  Varian Medical Systems has manufactured and recently put into operation a
clinically-applicable solution for image-guided radiation therapy. Cone-beam CT
imaging, one of the operation modes of the imaging unit of this device, aims at
high-quality volumetric reconstruction. To boost the image quality, the
dual-gain mode, a successful means for enhancing the dynamic range of the
flat-panel detectors and obtaining better results in the contrast of the
reconstructed image, was developed and successfully tested during the last few
years. The important steps in the calibration of this mode involve a correction
to the pulse widths associated with the X-ray production, the assessment of the
detector signal above which nonlinear effects become significant and the
determination of some properties of the detector pixels, namely, of dark
fields, flatness corrections, etc. Finally, the defect-pixel map is obtained,
containing dead and flickering pixels, as well as pixels with properties which
are sufficiently `out of range'. An effect observed in the offset correction of
raw images seems to originate in the mismatch between the measured and
extrapolated values of the dark field; a correction scheme is proposed to take
account of this effect. In the data processing, which is achieved on the basis
of the files produced in the calibration of the mode, the high-gain signal is
used whenever meaningful; otherwise, it is substituted by the low-gain signal,
properly scaled. Defect pixels are interpolated from their good neighbours; a
procedure achieving a correction in case of small defect-pixel clusters is
outlined.
