FI96381C - Measurement method for automatically determining the X-ray radiation cone's field shape with multi-electrode ionization chamber - Google Patents

Description

96381.

METHOD FOR AUTOMATIC DETERMINATION OF THE SHAPE OF THE FIELD OF THE RÖNTGEN TUBE'S RAIN BEAM WITH A MULTIPELEEL ELECTRODE DISEASE IN THE IONIZATION CHAMBER

The invention relates to a measuring method and a device for automatically determining the size and shape of the radiation field of X-ray equipment, which method is based on the dependence of the ratios of the different electrode currents 5 of the multi-electrode ionization chamber on the field size and shape.

In recent years, increasing attention has been paid to the radiation dose received by the patient in connection with the use of both X-ray diagnostic and treatment equipment. The automatic registration of the radiation dose significantly increases the comparability of 10 X-ray examinations and thereby helps to develop X-ray examinations that reduce the patient's radiation exposure without reducing the diagnostic value of the examinations.

The current radiation dose received by the patient is measured by the so-called

The surface area dosimeters comprise a flat two-electrode ionization chamber and an associated electronic measuring section. Ko. the equipment gives a reading substantially proportional to the field size and the irradiation input, but the field size itself cannot be determined separately. The size and shape of the radiation field is important, especially in determining the radiation exposure of various organs of a patient.

The method of the present invention can be used to automatically determine the size of the blinded radiation beam emitted by an X-ray tube by measuring the currents of different electrodes in a multi-electrode ionization chamber separately and automatically calculating the current ratios to determine the field size in different directions.

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In the above-mentioned ionization chamber, the conductive region corresponding to the electrons typically collecting positive ions in the conventional ionization chamber is divided into separate parts so that the ion-5 current accumulating from the different regions (electrodes) provides information about ionization of the gas space corresponding to these regions.

The ratio of the currents of the electrodes in the ionization chamber describes the field size relatively linearly, but for better accuracy it is advantageous to calibrate the ratio with a sufficient number of different values of the radiation field. The obtained calibration information is kept in the hardware's memory, for example, as an "spline" type approximation curve, the parameters of which are automatically calculated during calibration.

The shapes of the electrodes can also influence the ratios of the different electro-currents, the field dependencies of which can be made as linear as possible or suitably non-linear. Likewise, the part of the measuring chamber whose corresponding electrode area is not used to determine the field size, 20 is preferably used, especially at low irradiation values, for measuring the ionization current. At the same time, the "protection" electrodes of these regions improve the linearity of the edge regions of the electric fields generated by the polarization voltages.

In order to achieve the above effects, the invention is mainly characterized by the claims.

In the following, the invention is illustrated by way of example with the figures of the accompanying drawing, in which Figure 1 schematically shows an ionization chamber (5) in a typical operating environment, Figure 2 shows a top view of a chamber construction according to the invention.

3 96381 Fig. 3 shows an arrangement of chamber electrodes according to the invention and Fig. 4 shows the chamber of Fig. 2 from the side.

* ΐ Figure 1 shows a basic operating environment of the measuring method according to the invention, where the radiation (11) from the X-ray tube (10) is confined to the blinds (14) and absorbed in part (small part) into the chamber (5), where the absorbed radiation ions are transmitted by different polarization voltages to different electrodes.

The currents from the electrodes are integrated in the measuring device part (15) into readings which are proportional to the radiation penetrating the area corresponding to the sub-electrode of the chamber. The radiation further passes through the object (12) to be imaged to the image receptor (13). The reference electrode (1) of a chamber construction according to the invention 15 shown in Figures 2 and 4 is assumed to receive full radiation. The ratios of the current readings of the electrode (2) measuring the field in the X-direction and the electrode (3) measuring in the y-direction with the reading of the reference electrode are proportional to the size of the radiation field in the x- and y-20 directions, respectively. The areas (4) shown in Figures 2 and 4 and the area (6) wider in the x-direction illustrate typical shapes of radiation fields limited by blinds. Assuming that the radiation is evenly distributed. in the regions, the reading of the electrode (3) is found to be the same for both radiation field shapes (4) and (6) and the electrode (2) reading increases linearly as the radiation field shape (4) to shape (6) is proportional to the radiation field width in the x-direction. In this case, the ratios of the readings of the electrodes (2) and (3) with the readings of the reference electrode (1) are independent of the magnitude of the irradiation and vary according to the size of the radiation field.

Furthermore, Fig. 2 shows a "protection" electrode (7), from which the current reading can be added together with the readings of other electrodes, and the sum obtained corresponds to the reading given by a conventional (single-electrode) chamber.

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Figure 3 shows a possible chamber electrode arrangement according to the invention, in which the ratios of the current readings of the electrodes (2) and (3) measuring in the x and y directions and the reference electrode (1) are in principle directly equal to the size of the rectangular radiation beam in different directions.

It will be clear to a person skilled in the art that the invention is not limited to the embodiment shown in the description and figures, but can be modified within the scope of the claims. For example, the invention does not necessarily relate to use in connection with an X-ray source, but can also be applied in connection with other radiation sources for radiation therapy.

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Claims (3)

96301 A measuring method for determining the field shape of a radiation beam (11) of an ionizing radiation source automatically by means of a radiation measuring flat ionization chamber (5) and an associated electronic measuring part (15), characterized in that electrodes arranged in different directions in separate electrode regions (1,2,3) The ratios of (2,3) currents to the current of the reference electrode (1) located in the central region are substantially independent of the amount of radiation, and thus these ratios directly measure the shape and size of the radiation field. A method according to claim 1, characterized in that the apparatus according to the measurement method is calibrated with different large known radiation field shapes and the obtained calibration information is stored in the apparatus memory, e.g. as a "spline" type approximation curve, the parameters of which are automatically calculated during calibration. Multi-electrode ionization chamber (5) for carrying out the method according to claim 1, characterized in that in addition to the separate electrodes (2,3) measuring the size and shape of the radiation field and the reference electrode (1), the other electrode area of the chamber acts as a separate "protective" electrode (7) . 96381