  130  may also extend past the emission surface  110  into a wider notch  136  that terminates at an end  138  of a longest dimension of the emitter  100 . 
         [0026]    The emitter  100  may also include one or more v-shaped gaps  138  that partially separate portions of the emission surface  110  from one another. For example, the depicted embodiments shows two v-shaped, i.e., tapered, gaps  138  that separate left quadrants ( 140   a  and  140   b ) from right quadrants ( 142   a  and  142   b ) of the emission surface  110 . As illustrated, the v-shaped gaps  138  leave a single electrical path between the left quadrants  140  and the right quadrants  142 . In one embodiment, the v-shaped gaps  138  are aligned along an axis (e.g., a diameter axis). In another embodiment, the v-shaped gaps  138  are orthogonal to the channel  130 . 
         [0027]    The emitter  100  may also include temperature uniformity features that facilitate cooling or distribution of heat across the emission surface. For example, the size and shape of the passageways  124  may be selected to distribute heat. Passageways  146  may also be formed in the emitter  100  for this purpose. The passageways  146  may also be used as alignment holes for positioning the emitter  100  within the cathode assembly  66 . In addition, the channel  130  may include heat distribution features, such as a hole  148  formed in the center of the emission surface  110 . The hole  148  may be any suitable shape that facilitates regulating or smoothing the temperature. In one embodiment, the hole  148  has a diameter of about 550 μm. 
         [0028]      FIG. 5  is a perspective view of the emitter  100  showing the posts  160 . The post  160   a  is electrically coupled to the terminal  112   a  and provides electrical current. Similarly, the post  160   b  is coupled to the terminal  112   b . The post  160   c  is not electrically conductive and is coupled to the emitter  100  at junction  164 . This coupling, whether fixed or sliding, provides structural support to hold the emitter in-plane. It should be understood that the positions of the posts  160  and the terminals  112  may be exchanged. Further, the emitter  110  may be configured with a two-post arrangement rather than a three-post arrangement. The posts may be affixed in any suitable manner to the emitter  100 . In one embodiment, the posts  160  are laser welded to the emitter  100 . In one embodiment, the posts  160  are coupled to the emitter  100  at positions outside of the emission surface  110 . 
         [0029]    The posts  160  are coupled to the emitter  100  outside an area defining the emission surface  110 .  FIG. 6  illustrates the emission surface  110  as being within the circle defined by a dotted line  170 . In the depicted embodiment, the diameter d 1  of the emission surface may be at least about 7 mm or at least about 10 mm. The emitter  100  forms a circle with a diameter d 2  that extends outside of the emission surface  110 , such that the longest segment  120  is at least in part outside the emission surface  110 . In certain embodiments, the diameter d 1  is about 10 mm and the diameter d 2  is about 11.5 mm. 
         [0030]    As noted, the emitter may include one or more features that separate the generally round emitter  100  into different section or quadrants. For example, such features may include one or more v-shaped gaps  138 . The size and shape of the v-shaped gaps  138  may be selected to allow thermal expansion of the segments  120 . The emitter  100  is configured to expand within the one or more v-shaped gaps  138  when heated such that the one or more v-shaped gaps  138  decreases in size without permitting adjacent lobes or sections to touch one another. In particular, the v-shaped gaps may be generally wider as they extend radially away from the center of the emitter  100 . This allows longer segments  120  located towards the outer circumference of the emitter  100  to expand more than relatively shorter segments  120 . Shorter segments  120  may expand less, which facilitates a relatively narrower gap. The size of the v-shaped gaps  138  may be selected to permit expansion but also to minimize loss of emission area. 
         [0031]    The v-shaped gaps  138  taper towards the center of the emission surface  110  such that the gap length varies and is narrowest towards the hole  148 . At the widest point, the gap length l 1  may be 260 μm or less. In one embodiment, the v-shaped gap  138  may have a gap length that varies between about 120 μm to about 240 μm. Further, the v-shaped gap  138  may be characterized by a ratio of a widest gap length l 1  to a narrowest gap length of about 2 or more. That is, the widest point of the v-shaped gap  138  may be twice as wide or more as the narrowest point. The channel  130  may have a gap length l 2  that is a generally constant size. In one embodiment, the gap length l 2  of the channel  130  is less than about 240 μm. In another embodiment, the gap length l 2  of the channel  130  is between about 120 μm to about 240 μm. 
         [0032]    The size and shape of the emitter  100  may be selected based on suitable dimensions to be used in conjunction with the cathode assembly  66 . In a particular embodiment, the longer dimension l 3  of the emitter  100  may be about twice the diameter of the emission surface  110 . In one embodiment, the longer dimension l 3  may be within 1-2 mm, longer or shorter, than twice the diameter of the emission surface  110 . In another embodiment, the shorter dimension l 4  of the emitter  100  may be about the diameter of the emission surface  110 . 
         [0033]      FIG. 7  is an alternate embodiment of the emitter  100  in which the emission surface  110  is configured so that the electrical path is generally axial, which in the depicted embodiment results in a generally elliptical emission surface  110 . The segments  120  include turns  122  that define an electrical path that changes direction about 180 degrees at each turn  122 . 
         [0034]    The emitter  100  is capable of achieving emission temperatures with relatively larger emission surface diameters (e.g., at least 7 mm) with drive currents of about 7-9.5 Amps. This arrangement provides scaling up of emission surface diameter and improved electron emission characteristics without undesirable scaling up of the associated drive current. In one embodiment, the emission surface  110  may be any suitable shape or configuration that achieves this effect. For example, the emission surface  110  may be generally round, disc-shaped, circular, annular, elliptical, or rectangular. 
         [0035]    Regardless of the pattern used on forming the emitter  100 , the temperature distribution across the emission surface  100  is relatively uniform at operational drive currents. Table 1 shows the results of expected temperature profiles for the radial (10 mm diameter) and axial designs as modeled using thermal modeling software. 
         [0000]                                                      TABLE 1                       Drive Current (A)   Tmax (° C.)   ΔT (° C.)                                    Radial pattern   9.5   2601   124       Axial Pattern   7.5   2641   150                    
As shown, the temperature uniformity for the radial pattern remained consistent even at maximum drive currents. In one embodiment the emitter  100  maintains temperature uniformity across the emission surface  110  of less than about 10% or less than about 6% temperature difference from the maximum temperature.
 
         [0036]    This written description uses examples, including the best mode, and also to enable any person skilled in the art to practice the techniques, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.