A project aimed at the development of a robot-based patient positioning system forudhigh-precision proton radiotherapy is underway at iThemba LABS. Part of the projectudincluded the development of a portal digital x-ray imaging device that would be usedudto verify the patient treatment set-up. The imaging device consisted of a scintillationudscreen, front surface mirror and a high-resolution charged-couple device (CCD)udcamera. The total costs for the device were about 7 times less expensive than audcommercial amorphous silicon flat panel detector. To improve the efficiency of theudsystem, the CCD chip and scintillation screen were chosen so that the wavelength ofudthe light from the screen closely matched the wavelength at which the CCD sensorudhas the maximum quantum efficiency. The digital images compared favourably withudthose of x-ray film. Although the digital images were of lower resolution due to theudfinite resolution of the CCD chip, they were considered satisfactory. The use of pixeludbinning allowed for the use of lower exposure settings when compared to exposureudsettings for un-binned images. This resulted in a reduction of patient dose withoutudsignificantly compromising image quality. The device would not be used foruddiagnostic purposes, but only to verify patient position at treatment setup. As such, theuddigital images would be compared against digitally reconstructed radiographs (DRRs)udof the fields and/or the treatment position, which are created from the treatmentudplanning Computed Tomography (CT) images. In general, the spatial resolution of theudDRRs is also comparably lower than digital x-ray images, as the resolution of theudDRRs is limited to the voxel size of the CT images.
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