EP0059700B1 - Enregistrement électronique de rayons X - Google Patents

Enregistrement électronique de rayons X Download PDF

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Publication number
EP0059700B1
EP0059700B1 EP82870008A EP82870008A EP0059700B1 EP 0059700 B1 EP0059700 B1 EP 0059700B1 EP 82870008 A EP82870008 A EP 82870008A EP 82870008 A EP82870008 A EP 82870008A EP 0059700 B1 EP0059700 B1 EP 0059700B1
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Prior art keywords
ray
sheet
gas
insulating
potential
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German (de)
English (en)
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EP0059700A2 (fr
EP0059700A3 (en
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Marvin B. Bacaner
John H. Broadhurst
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J47/00—Tubes for determining the presence, intensity, density or energy of radiation or particles
    • H01J47/02—Ionisation chambers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J47/00—Tubes for determining the presence, intensity, density or energy of radiation or particles
    • H01J47/06—Proportional counter tubes
    • H01J47/062—Multiwire proportional counter tubes

Definitions

  • a shadowgram is defined as a two-dimensional spatial picture of the shadow, i.e., the absorption of x-rays by some object placed between an x-ray source and the recording system.
  • ionization detectors are very sensitive at x-ray energy levels.
  • the energy of an x-ray photon is the same as the energy required just to eject one of the electrons from an atom of the gas, then the gas becomes strongly absorbing, i.e., the x-ray photon energy is equal to an absorption edge energy of the gas.
  • Photons having an energy below the absorption edge of the gas are absorbed weakly and, therefore, are not detected.
  • Photons having an energy above the absorption edge of the gas emit secondary photons, as well as electrons, upon collision with a gas atom.
  • the secondary photon may itself be absorbed and cause electron emission, possibly from a different level. Electrons produced by secondary photons degrade x-ray images, and specialized techniques must be used to reduce their effect.
  • Ionization chamber detectors based on multi-wire proportional counters have also been described for gamma ray and nuclear particle detection and have been used in other applications as x-ray detectors. These latter detectors, however, if used to obtain x-ray shadowgrams, would require x-ray exposure periods on the order of fractions of a minute because the electronic read-out time is substantial.
  • Over-exposure of the patient can be avoided by reducing the intensity of the x-ray flux, but practical limits on the time a patient may remain free of voluntary and involuntary movement dictate that the exposure time be only a few seconds at most. Ideally, exposure time would be significantly less than one second so that heart movement could be frozen.
  • the present invention provides an apparatus which will permit the formation of x-ray shadowgram images of a quality comparable to that of photographic or fluoroscopic techniques while, at the same time, reducing patient exposure to harmful high energy x-ray radiation.
  • the method is a technique for optimizing the efficiency of position sensitive ionization detectors. Ionization detector structures which make routine diagnostic shadowgrams practical by acquiring image information with an exposure of only fractions of a second are also part of the present invention.
  • Ionization detectors employing high electric field gradients such as those employing a fine wire positive electrode, have the advantage that an electron released from a gas atom by an x-ray photon, once in the vicinity of the wire electrode, is strongly accelerated.
  • This high energy electron collides with other gas atoms and ionizes them, thereby creating a shower of electrons and positively charged gas ions.
  • the path of the electrons ends at the wire, whereas the positive ions return to the negative electrode.
  • This charge multiplication characteristic makes it much easier to detect single x-ray photons and thereby offers the potential of providing the same information as that of conventional photographic or fluoroscopic techniques while maintaining patient exposure at a much lower level.
  • One aspect of the present invention is maximizing the efficiency of the x-ray ionization detectors by matching the detector absorbing gas and x-ray source so that the energy level of the x-rays emitted by the source is just above the absorption edge of the gas. This keeps patient exposure to a minimum while achieving optimal detector efficiency. Furthermore, the problem of secondary photon emission causing false images is minimized, any secondary photons emitted having too low an energy to be strongly absorbed by the detector gas.
  • This method of matching the detector gas and the x-ray source in addition to lowering patient dosage by maximizing the efficiency of the detector, also permits lower patient exposures by use of lower energy x-rays for images of the patient's thinner extremities where body absorption of lower energy radiation is insufficient to seriously reduce the radiation flux reaching the detector.
  • a variation of the invention involves providing two or more matched gas/source combinations so that successive images obtained from x-rays of different energy levels can be compared to enhance contrast, as for instance, between bone and soft tissue. It is also possible to differentiate by this means between bone and either dye or calcium deposits.
  • subtractive techniques By employing subtractive techniques on images from two energy levels, it is possible to produce a deep body image of soft tissue in which the interfering bone structure can be significantly suppressed, if not eliminated.
  • Of particular utility would be the production of images of the thorax (heart, lungs and medial stinum) with minimal obstruction of ribs and vertabrae. This type of image could reduce the need of multiple x-ray views (i.e., lateral and oblique) when trying to detect soft tissue abnormalities.
  • a first detector has a multi-wire two-dimensional proportional counter which utilizes simultaneous read-out and storage - of one-dimensional x-ray collision information from each wire followed by processing of the information from all wires into a two-dimensional image.
  • This parallel read-out technique substantially decreases the amount of time necessary to obtain detailed image information. For instance, for an image of 256x256 dots, the acquisition time is approximately 0.125 seconds.
  • detectors are electro-mechanical ionization chamber detectors which create an ion density distribution, coresponding to the x-ray shadow image, on an insulating sheet and then electronically read the image after the x-ray ex- . posure has ceased.
  • the second is a simple variation of the first, having two insulating sheets and two read-out structures so that contrast information may be obtained by taking sequential images at different photon energies.
  • the insulating sheet is provided in belt form. The belt is driven past the read-out structures to simplify mechanization of the read-out operation.
  • Some embodiments of the electro-mechanical detector are portable cassettes which are free of external encumbrances or electrical connections.
  • the read-out means may be external to and separate from the cassette so that multiple cassettes may be employed with a single read-out machine. Such a cassette may be exposed in much the same manner as x-ray film cassettes currently used.
  • the present invention includes the method of minimizing patient x-ray exposure when producing a two-dimensional shadowgram image by detecting the image in an ionization gas chamber detector and matching the x-ray source energy with the ionization gas absorption edge so that the characteristic x-ray energy level is just above an absorption edge of the ionization gas.
  • An example of such a combination is a Praesodymium anode and Xenon detector gas.
  • the characteristic x-rays produced by the Praesodymium anode have an energy level just above the k absorption edge of the Xenon gas.
  • Krypton gas with a Yttrium anode x-ray tube is another example of such a gas/anode combination. Again, the Yttrium anode produces characteristic k x-ray emissions with energies just above the k x-ray absorption edge of the Krypton detector gas.
  • the invention need not be limited to matched combinations of x-ray sources and elemental gases. Molecular gases may also be useful. Neutron counting using Boron trifluoride is an established technique, as is the use of hydrocarbons, such as isobutane. However, no such literature exists for gaseous compounds of high Z materials. Of particular interest would be a gaseous compound of Erbium which would be the resonant material for Tungsten characteristic x-ray emission (Tungsten is the standard x-ray tube target material). Such a compound must not disassociate too easily with x-ray radiation and, more importantly, it should not form negative ions which would trap the initial electron and prevent multiplication (i.e., act as a quenching gas). Prospective gases can be tested for these characteristics by observation of their behavior in a proportional counter.
  • a second anode source is matched with a different cathode gas or different absorption edge of the same detector gas.
  • a Praesodymium anode/Xenon combination to produce one image
  • a Yttrium anode/ Krypton combination for a second image allows calcium, for which the ratio of absorption at the two energies is about 1:80, to be distinguished from carbon, for which the absorption ratio is 1:13. This enables image subtraction to be used for enhancing or suppressing bone tissue (calcium) relative to the soft tissue image (carbon).
  • This technique can also be used to enhance the identification of the cause of abnormal density changes in the soft tissue.
  • the aggregation of calcium in cancerous cell colonies will enable their direct differentiation from similar density benign cell tumors. It has been reported that the nuclei of breast cancer cells have such a high calcium concentration. It may be possible, therefore, to identify aggregates of breast cancer cells which are not visible by conventional mammography or palpation.
  • detection efficiency increases with increased concentration of gas atoms, it is not necessary to change detector gases between exposures when employing the method of comparing images obtained from different energy levels. If one is willing to operate the detector at higher pressure or to sacrifice some detector efficiency, mixtures of ionizing gases may be used. Alternatively, two absorption edges of a single gas, e.g. the k and I edges of Xenon, may be used in some applications.
  • the respective concentrations can be adjusted so that their absorption is approximately equal at the two x-ray energies of the target gases.
  • the Krypton concentration need only be about 10% of the total gas concentration.
  • a x-ray tube would be provided with a turret target having a mechanical rotation device to facilitate rapid charging of x-ray sources.
  • Other means for changing energies may also be employed including demountable tubes or a plurality of different x-ray tubes which may be successively directed at the subject.
  • each of these detectors permits an electronic image to be obtained while exposing the patient to the x-ray source for a minimal length of time.
  • Two-dimensional x-ray detectors of the proportional counter type have, in the past, used either a continuous position sensing element (for example, a long wire) or a two-dimensional matrix.
  • Such counters are economical to realize, but have the basic defect that only one measurement can be made within the counter resolving time over the whole area of the counter. This restriction makes its application to medical radiography impractical as the needed exposure times to produce an x-ray image are unreasonably long.
  • FIG. 1 A prior art two-dimensional type matrix type counter is shown in Figure 1.
  • An ionization chamber is constructed using a frame of insulating material 10 across which is strung a set of closely spaced fine wires 12. This chamber is closed and made gas tight by two plates 14 and 16 made of insulating material and carrying on their inner side a closely spaced array of conducting strips 18. One of these plates, 16, is made thin in order to permit x-rays (originating from a source shown generally as box 17) to enter the chamber.
  • this counter is filled with gas, usually at high pressure, and a positive potential applied to wires 12.
  • Incoming x-rays cause a sequence of events to occur, as is shown in Figures 2 and 3.
  • Incoming x-ray g interacts at point x with an atom of the counter gas filling, releasing a free electron e and possibly a further photon h of reduced energy.
  • the free electron is attracted to the nearest wire 12a by the positive potential applied and on its way, in the vicinity of the wire, creates a shower of further electrons e' by collision ionization processes.
  • the electrical outcome of this process is a pulse of current on wire 12a and on the metal strip 18 opposite the location of the event. These two pulses provide the electronic information of the X, Y location of the x-ray event, and can then be processed by standard digital electronics.
  • Figure 4 shows the multi-wire detector used to record x-ray information in this application.
  • a set of fine (25 um) conductors 22 are strung at 1 mm intervals. These spacings are illustrated and not fundamental to the design.
  • the "wire” is made of a highly resistive material, namely, a carbon film deposited on a quartz core. Electrically completing the counter are two sheets of aluminized polyester film marketed by DuPont under the trade mark "Mylar" 24, 26 which are assembled with their conducting sides toward the wires and mechanical end plates 28 and 30 which render the chamber gas-tight.
  • Plate 28 is made thin to allow the unrestricted entry of x-rays, while plate 30 is thick to absorb x-rays which have failed to interact in the counter.
  • Holes 32 admit gas to the counter, in this case Xenon, at a pressure of 3 atmospheres together with a 1 percent addition of chlorinated and fluorinated hydrocarbon known under the trade mark "Freon 13B1".
  • this chamber is identical to that of the prior art in that an incoming x-ray produces an electron e which flows at a given location to the nearest wire, being repelled from the Mylar sheets 24, 26 by virtue of a negative potential placed on them.
  • the electron close to the wire 22a the electron generates a cloud of electrons e' and ions due to collision ionization. This process is effective instantaneously, taking 10-11 seconds or less, and results electrically in a charge being placed instantaneously at a given point on the wires 22a.
  • This charge is not, however, able to flow away instantaneously to the wire ends, as the wire resistance, together with the capacitance between the wire and the aluminized Mylar plates, forms a distributed RC time constant.
  • the RC product will be proportional to the distance of the event from each end of the wire. Determination of those time constants will, therefore, determine one coordinate of the event, the other being determined by the physical location of the wire.
  • the data acquisition rate is now determined by the time to process N events simultaneously, where N is the number of wires in the counter (in the first realization 256). Assuming each position on the wire must process an average of 500 events and, for symmetry, the wire is divided into 256 distinct cells, then the acquisition time is 500x256xp, where p is the event processing time. In this realization, p is in the order of one microsecond, making the acquistion time for an x-ray image approximately 0.125 seconds.
  • the latched information from T I T 2 is then processed by normal digital techniques (not shown). Associated with each wire is memory of capacity 256 words of 12 bits. Each STORE request results in a read, add one, replace (RAO) cycle being executed at the address determined by the latched values of A o through A 7 .
  • This memory which totals 64 K words is also organized on normal data bus techniques to be part of the memory space of the associated computer and also to refresh the memory of the video display. It should be noted that each wire of the counter corresponds to one line of information of the video display, hence a minimal processing is needed to initially display the acquired image.
  • the total photon flux into the counter must be 2.56 ⁇ 10 9 photons per second for 100 percent counter efficiency.
  • the required flux is 5.12 ⁇ 10 10 photons per second. The x-ray tube intensity requirements are, therefore, minimal.
  • a conventional pierce electron gun system using an air cooled anode may be employed.
  • the preferred anode materials are, however, unique being a pellet of Praesodymium or Yttrium imbedded in a carbon block. These materials are chosen as they produce characteristic k x-rays of energy just above the required k absorption edge energy of Xenon and Krypton, respectively, the preferred chamber gas fillings. Usual filtering is employed to minimize the white background radiation.
  • the multi-wire detector may be used to obtain contrasting images by making successive exposures with different source/detector gas combinations. To do so, however, will normally require doubling of the initial memory capability because the second exposure will be most advantageously made before the data from the initial exposure can be processed into a composite image.
  • the ionization chamber used in this embodiment is shown in Figure 7. It consists of a frame 40 supporting taut closely strung wires 42. Wires 42 are typically 2 mil gold plated Tungsten strung at 10 mil intervals. On one side of this frame is a sheet of metal backed insulating material 44 which is typically copper-backed Mylar. Sheet 44 is separated from the wires by a small space, typically 1,26 cm. The whole assembly is mounted in a gas-tight container comprising rigid back plate 50, thin window 52 and gas retaining enclosing walls (not shown). The wire frame/ insulating sheet assembly must remain in mechanical alignment without distortion during pressurization of the chamber and, for this reason, this assembly is not attached rigidly to the container.
  • Bar 56 mounted on guides 58 and 60, as shown in Figure 8. Bar 56 may be mechanically moved along guides 58 and 60 by means of drive cable 62. Although not shown in Figure 7, guides 58 and 60 and drive cable 62 extend upwardly between the wire frame 40 and plate 44. The thickness of bar 56 is such that it will pass between the wires and the insulating plate 44.
  • the ionization chamber is filled with a suitable gas (typically Xenon) under pressure, to which may be added a stabilizing agent, such as Freon 13B1 (the combination being known in the literature as "magic gas").
  • a suitable gas typically Xenon
  • a stabilizing agent such as Freon 13B1 (the combination being known in the literature as "magic gas”).
  • the choice of front window thickness and gas pressure is chosen in order to minimize the fraction of x-ray photons absorbed in the window and to maximize the fraction absorbed in the Xenon gas filling.
  • a high positive potential typically plus 2000 V
  • the wires are not insulated from each other and are, therefore, energized together.
  • ions and electrons are formed as previously described.
  • the electrons are attracted to the wire, while the positive gas ions (Xenon) are attracted to the plate.
  • Xenon positive gas ions
  • the positive gas ions are repelled from the wire to the plate where they come to rest on the surface of the insulating layer of plate 44. Due to the insulating properties of this layer, these ions are immobilized at their landing site and, therefore, remain as a density distribution which reflects the x-ray intensity distribution, i.e., the x-ray image desired.
  • each plate 66 is about 0,043 cm 2 and is held by bar 56 about 0,025 cm from the insulator surface of plate 44.
  • Insulated plates 66 are each connected on the interior side, that is, the side remote from the insulating surface of sheet 44, to a current to voltage amplifier.
  • the interior sides of plates 66 form a set of contacts which connect to a group of current to voltage amplifiers which are guided down the bar. In principle, and in fact, this operation is similar to the beam and carriage operation of an X-Y recorder. The output of these amplifiers is transferred by flexible leads to connections passing through the pressure vessel.
  • a drive motor (not shown), associated with drive cable 62, causes the bar to traverse the insulator surface.
  • a displacement transducer (typically a linear potentiometer) is mounted along side bar guide 60.
  • the function of this device is to establish a uniform charge on the insulating surface of sheet 44 prior to x-ray exposure.
  • the high potential on wires 42 is removed and bar 56 is traversed at constant speed over the insulating surface.
  • the sensing capacitance plates 66 scan the surface of sheet 44, displacement currents flow as the charge shares between the capacitance formed by the sheet 44 surface to its metallic backing and the capacitance formed by the sheet 44 surface to the sensing electrode or plates 66. These currents, which are the differential of the charge pattern of the plate, are converted to potentials and led out of the chamber.
  • each pick-up plate 66 has its own amplifier, and an on-bar multiplexer enables all line images to be read out in one pass.
  • a group of amplifiers is mechanically switched to a group of pick-up plates, and several passes are made over the insulating surface. These details, however, merely affect the speed of the read-out.
  • a high potential is applied from a constant current supply to the sharp-edged electrode 74, establishing a steady corona discharge between it and the surrounding tube 70.
  • This discharge causes a conducting plasma to protrude from the tube slit 78 and contact the insulating surface of sheet 44. If this discharge is now traversed over the insulating layer of sheet 44 by moving bar 66, ions or electrons will land on it preferentially until the insulator surface is at plasma potential. In this manner, a uniform nearly zero charge is deposited on the insulator before commencing the next x-ray exposure.
  • the insulating sheet 44 may not be discharged evenly, and the gains and offsets of the amplifiers for the individual plates 66 may not be matched. This is overcome by reading first a discharged surface and then a surface after a uniform x-ray exposure. The value so obtained need not be retained by the associated computer and memory, but can be written out onto mass storage for later use as normalizing parameters. In this matter, systematic noise can be removed by renormalization of the data after recording.
  • the correct "exposure” can be obtained by integrating the displacement current to the metallic backing of the sheet 44 during the time the x-ray tube is emitting. This feature can be used to automatically control the x-ray flux to ensure an acceptable image with a minimal patient dose.
  • the reading speed limit is probably in the order of one second, assuming a typical slew rate for the bar of 20 inches per second. This would involve a 2.5 microsecond conversion rate and a current to voltage amplifier band width of about 2000 Hz, both of which are easily within the reach of modest electronics.
  • the chamber of this embodiment may be constructed using two insulating sheet read-out bar assemblies.
  • the second metallic backed insulating sheet 80 shown in Figure 10 is placed between the chamber window 52' and the wire frame 40'.
  • a second read-out beam and guide bar assembly as shown in Figure 8, is placed between sheet 80 and frame 40'.
  • sheet 44' is grounded, while sheet 80 is maintained at a potential of the wires 42'.
  • the positive ions formed in the vicinity of wires 42' when the x-rays enter the chamber, are attracted to sheet 44' and repelled from sheet 80.
  • the potential difference between the metallized surfaces of sheets 44' and 80° is reversed. That is, the metallic surface of sheet 80 is grounded, whereas the metallic surface of sheet 44' is maintained at the wire potential.
  • This double insulating sheet/read-out bar structure permits recording of two x-ray images, closely spaced in time, for subsequent processing for contrast information as previously described.
  • Portable cassette form of electro-mechanical x-ray imaging chamber
  • the portable cassette described in this section is an alternate embodiment of the electro-mechanical image chamber just described.
  • This embodiment is a transportable cassette which can replace film x-rays almost completely as the recording device has no electrical connections or other encumbrances. It may be extended to dental and industrial applications.
  • the image recording chamber is separate from the image reading machines, thus permitting a reading machine to be shared over multiple cassettes.
  • the basic construction of the cassette is shown in Figure 11. It consists of an enclosed gas-tight metal box having a metal front window 84, a frame 86 comprising side walls of the chamber, a grid or harp of fine wires 88 carried on the frame and an insulating rear window 90 transparent to ultraviolet light.
  • the wire grid is insulated from the box and provided for the connection to the outside.
  • a metallic back cover 92 is secured to the back of the box in order to provide protection for the rear window and to insure that the box is completely electrically conducting on the outer surface.
  • the gas in this chamber is retained by the transparent window, not the back cover.
  • a high quality capacitor 94 (polystyrene or similar dielectric), shown schematically in Figure 12, is connected between the wire grid and the outer ground potential surface of the box.
  • the back cover is attached by any suitable means, and a high voltage power supply is connected through a high impedance, such as 1 Megohm resistor 95 to the wire grid, and the high quality capacitor charged.
  • a high impedance such as 1 Megohm resistor 95 to the wire grid, and the high quality capacitor charged.
  • the high impedance provides both safety for the operator and prevents large currents from flowing.
  • the power supply is then disconnected.
  • the cassette is then ready for use.
  • a polystyrene capacitor has a leakage time of approximately 50 years, so the wire grid will remain charged to operating potential for an extended period.
  • the cassette is exposed in the same manner as a film x-ray cassette, i.e., placed behind the subject being x-rayed.
  • the electrical function is the same as described previously, the x-rays producing electrons, multiplication taking place near to the wires and positively charged ions accumulating on the inside surface of the transparent insulating sheet 90 where they are immobilized.
  • the front window 84 is maintained at the same potential, i.e., ground potential, as the insulating sheet 90, it is preferred that the wire grid be as close to window 84 as possible without causing arcing between the grid and window 84.
  • the potential on the grid typically will be on the order of 2 kilovolts. Moving the wire grid closer to front window 84 than to rear window 90 results in most photon-gas atom collisions occurring behind the wire mesh so that the positive ions resulting therefrom migrate toward insulating sheet 90, rather than toward metal front window 84.
  • the cassette is read out by removing the back cover 92 and placing the cassette with the exposed insulating plate over read-out bar equipment similar to that shown in Figure 8.
  • the read-out equipment contains a movable read out bar 56' essentially identical to bar 56 described in Figure 8. However, this equipment does not have the corona discharge tube 70, shown in Figure 8. Bar 56' traverses in close proximity to the outer surface of the insulating sheet and reads the displacement currents flowing to a row of sensors in a manner similar to that described for the electro-mechanical chamber.
  • the high voltage on the wire grid is removed by discharging the storage capacitor through the high impedance, and ultraviolet light is projected through the transparent insulating window 90.
  • This light produces photoelectrons which are attracted to the positively charged ions immobilized by the inner surface of window 90 and continues to neutralize them until the inner surface is uniformly charged slightly negative, thus preventing further electrons from landing.
  • a suitable material for the insulating is quartz, as this is both of high resistivity and transparent to ultraviolet light. However, materials satisfying both criteria may also be used. Since the read-out equipment must sense the capacitance differentials through the thickness of window 90, this window must be maintained at a minimum thickness, preferably about 10 mils or less.
  • the wire harp is preferably supported from the side of the enclosure by polystyrene insulators, not shown, as it is necessary to minimize leakage.
  • capacitor C, and resistor R" shown in Figure 12 shows these components on the exterior of the cassette chamber, it is contemplated that, in practice, these elements will be imbedded in the frame 86 of the cassette.
  • a recessed access port 97 can be used to provide means for making the necessary electrical connection to charge and discharge the capacitor.
  • Another access port 98 can be used to flush and fill the chamber with ionizing gas.
  • Figure 13 shows a top cut away view of another embodiment of the detector which may be more easily read out than the detectors shown in Figures 7, 10 and 11.
  • the transparent insulating window is formed into a continuous belt 90' supported by rollers 98 and 100 at both sides of the window in frame 86.
  • Belt 90' is on opposite sides of and surrounds the grid or harp of wires 88.
  • the outside surface of belt 90' is divided into two conductive portions positioned with one on each side of grid 88 when the belt is positioned to receive an x-ray shadowgram.
  • Rollers 98 and 100 have a conductive rubber surface which discharges any portion of the insulating surface of belt 90' brought into direct contact therewith.
  • the improved embodiment of Figure 13 is particularly suited to take two images in close time sequence. This feature is important when it is desired to take identical views of a subject using different intensities to differentiate between bone and dye or calcium.
  • the wire grid 88 is insulated from the remainder of the box and may be held at high potential by being connected to one of the conductive portions of belt 90' with a high quality capacitor 94 which has been charged through a high impedance resistor 95.
  • the other conductive portion is connected to the box reference potential to minimize interference with the ions in the detector.
  • the detector is then exposed by being placed behind the subject being x-rayed, and the inside dielectric surface of the lower portion of belt 90' receives the stored charge in accordance with the received radiation intensity to define an image.
  • a second image can be placed on the outer insulating surface of the belt by establishing the grid potential with respect to the other conductive backing and exposing the detector to the x-ray source.
  • the read-out of the image is accomplished by rotating the drums 98 and 100 in a clockwise manner, as viewed in Figure 14, to move the inner surfaces of the dielectric sheet 90' past the read-out heads 56A" and 56B" which are located in the vicinity of the roller 98.
  • the read-outs 56A” and 56B" are the equivalent of the read-outs 56 and 56' in Figures 8 and 12, but differ from them in that they remain fixed in a stationary manner relative to frame 86 during the entire read-out process, while the dielectric surfaces of belt 90' are transported past them by rotation of rollers 98 and 100.
  • the surface of the roller returns the dielectric surface to a nearly uncharged zero potential condition so that further images can be applied to the film as soon as it is read out.
  • both the top and botton inner surfaces of the dielectric belt 90' can be exposed at about the same time by selectively energizing the appropriate back conductor of the dielectric as desired before the read-out is accomplished.
  • the two inner surfaces of the belt are read out simultaneously by the detector arrays 56A" and 568" which are mounted on frame 86 in proximity with the inner face of belt 90'.
  • the two images are read and the belt is erased by drums 98 and 100 in a coordinated one-step operation as the information is stored in the storage means for further processing or use.
  • the improved embodiment of Figure 13 utilizes a secondary grid 96' which is deposited directly on the inner dielectric surface of belt 90'.
  • Figure 14 shows, in amplified detail, the surface of a portion of belt 90' with the conductive elements 96 of the secondary grid directly deposited on the inner surface of belt 90' on the opposite side of the belt from the conductive back portion 91.
  • the potential between the conductive grid 88 and the conductive back plate 91 of belt 90' is approximately 2000 volts with a potential maintained between the secondary grid and the back plate of approximately 100 volts.
  • the secondary grid 96 produces a potential distribution indicated by dotted lines 101 in Figure 14, thus preventing a charge build-up on the dielectric layer from repelling incoming charged particles and deflecting them off their ballistic path.
  • a charged particle is shown on its trajectory 97 in Figure 14.
  • a deflection due to charge build-up would result in an undesirable defocussing of the image.
  • the secondary grid also acts to provide a drain for excess charge build-up on the dielectric surface in the event of high intensity flow of charged particles. Excessive charge build-up can cause blooming of the x-ray image and a resultant loss of detail.
  • the net effect on applying the secondary grid 96 to the surface of the dielectric medium is to linearize the exposure versus charge relationship until the surface potential is equal to the voltage maintained on the secondary grid, at which point the charge versus exposure characteristic becomes flattened with virtually no further increase in the charge deposited on the dielectric after this threshold is reached.
  • the improved linearity and sharp cut-off of the exposure characteristics avoids the need of taking precautions to avoid large density differences between the object to be x-rayed and the background.

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  • Apparatus For Radiation Diagnosis (AREA)
  • Measurement Of Radiation (AREA)
  • Photoreceptors In Electrophotography (AREA)
  • Non-Silver Salt Photosensitive Materials And Non-Silver Salt Photography (AREA)
  • Analysing Materials By The Use Of Radiation (AREA)

Claims (16)

1. Appareil pour la production de radiographies aux rayons X en deux dimensions, comprenant une source à tube de rayons X (17) comportant une anode et un détecteur de rayons X par ionisation gazeuse, possédant un remplissage gazeux ionisa- ble par des photons de rayons X, caractérisé en ce que la première discontinuité d'absorption dudit gaz d'ionisation de détecteur et le niveau d'énergie des raies de rayons X caractéristiques émises par la source de rayons X sont mis en concordance de telle sorte que la source de rayons X produise des photons possédant un niveau d'énergie immédiatement supérieur à une discontinuité d'absorption du gaz.
2. Appareil suivant la revendication 1, caractérisé par une anode en praséodyme et du xénon en tant que gaz de détecteur.
3. Appareil suivant la revendication 1, caractérisé par une anode en yttrium et du krypton en tant que gaz de détecteur.
4. Appareil suivant la revendication 1, caractérisé par une multiplicité de tubes à rayons X émettant des raies de rayons X caractéristiques en concordance avec une multiplicité de discontinuités d'absorption du gaz de détecteur.
5. Appareil suivant la revendication 4, caractérisé par un détecteur qui contient un mélange de gaz ionisables possédant des discontinuités d'absorption différentes.
6. Appareil suivant la revendication 4, caractérisé par des tubes à rayons X qui sont mis en concordance avec différentes discontinuités d'absorption du même gaz d'ionisation.
7. Appareil suivant la revendication 4, caractérisé par une multiplicité de sources de rayons X montées sur une tourelle rotative pour le passage d'une source de rayons X à une autre.
8. Appareil suivant la revendication 1, dans lequel le détecteur comprend un récipient étanche au gaz possédant une fenêtre pénétrable par les rayons X (52,52', 86) et une harpe de fil métallique dans le récipient comportant un cadre (40,40', 86), définissant une ouverture alignée en association plane généralement parallèle avec la fenêtre et une multiplicité de fins fils métalliques pratiquement parallèles étroitement espacés (42, 42', 83) portés par le cadre dans l'ouverture de cadre, caractérisé en ce qu'il comprend une feuille généralement plane (44, 44', 90) de matière isolante alignée en association plane pratiquement parallèle avec la harpe, ladite feuille possédant une première surface isolante dans le récipient faisant face à la harpe et une seconde surface conductrice opposée, et des moyens pour appliquer un potentiel électrique positif aux fils métalliques par rapport à la seconde surface de la feuille isolante.
9. Chambre d'ionisation suivant la revendication 8, caractérisée en ce que lesdits moyens pour appliquer un potentiel positif comprennent:
une feuille conductrice généralement plane située au voisinage de et en alignement avec la seconde surface de ladite feuille de matière isolante, et
une source de potentiel électrique connectée de manière à établir un potentiel positif entre les fils métalliques de ladite harpe et laditefeuille conductrice.
10. Chambre d'ionisation suivant la revendication 9, caractérisée en ce que ladite feuille conductrice est un revêtement dorsal métallique sur ladite feuille de matière isolante.
11. Chambre d'ionisation suivant la revendication 10, comprenant aussi des moyens de grille secondaires maintenus à un potentiel positif par rapport à ladite seconde surface, qui est nettement inférieur au potentiel positif de ladite harpe en fil métallique par rapport à ladite seconde surface de ladite feuille de matière isolante, ladite grille secondaire étant située au voisinage immédiat de ladite première surface isolante afin de produire une répartition de potentiel sur ladite première surface isolante en empêchant ainsi une accumulation de charge sur ladite première surface isolante.
12. Chambre d'ionisation suivant ta revendication 11, caractérisée en ce que lesdits moyens de grille secondaire sont constitués par une multiplicité de rubans conducteurs liés à ladite première surface isolante.
13. Chambre d'ionisation suivant la revendication 12, caractérisée en ce que lesdits moyens de grille secondaire sont maintenus approximativement 100 volts au-dessus du potentiel de ladite seconde surface tandis que ladite harpe est maintenue à un potentiel approximativement 2000 volts supérieur à ladite seconde surface.
14. Appareil suivant la revendication 1, dans lequel le détecteur comprend un récipient pouvant être obturé de manière étanche possédant une fenêtre pénétrable par les rayons X (28), des moyens pour introduire un gaz d'ionisation dans le récipient, un cadre isolant (20) monté dans ledit récipient définissant une ouverture en alignement plan parallèle avec la fenêtre, caractérisé en ce qu'il comprend une paire de plaque conductrices de l'électricité (24, 26) alignées parallèlement audit cadre, lesdites plaques étant disposées sur des côtés opposés dudit cadre dans ladite chambre, une multiplicité de fils métalliques (28) hautement résistants étroitement espacés et pratiquement parallèles portés dans ledit cadre, des moyens pour appliquer un potentiel négatif auxdites plaques par rapport audit fil et des moyens de lecture électroniques connectés à chacun desdits fils pour déterminer l'emplacement d'un phénomène d'ionisation d'atome gazeux le long dudit fil métallique, lesdits moyens de lecture comprenant des moyens pour intégrer l'information de phénomène d'ionisation gazeuse à partir de chacun desdits fils métalliques pour procurer une information de radiographie en rayons X à deux dimensions.
15. Appareil suivant la revendication 1, dans lequel ledit détecteur d'ionisation gazeuse comprend une chambre d'ionisation gazeuse et un détecteur pour détecter une image radiographique en rayons X, qui comporte un récipient pouvant être obturé de manière étanche possédant une fenêtre pénétrable par les rayons X (52', 84), caractérisé en ce que dans le récipient pouvant être obturé de manière étanche est monté un cadre (40, 86) définissant une ouverture alignée avec la fenêtre, une multiplicité de fins fils métalliques pratiquement parallèles étroitement espacés (42', 88) portés par le cadre dans l'ouverture de cadre, des première et seconde feuilles isolantes (80, 44, 90') disposées sur des côtés opposées dudit cadre, chacune ayant une de ses faces, écartée des fils métalliques, revêtue d'une matière conductrice de l'électricité, lesdites feuilles isolantes étant alignées avec ledit cadre, de telle manière que leurs faces isolantes soient dirigées vers ledit cadre et les plaques définies par les feuilles soient généralement parallèles aux plans définis par le cadre et la fenêtre, des moyens pour appliquer en séquence un potentiel électrique positif aux fils métalliques par rapport au revêtement conducteur de ladite première ou seconde feuille, d'autres moyens pour appliquer en séquence un potentiel électrique aux surfaces conductrices desdites seconde et première feuilles isolantes afin de maintenir l'une desdites feuilles à un potentiel relativement négatif par rapport aux fils métalliques quand l'autre feuille se trouce à un potentiel positif, des moyens pour détecter la répartition de potentiel engendrée sur ladire première feuille isolante par des ions immobilisés sur ladite feuille quand le récipient est rempli avec un gaz ionisant, un potentiel positif est appliqué aux fils métalliques et à ladite seconde feuille, un potentiel relativement négatif est appliqué à la couche conductrice de ladite première feuille isolante et des photons de rayons X pénétrant dans le récipient sont absorbés par des atomes dudit gaz, et des moyens pour établir un potentiel uniforme sur ladite feuille isolante après y avoir détecté le charge.
16. Appareil suivant la revendication 15, caractérisé en ce que lesdits moyens pour déceler la répartition de potentiel comprennent, pour chaque feuille isolante:
une paire d'organes de guidage s'étendant entre ledit cadre à fils métalliques et ladite feuille isolante,
un barreau transversal coulissant s'étendant entre lesdits organes de guidage et porté par ceux-ci,
une multiplicité de plaque détectrices de capacité espacées uniformément montées sur ledit barreau face au côté isolant de ladite feuille isolante,
des moyens d'entraînement pour introduire un déplacement relatif entre le barreau et la surface isolante, et
des moyens amplificateurs connectés activement auxdites plaques afin de déceler séparément la charge sur la première surface de ladite feuille isolante alors qu'un déplacement relatif a lieu entre le barreau et la première surface de ladite feuille isolante.
EP82870008A 1981-03-02 1982-03-01 Enregistrement électronique de rayons X Expired EP0059700B1 (fr)

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AT82870008T ATE27382T1 (de) 1981-03-02 1982-03-01 Elektronische roentgenstrahlenregistrierung.

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US23931381A 1981-03-02 1981-03-02
US239313 1981-03-02

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EP0059700A3 EP0059700A3 (en) 1983-08-24
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Publication number Priority date Publication date Assignee Title
FR2538906B1 (fr) * 1983-01-04 1985-09-13 Commissariat Energie Atomique Procede d'examen de l'image radiographique d'un objet irradie a l'aide d'une source de rayonnements ionisants et chambre d'ionisation pour la mise en oeuvre du procede
NL8701122A (nl) * 1987-05-12 1988-12-01 Optische Ind De Oude Delft Nv Inrichting voor spleetradiografie met beeldharmonisatie.

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Publication number Priority date Publication date Assignee Title
US3786270A (en) * 1973-02-01 1974-01-15 Atomic Energy Commission Proportional counter radiation camera
DE2529037C3 (de) * 1975-06-28 1978-03-09 Philips Patentverwaltung Gmbh, 2000 Hamburg Elektroradiographische Vorrichtung
FR2363117A1 (fr) * 1976-08-26 1978-03-24 Anvar Perfectionnements aux dispositifs de detection et de localisation de rayonnements
GB2057184B (en) * 1979-08-10 1983-05-18 Emi Ltd Radiography

Non-Patent Citations (1)

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Title
Concise Dictionary of Physics, 2nd Edition, 1979, page 363 *

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EP0059700A3 (en) 1983-08-24
ATE27382T1 (de) 1987-06-15
DE3276401D1 (en) 1987-06-25

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