Patent Application
20120045036
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 is arranged 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. No. 4,274,005 and U.S. Pat. No. 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 an emitter support block. An electron-emitting region may be formed on the support block and arranged to emit electrons. An electrical connector may be arranged to connect a source of electric current to the electron-emitting region. Heating means may be arranged to heat the support block.
The present invention further provides a control system for an X-ray scanner. The system may comprise an input arranged to receive an input signal identifying which of a plurality of electron emitters is to be active. The system may be arranged to produce a plurality of outputs each arranged to control operation of one of the emitters. In some embodiments each of the outputs can be in a first state arranged to activate its respective emitter, a second state arranged to de-activate said emitter, or a third state arranged to put said emitter into a floating state.
The present invention further provides a control system for an X-ray scanner, the system comprising an input arranged to receive an input signal identifying which of a plurality of electron emitters is to be active, and to produce a plurality of outputs each arranged to control operation of one of the emitters. The system may further comprise output monitoring means arranged to monitor each of the outputs, and the monitoring means may be arranged to generate a feedback signal indicating if any of the outputs exceeds a predetermined threshold.
The present invention further provides a control system for an X-ray scanner, the system comprising an input arranged to receive an input signal identifying which of a plurality of electron emitters is to be active, and to produce a plurality of outputs each arranged to control operation of one of the emitters, wherein each of the outputs can be in a first state arranged to activate its respective emitter, and a second state arranged to de-activate said emitter. The system may further comprise blanking means arranged to fix all of the outputs in the second state irrespective of which state the input signal indicates they should nominally be in.
Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:
a is top perspective view of an emitter element of the scanner of
b is a bottom perspective view of the emitter element of
a and 11b are top and bottom perspective views of an emitter element according to a further embodiment of the invention; and
Referring to
Referring to
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 replace the injecting contact with a blocking contact. This may be achieved, for example, by growing an aluminium oxide layer on the surface of the AlN substrate 120 prior to fabrication of the Pt metallisation. The provision of an oxide layer between the AlN and the Pt emitter forms a suitable blocking contact.
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.
To form the heater element 122 of this embodiment the Pt metal is formed into a track of 1-3 mm wide with a thickness of 10-200 microns to give a track resistance at room temperature in the range 5 to 200 ohms. It is advantageous to limit the heater voltage to below 100V to avoid electrical cross talk to the emitter pads 118 on the upper surface 120 of the substrate. 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.
The emitter pads 118, heater element 122, and connecting strips 123, are applied to the surface of the substrate block 117 in the required pattern by printing. The connector pads 124 are formed by applying several layers of ink by means of multiple printing so that they are thicker than the connecting strips 123. The connectors at the ends of the heater element 122 are built up in the same way. The substrate block 117 is then heated to around 1100 C to sinter the ink into the surface of the substrate block 117. The emitter pads 118 are then coated with a Ba:Sr:Ca carbonate material in the form of an emulsion with an organic binder. This coating can be applied using electrophoretic deposition or silk screen printing. When the emitter element 116 is installed, before it is used, the heater element 122 is used to heat the substrate block 117 to over 700 C, which causes the carbonate material firstly to eject the organic binder material, and then to convert from the carbonate to the oxide form. This process is known as activation. The most active material remaining in the emitter pad coating is then barium oxide, and electron emission densities in excess of 1 mA/mm2 can be achieved at operating temperatures of around 850-950 C.
Referring to
The emitter element 116 is connected to the circuit board 310 by means of sprung connection elements 316. These provide physical support of the emitter element over the circuit board 310, and also each connection element 316 provides electrical connection between a respective one of the connector pads 124 on the emitter element 116 and a respective connector on the circuit board 310. Each connection element 316 comprises an upper tube 318 connected at its upper end to the emitter element so that it is in electrical contact with one of the connector pads 124, and a lower tube 320 of smaller diameter, mounted on the circuit board 310 with its lower end in electrical contact with the relevant contact on the circuit board 310. The upper end of the lower tube 320 is slidingly received within the lower end of the upper tube 318, and a coil spring 322 acts between the two tubes to locate them resiliently relative to each other, and therefore to locate the emitter element 116 resiliently relative to the circuit board 310.
The connector elements 316 provide electrical connection to the connector pads 124, and hence to the emitter pads 118, and mechanical connection to, and support of, the AlN substrate. Preferably the springs 322 will be made of tungsten although molybdenum or other materials may be used. These springs 322 flex according to the thermal expansion of the electron emitter assembly 116, providing a reliable interconnect method. The grid 312 and focussing elements 324 are less affected by thermal expansion and therefore provide a fixed location. The top of the emitter element 116 is kept at a fixed distance from the grid 312 by spacers in the form of sapphire spheres 317. Hence the emitter pads 118 are held stationary by being clamped against the grid 312, via the sapphire spacers 317, during any thermal expansion or contraction of the emitter assembly 116. The potential of each of the emitter pads 118 can therefore be switched between an emitting potential, which is lower than that of the grid 312 such that electrons will be extracted from the emitter 118 towards the grid 312, and a blocking, or non-emitting, potential, which is higher than that of the grid, so that electrons will tend not to leave the surface of the emitter 118, or if they do, will be attracted back towards the emitter.
Referring also to
The focusing elements 314 extend one along each side of the emitter element 116. Each focusing element 314 is mounted on isolating mountings 323 so as to be electrically isolated from the grid 312 and the emitter element 116. It includes a flat lower portion 324 that extends parallel to, and spaced from, the side portions of the grid 312, and a curved portion 326 that extends upwards from the lower portion 324 beyond the grid upper portion, over in a curved cross section, and back towards the grid 312, with its inner edge 328 extending along the length of the emitter, spaced from the grid 312 and approximately level, in the lateral direction, with the edge of the emitter pads 118. This leaves a gap between the two focusing elements 314 that is approximately equal in width to the emitter pads 118 and the apertured areas of the grid 312. The focusing elements 314 are both held at an electric potential that is negative with respect to the grid 312, and this causes an electric field that focuses in the lateral direction the electrons extracted from the emitters. The focusing elements form a further, outer heat shield, spaced from the grid 312, which further reduces the radiation of heat away from the emitter elements 116.
Referring to
Referring back to
Referring to
Referring to
The processor 601 is arranged to receive a control signal CTRL as well as the data signal Din and a clock signal SCLK, and to output a number of signals that control operation of the shift registers, and other functions of the device 600.
One of the registers is a data register 602, in the form of 32 bit serial-in-parallel-out (SIPO) shift register, and is arranged to receive the serial input signal Din, which includes data indicating the required state of each of the emitters 118 for a particular cycle, to load that data under control of a signal ld_dat from the processor 601 and a clock signal SCLK. It is arranged to output the 32 required states to the inputs of a parallel-in-parallel-out data register 604, which loads them under control of a clock signal XCLK. The data register 604 presents the loaded data at its parallel outputs to one of three inputs to respective NAND gates 606. Assuming for now that the other inputs to the NAND gates 606 are all high, the outputs of each NAND gate 606 will be low if its respective emitter 118 is to be active, and high if it is to be inactive. The output from each NAND gate 606 is fed to one input of an exclusive-OR (EOR) gate 609, the other input of which is arranged to receive a polarity signal POL. The output of each EOR gate 609 is input to a respective output stage 500, each of which is as shown in
A tri-state register 608, in the form of a second 32 bit SIPO register, is arranged to receive the serial input signal Din which also includes data indicating which of the outputs should be set to the tri-state (or floating state) condition. This data is read from the input signal and loaded into the tri-state register 608 under the control of the signal ld_en and the clock signal SCLK. This data is then output in parallel to the respective output stages 500, with the output en being high if the output stage 500 is to be switched to the tri-state condition, and low if the output stage is to be set to the high or low level as determined by the output from the respective NAND gate 606. Referring back to
Each NAND gate 606 also has one input connected to a blanking signal BLA. Therefore if the blanking signal BLA is high, the output of the NAND gate 606 will be low regardless of the output from the data register 602. The blanking signal can therefore be used to set the outputs of any of the NAND gates to a blanked state, in which they are constant or at least independent of the input data, or an active state, in which they are controlled by the input data. A further chip select input CS is provided to all of the NAND gates and can be used to activate or de-activate the whole control chip 600.
Each HV output HVout is input to a respective comparator 612 which is arranged to compare it to a threshold signal VREF, and produce a feedback output indicative of whether the output drive signal is above or below the threshold. This feedback data, for all 32 output signals, is input to a parallel-in-serial-out feedback register 614, under control of a signal rd_fb from the processor 601, and the feedback register 614 converts it to a serial feedback output 616. This output 616 therefore indicates if any of the outputs is supplying excessive current, which can be used as an indication of, for example, a short circuit problem. The level of the reference signal VREF is set by the processor 601.
A serial output 618 is also provided from the data register 602 which is indicative of whether each of the output signals is nominally at the high or low level. These two serial outputs are multiplexed by a multiplexer 620, under the control of a multiplexing control signal mux from the processor 601, to produce a single serial digital output signal Dout. This allows the expected output values to be checked from 618, for example to check the programming of the device, and the actual values to be checked from 616 to check that the correct outputs have actually been achieved.
The control device 600 is arranged to operate in three different modes: a sequential access mode, a random access mode, and a non-scanning or reset mode. In the sequential access mode, the X-ray beam is scanned around the X-ray sources sequentially. Therefore in each full scan of all emitters, each control device will be active for a single period within the scan, and during that period, will activate each of the emitters it controls in sequence for respective activation periods. In the random access mode, the X-ray source is moved around the X-ray source array in a pseudo-random manner. Therefore, in each scan of all emitters, each control device will activate one of its emitters for one activation period, and then will be inactive for a number of activation periods while emitters controlled by other devices 600 are active, and will then be active again for a further activation period when another of its emitters is active. Some of the control inputs for the sequential access mode are shown in
Referring to
The format of the data input signal Din is a 5-byte programming pattern having the following format:
The control word has a bit configuration such as:
Therefore one 5-byte input signal is required for each emitter activation period, and the signal indicates by means of the four data/status bytes which emitter is to be active, and by means of the control byte which mode the system is in. The emitter control block 64 sends a serial data input signal to a control device 600 for each of the emitter units 25 of the scanner so as to coordinate operation of all of the emitters in the scanner.
In this embodiment as shown in
In operation, an object to be scanned is passed along the Z axis, and the X-ray beam is generated by controlling the emitter pad potentials so that electrons from each of the emitter pads 118 in turn are directed at respective target positions on the anode 311 in turn, and the X-rays passing through the object from each X-ray source position in each unit detected by the sensors 52. As described above, for some applications the beam is arranged to scan along the emitter in discrete steps, and for some it is arranged to switch between the emitter pads 118 in a pseudo-random manner to spread the thermal load on the emitter. Data from the sensors 52 for each X-ray source point in the scan is recorded as a respective data set. The data set from each scan of the X-ray source position can be analysed to produce an image of a plane through the object. The beam is scanned repeatedly as the object passes along the Z axis so as to build up a three dimensional tomographic image of the entire object.
In an alternative embodiment the connector elements 316 of
As an alternative to the wraparound interconnects 124 of the embodiment of
It will be appreciated 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.
Referring to
Referring to
On the opposite side 818 of the substrate from the emitter pads, a heating element in the form of a continuous conductive film 820 is applied, which in this case covers the whole of the rear side 818 of the emitter element 810. The heating element is also formed by means of sputter coating, and at each end of the emitter element, the conductive film is made thicker, by further sputter coating, to form contact areas 822, 824. Clearly since the substrate is electrically non-conducting, the heating element 820 is electrically isolated from the emitter pads, which in turn are electrically isolated from each other.
Referring to
The side elements 832, 834 and the cross members 836, 838 are formed from silica plates, which are formed into interlocking shapes by laser cutting. These plates therefore interlock to form a stable mechanical structure. The silica material is coated on one side, the side facing the emitter element 810, with a high reflectance low emissivity material, such as Au or Ti. Alternatively the silica may be coated with a multi-layer infra-red mirror.
A series of connecting wires 842 each have one end connected to a respective one of the emitter pads 814, and extend around the outside of the heat shield structure 830, having their other ends connected to respective connectors on the circuit card 840. The interconnecting circuit card 840 is used to transfer signals from outside the vacuum envelope of the scanner, either directly through a hermetic seal or indirectly through a metal contact which engages with a hermetic electrical feedthru.
A grid 844, similar to that of
As with the embodiment of
| Number | Date | Country | Kind |
|---|---|---|---|
| 0901338.4 | Jan 2009 | GB | national |
The present application is a national stage application of PCT Application Number PCT/GB2010/050125, which was filed on Jan. 27, 2010, and relies on Great Britain Patent Application No. 0901338.4 filed on Jan. 28, 2009.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2010/050125 | 1/27/2010 | WO | 00 | 11/10/2011 |
