  We propose a method to achieve the geometric calibration of cone-beam imaging
and delivery systems in radiation therapy; our approach applies to devices
where an X-ray source and a flat-panel detector, facing each other, move in
circular orbits around the irradiated object. In order to extract the
parameters of the geometry from the data, we use a light needle phantom which
is easy to manufacture. A model with ten free parameters (spatial lengths and
distortion angles) has been put forth to describe the geometry and the
mechanical imperfections of the units being calibrated; a few additional
parameters are introduced to account for residual effects (small effects which
lie beyond our model). The values of the model parameters are determined from
one complete scan of the needle phantom via a robust optimisation scheme. The
application of this method to two sets of five counterclockwise (ccw) and five
clockwise (cw) scans yielded consistent and reproducible results. A number of
differences have been observed between the ccw and cw scans, suggesting a
dissimilar behaviour of the devices calibrated in the two rotation modes. The
description of the geometry of the devices was achieved at the sub-pixel level.
