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X-ray FacilityIn order to make full use of the geometrical precision of the therapy beam, the patient has to be positioned in the irradiation room with a similar accuracy. Therefore, to check the position of the target point in three dimensions, two orthogonal x-ray images are taken and compared to digitally reconstructed radiographs from the treatment planning prior to each irradiation. Deviations well below 1 mm can be detected on the x-ray projections, on which the osseous structures appear magnified. Since 2002 two x-ray image intensifiers are in operation which have a number of advantages for patients and staff when compared to the formely used x-ray films:
• A sufficient resolution is achieved with significantly less dose.
• The x-ray image is available instantly after exposure. The time the patient has to stay in the mask is reduced by several minutes, larger patient numbers are possible.
• There is no more need to deal with heavy film cassettes and with fotographic chemicals.
• The digital images can directly be used for further computer processing.
A control unit developed and built at GSI supplies the power for the intensifiers and their CCD cameras and manages tasks like triggering, gain setting, zoom and image hold. The software to display and archive the images has also been written at GSI.
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