X-ray or gamma ray systems or devices – Electronic circuit – Exposure timer
Reexamination Certificate
1999-03-16
2001-05-22
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
Exposure timer
C250S385100
Reexamination Certificate
active
06236711
ABSTRACT:
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.
REFERENCES:
patent: 3988584 (1976-10-01), Lange et al.
patent: 4230944 (1980-10-01), Wiegman et al.
patent: 5264701 (1993-11-01), Crain
patent: 1082989 (1960-11-01), None
patent: 0562762A1 (1993-09-01), None
patent: 0123456 A2 (2000-01-01), None
Dunn Drew A.
Kim Robert H.
U.S. Philips Corporation
Vodopia John F.
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