GB1600440A - Multi-channel x-ray detector - Google Patents

Description

(54) MULTI-CHANNEL X-RAY DETECTOR (71) We, N. V. PHILIPS' GLOEILAMPENFABRIEKEN, a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands, 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: The invention relates to an X-ray detector, comprising a plurality of plate-like high-voltage electrodes which are arranged substantially parallel in a gastight housing containing an ionisable gas, and a corresponding plurality of plate-like signal electrodes which are arranged substantially parallel to and to lie between, the highvoltage electrodes, at least part of said electrodes being arranged on a supporting plate.

An X-ray detector of this kind is particularly suitable for use in a device for computer tomography in which a section of the body of a patient to be examined is irradiated by an X-ray beam from different directions in the sectional plane. The locally transmitted radiation is measured, and from the measurement data thus obtained the density distribution in the body section of the patient. is computed by a computer and displayed, for example, on a television monitor. It is desirable to execute the measurement quickly in order to reduce measurement errors caused by movement of the patient. To achieve this, use is preferably made of a fan-shaped X-ray beam which extends in the sectional examination plane over the full width of the object section to be measured in at least one direction. The transmitted radiation is then measured in a location dependent manner throughout the angular extent of the fan beam by means of a detector array which comprises a large plurality of detection elements. In order to obtain an adequate number of measurements for computing the desired density distribution, the irradiation direction of the fan-shaped X-ray beam is varied in the sectional plane of the patient during examination.

An X-ray detector array of the kind hereinbefore referred to is known from Dutch Published Patent Application No.

76.02.007. That application discloses an ray detector array whose high voltage electrodes, referred to therein as cathodes, are constructed as metal plates, the signal electrodes, referred to therein as anodes, being provided on a sheet of dielectric material. Each sheet of dielectric material is provided with a signal electrode on both major surfaces thereof. The signal electrodes and the high voltage electrodes are made of metal having a high atomic number, such as molybdenum, tantalum or tungsten, and have a thickness of approximately 50,us. The sheet of dielectric material consists of a material such as ceramic, mica or a synthetic resin which is commercially available as "Mylar" (Registered Trade Mark). The X-ray detector is of the type comprising a gasfilled ionisation chamber, so that it is particularly suitable for said application to computed tomography, because such an X ray detector has a large dynamic range and high accuracy. The ionisation chambers are accommodated in a common gastight housing so that chambers contain the same gas at the same pressure and mutual differences in sensitivity can be made small.

In order to achieve a high spatial resolution in a direction lateral to the beam direction in such an application, the detector array comprises a large number of individual measurement chambers. To achieve this, the electrodes are arranged to be comparatively near to each other and, in order to prevent loss of sensitivity, the dielectric supports and the electrodes are constructed to be thin. As a result, in such detectors the susceptibility to vibration, and the mutual interaction between the ionisation chambers, referred to hereinafter as cross-talk, may be comparatively high.

Vibration of the thin electrodes affects the comparatively high electrical capacitance between the electrodes which are mounted near to each other, and thus causes an interference signal to appear at the output of the detector.

It is an object of the invention to provide an improved X-ray detector array of the kind referred to in which such effects can be reduced.

According to the invention there is provided an X-ray detector, comprising a gastight housing containing an ionisable gas, a plurality of plate-like high-voltage electrodes arranged therein to be substantially parallel, as herein defined, and a corresponding plurality of plate-like signal electrodes each arranged to be substantially parallel to, as herein defined, and to lie between corresponding said high-voltage electrodes, at least a major portion of each of said electrodes being arranged on a supporting backing plate, wherein each said supporting backing plate is at least 0.2 mm thick and is formed from a material having vibration damping properties and comprising fibres and a binder, the arrangement being such that the tendencv for said plate-like electrodes to vibrate is reduced by the presence of said supporting backing plates.

The term substantially parallel when applied to the plate-like electrodes of the detector is intended herein to include the case of said plate electrodes when directed radially towards the focal point of a cooperating X-ray source in an X-ray absorption measurement assembly.

When electrodes are arranged on a supporting backing plate of such a material, exhibing significant vibration-damping properties, the amplitude of electrode vibration, for example due to movement of the X-ray detector array during an examination, can be substantially reduced.

Furthermore, due to the inhomogeneous structure of the material, the resonant frequencies of the electrode vibrations will be shifted to higher values. The effect on the output signal of the X-ray detector of electrode vibration will be reduced because interference signals of sufficiently high frequency will be at least substantially cancelled by averaging during the time within which a measurement is performed in an ionisation chamber. The undesired effect on the output signal of the X-ray detector array produced by vibration of the apparatus can thus be substantially reduced.

The described steps are most effective in an embodiment of the invention in which the whole of each electrode is arranged on a corresponding supporting backing plate.

In a further embodiment of the invention the electrodes include a layer of vibrationdamping, relatively good X-radiation absorbing material, so that additional damping is provided thereby and so that cross-talk between ionisation chambers can thereby be reduced. Said layer preferably contains at least one of the elements Sn, In.

Cd, Ag Pd. The cross-talk is caused by Kradiation, which is generated in the detector gas by the X-radiation to be detected, said radiation penetrating the thin electrodes, to be measured in neighbouring ionisation chambers. When Xe is used as the detector gas, K-radiation of Xe is the main cause of such cross-talk. In the present embodiment of the K-radiation of Xe can be well absorbed by the said elements includes in the layer applied to the corresponding electrodes, because the absorption of Xradiation by these elements exhibits an absorption edge which is situated at a wavelength which is just slightly longer than the wavelength of the K-radiation of Xe. The linear absorption coefficient of these elements in respect of the K-radiation of Xe is therefore equal to or greater than that of heavy metals such as W, Ta, Mo or Pb.

Solder, containing 60 ö Sn and 400, Pb, is also a very suitable material for the vibration damping and X-radiation absorbing layer.

In a further embodiment of the invention, a signal electrode which is arranged on a supporting backing plate, comprises a plurality of electrically conductive portions which are insulated with respect to each other and each of which is capable of supplying a detector output signal. The electrons and ions formed by the ionizing effect of the X-radiation to be detected, travel along electrical field lines in the ionisation chamber. When the signal electrode is subdivided into a number of distinct electrically conductive portions which are insulated from each other, a corresponding number of ionisation chamber measurement sub-regions will be formed. When the ionisation chamber subregions are arranged one behind the other, as viewed along the direction of propagation of the X-radiation to be detected, the X-radiation will be detected in a wavelength-dependent manner. Radiation having a comparatively short wavelength, i.e. the hard radiation will be measured in the rearmost ionisation chamber sub-region, radiation having a comparatively long wavelength, i.e. the soft radiation, being measured in the foremost ionisation chamber sub-region. The ratio of the output signals from the respective-ionisation chamber sub-regions, enables inter alia the degree of relative hardening of the Xradiation by the object to be determined.

In order that the invention may be clearly understood and readily carried into effect, embodiments thereof will now be described by way of example. with reference to the accompanying drawings, of which: Figure I shows diagrammatically a device for computer tomography: Figure 2 is a diagrammatic perspective view showing a portion of a detector embodying the invention in partial front view, partly in section: Figure 3 is a view of a major surface of a supporting plate of the detector of Figure 2 with a signal electrode, and Figure 4 is a view of a major surface of a supporting plate of the detector of Figure 2 with a high voltage electrode.

Figure I shows a device 1 for computer tomography in which a fan-shaped X-ray beam 3, generated by an X-ray source 2, irradiates a section 4 of the body of a patient to be examined. The X-ray beam 3 has an angle of spread of, for example, approximately 60O in the plane of the drawing and is comparatively flat, with a thickness of, for example, 15 mm, in a direction perpendicular thereto. The beam spans the entire body section in the width direction. The transmitted radiation is measured by means of an X-ray detector array 5 which comprises a large number of ionisation chambers 6 in order to enable an examination to be performed quickly. The figure only shows 15 ionisation chambers for the sake of clarity, but in practice there may be for example 300 ionisation chambers. The ionisation chambers are connected to a signal processing circuit 7 in which the detector measurement output signals are processed to form computer input signals. In order to obtain an adequate amount of measurement data, the X-ray beam 3 together with the detector array 5 are rotated about the patient during an examination, by means of a drive 8. A computer 9 is used to compute the density distribution in the section of the body being examined: this distribution is displayed, for example, on a television monitor 10 for evaluation. The X-ray detector array 5 will be described in more detail hereinafter with reference to Figure 2.

Figure 2 shows a portion of the X-ray detector array 5 which comprises a gastight housing 11 made. for example, of steel and filled, for example, with Xe gas at a pressure of 20 atm. Through an X-radiationtransparent window 12, for example, made of aluminium or a composite of graphite in an epoxy resin binder, X-radiation passes into ionisation chambers 14 in a direction 13. The ionisation chambers 14 are separated from each other by partitions 15 and 16, the construction of which is shown in more detail in Figures 3 and 4, respectively. Holders 17 and 18 of a synthetic material provide a substantially vibration-free suspension for the partitions 15, 16 in the gastight housing 11. Electrodes which are arranged on the partitions 15 and 16, and which are not shown in Figure 2 are connected to the signal processing circuit shown in Figure 1 via gastight-insulating seals 19.

Figure 3 shows one side of the partition 15, comprising a supporting backing plate 20 with a signal electrode 21 and to either side of the electrode 21, a respective further electrode 22 which acts as a guard electrode to intercept any leakage currents present on the holders 17 and 18 which are made of synthetic material and are shown in Figure 2.

Figure 4 shows one side of the partition 16, comprising a supporting backing plate 24 with a high voltage electrode 25.

The supporting backing plates 20 and 24 are made of a material exhibiting comparatively strong vibration-damping properties, such as glass-fibre reinforced epoxy resin must have a thickness of not less than 0.2 mm and preferably not greater than 0.6 mm. In one example the thickness of the backing plate was approximately 0.4 mm and the electrodes 21, 22 and 25 consist of a copper layer which had a thickness of approximately 35,am and which was covered, for .the purpose of additional vibration damping, with a layer of solder consisting of 60% Sn and 40% Pb having a thickness of approximately 50cm. An additional advantage of the latter step is that the partitions 15 and 16 of Figure 2 are thereby rendered substantially impermeable to Kradiation of Xe gas generated from the Xe gas by the X-radiation to be detected, so that cross-talk between ionisation chambers resulting from this cause, is significantly reduced. As an alternative to solder, any one of the elements Sn, In, Cd, Ag and Pd can be used for covering the electrodes in order to obtain additional vibration damping and to reduce the said cross-talk between ionisation chambers In a simple embodiment of the X-ray detector, the electrodes are themselves made of Ag and have a thickness of 50cm.

The side of the partitions 15 and 16 which is not shown in Figures 3 and 4, respectively, is constructed in similar manner to the side shown. However, it is alternatively possible for the side of the partition which is not shown in Figure 3 to be provided with a high-voltage electrode as shown in Figure 4, and for the side of the partition 16 which is not shown in Figure 4 to be provided with a signal electrode as shown in Figure 3. The partitions 15 and 16 are arranged in the detector 5 so that the ionisation chambers 14 are each bounded by a high voltage electrode 25 and a signal electrode 21.

In order to prevent the comparatively thick holders 17 and 18 of Figure 2, made of a synthetic material, from being degraded by the X-radiation to be detected, a shielding plate of, for example, brass (not shown in the Figures) is arranged as an extension of each holder 17, 18 in the direction towards the incident X-ray beam The collimator formed by these shielding plates moreover, substantially reduces Xradiation which is scattered by the object being examined and whose direction deviates from that of the unscattered radiation beam. from penetrating into the detector and hence contributing to unwanted background signals.

Consequently, if a collimator is used, as is often the case, it is advantageous to arrange the elements of this collimator in register with the holders 17, 18, because the holders are necessarily comparatively thick. When the collimator is accurately aligned with respect to the supporting plates for example, by means of continuous mounting plates to which the respective positions of the holders and the collimator plates can be referred, any further reduction in sensitivity and irregularities of measurement due to the uncertain definition of the input aperture of each of the chambers can be reduced.

WHAT WE CLAIM IS: 1. An X-ray detector, comprising a gastight housing containing an ionisable gas, a plurality gas, a plurality of plate-like highvoltage electrodes arranged therein to be substantially parallel, as herein defined, and a corresponding plurality of plate-like signal electrodes each arranged to be substantially parallel to, as herein defined, and to lie between corresponding said high-voltage electrodes, at least a major portion of each of said electrodes being arranged on a supporting backing plate, wherein each said supporting backing plate is at least 0.2 mm thick and is formed from a material having vibration damping properties and comprising fibres and a binder, the arrangement being such that the tendency for said plate-like electrodes to vibrate is reduced by the presence of said supporting backing plates.

2. An X-ray detector as claimed in Claim 1, wherein the supporting backing plate Is made of a synthetic resin reinforced with glass fibres.

3. An X-ray detector as claimed in Claim 1 or Claim 2, wherein the supporting backing plate has a thickness lying in the range from 0.2 mm to 0.6 mm 4. An X-ray detector as claimed in any one of the preceding claims wherein the whole of each said electrode is arranged on a said supporting backing plate.

5. An X-ray detector as claimed in anv one of the preceding claims. wherein the electrodes comprise a layer of vibrationdamping material having a relatively high Xradiation absorption coefficient to Xradiation generated in a gas present in said detector as a result of incident radiation.

6. An X-ray detector as claimed in Claim 5, wherein the vibration-damping electrode layer contains at least one of the elements Sn, In, Cd, Ag, or Pd.

7. An X-ray detector as claimed in Claim 5, wherein the vibration-damping electrode layer consists mainly of solder.

8. An X-ray detector as claimed in any one of the Claims 1, 2, 3 or 4, wherein the electrodes mainly comprise silver.

9. An X-ray detector as claimed in any one of the preceding claims, wherein a signal electrode in which is arranged on a said supporting backing plate comprises a plurality of distinct electrically conductive portions which are insulated from each other and each of which is arranged to supply a corresponding detector output signal.

10. An X-ray detector substantially as herein described with reference to figures 2, 3 and 4 of the accompanying drawings.

11. A device for computer tomography, in which an object to be examined is irradiated from different directions by means of a fanshaped X-ray beam and in which the Xradiation transmitted by the object is measured by means of an X-ray detector as claimed in any one of the preceding claims.

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

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. thick holders 17 and 18 of Figure 2, made of a synthetic material, from being degraded by the X-radiation to be detected, a shielding plate of, for example, brass (not shown in the Figures) is arranged as an extension of each holder 17, 18 in the direction towards the incident X-ray beam The collimator formed by these shielding plates moreover, substantially reduces Xradiation which is scattered by the object being examined and whose direction deviates from that of the unscattered radiation beam. from penetrating into the detector and hence contributing to unwanted background signals. Consequently, if a collimator is used, as is often the case, it is advantageous to arrange the elements of this collimator in register with the holders 17, 18, because the holders are necessarily comparatively thick. When the collimator is accurately aligned with respect to the supporting plates for example, by means of continuous mounting plates to which the respective positions of the holders and the collimator plates can be referred, any further reduction in sensitivity and irregularities of measurement due to the uncertain definition of the input aperture of each of the chambers can be reduced. WHAT WE CLAIM IS:

1. An X-ray detector, comprising a gastight housing containing an ionisable gas, a plurality gas, a plurality of plate-like highvoltage electrodes arranged therein to be substantially parallel, as herein defined, and a corresponding plurality of plate-like signal electrodes each arranged to be substantially parallel to, as herein defined, and to lie between corresponding said high-voltage electrodes, at least a major portion of each of said electrodes being arranged on a supporting backing plate, wherein each said supporting backing plate is at least 0.2 mm thick and is formed from a material having vibration damping properties and comprising fibres and a binder, the arrangement being such that the tendency for said plate-like electrodes to vibrate is reduced by the presence of said supporting backing plates.

2. An X-ray detector as claimed in Claim 1, wherein the supporting backing plate Is made of a synthetic resin reinforced with glass fibres.

3. An X-ray detector as claimed in Claim 1 or Claim 2, wherein the supporting backing plate has a thickness lying in the range from 0.2 mm to 0.6 mm

4. An X-ray detector as claimed in any one of the preceding claims wherein the whole of each said electrode is arranged on a said supporting backing plate.

5. An X-ray detector as claimed in anv one of the preceding claims. wherein the electrodes comprise a layer of vibrationdamping material having a relatively high Xradiation absorption coefficient to Xradiation generated in a gas present in said detector as a result of incident radiation.

6. An X-ray detector as claimed in Claim 5, wherein the vibration-damping electrode layer contains at least one of the elements Sn, In, Cd, Ag, or Pd.

7. An X-ray detector as claimed in Claim 5, wherein the vibration-damping electrode layer consists mainly of solder.

8. An X-ray detector as claimed in any one of the Claims 1, 2, 3 or 4, wherein the electrodes mainly comprise silver.

9. An X-ray detector as claimed in any one of the preceding claims, wherein a signal electrode in which is arranged on a said supporting backing plate comprises a plurality of distinct electrically conductive portions which are insulated from each other and each of which is arranged to supply a corresponding detector output signal.

10. An X-ray detector substantially as herein described with reference to figures 2, 3 and 4 of the accompanying drawings.

11. A device for computer tomography, in which an object to be examined is irradiated from different directions by means of a fanshaped X-ray beam and in which the Xradiation transmitted by the object is measured by means of an X-ray detector as claimed in any one of the preceding claims.