GB1561174A - Ray detectors - Google Patents

Description

(54) IMPROVEMENTS IN X-RAY DETECTORS (71) We, GENERAL ELECTRIC COM PANY, a corporation organized and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12305, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to ionization chamber, x-ray detectors. More specifically, this invention relates to multi-cellular detectors comprising high pressure gas for use in computerized tomography systems.

In a computerized. x-ray tomographaspa- tial distribution of x-ray intensities must be translated into electrical signals which are processed to yield image information. Detectors for use in such systems must efficiently detect x-ray electromagnetic energy with a high degree of spatial resolution. The x-ray pulse repetition rate in tomograph systems is generally limited by the recovery time of the x-ray detectors. It is desirable, therefore, to utilize x-ray detectors characterized by fast recovery times, high sensivitity, and fine spatial resolution. Proposed x-ray tomography systems employ hundreds of such x-ray detectors. A multicellular construction, wherein multiple, spatially separated detection cells are incorporated in a single detector assembly, provides an economic means for the production of such systems.

The present invention is a modification or an improvement of the invention described in our copending Patent Application No.

5331/76 now Patent No. 1543651 which claims an x-ray detector comprising: a gaseous medium of the type characterized as being substantially opaque to electromagnetic radiation at x-ray frequencies; a plurality of substantially planar sheet anodes, comprising material which is substantially opaque to electromagnetic radiation at x-ray frequencies, disposed in said gaseous medium; a plurality of substantially planar sheet cathodes, comprising material which is substantially opaque to electromagnetic radiation at x-ray frequencies, disposed in said gaseous medium, each of said cathodes lying approximately equi-distant between two of the said anodes; means for applying direct current electric potential between said cathodes and said anodes whereby an electric field is impressed between said cathodes and said anodes and said detector is caused to operate in the ionization chamber mode when a beam of x-ray photons is caused to impinge on said gaseous medium in directions substantially parallel to the planes of adjacent said sheet anodes and said sheet cathodes; and means for separately measuring the electric current flow from each of said anodes, said measuring means being connected in series between said anodes and said means for applying said potential.

The modification or improvement provided by the present invention comprises the addition of dielectric support means disposed between said anodes and said cathodes, and guard rings disposed on a surface of said support means between adjacent anodes and cathodes, the guard rings being connected to said means for applying said potential so as to drain surface leakage currents which might otherwise flow between said anodes and said cathodes when the direct current is applied.

The electrons and positive ions which are produced by the interaction of the x-ray photons and the gas drift along the electric field lines and are collected respectively on the positive and negative electrodes. Substantially all of the electrons and ions produced by the interaction of an x-ray pulse with the gas must be collected and removed from the detector before a subsequent x-ray pulse may be unambiguously detected. High pulse repetition rates are required for efficient computerized tomography so that detectors with short ionelectron collection times are desirable for use in such equipment. Preferably the x-ray detector comprises a high pressure ionization chamber having a plurality of closely spaced parallel plate electrodes which lie substantially parallel to an incident x-ray beam. This electrode configuration allows prompt removal of the electron-ion pairs and permits the use of high x-ray pulse repetition rates at relatively low electrode potentials.

Heavy gas atoms, which are used in the ionization x-ray detectors of the present invention, tend to fluoresce; radiating photons at low energy, x-ray frequencies.

These low energy, x-ray photons have a relatively long range in the detector gas and tend to degrade detector spatial resolution.

The parallel plate electrodes may be constructed of high atomic weight material which acts to absorb those low energy, secondary photons at the detector cell boundaries and, thus, improve the spatial resolution of the detector.

Highly efficient x-ray detectors are required to make maximum use of the information available from each x-ray exposure and to thereby minimize the total radiation exposure. Tomography detectors must, therefore, detect at least 50 percent of incident x-ray photons. Safe and efficient system operation typiclly requires detectors capable of detecting more than 70 percent of the incident x-ray beam which typically has an energy in the range from 30KEV to 100 KEV.

The present invention will be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is an embodiment of a detector of the invention described in Patent Application No. 5331/76 now Patent No. 1543651 which incorporates parallel plate cathodes and anodes; Figure la is a top view of the detector of Figure 1; Figure 2 is an alternate anode structure for the detector of Figure 1; Figure 3 is a structure incorporating the anodes of Figure 2 in a detector of the type indicated in Figure la; and Figure 4 is an electrode embodiment for use in the detector of Figure 1 to bring the detector within the scope of the present invention.

X-ray photons will interact with atoms of a heavy detector gas to produce electron-ion pairs. The x-ray photons are, generally, absorbed by a gas atom which emits a photoelectron from one of its electronic levels. The photoelectrons move through the gas interacting with an ionizing other gas atoms to produce a shower of electrons and positive ions which may be collected on suitable electrodes to produce an electric current flow. If, for example, xenon gas at approximately 10 atmospheres pressure is irradiated with 60 KEV x-ray photons, photoelectrons will be ejected from the 34.5 KEV xenon k shell at approximately 25.5 KEV. The 25.5 KEV photoelectrons, having a range of approximately 0.1 mm in the xenon, will produce approximately 800 electron-ion pairs each. If these electron-ion pairs are produced in a region between two electrodes of opposite polarity, they will drift along electric field lines to the electrodes and yield a net electric current flow between them. The electric current flow between the electrodes is thus a function of the total number of x-ray photons interacting in the vicinity of those electrodes The probability of detection of an x-ray photon is a function of the atomic number of the gas and of the number of gas atoms lying between the collector electrodes. Thus, high sensitivity detectors may be constructed from a gas of high atomic weight at a relatively high pressure. Detector sensitivity may also be increased by increasing the spacing, and therefore the number of gas molecules, between the electrodes. Increased electrode spacing, however, increases the distance the electron-ion pairs must drift for collection and thus tends to increase the recovery time of the detector.

An increased electric field gradient between the electrodes will tend to increase the electron-ion drift velocity and thus somewhat shorten the detector recovery time; the drift velocity, however, increases in relatively small proportion with electrode voltage increases. Furthermore, it is well known that an excessive electric field gradient will cause avalanche gas breakdown and will create highly nonlinear responses in detection sensitivity.

The present detectors operate with electric field gradients which are insufficient to cause electron multiplication: that is, they may be characterized as ionization chambers and not as proportional counters. The production of electron-ion pairs described above is attributable solely to energy transfer from the ejected k-shell photoelectrons and is not caused by collisions of electrons or ions moving under the influence of the impressed electric field. The values of electric field gradients which are suitable for use in ionization chamber detectors are well known in the art and are more fully described in Medical Radiation Physics, W.R.

Hendee, Year Book Medical Publishers, Chicago, at chapters 4 and 17. The present detectors operate with electric field gradients between approximately 10 v/mm and approximagely 1000 v/mm.

An shell electron will generally drop to fill the opening which is produced by the emission of the k shell photoelectron from a heavy gas atom. The energy difference resulting from the drop of the electron from the f to the k shell levels is radiated in the form of a secondary x-ray photon. In xenon gas, for example, the f to k energy level shift produces 29 KEV x-ray photons. The range of these secondary photons in the high pressure gas is generally much larger than the range of the photoelectrons. By way of example, in xenon at 10 atmospheres pressure 25.5 KEV photoelectrons have a range of approximately 1 mm while 29 KEV x-ray photons have a range of approximately 20 mm.

The secondary photons which are produced by the fluorescence of the heavy gas atoms upon excitation by incident x-ray photons will be absorbed by other heavy gas molecules in the detector and are indistinguishable from the incident x-ray photons Thus, photons which are produced by fluorescence in the region of one electrode cell may travel through a multicell detector to the region of another electrode cell where they will be detected in the same manner as incident x-rays. The k-shell fluorescence effect may, therefore, be seen to contribute to the degradation of spatial resolution in multicell, ionization chamber detectors.

Figures 1 and la illustrate an embodiment of a multicell, x-ray detector of the invention described in Patent Application No.

5331/76 now Patent No. 1543651. A pressure vessel 10 contains a detector gas 12 at high pressure. One side of the pressure vessel 10 defines a thin window 14 which is substantially transparent to electromagnetic radiation at x-ray frequencies. The window 14 may be constructed from any of the materials which are well known and commonly used for that purpose in the radiation detection arts; for example aluminium, plastics resin, or a matrix of plastic resin reinforced by low atomic number metals.

The term "substantially transparent", as used herein, means that the probability of x-ray radiation interacting with the window material is much less than the probability of that x-ray radiation interacting with the detector gas 12.

The detector gas 12 fills the pressure vessel 10 and is chosen to be substantially opaque to electromagnetic radiation at x-ray frequencies. As used herein, the term "substantially opaque" means that the probability of x-rav radiation interacting with the detector gas 12 is much greater than the probability of that electromagnetic radiation interacting with the window 14. The gas type, gas pressure, and electrode spacing are chosen using methods well known to the art so that a large fraction (typically more than 70 percent) of the incident x-ray photons are absorbed within the gas. The detector gas 12 may, typically, comprise a rare gas of high atomic number, for example, xenon, krypton, argon, or a molecular gas comprising atoms having an atomic weight greater than that of argon (i.e., 39.9); at a pressure from approximately 10 atmospheres to approximately 50 atmospheres.

A plurality of flat anodes 42 are aligned within the pressure vessel 10 in a direction substantially perpendicular to the window 14. The anodes 42 are individually connected to a plurality of leads 22 which pass through the pressure vessel on dielectric feed-throughs 24. A respective metal plate cathode 38 is positioned equidistant between each adjacent pair of the anodes 42.

The cathodes 38 are connected in parallel by a lead 30 which passes through the pressure vessel 10 on a dielectric feed-through 40.

The anode plates 42 and the cathode plates 38 are constructed from metals which are substantially opaque to electromagnetic radiation at x-ray frequencies. Metals of high atomic number, for example, molybdenum, tantalum, or tungsten, are suitable for use as the anodes 42 and the cathodes 38. By way of illustration only, in a typical detector the anode and cathode plates are constructed from 0.05 mm molybdenum or tungsten sheets. The cathode lead 30 and the anode leads 22 are electrically connected to a signal processor 26 for separately measuring the electric current flow from each of the said anodes 42 and to a potential source 28 so that the potential source 28 is connected in series between the cathode lead 30 and the anode leads 22 to produce an electric field between the anodes 42 and the cathodes 38. In typical detectors the electric field gradient is between approximately 100 v/mm. and approximately 300 v/mm. As shown in Figure 1, the signal processor 26 is connected in series between the anodes 42 and the potential source 28.

Photons of x-ray radiation 32 enter the detector through the window 14 in directions substantially parallel to the anode plates 42 and the cathode plates 38. The photons interact with the fill gas 12 in the regions between the anode plates 42 and the cathode plates 38. Electron-ion pairs which are produced by interaction of the gas 12 with the photons 32 drift along electric field lines between the anodes and cathodes and are collected thereon to produce electric current signals. The electric current flowing from a particular anode 42 is proportional to the number of x-ray photons interacting with the gas 12 in the space between that anode and the adjacent pair of cathodes 38.

The signals from the anodes may be com bined in the signal processor 26, using techniques well known to the tomography art, to yield an image from the x-ray intensity along the line of anodes.

This detector is insensitive to the resolu tion limiting effects of k-band x-ray fluoresc ence. Any x-ray photons which are pro duced by fluorescence in the region between an anode plate 42 and a cathode plate 38 must pass through a cathode plate 38 before they would be capable of producing elec tron-ion pairs which would drift to an adjacent anode. As indicated above, the cathode plates 38 are constructed from material which is substantially opaque to x-ray photons and the incidence of fluores cent x-ray photons with sufficient range to produce current in adjacent anode cells is thereby greatly reduced. The anode 42 and cathode 38 structures of the present embodi ment lie parallel to the direction of photon incidence. The plates of the anodes 42 and the cathodes 38 may, therefore, be spaced relatively close together yielding a detector with a short recovery time, while the length of the plates may be increased to produce a detector of high sensitivity. By way of illustration only, in a typical detector, the anode and cathode plates are mounted on 2 mm centers. The parallel electrode plates of this detector also serve to absorb incident photons which are scattered from external objects (i.e., tissue under examination) and which enter the detector at an oblique angle.

Figure 2 illustrates an alternative arrange ment of the anode plates 42 which may be utilized in the detector of Figure 1. In this arrangement, each anode plate comprises a thin dielectric sheet 46; which may, by way of illustration, be constructed from ceramic, mica. or Mvlar plastic resin sheet (Mylar is a Registered Trade Mark). A pair of elec trodes 44. constructed from metal which is substantially opaque to electromagnetic radiation at x-rav frequencies, are disposed on opposite sides of the dielectric sheet 46.

Separate leads 22 are connected to each metal electrode 44 and pass through the pressure vessel 10 on separate dielectric feed-throughs 23. Electron currents flowing to opposite sides of the anode plate 42 are thus collected on the separate metal sheets 44 and transmitted separately to the signal processor 26 (of Figure 1). The spatial resolution of the detector is thereby increased by a factor of two.

A method of construction of an assembly of anode and cathode plates is illustrated in Figure 3. The anode plates 42 and the cathode plates 38 are alternately stacked on a plurality of insulating bolts 48. A series of tubular insulators 50 are threaded on the bolts 48 between the anode plates 42 and the cathode plates 38 and serve to position the plates. The plates may be mounted in parallel alignment for detection of a collimated x-ray beam or the thickness of the insulators 50 may be varied to produce a curved plate alignment suitable for detection of a diverging x-ray beam.

The electron-ion current flowing within these ionization chambers is typically very small and may be of the same order of magnitude as leakage currents which flow on the structures. By using the present invention, these leakage currents which may induce noise in or interfere with the operation of detector amplifier electronics are drained from the detector circuit on guard rings which are spaced on the electrode support structures between adjacent electrodes and are operated at anode potential.

Figure 4 is an embodiment of electrode structures for use in the detector of Figure 1 to enable that detector to take advantage of the present invention. Guard ring elements 52 are disposed on a surface of dielectric support rods or insulators 50 between the cathode plates 38 and the anode plates 42 to drain surface leakage currents which might otherwise flow between them. The guard rings are connected to the positive terminal of the potential source 28 in parallel with the signal processor 26.

The x-ray detector structures described above produce electrical signals in response to a linear space distribution of x-ray intensities. The structures allow the construction of detectors having high sensitivity, short recovery time, and fine spatial resolution and which are relatively insensitive to the adverse effects of k shell, x-ray fluorescence.

The electrodes in the above description have, for ease of description, been referred to as "cathodes" and "anodes". It is to be understood, however, that the polarity of the electric potentials applied to these detectors may be reversed without affecting the principles of operation of the disclosed invention and that the "anode" structures may be operated at an applied potential which is negative with respect to the "cathode" potential. The terms "cathode" and "anode" as used herein and in the appended claims mean electrodes of opposite polarity.

WHAT WE CLAIM IS: 1. An X-ray detector comprising; a gaseous medium of the type characterized as being substantially opaque to electromagnetic radiation at X-ray frequencies; a plurality of substantially planar sheet anodes, comprising material which is substantially opaque to electromagnetic radiation at x-ray frequencies, disposed in said

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. with the gas 12 in the space between that anode and the adjacent pair of cathodes 38. The signals from the anodes may be com bined in the signal processor 26, using techniques well known to the tomography art, to yield an image from the x-ray intensity along the line of anodes. This detector is insensitive to the resolu tion limiting effects of k-band x-ray fluoresc ence. Any x-ray photons which are pro duced by fluorescence in the region between an anode plate 42 and a cathode plate 38 must pass through a cathode plate 38 before they would be capable of producing elec tron-ion pairs which would drift to an adjacent anode. As indicated above, the cathode plates 38 are constructed from material which is substantially opaque to x-ray photons and the incidence of fluores cent x-ray photons with sufficient range to produce current in adjacent anode cells is thereby greatly reduced. The anode 42 and cathode 38 structures of the present embodi ment lie parallel to the direction of photon incidence. The plates of the anodes 42 and the cathodes 38 may, therefore, be spaced relatively close together yielding a detector with a short recovery time, while the length of the plates may be increased to produce a detector of high sensitivity. By way of illustration only, in a typical detector, the anode and cathode plates are mounted on 2 mm centers. The parallel electrode plates of this detector also serve to absorb incident photons which are scattered from external objects (i.e., tissue under examination) and which enter the detector at an oblique angle. Figure 2 illustrates an alternative arrange ment of the anode plates 42 which may be utilized in the detector of Figure 1. In this arrangement, each anode plate comprises a thin dielectric sheet 46; which may, by way of illustration, be constructed from ceramic, mica. or Mvlar plastic resin sheet (Mylar is a Registered Trade Mark). A pair of elec trodes 44. constructed from metal which is substantially opaque to electromagnetic radiation at x-rav frequencies, are disposed on opposite sides of the dielectric sheet 46. Separate leads 22 are connected to each metal electrode 44 and pass through the pressure vessel 10 on separate dielectric feed-throughs 23. Electron currents flowing to opposite sides of the anode plate 42 are thus collected on the separate metal sheets 44 and transmitted separately to the signal processor 26 (of Figure 1). The spatial resolution of the detector is thereby increased by a factor of two. A method of construction of an assembly of anode and cathode plates is illustrated in Figure 3. The anode plates 42 and the cathode plates 38 are alternately stacked on a plurality of insulating bolts 48. A series of tubular insulators 50 are threaded on the bolts 48 between the anode plates 42 and the cathode plates 38 and serve to position the plates. The plates may be mounted in parallel alignment for detection of a collimated x-ray beam or the thickness of the insulators 50 may be varied to produce a curved plate alignment suitable for detection of a diverging x-ray beam. The electron-ion current flowing within these ionization chambers is typically very small and may be of the same order of magnitude as leakage currents which flow on the structures. By using the present invention, these leakage currents which may induce noise in or interfere with the operation of detector amplifier electronics are drained from the detector circuit on guard rings which are spaced on the electrode support structures between adjacent electrodes and are operated at anode potential. Figure 4 is an embodiment of electrode structures for use in the detector of Figure 1 to enable that detector to take advantage of the present invention. Guard ring elements 52 are disposed on a surface of dielectric support rods or insulators 50 between the cathode plates 38 and the anode plates 42 to drain surface leakage currents which might otherwise flow between them. The guard rings are connected to the positive terminal of the potential source 28 in parallel with the signal processor 26. The x-ray detector structures described above produce electrical signals in response to a linear space distribution of x-ray intensities. The structures allow the construction of detectors having high sensitivity, short recovery time, and fine spatial resolution and which are relatively insensitive to the adverse effects of k shell, x-ray fluorescence. The electrodes in the above description have, for ease of description, been referred to as "cathodes" and "anodes". It is to be understood, however, that the polarity of the electric potentials applied to these detectors may be reversed without affecting the principles of operation of the disclosed invention and that the "anode" structures may be operated at an applied potential which is negative with respect to the "cathode" potential. The terms "cathode" and "anode" as used herein and in the appended claims mean electrodes of opposite polarity. WHAT WE CLAIM IS:

1. An X-ray detector comprising; a gaseous medium of the type characterized as being substantially opaque to electromagnetic radiation at X-ray frequencies; a plurality of substantially planar sheet anodes, comprising material which is substantially opaque to electromagnetic radiation at x-ray frequencies, disposed in said

gaseous medium; a plurality of substantially planar sheet cathodes, comprising material which is substantially opaque to electromagnetic radiation at x-ray frequencies, disposed in said gaseous medium each of said cathodes lying approximately equi-distant between two of said anodes; means for applying direct current electric potential between said cathodes and said anodes whereby an electric field is impressed between said cathodes and said anodes and said detector is caused to operate in the ionization chamber mode when a beam of x-ray photons is caused to impinge on said gaseous medium in directions substantially parallel the planes of adjacent said sheet anodes and said sheet cathodes; means for separately measuring the electric current flow from each of said anodes, said measuring means being connected in series between said anodes and said means for applying said potential; dielectric support means disposed between said anodes and said cathodes, and guard rings disposed on a surface of said support means between adjacent anodes and cathodes, the guard rings being connected to said means for applying said potential so as to drain surface leakage currents which might otherwise flow between said anodes and said cathodes when the direct current is applied.

2. A detector as claimed in claim 1 wherein said anodes are substantially parallel.

3. A detector as claimed in claim 1 or claim 2 wherein said anodes are spaced equi-distant, one from another.

4. A detector as claimed in any one of claims 1 to 3, wherein each of said anodes comprises: a flat dielectric plate having two sides and two metallic electrodes disposed on the sides of said dielectric plate.

5. A detector as claimed in any one of claims 1 to 4, further comprising a pressure vessel disposed about and containing said anodes, said cathodes, and said gaseous medium.

6. A detector as claimed in claim 5, wherein said pressure vessel further comprises a window perpendicular to said cathodes, said window being characterized as substantially transparent to electromagnetic radiation at x-ray frequencies.

7. A detector as claimed in claim 6.

wherein said window comprises aluminium or plastics resin.

8. A detector as claimed in any one of claims 1 to 7, wherein said source of electric potential is adapted to impress an electric field gradient between 10v/mm and 1000 v/mm in the regions separating said anodes and said cathodes.

9. A detector as claimed in any one of the preceding claims, wherein said gaseous medium comprises elements having an atomic weight greater than or equal to the atomic weight of argon.

10. A detector as claimed in claim 9, wherein said gaseous medium is argon, krypton, or xenon.

11. A detector as claimed in any one of the preceding claims, wherein said gaseous detecting medium has a pressure between 10 atmospheres and 50 atmospheres.

12. A detector as claimed in any one of the preceding claims, wherein the material of said anodes and cathodes is tantalum, tungsten or molybdenum.

13. An x-ray detector as claimed in claim 1 substantially as hereinbefore described with reference to and as shown in the accompanying drawings.