Patent Publication Number: US-2019189384-A1

Title: Bipolar grid for controlling an electron beam in an x-ray tube

Description:
BACKGROUND 
     The present disclosure generally relates to X-ray tubes, including embodiments relating to controlling electron beams generated in X-ray tubes. 
     X-ray tubes are used in a variety of industrial and medical applications. For example, X-ray tubes are employed in medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and material analysis. More specifically, X-ray tubes are often used in computed tomography (CT) or X-ray imaging systems to analyze patients in medical imaging procedures or objects during package scanning. 
     During operation of a typical X-ray tube, an electrical current may be supplied to an electron emitter or filament of a cathode. This causes electrons to be formed on the emitter via a process known as thermionic emission. The electrons accelerate from the emitter toward a target track formed on the anode in the presence of a high voltage differential applied between the anode and the cathode. Upon striking the anode, some of the resulting kinetic energy from the striking electrons is converted into X-rays. The region of the anode upon which the majority of the electrons collide is generally referred to as a “focal spot.” The resulting X-rays may then pass through an X-ray transmissive window and are directed towards patient or other object to be examined. In a typical environment, an image is provided based on the X-rays that pass through the patient/object. 
     The claimed subject matter is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. This background is only provided to illustrate examples of where the present disclosure may be utilized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of an example X-ray tube in which one or more embodiments described herein may be implemented. 
         FIG. 1B  is a side view of the X-ray tube of  FIG. 1A . 
         FIG. 1C  is a cross-sectional view of the X-ray tube of  FIG. 1A . 
         FIG. 2A  is a top perspective view of an example of a cathode assembly. 
         FIG. 2B  is a bottom perspective view of the cathode assembly of  FIG. 2A . 
         FIG. 2C  is a perspective view of a portion of the cathode assembly of  FIG. 2A . 
         FIG. 2D  is a perspective view of another portion of the cathode assembly of  FIG. 2A . 
         FIGS. 3A-3C  are schematic representations of electron beams. 
         FIG. 4  is an example emission chart illustrating the relationship between emitter temperature, grid voltage, and beam current. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will be made to the drawings and specific language will be used to describe various aspects of the disclosure. Using the drawings and description in this manner should not be construed as limiting its scope. Additional aspects may be apparent in light of the disclosure, including the claims, or may be learned by practice. 
     In an X-ray tube, electrons are typically generated using an electron emitter. In the presence of a voltage differential, the electrons may then be directed to a focal spot or a target on an anode, and upon striking the target, some of the resulting energy generated from the electron collision with the anode is converted into X-rays. The X-rays generated by the X-ray tube may then be directed to a patient or an object for analysis or treatment. 
     In some circumstances, it may be desirable to control the amount of X-rays provided to a patient or an object. This may be accomplished by turning the X-ray tube on and off over a specific time period. However, there may be some delay between the time the X-ray tube is turned on and X-rays are produced, and between the time the X-ray tube is turned off and X-rays cease. Accordingly, precisely controlling the dosage of X-rays provided to a patient or an object may depend on how quickly an X-ray tube responds to being turned on and off In particular, if an X-ray tube quickly starts and stops producing X-rays, the X-ray dosage provided to a patient or an object may be more accurately controlled. 
     Typically, the electron beam from an X-ray tube is turned on and off by changing the electrical current through the electron emitter. For example, an electrical current may be provided through the electron emitter to turn the electron beam on, and the electrical current may be turned off to turn the electron beam off. However, in such configurations, the response speed of the X-ray tube depends on thermal characteristics of the electron emitter. In particular, the electron emitter needs to heat up above a specific temperature threshold to emit electrons, typically referred to as thermionic emission. Thus, there is a delay between the time electrical current is provided through the electron emitter and the time the electron beam is emitted, because of the time it takes for the electron emitter to be heated above the temperature threshold. Similarly, the electron emitter needs to cool down below the temperature threshold to cease emitting electrons. Accordingly, there may be a delay between the time electrical current is turned off and the time the electron beam is emitted, because of the time it takes for the electron emitter to cool down below the temperature threshold. 
     The delay for the electron emitter to heat up and cool down depends on the thermal time constant of the electron emitter, which in turn depends on various characteristics such as the material, shape, and/or surface area of the electron emitter. 
     One type of emitter used in X-ray tubes is a coil filament. A coil filament is typically formed of a wire arranged in a spiral or helical configuration. Advantages of coil filaments include lower cost and widespread use in X-ray tubes. Another type of emitter that may be used in X-ray tubes is a flat or planar emitter. X-ray tube configurations with flat or planar emitters may be more expensive, but they generally have better X-ray emission characteristics. In particular, planar emitters typically have larger emission surface areas, and therefore are capable of generating more electrons and a more uniform electron density distribution in the electron beam. In addition, planar emitters may create better focal spots, which may improve imaging characteristics. Other types of emitters that may be implemented include bulk emitters, dispenser cathodes, indirectly heated or bombarded emitters, or field emission sources (such as nanotubes or carbon nanotubes). 
     Some X-ray tubes may include a grid configured to receive a grid voltage to control the electron beam emitted by the electron emitter. The grid may be used to focus and/or steer electrons emitted by the electron emitter. Additionally or alternatively, the grid may be used to “cut off” the electron beam by providing a sufficiently large voltage difference to prevent the electron beam from reaching the target and/or the focal spot. In particular, the grid voltage may stop the electrons from flowing by isolating the electron emitter from the high voltage field that the electron beam travels through. Thus, the grid may be used to reduce the electron beam current by limiting the number of electrons that are able to reach the anode. In some circumstances, grid cutoff may be used to turn the electron beam on and off rather than turning the electron emitter on and off, which may have a longer delay because of the thermal characteristics of the emitter. Accordingly, “cutting off” the electron beam may be used for controlling the amount of total X-rays received by a patient or object during an X-ray scan. 
     Typically, a grid needs to be positioned relative to the emitter in a manner to provide suitable electron beam cutoff without interfering with the electron beam when the X-ray tube is operating. Because of the shape and size of coil filaments, and the slots in the cathode that receive coil filaments, the grid may be positioned relatively close to a coil filament without interfering with the electron beam, and in such configurations the grid voltage necessary to cut off the electron beam may be relatively low. In contrast, the surface area of flat or planar emitters is generally larger, and the openings in the cathode that receive such emitters are also generally larger. Accordingly, the grid would generally need to be positioned further from the flat or planar emitter to avoid interfering with the electron beam during operation, and would therefore require a much higher grid voltage to cut off the electron beam. However, in some circumstances, it may not be practicable to increase the grid voltage because of limitations on the power supply (e.g., a generator) or the electrical couplings that transmit the voltage to the X-ray tube (e.g., electrical couplings extending into a vacuum envelope of the X-ray tube). 
     The disclosed embodiments may facilitate using a grid positioned between the cathode and the anode to provide electron beam cutoff. In some aspects, the grid may provide suitable electron beam cutoff without influencing the electron beam when the X-ray tube is operating. In addition, the described aspects may be implemented for cathodes with any suitable emitter types, including coil filaments, flat or planar emitters, bulk emitters, indirectly heated emitters, bombarded emitters, field emission emitters, or others. Accordingly, aspects of the disclosure address the challenges in providing a grid for electron beam cutoff for flat or planar emitter cathodes or other high electron density emission sources (e.g. dispenser cathodes). For example, the disclosed embodiments may be implemented in flat or planar emitter cathodes without requiring relatively high grid voltages to cut off the electron beam. 
     Aspects of the disclosure include a bipolar grid positioned in front of a cathode head to provide beam cutoff In some circumstances, a grid positioned in front of a cathode head may cause undesired focusing effects or the grid may otherwise interfere with the electron beam. However, in the disclosed embodiments the grid may be bipolar such that the voltage of the grid may be either positive or negative with respect to the voltage of the cathode. In such configurations, the grid may be subject to a positive voltage while the X-ray tube is operating to generate an electron beam. The positive voltage of the grid may correspond with the voltage in the electric field between the cathode and the anode such that the grid voltage matches the typical X-ray tube voltage and therefore does not interfere with the electron beam. The voltage of the grid may be changed to a negative voltage to cutoff the electron beam by isolating the electron emitter from the electric field between the cathode and the anode. 
     Such configurations permit the grid to be positioned proximate the electron emitter on the cathode without interfering with the electron beam. In effect, the grid may be “hidden” in the electric field because the voltage of the grid matches the voltage of the electric field. As the electrons pass proximate the grid, they are not disrupted by the grid and continue to travel to the anode uninterrupted. In addition, the grid may be positioned relatively close to the electron emitter without interfering with the electron beam, therefore a relatively high grid voltage is not required to cut off the electron beam. 
     The disclosed embodiments may permit the electron beam from the X-ray tube to be turned on and off more quickly than X-ray tubes that are controlled by changing the electrical current through the electron emitter. Such configurations may permit more accurate control of X-ray dosage provided to a patient or an object. Specifically, the disclosed embodiments avoid the delay in turning the electron emitter on and off caused when the electron emitter warms up and cools down. 
       FIGS. 1A-1C  are views of one example of an X-ray tube  100  in which one or more embodiments described herein may be implemented. Specifically,  FIG. 1A  is a perspective view of the X-ray tube  100  and  FIG. 1B  is a side view of the X-ray tube  100 , while  FIG. 1C  is a cross-sectional view of the X-ray tube  100 . The X-ray tube  100  illustrated in  FIGS. 1A-1C  represents an example operating environment and is not meant to limit the embodiments described herein. 
     Generally, X-rays are generated within the X-ray tube  100 , some of which then exit the X-ray tube  100  to be utilized in one or more applications. The X-ray tube  100  may include a vacuum enclosure structure  102  which may act as the outer structure of the X-ray tube  100 . The vacuum enclosure structure  102  may include a cathode housing  104  and an anode housing  106 . The cathode housing  104  may be secured to the anode housing  106  such that an interior cathode volume  103  is defined by the cathode housing  104 , and an interior anode volume  105  is defined by the anode housing  106 , each of which are joined so as to define the vacuum enclosure  102 . 
     In some embodiments, the vacuum enclosure structure  102  is disposed within an outer housing (not shown) within which a coolant, such as liquid or air, is circulated so as to dissipate heat from the external surfaces of the vacuum enclosure  102 . An external heat exchanger (not shown) is operatively connected so as to remove heat from the coolant and recirculate it within the outer housing. 
     The X-ray tube  100  of  FIGS. 1A-1C  includes a shield component  107  (e.g., sometimes referred to as an electron shield, aperture, or electron collector) that is positioned between the anode housing  106  and the cathode housing  104  so as to further define the vacuum enclosure  102 . The cathode housing  104  and the anode housing  106  may each be welded, brazed, or otherwise mechanically coupled to the shield  107 . 
     The X-ray tube  100  may also include an X-ray transmissive window  108 . Some of the X-rays that are generated in the X-ray tube  100  may exit through the window  108 . The window  108  may be composed of beryllium or another suitable X-ray transmissive material. 
     With specific reference to  FIG. 1C , the cathode housing  104  forms a portion of the X-ray tube referred to as a cathode assembly  110 . The cathode assembly  110  generally includes components that relate to the generation of electrons that together form an electron beam, denoted at  112 . The cathode assembly  110  may also include the components of the X-ray tube between an end  116  of the cathode housing  104  and an anode  114 . For example, the cathode assembly  110  may include a cathode head  115  having an electron emitter, generally denoted at  122 , disposed at an end of the cathode head  115 . As will be further described, in some embodiments the electron emitter  122  can be configured as a planar electron emitter. When an electrical current is applied to the electron emitter  122 , the electron emitter  122  is configured to emit electrons via thermionic emission, that together form a laminar electron beam  112  that accelerates towards the anode target  128 . 
     The cathode assembly  110  may additionally include an acceleration region  126  further defined by the cathode housing  104  and adjacent to the electron emitter  122 . The electrons emitted by the electron emitter  122  form an electron beam  112  and traverse through the acceleration region  126  and accelerate towards the anode  114  due to a suitable voltage differential. More specifically, according to the arbitrarily-defined coordinate system included in  FIGS. 1A-1C , the electron beam  112  may accelerate in a z-direction, away from the electron emitter  122  in a direction through the acceleration region  126 . 
     The cathode assembly  110  may additionally include at least part of a drift region  124  defined by a neck portion  124   a  of the cathode housing  104 . In this and other embodiments, the drift region  124  may also be in communication with an opening  150  provided by the shield  107 , thereby allowing the electron beam  112  emitted by the electron emitter  122  to propagate through the acceleration region  126 , the drift region  124  and opening  150  until striking the anode target surface  128 . In the drift region  124 , a rate of acceleration of the electron beam  112  may be reduced from the rate of acceleration in the acceleration region  126 . As used herein, the term “drift” describes the propagation of the electrons in the form of the electron beam  112  through the drift region  124 . 
     Positioned within the anode interior volume  105  defined by the anode housing  106  is the anode  114 . The anode  114  is spaced apart from and opposite to the cathode assembly  110  at a terminal end of the drift region  124 . Generally, the anode  114  may be at least partially composed of a thermally conductive material or substrate, denoted at  160 . For example, the conductive material may include tungsten or molybdenum alloy. The backside of the anode substrate  160  may include additional thermally conductive material, such as a graphite backing, denoted at  162 . 
     The anode  114  may be configured to rotate via a rotatably mounted shaft positioned in a bearing housing, denoted here as  164 , which rotates via an inductively induced rotational force on a rotor assembly via ball bearings, liquid metal bearings or other suitable structure. As the electron beam  112  is emitted from the electron emitter  122 , electrons impinge upon the target surface  128  of the anode  114 . The target surface  128  is annular or ring-shaped and may be positioned around the rotating anode  114 . The location in which the electron beam  112  impinges on the target surface  128  is known as a focal spot (not shown). The target surface  128  may be composed of tungsten or a similar material having a high atomic (“high Z”) number. A material with a high atomic number may be used for the target surface  128  so that the material will correspondingly include electrons in “high” electron shells that may interact with the impinging electrons to generate X-rays. 
     During operation of the X-ray tube  100 , the anode  114  and the electron emitter  122  are connected in an electrical circuit. The electrical circuit allows the application of a high voltage potential between the anode  114  and the electron emitter  122 . Additionally, the electron emitter  122  is connected to a power source such that an electrical current is passed through the electron emitter  122  to cause electrons to be generated by thermionic emission. The application of a high voltage differential between the anode  114  and the electron emitter  122  causes the emitted electrons to form an electron beam  112  that accelerates through the acceleration region  126  and the drift region  124  towards the target surface  128 . Specifically, the high voltage differential causes the electron beam  112  to accelerate through the acceleration region  126  and then drift through the drift region  124 . As the electrons within the electron beam  112  accelerate, the electron beam  112  gains kinetic energy. Upon striking the target surface  128 , some of this kinetic energy is converted into electromagnetic radiation having a high frequency, i.e., X-rays. The target surface  128  is oriented with respect to the window  108  such that the X-rays are directed towards the window  108 . At least some portion of the X-rays then exit the X-ray tube  100  via the window  108 . 
     In some embodiments, the X-ray tube  100  may include one or more electron beam manipulation components. Such components can be implemented to “focus,” “steer” and/or “deflect” the electron beam  112  before it traverses the region  126 , thereby manipulating or “toggling” the dimension and/or the position of the focal spot on the target surface  128 . Additionally or alternatively, a manipulation component or system can be used to alter or “focus” the cross-sectional shape (e.g., length and/or width) of the electron beam and thereby change the shape and dimension of the focal spot on the target  128 . In some configurations, the components configured to “focus,” “steer” and/or “deflect” the electron beam may be located on the cathode head  115  and/or the cathode assembly  110 . In the illustrated embodiments electron beam focusing and steering are provided by way of a magnetic system denoted generally at  180 . 
     The magnetic system  180  may include various combinations of focusing quadrupoles, steering quadrupoles, steering coils, and steering dipoles implementations that are disposed so as to impose magnetic forces on the electron beam  112  so as to focus and/or steer the beam. One example of the magnetic system  180  is shown in  FIGS. 1A-1C . In this embodiment, the magnetic system  180  is implemented as two magnetic cores  182 ,  184  disposed in the electron beam path  112  of the X-ray tube  100 . The combination of the two cores  182 ,  184  are configured to (a) focus in both directions perpendicular to the beam path, and (b) to steer the beam in both directions perpendicular to the beam path. In this way, the two cores  182 ,  184  can have quadrupoles that act together to form a magnetic lens (sometimes referred to as a “doublet”), and the focusing and steering is accomplished as the electron beam passes through the quadrupole “lens.” The “focusing” provides a desired focal spot shape and size. Additionally, the magnetic system  180  can be configured with at least one coil or a pair of coils that have an AC offset, and preferably two perpendicular pairs of coils that have an AC offset, used for steering. The steering can be implemented by configurations of the two or more cores  182  and  184 . The “steering” affects the positioning of the focal spot on the anode target surface  128 . The magnetic system  180  may be substituted with any of the other suitable focusing or steering configuration. 
     The embodiments described herein may be implemented with any suitable focusing or steering structures, such as a spatial, magnetic, electrostatic, or combination thereof. The described embodiments may be implemented using a single electrostatic focusing grid or multiple grid configurations (e.g., dual grids). In other configurations, embodiments may not include electrostatic focusing and may rely on other suitable focusing structures, such as spatial and/or magnetic. 
       FIGS. 2A-2D  are views of an example of a cathode assembly  200 . In some configurations, the cathode assembly  200  may be implemented in the X-ray tube  100  of  FIGS. 1A-1C , for example, instead of the cathode assembly  110 . Any suitable aspects of the cathode assembly  200  may be included in the cathode assembly  110 , or vice versa. Specifically,  FIG. 2A  is a top perspective view of the cathode assembly  200 ,  FIG. 2B  is a bottom perspective view of the cathode assembly  200 , and  FIGS. 2C-2D  are perspective views of portions of the cathode assembly  200 . 
     As illustrated for example in  FIG. 2A-2B , the cathode assembly  200  may include a cathode head  202 , a housing  203 , and a grid  204 . The grid  204  may be define an opening  206  sized and shaped to permit electrons generated by the cathode assembly  200  to travel therethrough. The grid  204  may be configured to control an electron beam generated at the cathode assembly  200 . The cathode assembly  200  may include electrical couplings  208   a ,  208   b , and  208   c . A power source may be electrically coupled to the cathode assembly  200  via the electrical couplings  208   a - c.    
       FIG. 2C  is a perspective view of the cathode assembly  200  with the grid  204  not shown, and  FIG. 2D  is a perspective view of the cathode assembly  200  with the grid  204  and the housing  203  not shown. With attention to  FIGS. 2C and 2D , additional aspects of the cathode assembly  200  will be described in further detail. In the illustrated configuration, the cathode assembly  200  includes a planar electron emitter  210  on an end of the cathode head  202 . The electron emitter  210  may be oriented toward an anode, such the as the anode  114  of  FIGS. 1A-1C , and may be configured to generate electrons or an electron beam directed to the anode  114 . Although a planar electron emitter  210  is illustrated in this embodiment, any suitable emitter may be implemented. For example, in other configurations a spiral, helical, or coil filament may be implemented. 
     The electrical couplings  208   a - c  may extend at least partially through the cathode head  202  and be coupled to the electron emitter  210  and the grid  204 . In particular, the electrical couplings  208   a  and  208   b  may be electrically coupled to the electron emitter  210  and the electrical coupling  208   c  may be electrically coupled to the grid  204 . As shown, the electrical coupling  208   c  may extend entirely through the cathode head  202  to the grid  204 . 
     The cathode head  202  may include a head surface  212  that has an emitter region  214 . The emitter region  214  can have various configurations, for example, in the illustrated configuration the emitter region  214  is a recess defined in the cathode head  202 . As shown, the recess may be sized and shaped to receive the electron emitter  210 . In other configurations, the emitter region  214  may be a surface with an electron emitter positioned above or proximate the surface. The cathode head  202  may define lead openings  216   a  and  216   b . The lead openings  216   a ,  216   b  may permit the electrical couplings  208   a ,  208   b  to extend through to cathode head  202  to the electron emitter  210 . The electron emitter  210  includes an emitter body that extends continuously between the electrical coupling  208   a  and the  208   b  electrical coupling. The cathode head  202  may include electron beam focusing elements  218  positioned on the surface  212  on opposite sides of the electron emitter  210 . The focusing elements  218  may be configured to focus an electron beam generated by the electron emitter  210 . 
     In the illustrated configuration, the electron emitter  210  extends in a spiraling rectangular configuration, although any suitable pattern or configuration may be implemented. The pattern of the electron emitter  210  may be two-dimensional so as to form a planar emitter surface, where different regions of the electron emitter  210  cooperate to form the planar emitter surface. In the illustrated configuration, gaps (e.g., illustrated by lines between members) may be positioned between different regions of the electron emitter  210 . The gaps may form a first continuous gap from a first end of the electron emitter  210  to a middle region of the electron emitter  210 . The gaps may also form a second continuous gap from the middle region to a second end of the electron emitter  210 . As shown, the electron emitter  210  is continuous and patterned so that electrical current flows from a first end of the electron emitter  210  to a second end. However, other arrangements, configurations, or patterns may be implemented. 
     In some configurations, the electron emitter  210  may include a tungsten foil, and alloy of tungsten, or other suitable material. The emitting surface of the electron emitter  210  may be coated with a composition that reduces the emission temperature. For example, the coating may include tungsten, tungsten alloys, thoriated tungsten, doped tungsten (e.g., potassium doped), zirconium carbide mixtures, barium mixtures or other coatings can be used to decrease the emission temperature. 
     As mentioned, the grid  204  may be configured to control an electron beam generated by the electron emitter  210 . In particular, the grid  204  may be used to focus, direct, and/or cut off the electron beam from the electron emitter  210 . The grid  204  may be electrically isolated from the cathode head  202  and may be configured to receive a grid voltage (e.g., via the electrical coupling  208   c ) to focus and/or steer electrons emitted by the electron emitter  210 . Particularly, the grid  204  may focus the electron beam in one or more directions as the electron beam passes through the opening  206 , and/or steer the electron beam in one or more directions. The voltage through the grid  204  may be modulated so as to provide an electron beam with a given dimension. Specifically, the voltage difference between the grid  204  and the electron emitter  210  may be modulated to change one or more cross-sectional dimension of the electron beam. 
     In some circumstances, the grid  204  may be used to cut off the electron beam by providing a sufficiently large voltage difference to prevent the electron beam from reaching the target and/or the focal spot. Cutting off the electron beam may be used for controlling the amount of total X-rays received by a patient or object during an X-ray scan. For example, cutting off the electron beam may be used to limit the amount of X-rays a patient or object receives during a scan. This may be useful, for example, during cardiac scanning of a patient. Accordingly, the grid  204  may be used to control the emission of X-rays from the X-ray tube by cutting off electron beams from the electron emitter  210 . 
     The grid  204  may be a bipolar grid positioned in front of the cathode head  202  to provide beam cutoff for the electron beam generated by the electron emitter  210 . In particular, the grid  204  may receive both a positive and a negative voltage. The grid  204  may be configured to have a positive voltage while the electron emitter  210  generates an electron beam. The positive voltage of the grid  204  may correspond with the voltage in the electric field between the cathode and the anode such that the grid voltage matches the electric field voltage and therefore does not interfere with the electron beam from the electron emitter  210 . The voltage of the grid  204  may be changed to a negative voltage to cut off the electron beam from the electron emitter  210  by isolating the electron emitter  210  from the electric field between the cathode and the anode. Additionally or alternatively, the voltage of the grid  204  may be changed to a negative voltage to inhibit the flow of electrons and/or reduce electron density of the electron beam generated by the electron emitter. 
     Such configurations may permit the grid  204  to be positioned proximate the electron emitter  210  on the cathode head  202  without interfering with the electron beam. In effect, the grid  204  may be “hidden” in the electric field because the voltage of the grid  204  matches the voltage of the electric field. As the electrons pass proximate the grid  204 , they are not disrupted by the grid  204  and continue to travel to the anode uninterrupted. In addition, the grid  204  may be positioned relatively close to the electron emitter  210  with interfering with the electron beam, therefore a relatively high grid voltage is not required to cut off the electron beam. 
     The illustrated configuration may permit the electron beam to be turned on and off more quickly than X-ray tubes that are controlled by changing the electrical current through the electron emitter  210 . Such configurations may permit more accurate control of X-ray dosage provided to a patient or an object. Specifically, such configurations avoid the delay in turning the electron emitter  210  on and off caused when the electron emitter  210  warms up and cools down. In some circumstances, the delay to turn the electron beam  210  on and off may be reduced to be in the range of tens of microseconds. The grid  204  may provide suitable electron beam cutoff without influencing the electron beam when the X-ray tube is operating. The grid  204  may be implemented for cathodes with any suitable emitter types, including coil filaments and flat or planar emitters. 
     In some configurations, a power supply may be configured to swing the voltage of the grid  204  between a negative voltage and a positive voltage. In one example, a power supply may be configured to swing the voltage of the grid  204  between −4 kilovolts (kV) and +4 kV. In another example, a power supply may be configured to swing the voltage of the grid  204  between −10 kilovolts (kV) and +10 kV, although other configurations maybe implemented. In some circumstances, the voltage of the grid  204  may swing from a negative voltage and a positive voltage, and vice versa, in less than  100  microseconds. 
     The grid  204  may be formed of a material suitable for withstanding the operating conditions inside of the X-ray tube. For example, the grid  204  may include a material that is structurally robust and tolerant of the relatively large temperature changes in an X-ray tube. Additionally or alternatively, the grid  204  may include an electrically conductive material suitable for receiving the grid voltage from the power supply. In some configurations, the grid  204  may include nickel, stainless steel, tungsten, a tungsten alloy, molybdenum, a molybdenum alloy, niobium and/or other suitable materials. In some configurations, the grid  204  may include a refractory metal, in other configurations, a non-refractory metal may be implemented. 
     A thickness of the grid  204  may be selected to be sufficiently narrow to avoid electric field lens effects, where the grid  204  acts as a lens and distorts the electron beam as it travels through the opening  206 . Additionally or alternatively, the thickness of the grid  204  may be selected to be sufficiently wide to be structurally robust and/or to withstand heat effects (e.g., expansion caused by temperature changes in the X-ray tube). In some configurations, the thickness of the grid  204  may be between 0.1 millimeters (mm) and 3 mm, between 1 millimeters (mm) and 2 mm, or any other suitable configurations. 
     In some configurations, size and shape of the opening  206  may be selected to correspond to the electron emitter  210  and/or the electron beam formed by the electron emitter  210 . For example, in the illustrated configuration the electron emitter  210  is a flat emitter with a rectangular or square configuration, and thus the opening  206  includes a rectangular or square configuration, although other suitable configurations may be implemented. Similarly, the dimensions of the opening  206  may correspond to the dimensions of the electron emitter  210  and/or the electron beam. In some configurations, the size, shape, and position of the opening  206  may be selected such that electrons from the electron emitter  210  do not impact the grid  204 . Such configurations of the grid  204  may be referred to as a “non-intercepting grid.” In contrast, an “intercepting grid” disturbs the electron beam and intercepts electrons traveling therethrough. An example of an intercepting grid is a mesh placed between a cathode and an anode. The mesh may not include an opening, such as the opening  206 . 
     The grid  204  may be spaced from the cathode head  202  or the electron emitter  210 . The distance or spacing may be based on the magnitude of the electric field between the cathode head  202  and an anode. The distance or spacing may be selected to be close enough the electron emitter  210  such that the grid voltage required to cut off or otherwise control the electron beam is not too large. Further, the distance or spacing may be selected to such that electrons from the electron emitter  210  do not impact the grid  204 . In one example, the grid  204  may be positioned 2 mm from the cathode head  202 . In another example, the grid  204  may be positioned between 0.01 and 10 mm from the cathode head  202 . In some configurations, the grid  204  may be spaced from the cathode head  202  by standoff member(s) that retain the grid  204  with respect to the cathode head  202  while electrically insulating the grid  204  from the cathode head  202 . 
     Additionally or alternatively, the geometry and the spacing of the grid  204  and the opening  206  may be based on the desired beam characteristics of the electron beam formed by the electron emitter  210 . For example, the configuration of the grid  204  and the opening  206  may be different converging electron beams, diverging electron beams, and/or laminar flow electron beams. 
     In one example configuration, the opening  206  may include one or more dimensions of 8 mm (e.g., length, width, or diameter), the electron emitter  210  may include one or more dimensions of 5 mm, and the grid  204  may be positioned 2 mm from the cathode head  202 . 
     In the illustrated configuration, the size and shape of the grid  204  generally corresponds to the size and shape of the cathode head  202 , however, other configurations may be implemented. The size and shape of the grid  204  may be selected to suitably control the electron beam without necessarily corresponding to the size and shape of the cathode head  202 . 
       FIGS. 3A-3C  are schematic representations of electron paths according to embodiments described herein. In particular,  FIGS. 3A-3C  include a cathode  300 , an anode  320 , and a grid  304  positioned therebetween. The cathode  300  includes an electron emitter  302  that is configured to generate electrons, or an electron beam. The grid  304  defines an opening  306  sized and shaped to permit electrons to travel between the emitter  302  and the anode  320 . The cathode  300 , anode  320 , and grid  304  may include any suitable features described above. 
     In the configuration shown in  FIG. 3A , the grid  304  is set to a positive potential relative to the cathode voltage. As shown, the grid  304  does not interrupt an electron beam  310   a  generated by the electron emitter  302 . The electron beam  310   a  travels from the electron emitter  302 , through the opening  306  defined in the grid  304 , to a focal spot  312   a  on the anode  320 . The voltage or potential of the grid  304  may be selected to correspond to or match the voltage of the electric field between the electron emitter  302  and the anode  320 . In effect, the grid  304  may be “hidden” in the electric field in a manner not to disrupt the electron beam  310   a.    
     In one example of the configuration of  FIG. 3A , the tube voltage may be 80 kilovolts (kV), the grid voltage may be 4 kV relative to the cathode voltage, the temperature of the electron emitter  302  may be 2370 degrees Celsius (° C.), and the electron beam current may be 236 milliamps (mA). 
       FIG. 3B  illustrates a configuration similar to  FIG. 3A , except with a higher beam current and increased electron emitter temperature. In  FIG. 3B , the grid  304  is also set to a positive potential relative to the cathode voltage and does not interrupt an electron beam  310   b  generated by the electron emitter  302 . The electron beam  310   b  travels from the electron emitter  302 , through the opening  306  defined in the grid  304 , to a focal spot  312   a  on the anode  320 . The voltage or potential of the grid  304  may be selected to correspond to or match the voltage of the electric field between the electron emitter  302  and the anode  320 . In effect, the grid  304  may be “hidden” in the electric field in a manner not to disrupt the electron beam  310   b.    
     In one example of the configuration of  FIG. 3B , the tube voltage may be 80 kV, the grid voltage may be 4 kV relative to the cathode voltage, the temperature of the electron emitter  302  may be 2510° C., and the electron beam current may be 715 mA. 
       FIG. 3C  illustrates a configuration similar to  FIG. 3B , except the grid  304  is set to a negative potential to inhibit or cut off electron flow. As shown, the grid  304  cuts off the electron beam by providing a sufficiently large voltage difference to prevent the electron beam from reaching the anode  320 . Accordingly, no electrons reach a focal spot on the anode  320 . 
     In one example of the configuration of  FIG. 3C , the tube voltage may be 80 kV, the grid voltage may be −4 kV (a negative voltage) relative to the cathode voltage, the temperature of the electron emitter  302  may be 2550° C., and the electron beam current may be 0 mA. 
     In such configurations, although electrons are not flowing from the electron emitter  302 , the temperature of electron emitter  302  is still sufficiently high to generate electrons. Accordingly, the electron emitter  302  does not need to be heated up to be turned on or to generate electrons. Instead, the grid voltage must be changed from a negative voltage to a positive voltage to permit electrons to flow (for example, from −4 kV to +4 kV or from −10 kV to +10 kV). Thus, the delay to turn the electron beam on and off is much shorter, because it does not depend on the thermal characteristics of the electron emitter  302  (e.g., the time it takes for the electron emitter  302  to heat up and cool down). Rather, the delay to turn the electron beam on and off depends on how quickly the grid voltage may be changed from a suitable negative voltage to a positive voltage, or vice versa. 
     Although various example configurations for tube voltage, grid voltage, emitter temperature, and electron beam current are provided with respect to  FIGS. 3A-3C , any suitable parameters may be implemented. For example, in other configurations the tube voltage of an X-ray tube may be 70 kV, 100 kV, 120 kV, 140 kV or any other suitable value. In another example, the tube voltage of an X-ray tube may be between 0 and 100 kV. Furthermore, in the illustrated configuration the grid voltage swings between −4 kV to +4 kV, however other suitable configurations may be implemented. In some circumstances, electron emitters with larger surface areas may require larger grid voltage swings to “hide” the grid and/or provide suitable electron beam cutoff. Accordingly, grids may be implemented with other suitable operational grid voltages and/or cutoff voltage. As used herein, an “operational grid voltage” may be grid voltage or range of voltages that is suitable for permitting electrons to travel from the electron emitter to the anode. A “cutoff voltage” may be grid voltage or range of voltages that is suitable for cutting off the electron beam. As explained above, the grids described herein may be configured to switch between the operational grid voltage and the cutoff voltage. 
     For some medical applications, the X-ray tube voltage may be between 60 kV and 150 kV. For other applications, the X-ray tube may include other suitable X-ray tube voltage configurations. For example, for some industrial applications the X-ray tube voltage may be between 200 kV and 500 kV, although other suitable configurations may be implemented depending on the application. Accordingly, in various configurations the X-ray tube voltage may range between 1 and 500 kV. 
       FIG. 4  is an example emission chart illustrating the relationship between emitter temperature, grid voltage, and beam current.  FIG. 4  may be representative of the relationship between emitter temperature, grid voltage, and beam current for the configuration of  FIGS. 3A-3C . In  FIG. 4 , beam current is expressed in mA, emitter temperature is expressed in ° C., and grid voltage is expressed in volts (V) with respect to the cathode voltage. As shown, the beam current increases as emitter temperature increases. The grid voltage does not affect beam current when the grid voltage is set to 4000 V. However, when the voltage is set to −4000 V, the beam current drops to 0 mA, indicating electron beam cutoff. 
     Accordingly, in the configurations described herein, the grid voltage may be swung between a positive and a negative voltage while maintaining a desired temperature in the electron emitter. This permits the electron beam to be cut off or otherwise controlled rapidly, without significant delays when the electron beam is turned on and off. 
     As mentioned, the delay for the electron emitter to heat up and cool down depends on the thermal time constant of the electron emitter, which in turn depends on various characteristics such as the material, shape, and/or surface area of the electron emitter. In some configurations, the delay for the electron emitter to heat up and/or cool down may be in the range of tens of milliseconds. The disclosed embodiments may permit the electron beam from the X-ray tube to be turned on and off more quickly than X-ray tubes that are controlled by changing the electrical current through the electron emitter (e.g., with a delay in the range of tens of milliseconds). Such configurations may permit more accurate control of X-ray dosage provided to a patient or an object. Specifically, the disclosed embodiments avoid the delay in turning the electron emitter on and off caused when the electron emitter warms up and cools down. In some circumstances, the delay to turn the electron beam on and off may be reduced to be in the range of tens of microseconds (e.g., a full order of magnitude faster). 
     Furthermore, in such configurations the voltage needed to shut off the electron beam may be lower than what would otherwise be required for ordinary, non-bipolar grids. For example, if the grid voltage swings between −4 kV and +4 kV, the total change in voltage is 8 kV. However, only a 4 kV maximum voltage needs to be supplied to the X-ray tube. If only a positive voltage is supplied to non-bipolar grid, the grid may require a voltage of 8 kV. However, it may be relatively difficult to manufacture X-ray tubes that handle larger voltages. In particular, various components of the X-ray tube, such as the electrically connections, may have to be more robust to handle larger voltages. Accordingly, the configurations described herein may include X-ray tubes with improved electron beam control and response, without requiring increased voltage handling and associated design complexity. 
     In some configurations, an X-ray tube ( 100 ) may include a cathode ( 110 ,  200 ,  300 ) including an electron emitter ( 122 ,  210 ,  302 ), an anode ( 114 ,  320 ) spaced apart from the cathode ( 110 ,  200 ,  300 ), a grid ( 204 ,  304 ) positioned between the cathode ( 110 ,  200 ,  300 ) and the anode ( 114 ,  320 ); and a power supply electrically coupled to the grid ( 204 ,  304 ). The power supply may be configured to provide a positive grid voltage and a negative grid voltage to the grid ( 204 ,  304 ). 
     The positive grid voltage may correspond to a voltage in the electric field between the cathode ( 110 ,  200 ,  300 ) and the anode ( 114 ,  320 ) such that the grid ( 204 ,  304 ) does not interfere with an electron beam generated by the electron emitter ( 122 ,  210 ,  302 ). The negative grid voltage may inhibit electron flow and/or reduce electron density of the electron beam generated by the electron emitter ( 122 ,  210 ,  302 ). The negative grid voltage may isolate the electron emitter ( 122 ,  210 ,  302 ) such that an electron beam does not reach the anode ( 114 ,  320 ). The negative grid voltage may be between 0 and −10 kilovolts (kV) and the positive grid voltage may be between 0 and 10 kV. 
     The grid ( 204 ,  304 ) may define an opening sized and shaped to permit electrons generated by the electron emitter ( 122 ,  210 ,  302 ) to pass therethrough. The grid ( 204 ,  304 ) may be electrically isolated from the cathode ( 110 ,  200 ,  300 ). The electron emitter ( 122 ,  210 ,  302 ) may include a planar emitter or a coil filament. The voltage of the X-ray tube ( 100 ) may be between 1 and 100 kilovolts (kV). 
     In some configurations, a bipolar grid ( 204 ,  304 ) may be positioned between a cathode ( 110 ,  200 ,  300 ) and an anode ( 114 ,  320 ). The bipolar grid ( 204 ,  304 ) may be configured to receive a positive grid voltage that corresponds to a voltage in an electric field between the cathode ( 110 ,  200 ,  300 ) and the anode ( 114 ,  320 ) such that the grid ( 204 ,  304 ) does not interfere with an electron beam generated by an electron emitter ( 122 ,  210 ,  302 ) of the cathode ( 110 ,  200 ,  300 ). The bipolar grid ( 204 ,  304 ) may be configured to receive a negative grid voltage to isolate the electron emitter ( 122 ,  210 ,  302 ) such that the electron beam does not reach the anode ( 114 ,  320 ). 
     The negative grid voltage may be between 0 and −10 kilovolts (kV) and the positive grid voltage may be between 0 and 10 kilovolts (kV). The bipolar grid ( 204 ,  304 ) may define an opening sized and shaped to permit electrons generated by the electron emitter ( 122 ,  210 ,  302 ) to pass therethrough. The size and shape of the opening may correspond to the electron emitter ( 122 ,  210 ,  302 ). The bipolar grid ( 204 ,  304 ) may be electrically isolated from the cathode ( 110 ,  200 ). The bipolar grid ( 204 ,  304 ) may be spaced apart from the cathode ( 110 ,  200 ,  300 ) between 0 and 10 mm. 
     In some configurations, a bipolar grid ( 204 ,  304 ) may be positioned between a cathode ( 110 ,  200 ,  300 ) and an anode ( 114 ,  320 ). The bipolar grid ( 204 ,  304 ) may be configured to swing between a positive grid voltage and a negative grid voltage. The positive grid voltage may correspond to a voltage in an electric field between a cathode ( 110 ,  200 ,  300 ) and an anode ( 114 ,  320 ) such that the bipolar grid ( 204 ,  304 ) does not interfere with an electron beam generated by an electron emitter ( 122 ,  210 ,  302 ) of the cathode ( 110 ,  200 ,  300 ). 
     The negative grid voltage may isolate the electron emitter ( 122 ,  210 ,  302 ) such that the electron beam does not reach the anode ( 114 ,  320 ). The negative grid voltage may be between 0 and −10 kilovolts (kV) and the positive grid voltage may be between 0 and 10 kV. The grid ( 204 ,  304 ) may define an opening sized and shaped to permit electrons generated by the electron emitter ( 122 ,  210 ,  302 ) to pass therethrough. 
     A method of operating the bipolar grid ( 204 ,  304 ) positioned between a cathode ( 110 ,  200 ,  300 ) and an anode ( 114 ,  320 ) may include providing a positive grid voltage to the bipolar grid ( 204 ,  304 ). The positive grid voltage may correspond to a voltage in an electric field between the cathode ( 110 ,  200 ,  300 ) and the anode ( 114 ,  320 ) such that the bipolar grid ( 204 ,  304 ) does not interfere with an electron beam generated by an electron emitter ( 122 ,  210 ,  302 ) of the cathode ( 110 ,  200 ,  300 ). The method may include swinging the bipolar grid ( 204 ,  304 ) to a negative grid voltage, wherein the negative grid voltage reduces electron density of the electron beam generated by the electron emitter ( 122 ,  210 ,  302 ). The negative grid voltage may isolate the electron emitter ( 122 ,  210 ,  302 ) such that the electron beam does not reach the anode ( 114 ,  320 ). The negative grid voltage may be between 0 and −10 kilovolts (kV) and the positive grid voltage is between 0 and 10 kV. The bipolar grid may define an opening sized and shaped to permit electrons generated by the electron emitter ( 122 ,  210 ,  302 ) to pass therethrough. 
     The terms and words used in this description and claims are not limited to the bibliographical meanings, but, are merely used to enable a clear and consistent understanding of the disclosure. It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those skilled in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
     Aspects of the present disclosure may be embodied in other forms without departing from its spirit or essential characteristics. The described aspects are to be considered in all respects illustrative and not restrictive. The claimed subject matter is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.