Radiation measuring device comprising an ionization chamber

The invention relates to an ionization chamber which includes a plurality of measuring field electrodes (131 . . . 133) which are arranged on a substrate (120) at a distance from one another and are provided with supply leads (134), and at least one electrode (180) which is arranged at a distance from and faces the substrate and emits charge carriers under the influence of X-rays. An insulating layer (140; 190) provided on the supply leads and/or on the electrode, at least at the area of the supply leads, prevents the signals from the ionization chamber from being falsified by the charge carriers incident on the supply leads. The electrically insulating (layers) have such a high X-ray transparency that they are practically not reproduced in an X-ray image.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to an ionization chamber which comprises a plurality
 of measuring field electrodes which are arranged on a substrate at a
 distance from one another and are provided with supply leads, and also at
 least one electrode which is arranged at a distance from and faces the
 substrate and emits charge carriers under the influence of X-rays.
 2. Description of Related Art
 Ionization chambers of this kind are known from EP-A 562 762 and from DE-PS
 1 082 989 and are used in an X-ray system so as to switch off the X-rays
 after a given dose has been reached during an X-ray exposure. They are
 arranged between an X-ray image detector and the patient to be examined,
 so that it is important that the ionization chamber absorbs a minimum
 amount of X-rays and that the spatial absorption differences within the
 ionization chamber are as small as possible so as to avoid reproduction of
 the ionization chamber.
 The space between the substrate and the electrode in the ionization chamber
 according to EP-A 562 762 is filled with a foam insert which has a
 thickness of several millimeters and is provided with windows only at the
 area of the measuring field electrodes, so that an air volume is present
 in the zone between a measuring field electrode and the facing part of the
 electrode. Therefore, charge carriers from the electrode can reach the
 measuring field electrode only at the area of the windows.
 The foam insert serves to prevent the supply leads for the measuring field
 electrodes from being struck by charge carriers during an X-ray exposure,
 as otherwise the measurement would be falsified as in the ionization
 chamber disclosed in DE-PS 1 082 989. Moreover, the foam insert enhances
 the mechanical stability of the ionization chamber. The absorption of
 X-rays by the foam insert is greater than that of the air at the area of
 the measuring field electrodes, even when the foam insert has a small
 thickness only. In the case of soft X-rays, i.e. in the case of low
 voltages (for example, 40 kV) applied to the X-ray tube generating the
 X-rays, such a difference in absorption may cause reproduction of the
 ionization chamber in the X-ray image; therefore, conventional Bucky
 exposures are often performed without an automatic exposure control system
 or without an ionization chamber.
 Contemporary X-ray image converters, comprising electrically readable
 sensors (digital image detectors), moreover, are capable of reproducing
 absorption differences in the X-ray image which are much smaller than
 those reproduced by systems used thus far which utilize an X-ray film in
 combination with an intensifier foil. The risk of reproduction of the
 ionization chamber is then particularly high.
 SUMMARY OF THE INVENTION
 Therefore, it is an object of the present invention to construct an
 ionization chamber of the kind set forth in such a manner that on the one
 hand its reproduction in an X-ray image is precluded to a high degree and
 that on the other hand the supply leads to the measuring field electrodes
 do not influence the signals supplied by the ionization chamber. This
 object is achieved according to the invention in that an electrically
 insulating layer is provided on the side of the supply leads facing the
 electrode and/or on the side of the electrode facing the measuring field
 electrodes, the thickness of said insulating layer being small in
 comparison with the distance between the substrate and the electrode.
 Providing an electrically insulating layer on the electrode, at least at
 the area of the supply leads but preferably on the entire electrode with
 the exception of the regions facing the measuring field electrodes,
 prevents charge carriers which are generated in the electrode and
 constitute the essential part of the ionization current from being emitted
 outside the region of the measuring field electrodes. A layer provided on
 the supply leads, moreover, prevents charge carriers generated at the area
 of the supply leads, for example in the air volume over said leads, from
 reaching the supply leads. Such insulating layers can be constructed to be
 so thin that they are practically not reproduced in the X-ray image.
 The measuring field electrodes themselves will not be reproduced in the
 X-ray image when they comprise a layer of conductive lacquer preferably
 containing graphite.
 The absorption of the X-rays by the spatially homogeneous electrode does
 not lead to its reproduction in the X-ray image (the ionization chamber is
 larger than the X-ray image detector), but decreases the radiation load
 for the patient in proportion to the part of the X-rays absorbed by the
 electrode. Use of an electrode including a homogenous layer containing a
 metal with an atomic number of at least 40; can achieve a low absorption
 by using a suitable substrate and a thin electrode layer. Because the
 layer contains a metal having an atomic number of at least 40, charge
 carriers are emitted thereby under the influence of X-rays for as long as
 the electrode is not covered by an electrically insulating layer. Having
 the outer side of the substrates provided with a conductive layer
 preferably containing graphite ensures electrical shielding of the
 ionization chamber when the substrates are made of an electrically
 insulating material.
 Adequate mechanical stability is achieved for the ionization chamber by
 interconnecting the substrates by way of frames.
 The risk of reproduction of the measuring fields in the X-ray image is
 reduced further by providing and insulating lay er on the electrode having
 opening whose dimensions deviate slightly from those of the measuring
 field electrodes.
 This invention also includes an X-ray system comprising an X-ray tube, an
 X-ray generator, an X-ray detector, and an automatic exposure control
 device including an ionization chamber according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 The reference numeral 1 in FIG. 1 denotes an ionization chamber which is
 arranged between an X-ray source 2 and an X-ray image detector 3 or
 between a patient 4 to be examined and the X-ray image detector 3. The
 ionization chamber 1 is larger than the image detector 3, so that its
 outer contours cannot be imaged on the X-ray image detector. It comprises
 a plurality of measuring fields in which the X-ray dose is measured and
 one (or more) of which can be selected for the dose measurement.
 The X-ray source 2 is fed by an X-ray generator which comprises a
 high-voltage generator 5 and a control unit 6. During an X-ray exposure,
 the ionization currents generated by the X-ray flow across the associated
 measuring field electrode in the previously selected measuring field of
 the ionization chamber 1. These ionization currents are integrated by the
 control unit 6 and ensure that the X-ray exposure is automatically
 terminated when a given integral value is reached, i.e. a given dose in
 the selected measuring field.
 The construction of the ionization chambers will be described in detail
 hereinafter with reference to the FIGS. 2A and 2B; it is to be noted that
 FIG. 2B does not show the construction of the chamber at the correct
 scale. The ionization chamber consists of a flat housing with plane,
 square side walls, one of which supports the measuring field electrodes
 whereas the other supports the large-area electrode which carries a
 negative potential with respect to the measuring field electrodes in the
 operating condition, so that the electrons released in the electrode by
 the X-rays can reach the measuring field electrodes.
 As appears from FIG. 2B, the lower wall of the ionization chamber housing
 comprises a substrate 120 of an insulating material, for example a
 plexiglass plate having a thickness of from 1 to 2 mm. The outer side of
 the substrate 120 is provided with a thin, conductive layer 110, for
 example a graphite layer, which can be formed by deposition of a
 conductive lacquer with a thickness of, for example 0.01 mm by means of a
 screen printing method. It electrically insulates the ionization chamber
 from the environment.
 The measuring field electrodes are provided in a layer 130 on the inner
 side of the substrate 120. As appears from FIG. 2A, showing the layer 130,
 there are provided a central measuring field with a measuring field
 electrode 131, two measuring fields which are situated above the
 horizontal central line, symmetrically with respect to the vertical
 central line (for example for chest exposures) which comprise measuring
 field electrodes 132, and three smaller measuring fields which are
 90.degree. offset relative to one another about the center (for example,
 for extremity exposures} and each of which comprises a measuring field
 electrode 133. Each measuring field electrode is connected, via a supply
 lead 134 provided on the substrate, to a respective integrator circuit
 which is provided in the control unit and has a high-ohmic input so as to
 integrate the ionization currents flowing to the measuring field
 electrodes. The supply leads, having a width of approximately 3 mm, and
 the measuring field electrodes 131 . . . 133 are enclosed by a grounded
 drain electrode 135 which is situated at a distance of approximately 6 mm
 therefrom. The electrically conductive layer 130, consisting of the
 components 131 . . . 135, is a layer having a thickness of approximately
 0.01 mm, like the layer 110, which is formed by deposition of a conductive
 lacquer layer, containing graphite, by means of a screen printing process.
 The lines of sight A-A' in FIG. 2A define the plane whose cross-section is
 shown in FIG. 2B.
 At the area of the supply leads 134 the layer 130 is provided with an
 insulating layer 140 which covers the supply leads and the intermediate
 spaces to the grounded drain electrode 135. The layer is formed by
 deposition of an insulating lacquer having a high X-ray transparency (or
 low X-ray absorption). It has a thickness of from 5 to 6 .mu.m. The layer
 may also be larger; however, it is important that it does not cover the
 measuring field electrodes.
 The second chamber wall comprises a substrate 160 of the same material and
 the same thickness as the substrate 120. The outer side of this substrate
 is provided with a conductive layer 170 which has the same function as the
 layer 110 and is formed in the same way. On the inner side of the
 substrate there is provided a locally uniform, electrically conductive
 layer 180 which contains a metal having an atomic number amounting to at
 least 40, for example silver or lead. The layer 180 can be formed by
 printing on the substrate, for example using a silver emulsion of the type
 "Electrodog 1415 M" from Acheson, 89160 Dornstadt Del. The layer 180 has a
 thickness of from 5 to 6 .mu.m which suffices to generate an adequate
 number of free electrons under the influence of X-rays, but is thin enough
 to cause only a slight overall attenuation of the X-rays.
 On the electrode layer 180 there is provided, in the same way as and using
 the same material as for the layer 140, an insulating layer 190 which is
 provided with openings 191 in the region facing the measuring field
 electrodes, the charge carriers generated in the electrode in this region
 can emerge through said openings and reach, after charge carrier
 multiplication in the intermediate air space, the oppositely situated
 measuring field electrode. These openings may have the same dimensions as
 the facing measuring field electrodes, or dimensions which slightly
 deviate therefrom, for example slightly smaller dimensions.
 The insulating layers 190, 140 effectively ensure that charge carriers are
 not emitted by the electrode 180 at the area of the supply leads or that
 they cannot strike or be incident on the supply leads 134. The X-ray
 transparency of these insulating layers is so high that reproduction of
 the pattern formed by the layers in the X-ray image is practically
 precluded. It is a further advantage of the measuring chamber according to
 the invention that it can be so simply manufactured (multiple printing of
 substrates with conductive and insulating layers, so that the cost of
 manufacture of such an ionization chamber is substantially reduced.
 If one of the insulating layers were to be omitted, a usable ionization
 chamber could still be obtained. However, better results are obtained when
 both layers are used, because they prevent on the one hand the emission of
 charge carriers from the electrode 80 and on the other hand the incidence
 of charge carriers on the supply leads.
 The described embodiment of an ionization chamber includes merely one
 electrode and the substrate for the measuring field electrodes constitutes
 one of the side walls for the ionization chamber. However, like in the
 ionization chamber according to DE OS 1 082 983, it is also possible to
 provide an ionization chamber with two electrodes which are arranged to
 both sides of the auxiliary electrode and constitute the side walls of the
 chamber in conjunction with the substrates on which they are provided. The
 measuring field electrodes should then be provided to both sides of a thin
 substrate situated halfway between the two electrodes.
 All references cited herein are incorporated herein by reference in their
 entirety and for all purposes to the same extent as if each individual
 publication or patent or patent application was specifically and
 individually indicates to by incorporated by reference in its entirety for
 all purposes.