A project aimed at the development of a robot-based patient positioning system for
high-precision proton radiotherapy is underway at iThemba LABS. Part of the project
included the development of a portal digital x-ray imaging device that would be used
to verify the patient treatment set-up. The imaging device consisted of a scintillation
screen, front surface mirror and a high-resolution charged-couple device (CCD)
camera. The total costs for the device were about 7 times less expensive than a
commercial amorphous silicon flat panel detector. To improve the efficiency of the
system, the CCD chip and scintillation screen were chosen so that the wavelength of
the light from the screen closely matched the wavelength at which the CCD sensor
has the maximum quantum efficiency. The digital images compared favourably with
those of x-ray film. Although the digital images were of lower resolution due to the
finite resolution of the CCD chip, they were considered satisfactory. The use of pixel
binning allowed for the use of lower exposure settings when compared to exposure
settings for un-binned images. This resulted in a reduction of patient dose without
significantly compromising image quality. The device would not be used for
diagnostic purposes, but only to verify patient position at treatment setup. As such, the
digital images would be compared against digitally reconstructed radiographs (DRRs)
of the fields and/or the treatment position, which are created from the treatment
planning Computed Tomography (CT) images. In general, the spatial resolution of the
DRRs is also comparably lower than digital x-ray images, as the resolution of the
DRRs is limited to the voxel size of the CT images
