Radiant energy – Invisible radiant energy responsive electric signalling – Including a radiant energy responsive gas discharge device
Patent
1996-11-19
1998-06-02
Wong, Don
Radiant energy
Invisible radiant energy responsive electric signalling
Including a radiant energy responsive gas discharge device
2503851, G01T 1105
Patent
active
057604045
DESCRIPTION:
BRIEF SUMMARY
The object of the invention is a method for determining the size and shape of (he field of a radiation beam of an ionizing radiation source by means of an ionization chamber.
In recent years, increasing attention has been paid to the X-radiation doses patients are subjected to in connection with both X-ray diagnostics and the use of treatment equipment. Automatic recording of radiation doses significantly increases the possibilities for comparing X-ray examinations, thus facilitating the development of X-ray examinations which reduce the radiation stress on the patient without, however, diminishing the diagnostic value of the examinations.
At present, the radiation dose the patient is subjected to is measured by means of a so-called area-dosimeter which consists of a flat, bielectrode ionization chamber and an electronic measuring part relating to it. This equipment gives a reading essentially proportional to the product of field size and radiation, but the field size itself cannot be separately determined. The size and shape of the radiation field are, however, extremely significant, especially in determining the predisposition of the patient's different organs to radiation.
The aim of the present invention is to determine the size and shape of the X-radiation field in a new way. It is characteristic of the method relating to the invention electrode areas, so that the first area, or the reference electrode area, is located mainly in the centre part of the radiation field being measured, area, or the electrode area measuring the size and shape of the field, the first electrode area with the current of the second electrode area.
By means of the method relating to the invention, the size of the useful beam of the radiation transmitted from the X-ray tube and limited with screens can be determined by measuring separately the currents of different electrodes in a multi-electrode ionization chamber. By calculating the proportions of the currents, the size of the field in different directions can be determined.
The object of the invention also comprises a device for determining the size and shape of the field of a radiation beam of an ionizing radiation source by means of an ionization chamber. At is characteristic of the device relating to the invention namely a first electrode, or reference electrode, and a second electrode, or the electrode measuring the size and shape of the field, radiation field being measured, and the second electrode is located so that the edge of the radiation area can be set to pass through this electrode, field by comparing the current of the first electrode with the current of the second electrode.
The ionization chamber relating to the invention differs from a conventional ionization chamber in that the conductive area, corresponding to an electrode gathering positive ions in a conventional ionization chamber, is divided into separate parts. The ionic current gathered from these areas, that is, from the different electrodes, gives information on the ionization of the air or gas space corresponding to these areas, and thus at the same time on the radiation energy passing through the said areas.
The ratio of the currents of the electrodes in the ionization chamber describes the size of the field more or less linearly. When aiming at greater accuracy, however, it is advantageous to calibrate the ratio with a sufficient number of different values of the radiation field. The calibration data obtained is stored in the apparatus' memory, for example, as a "spline"-type approximation curve, the parameters of which are calculated automatically in connection with calibration.
The ratios between different electrode currents can also be influenced by changing the shape of the electrodes. Their field dependencies can be made as linear as possible or appropriately non-linear. Similarly, it is advantageous to use that part of the measuring chamber, of which the corresponding electrode area is not used for determining the size of the field, to measure the ionization current, especially at low radia
REFERENCES:
patent: 3942012 (1976-03-01), Boux
patent: 4206355 (1980-06-01), Boux
patent: 5396889 (1995-03-01), Ueda et al.
Helenius Kari
Rannikko Simo
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