Patent Publication Number: US-7903789-B2

Title: X-ray tube electron sources

Description:
CROSS-REFERENCE 
     The present application is a continuation of U.S. patent application No. 10/554,975, filed on Aug. 2, 2006 issued as U.S. Pat. No. 7,512,215, which is a national stage application of PCT/GB2004/01741, filed on Apr. 23, 2004 and which, in turn, relies on Great Britain Application Number 0309383.8, filed on Apr. 25, 2003, for priority. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to X-ray tubes, to electron sources for X-ray tubes, and to X-ray imaging systems. 
     X-ray tubes include an electron source, which can be a thermionic emitter or a cold cathode source, some form of extraction device, such as a grid, which can be switched between an extracting potential and a blocking potential to control the extraction of electrons from the emitter, and an anode which produces the X-rays when impacted by the electrons. Examples of such systems are disclosed in U.S. Pat. Nos. 4,274,005 and 5,259,014. 
     With the increasing use of X-ray scanners, for example for medical and security purposes, it is becoming increasingly desirable to produce X-ray tubes which are relatively inexpensive and which have a long lifetime. 
     Accordingly the present invention provides an electron source for an X-ray scanner comprising electron emitting means defining a plurality of electron source regions, an extraction grid defining a plurality of grid regions each associated with at least a respective one of the source regions, and control means arranged to control the relative electrical potential between each of the grid regions and the respective source region so that the position from which electrons are extracted from the emitting means can be moved between said source regions. 
     The extraction grid may comprise a plurality of grid elements spaced along the emitting means. In this case each grid region can comprise one or more of the grid elements. 
     The emitting means may comprise an elongate emitter member and the grid elements may be spaced along the emitter member such that the source regions are each at a respective position along the emitter member. 
     Preferably the control means is arranged to connect each of the grid elements to either an extracting electrical potential which is positive with respect to the emitting means or an inhibiting electrical potential which is negative with respect to the emitting means. More preferably the control means is arranged to connect the grid elements to the extracting potential successively in adjacent pairs so as to direct a beam of electrons between each pair of grid elements. Still more preferably each of the grid elements can be connected to the same electrical potential as either of the grid elements which are adjacent to it, so that it can be part of two different said pairs. 
     The control means may be arranged, while each of said adjacent pairs is connected to the extracting potential, to connect the grid elements to either side of the pair, or even all of the grid elements not in the pair, to the inhibiting potential. 
     The grid elements preferably comprise parallel elongate members, and the emitting member, where it is also an elongate member, preferably extends substantially perpendicularly to the grid elements. 
     The grid elements may comprise wires, and more preferably are planar and extend in a plane substantially perpendicular to the emitter member so as to protect the emitter member from reverse ion bombardment from the anode. The grid elements are preferably spaced from the emitting means by a distance approximately equal to the distance between adjacent grid elements. 
     The electron source preferably further comprises a plurality of focusing elements, which may also be elongate and are preferably parallel to the grid elements, arranged to focus the beams of electrons after they have passed the grid elements. More preferably the focusing elements are aligned with the grid elements such that electrons passing between any pair of the grid elements will pass between a corresponding pair of focusing elements. 
     Preferably the focusing elements are arranged to be connected to an electric potential which is negative with respect to the emitter. Preferably the focusing elements are arranged to be connected to an electric potential which is positive with respect to the grid elements. 
     Preferably the control means is arranged to control the potential applied to the focusing elements thereby to control focusing of the beams of electrons. 
     The focusing elements may comprise wires, and may be planar, extending in a plane substantially perpendicular to the emitter member so as to protect the emitter member from reverse ion bombardment from an anode. 
     The grid elements are preferably spaced from the emitter such that if a group of one or more adjacent grid elements are switched to the extracting potential, electrons will be extracted from a length of the emitter member which is longer than the width of said group of grid elements. For example the grid elements may be spaced from the emitter member by a distance which is at least substantially equal to the distance between adjacent grid elements, which may be of the order of 5 mm. 
     Preferably the grid elements are arranged to at least partially focus the extracted electrons into a beam. 
     The present invention further provides an X-ray tube system comprising an electron source according to the invention and at least one anode. Preferably the at least one anode comprises an elongate anode arranged such that beams of electrons produced by different grid elements will hit different parts of the anode. 
     The present invention further provides an X-ray scanner comprising an X-ray tube according to the invention and X-ray detection means wherein the control means is arranged to produce X-rays from respective X-ray source points on said at least one anode, and to collect respective data sets from the detection means. Preferably the detection means comprises a plurality of detectors. More preferably the control means is arranged to control the electric potentials of the source regions or the grid regions so as to extract electrons from a plurality of successive groupings of said source regions each grouping producing an illumination having a square wave pattern of a different wavelength, and to record a reading of the detection means for each of the illuminations. Still more preferably the control means is further arranged to apply a mathematical transform to the recorded readings to reconstruct features of an object placed between the X-ray tube and the detector. 
     The present invention further provides an X-ray scanner comprising an X-ray source having a plurality of X-ray source points, X-ray detection means, and control means arranged to control the source to produce X-rays from a plurality of successive groupings of the source points each grouping producing an illumination having a square wave pattern of a different wavelength, and to record a reading of the detection means for each of the illuminations. Preferably the source points are arranged in a linear array. Preferably the detection means comprises a linear array of detectors extending in a direction substantially perpendicular to the linear array of source points. More preferably the control means is arranged to record a reading from each of the detectors for each illumination. This can enable the control means to use the readings from each of the detectors to reconstruct features of a respective layer of the object. Preferably the control means is arranged to use the readings to build up a three dimensional reconstruction of the object. 
     The present invention further comprises an X-ray scanner comprising an X-ray source comprising a linear array of source points, and X-ray detection means comprising a linear array of detectors, and control means, wherein the linear arrays are arranged substantially perpendicular to each other and the control means is arranged to control either the source points or the detectors to operate in a plurality of successive groupings, each grouping comprising groups of different numbers of the source points or detectors, and to analyse readings from the detectors using a mathematical transform to produce a three-dimensional image of an object. Preferably the control means is arranged to operate the source points in said plurality of groupings, and readings are taken simultaneously from each of the detectors for each of said groupings. Alternatively the control means may be arranged to operate the detectors in said plurality of groupings and, for each grouping, to activate each of the source points in turn to produce respective readings. 
    
    
     
       Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  shows an electron source according to the invention; 
         FIG. 2  shows an X-ray emitter unit including the electron source of  FIG. 1 ; 
         FIG. 3  is a transverse section through the unit of  FIG. 2  showing the path of electrons within the unit; 
         FIG. 4  is a longitudinal section through the unit of  FIG. 2  showing the path of electrons within the unit; 
         FIG. 5  is a diagram of an X-ray imaging system including a number of emitter units according to the invention; 
         FIG. 6  is a diagram of a X-ray tube according to a second embodiment of the invention; 
         FIG. 7  is a diagram of an X-ray tube according to a third embodiment of the invention; 
         FIG. 8  is a perspective view of an X-ray tube according to a fourth embodiment of the invention; 
         FIG. 9  is a section through the X-ray tube of  FIG. 8   
         FIG. 10  is a section through an X-ray tube according to a fifth embodiment of the invention; 
         FIG. 11  shows an emitter element forming part of the X-ray tube of  FIG. 10 ; 
         FIG. 12  is a section through an X-ray tube according to a sixth embodiment of the invention; 
         FIG. 12   a  is a longitudinal section through an X-ray tube according to a seventh embodiment of the invention; 
         FIG. 12   b  is a transverse section through the X-ray tube of  FIG. 12   a;    
         FIG. 12   c  is a perspective view of part of the X-ray tube of  FIG. 12   a;    
         FIG. 13  is a schematic representation of an X-ray scanning system according to an eighth embodiment of the invention; 
         FIGS. 14   a ,  14   b  and  14   c  show operation of the system of  FIG. 13 ; 
         FIG. 15  is a schematic representation of an X-ray scanning system according to a ninth embodiment of the invention; 
         FIGS. 16   a  and  16   b  show an emitter layer and a heater layer of an emitter according to a tenth embodiment of the invention; 
         FIG. 17  shows an emitter element including the emitter layer and heater layer of  FIGS. 16   a  and  16   b ; and 
         FIG. 18  shows an alternative arrangement of the emitter element shown in  FIG. 17 . 
     
    
    
     Referring to  FIG. 1 , an electron source  10  comprises a conductive metal suppressor  12  having two sides  14 ,  16 , and an emitter element  18  extending along between the suppressor sides  14 ,  16 . A number of grid elements in the form of grid wires  20  are supported above the suppressor  12  and extend over the gap between its two sides  14 ,  16  perpendicular to the emitter element  18 , but in a plane which is parallel to it. In this example the grid wires have a diameter of 0.5 mm and are spaced apart by a distance of 5 mm. They are also spaced about 5 mm from the emitter element  18 . A number of focusing elements in the form of focusing wires  22  are supported in another plane on the opposite side of the grid wires to the emitter element. The focusing wires  22  are parallel to the grid wires  20  and spaced apart from each other with the same spacing, 5 mm, as the grid wires, each focusing wire  22  being aligned with a respective one of the grid wires  20 . The focusing wires  22  are spaced about 8 mm from the grid wires  20 . 
     As shown in  FIG. 2 , the source  10  is enclosed in a housing  24  of an emitter unit  25  with the suppressor  12  being supported on the base  24   a  of the housing  24 . The focusing wires  22  are supported on two support rails  26   a ,  26   b  which extend parallel to the emitter element  18 , and are spaced from the suppressor  12 , the support rails being mounted on the base  24   a  of the housing  24 . The support rails  26   a ,  26   b  are electrically conducting so that all of the focusing wires  22  are electrically connected together. One of the support rails  26   a  is connected to a connector  28  which projects through the base  24   a  of the housing  24  to provide an electrical connection for the focusing wires  22 . Each of the grid wires  20  extends down one side  16  of the suppressor  12  and is connected to a respective electrical connector  30  which provide separate electrical connections for each of the grid wires  20 . 
     An anode  32  is supported between the side walls  24   b ,  24   c  of the housing  24 . The anode  32  is formed as a rod, typically of copper with tungsten or silver plating, and extends parallel to the emitter element  18 . The grid and focusing wires  20 ,  22  therefore extend between the emitter element  18  and the anode  32 . An electrical connector  34  to the anode  32  extends through the side wall  24   b  of the housing  24 . 
     The emitter element  18  is supported in the ends  12   a ,  12   b  of the suppressor  12 , but electrically isolated from it, and is heated by means of an electric current supplied to it via further connectors  36 ,  38  in the housing  24 . In this embodiment the emitter  18  is formed from a tungsten wire core which acts as the heater, a nickel coating on the core, and a layer of rare earth oxide having a low work function over the nickel. However other emitter types can also be used, such as simple tungsten wire. 
     Referring to  FIG. 3 , in order to produce a beam of electrons  40 , the emitter element  18  is electrically grounded and heated so that it emits electrons. The suppressor is held at a constant voltage of typically 3-5V so as to prevent extraneous electric fields from accelerating the electrons in undesired directions. A pair of adjacent grid wires  20   a ,  20   b  are connected to a potential which is between 1 and 4 kV more positive than the emitter. The other grid wires are connected to a potential of −100V. All of the focusing wires  22  are kept at a positive potential which is between 1 and 4 kV more positive than the grid wires. 
     All of the grid wires  20  apart from those  20   a ,  20   b  in the extracting pair inhibit, and even substantially prevent, the emission of electrons towards the anode over most of the length of the emitter element  18 . This is because they are at a potential which is negative with respect to the emitter  18  and therefore the direction of the electric field between the grid wires  20  and the emitter  18  tends to force emitted electrons back towards the emitter  18 . However the extracting pair  20   a ,  20   b , being at a positive potential with respect to the emitter  18 , attract the emitted electrons away from the emitter  18 , thereby producing a beam  40  of electrons which pass between the extracting wires  20   a ,  20   b  and proceed towards the anode  32 . Because of the spacing of the grid wires  20  from the emitter element  18 , electrons emitted from a length x of the emitter element  18 , which is considerably greater than the spacing between the two grid wires  20   a ,  20   b , are drawn together into the beam which passes between the pair of wires  20   a ,  20   b . The grid wires  20  therefore serve not only to extract the electrons but also to focus them together into the beam  40 . The length of the emitter  18  over which electrons will be extracted depends on the spacing of the grid wires  20  and on the difference in potential between the extracting pair  20   a ,  20   b  and the remaining grid wires  20 . 
     After passing between the two extracting grid wires  20   a ,  20   b , the beam  40  is attracted towards, and passes between the corresponding pair of focusing wires  22   a ,  22   b . The beam converges towards a focal line f 1  which is between the focusing wires  22  and the anode  32 , and then diverges again towards the anode  32 . The positive potential of the focus wires  22  can be varied to vary the position of the focal line f 1  thereby to vary the width of the beam when it hits the anode  32 . 
     Referring to  FIG. 4 , viewed in the longitudinal direction of the emitter  18  and anode  32 , the electron beam  40  again converges towards a focal line f 2  between the focus wires  22  and the anode  32 , the position of the focal line f 2  being mainly dependent on the field strength produced between the emitter  18  and anode  32 . 
     Referring back to  FIG. 2 , in order to produce a moving beam of electrons successive pairs of adjacent grid wires  20  can be connected to the extracting potential in rapid succession thereby to vary the position on the anode  32  at which X-rays will be produced. 
     The fact that the length x of the emitter  18  from which electrons are extracted is significantly greater than the spacing between the grid wires  20  has a number of advantages. For a given minimum beam spacing, that is distance between two adjacent positions of the electron beam, the length of emitter  18  from which electrons can be extracted for each beam is significantly greater than the minimum beam spacing. This is because each part of the emitter  18  can emit electrons which can be drawn into beams in a plurality of different positions. This allows the emitter  18  to be run at a relatively low temperature compared to a conventional source to provide an equivalent beam current. Alternatively, if the same temperature is used as in a conventional source, a beam current which is much larger, by a factor of up to seven, can be produced. Also the variations in source brightness over the length of the emitter  18  are smeared out, so that the resulting variation in strength of beams extracted from different parts of the emitter  18  is greatly reduced. 
     Referring to  FIG. 5 , an X-ray scanner  50  is set up in a conventional geometry and comprises an array of emitter units  25  arranged in an arc around a central scanner Z axis, and orientated so as to emit X-rays towards the scanner Z axis. A ring of sensors  52  is placed inside the emitters, directed inwards towards the scanner Z axis. The sensors  52  and emitter units  25  are offset from each other along the Z axis so that X-rays emitted from the emitter units pass by the sensors nearest to them, through the Z axis, and are detected by the sensors furthest from them. The scanner is controlled by a control system which operates a number of functions represented by functional blocks in  FIG. 5 . A system control block  54  controls, and receives data from, an image display unit  56 , an X-ray tube control block  58  and an image reconstruction block  60 . The X-ray tube control block  58  controls a focus control block  62  which controls the potentials of the focus wires  22  in each of the emitter units  25 , a grid control block  64  which controls the potential of the individual grid wires  20  in each emitter unit  25 , and a high voltage supply  68  which provides the power to the anode  32  of each of the emitter blocks and the power to the emitter elements  18 . The image reconstruction block  60  controls and receives data from a sensor control block  70  which in turn controls and receives data from the sensors  52 . 
     In operation, an object to be scanned is passed along the Z axis, and the X-ray beam is swept along each emitter unit in turn so as to rotate it around the object, and the X-rays passing through the object from each X-ray source position in each unit detected by the sensors  52 . Data from the sensors  52  for each X-ray source point in the scan is recorded as a respective data set. The data sets from each rotation of the X-ray source position can be analysed to produce an image of a plane through the object. The beam is rotated repeatedly as the object passes along the Z axis so as to build up a three dimensional tomographic image of the entire object. 
     Referring to  FIG. 6 , in a second embodiment of the invention the grid elements  120  and the focusing elements  122  are formed as flat strips. The elements  120 ,  122  are positioned as in the first embodiment, but plane of the strips lies perpendicular to the emitter element  118  and anode  132 , and parallel to the direction in which the emitter element  118  is arranged to emit electrons. An advantage of this arrangement is that ions  170  which are produced by the electron beam  140  hitting the anode  132  and emitted back towards the emitter are largely blocked by the elements  120 ,  122  before they reach the emitter. A small number of ions  172  which travel back directly along the path of the electron beam  140  will reach the emitter, but the total damage to the emitter due to reverse ion bombardment is substantially reduced. In some cases it may be sufficient for only the grid elements  120  or only the focusing elements  122  to be flat. 
     In the embodiment of  FIG. 6  the width of the strips  120 ,  122  is substantially equal to their distance apart, i.e. approximately 5 mm. However it will be appreciated that they could be substantially wider. 
     Referring to  FIG. 7 , in a third embodiment of the invention the grid elements  220  and the focusing elements  222  are more closely spaced than in the first embodiment. This enables groups of more than two of the grid elements  220   a ,  220   b ,  220   c , three in the example shown, can be switched to the extracting potential to form an extracting window in the extracting grid. In this case the width of the extracting window is approximately equal to the width of the group of three elements  220 . The spacing of the grid elements  220  from the emitter  218  is approximately equal to the width of the extracting window. The focusing elements are also connected to a positive potential by means of individual switches so that each of them can be connected to either the positive potential or a negative potential. The two focusing elements  222   a    222   b  best suited to focusing the beam of electrons are connected to the positive focusing potential. The remaining focusing elements  222  are connected to a negative potential. In this case as there is one focusing element  222   c  between the two required for focusing, that focusing element is also connected to the positive focusing potential. 
     Referring to  FIGS. 8 and 9 , an electron source according to a fourth embodiment of the invention comprises a number of emitter elements  318 , only one of which is shown, each formed from a tungsten metal strip which is heated by passing an electrical current through it. A region  318   a  at the centre of the strip is thoriated in order to reduce the work function for thermal emission of an electron from its surface. A suppressor  312  comprises a metallic block having a channel  313  extending along its under side  314  in which the emitter elements  318  are located. A row of apertures  315  are provided along the suppressor  312  each aligned with the thoriated region  318   a  of a respective one of the emitter elements  318 . A series of grid elements  320 , only one of which is shown, extend over the apertures  315  in the suppressor  312 , i.e. on the opposite side of the apertures  315  to the emitter elements  318 . Each of the grid elements  320  also has an aperture  321  through it which is aligned with the respective suppressor aperture  315  so that electrons leaving the emitter elements  318  can travel as a beam through the apertures  315 ,  320 . The emitter elements  318  are connected to electrical connectors  319  and the grid elements  320  are connected to electrical connectors  330 , the connectors  320 ,  330  projecting through a base member  324 , not shown in  FIG. 8 , to allow an electrical current to be passed through the emitter elements  318  and the potential of the grid elements  20  to be controlled. 
     In operation, due to the potential difference between the emitter elements  318  and the surrounding suppressor electrode  312 , which is typically less than 10V, electrons from the thoriated region  318   a  of the emitter elements  318  are extracted. Depending on the potential of the respective grid element  320  located above the suppressor  312 , which can be controlled individually, these electrons will either be extracted towards the grid element  320  or they will remain adjacent to the point of emission. 
     In the event that the grid element  320  is held at positive potential (e.g. +300V) with respect to the emitter element  318 , the extracted electrons will accelerate towards the grid element  318  and the majority will pass through a aperture  321  placed in the grid  320  above the aperture  315  in the suppressor  312 . This forms an electron beam that passes into the external field above the grid  320 . 
     When the grid element  320  is held at a negative potential (e.g. −300V) with respect to the emitter  318  the extracted electrons will be repelled from the grid and will remain adjacent to the point of emission. This cuts to zero any external electron emission from the source. 
     This electron source can be set up to form part of a scanner system similar to that shown in  FIG. 5 , with the potential of each of the grid elements  330  being controlled individually. This provides a scanner including a grid-controlled electron source where the effective source position of the source can be varied in space under electronic control in the same manner as described above with reference to  FIG. 5 . 
     Referring to  FIG. 10 , in the fifth embodiment of the invention an electron source is similar to that of  FIGS. 8 and 9  with corresponding parts indicated by the same reference numeral increased by 100. In this embodiment the emitter elements  318  are replaced by a single heated wire filament  418  placed within a suppressor box  412 . A series of grid elements  420  are used to determine the position of the effective source point for the external electron beam  440 . Due to the potential difference that is experienced along the length of the wire  318  because of the electric current being passed through it, the efficiency of electron extraction will vary with position. 
     To reduce these variations, it is possible to use a secondary oxide emitter  500  as shown in  FIG. 11 . This emitter  500  comprises a low work function emitter material  502  such as strontium-barium oxide coated onto an electrically conductive tube  504 , which is preferably of nickel. A tungsten wire  506  is coated with glass or ceramic particles  508  and then threaded through the tube  504 . When used in the source of  FIG. 10 , the nickel tube  504  is held at a suitable potential with respect to the suppressor  412  and a current passed through the tungsten wire  506 . As the wire  506  heats up, radiated thermal energy heats up the nickel tube  504 . This in turn heats the emitter material  502  which starts to emit electrons. In this case, the emitter potential is fixed with respect to the suppressor electrode  412  so ensuring uniform extraction efficiency along the length of the emitter  500 . Further, due to the good thermal conductivity of nickel, any variation in temperature of the tungsten wire  506 , for example caused by thickness variation during manufacture or by ageing processes, is averaged out resulting in more uniform electron extraction for all regions of the emitter  500 . 
     Referring to  FIG. 12 , in a sixth embodiment of the invention a grid controlled electron emitter comprises a small nickel block  600 , typically 10×3×3 mm, coated on one side  601  (e.g. 10×3 mm) by a low work function oxide material  602  such as strontium barium oxide. The nickel block  600  is held at a potential of, for example, between +60V and +300V with respect to the surrounding suppressor electrode  604  by mounting on an electrical feedthrough  606 . One or more tungsten wires  608  are fed through insulated holes  610  in the nickel block  600 . Typically, this is achieved by coating the tungsten wire with glass or ceramic particles  612  before passing it through the hole  610  in the nickel block  600 . A wire mesh  614  is electrically connected to the suppressor  604  and extends over the coated surface  601  of the nickel block  600  so that it establishes the same potential as the suppressor  604  above the surface  601 . 
     When a current is passed through the tungsten wire  608 , the wire heats and radiates thermal energy into the surrounding nickel block  600 . The nickel block  600  heats up so warming the oxide coating  602 . At around 900 centigrade, the oxide coating  602  becomes an effective electron emitter. 
     If, using the insulated feedthrough  606 , the nickel block  600  is held at a potential that is negative (e.g. −60V) with respect to the suppressor electrode  604 , electrons from the oxide  602  will be extracted through the wire mesh  614  which is integral with the suppressor  604  into the external vacuum. If the nickel block  600  is held at a potential which is positive (e.g. +60V) with respect to the suppressor electrode  604 , electron emission through the mesh  614  will be cut off. Since the electrical potentials of the nickel block  600  and tungsten wire  608  are insulated from each other by the insulating particles  612 , the tungsten wire  608  can be fixed at a potential typically close to that of the suppressor electrode  604 . 
     Using a plurality of oxide coated emitter blocks  600  with one or more tungsten wires  608  to heat the set of blocks  600 , it is possible to create a multiple emitter electron source in which each of the emitters can be turned on and off independently. This enables the electron source to be used in a scanner system, for example similar to that of  FIG. 5 . 
     Referring to  FIGS. 12   a ,  12   b  and  12   c , in a seventh embodiment of the invention, a multiple emitter source comprises an assembly of insulating alumina blocks  600   a ,  600   b ,  600   c  supporting a number of nickel emitter pads  603   a  which are each coated with oxide  602   a . The blocks comprise a long rectangular upper block  600   a , and a correspondingly shaped lower block  600   c  and two intermediate blocks  600   b  which are sandwiched between the upper and lower blocks and have a gap between them forming a channel  605   a  extending along the assembly. A tungsten heater coil  608   a  extends along the channel  605   a  over the whole length of the blocks  600   a ,  600   b ,  600   c . The nickel pads  603   a  are rectangular and extend across the upper surface  601   a  of the upper block  600   a  at intervals along its length. The nickel pads  603   a  are spaced apart so as to be electrically insulated from each other. 
     A suppressor  604   a  extends along the sides of the books  600   a ,  600   b ,  600   c  and supports a wire mesh  614   a  over the nickel emitter pads  603   a . The suppressor also supports a number of focusing wires  616   a  which are located just above the mesh  614   a  and extend across the source parallel to the nickel pads  603   a , each wire being located between two adjacent nickel pads  603   a . The focusing wires  616   a  and the mesh  614   a  are electrically connected to the suppressor  604   a  and are therefore at the same electrical potential. 
     As with the embodiment of  FIG. 12 , the heater coil  608   a  heats the emitter pads  603   a  such that the oxide layer can emit electrons. The pads  603   a  are held at a positive potential, for example of +60V, with respect to the suppressor  604   a , but are individually connected to a negative potential, for example of −60V, with respect to the suppressor  604   a  to cause them to emit. As can best be seen in  FIG. 12   a , when any one of the pads  603   a  is emitting electrons, these are focused into beam  607   a  by the two focusing wires  616   a  on either side of the pads  603   a . This is because the electric field lines between the emitter pads  603   a  and the anode are pinched inwards slightly where they pass between the focusing wires  616   a.    
     Referring to  FIG. 13 , in an eighth embodiment of the invention, an X-ray source  700  is arranged to produce X-rays from each of a series of X-ray source points  702 . These can be made up of one or more anodes and a number of electron sources according to any of the embodiments described above. The X-ray source points  702  can be turned on and off individually. A single X-ray detector  704  is provided, and the object  706  to be imaged is placed between the X-ray source and the detector. An image of the object  706  is then built up using Hadamard transforms as described below. 
     Referring to  FIGS. 14   a  to  14   c , the source points  702  are divided into groups of equal numbers of adjacent points  702 . For example in the grouping shown in  FIG. 14   a , each group consists of a single source point  702 . The source points  702  in alternate groups are then activated simultaneously, so that in the grouping of  FIG. 14   a  alternate source points  702   a  are activated, while each source point  702   b  between the activated source points  702   a  is not activated. This produces a square wave illumination pattern with a wavelength equal to the width of two source points  702   a ,  702   b . The amount of X-ray illumination measured by the detector  704  is recorded for this illumination pattern. Then another illumination pattern is used as shown in  FIG. 14   b  where each group of source points  702  comprises two adjacent source points, and alternate groups  702   c  are again activated, with the intervening groups  702   d  not being activated. This produces a square wave illumination pattern as shown in  FIG. 14   b  with a wavelength equal to the width of four of the source points  702 . The amount of X-ray illumination at the detector  704  is again recorded. This process is then repeated as shown in  FIG. 14   c  with groups of four source points  702 , and also with a large number of other group sizes. When all of the group sizes have been used and the respective measurements associated with the different square wave illumination wavelengths taken, the results can be used to reconstruct a full image profile of the 2D layer of the object  706  lying between the line of source points  702  and the detector  704  using Hadamard transforms. It is an advantage of this arrangement that, instead of the source points being activated individually, at any one time half of the source points  702  are activated and half are not. Therefore the signal to noise ratio of this method is significantly greater than in methods where the source points  702  are activated individually to scan along the source point array. 
     A Hadamard transform analysis can also be made using a single source on one side of the object and a linear array of detectors on the other side of the object. In this case, instead of activating the sources in groups of different sizes, the single source is continually activated and readings from the detectors are taken in groups of different sizes, corresponding to the groups of source points  702  described above. The analysis and reconstruction of the image of the object are similar to that used for the  FIG. 13  arrangement. 
     Referring to  FIG. 15 , in a modification to this arrangement the single detector of  FIG. 13  is replaced by a linear array of detectors  804  extending in a direction perpendicular to the linear array of source points  802 . The arrays of source points  802  and detectors  804  define a three dimensional volume  805  bounded by the lines  807  joining the source points  802   a    802   b  at the ends of the source point array to the detectors  804   a ,  804   b  at the ends of the detector array. This system is operated exactly as that in  FIG. 13 , except that for each square wave grouping of source points illuminated, the X-ray illumination at each of the detectors  804  is recorded. For each detector a two dimensional image of a layer of the object  806  within the volume  805  can be reconstructed, and the layers can then be combined to form a fully three dimensional image of the object  806 . 
     Referring to  FIGS. 16   a  and  16   b ,  17  and  18 , in a further embodiment, the emitter element  916  comprises an AlN emitter layer  917  with low work function emitters  918  formed on it and a heater layer  919  made up of Aluminium Nitride (AlN) substrate  920  and a Platinum (Pt) heater element  922 , connected via interconnecting pads  924 . Conducting springs  926  then connect the AlN substrate  920  to a circuit board  928 . Aluminium nitride (AlN) is a high thermal conductivity, strong, ceramic material and the thermal expansion coefficient of AlN is closely matched to that of platinum (Pt). These properties lead to the design of an integrated heater-electron emitter  916  as shown in  FIGS. 16   a  and  16   b  for use in X-ray tube applications. 
     Typically the Pt metal is formed into a track of 1-3 mm wide with a thickness of 10-100 microns to give a track resistance at room temperature in the range 5 to 50 ohms. By passing an electrical current through the track, the track will start to heat up and this thermal energy is dissipated directly into the AlN substrate. Due to the excellent thermal conductivity of AlN, the heating of the AlN is very uniform across the substrate, typically to within 10 to 20 degrees. Depending on the current flow and the ambient environment, stable substrate temperatures in excess of 1100 C can be achieved. Since both AlN and Pt are resistant to attack by oxygen, such temperatures can be achieved with the substrate in air. However, for X-ray tube applications, the substrate is typically heated in vacuum. 
     Referring to  FIG. 17 , heat reflectors  930  are located proximate to the heated side of the AlN substrate  920  to improve the heater efficiency, reducing the loss of heat through radiative heat transfer. In this embodiment, the heat shield  930  is formed from a mica sheet coated in a thin layer of gold. The addition of a titanium layer underneath the gold improves adhesion to the mica. 
     In order to generate electrons, a series of Pt strips  932  are deposited onto the AlN substrate  920  on the opposite side of the AlN substrate to the heater  922  with their ends extending round the sides of the substrate and ending in the underside of the substrate where they form the pads  924 . Typically these strips  932  will be deposited using Pt inks and subsequent thermal baking. The Pt strips  932  are then coated in a central region thereof with a thin layer of Sr;Ba;Ca carbonate mixture  918 . When the carbonate material is heated to temperatures typically in excess of 700 C, it will decompose into Sr:Ba:Ca oxides—low work function materials that are very efficient electron sources at temperatures of typically 700-900 C. 
     In order to generate an electron beam, the Pt strip  932  is connected to an electrical power source in order to source the beam current that is extracted from the Sr:Ba:Ca oxides into the vacuum. In this embodiment this is achieved by using an assembly such as that shown in  FIG. 17 . Here, a set of springs  926  provides electrical connection to the pads  924  and mechanical connection to the AlN substrate. Preferably these springs will be made of tungsten although molybdenum or other materials may be used. These springs  926  flex according to the thermal expansion of the electron emitter assembly  916 , providing a reliable interconnect method. 
     The bases of the springs are preferably located into thin walled tubes  934  with poor thermal conductivity but good electrical conductivity that provide electrical connection to an underlying ceramic circuit board  928 . Typically, this underlying circuit board  928  will provide vacuum feedthrus for the control/power signals that are individually controlled on an emitter-by-emitter basis. The circuit board is best made of a material with low outgassing properties such as alumina ceramic. 
     An alternative configuration inverts the thin walled tube  934  and spring assembly  926  such that the tube  934  runs at high temperature and the spring  926  at low temperature as shown in  FIG. 18 . This affords a greater choice of spring materials since creeping of the spring is reduced at lower temperatures. 
     It is advantageous in this design to use wraparound or through-hole Pt interconnects  924  on the AlN substrate  920  between the top emission surface and the bottom interconnect point  924  as shown in  FIGS. 16   a  and  16   b . Alternatively, a clip arrangement may be used to connect the electrical power source to the top surface of the AlN substrate. 
     It is clear that alternative assembly methods can be used including welded assemblies, high temperature soldered assemblies and other mechanical connections such as press-studs and loop springs. 
     AlN is a wide bandgap semiconductor material and a semiconductor injecting contact is formed between Pt and AlN. To reduce injected current that can occur at high operating temperatures, it is advantageous to convert the injecting contact to a blocking contact. This may be achieved, for example, by growing an aluminium oxide layer on the surface of the AlN substrate  920  prior to fabrication of the Pt metallisation. 
     Alternatively, a number of other materials may be used in place of Pt, such as tungsten or nickel. Typically, such metals may be sintered into the ceramic during its firing process to give a robust hybrid device. 
     In some cases, it is advantageous to coat the metal on the AlN substrate with a second metal such as Ni. This can help to extend lifetime of the oxide emitter or control the resistance of the heater, for example. 
     In a further embodiment the heater element  922  is formed on the back of the emitter block  917  so that the underside of the emitter block  917  of  FIG. 16   a  is as shown in  FIG. 16   b . The conductive pads  924  shown in  FIGS. 16   a  and  16   b  are then the same component, and provide the electrical contacts to the connector elements  926 .