Imaging panel skew correction and auto-focusing in radiotherapy cone beam imaging

Digital transducers, notably amorphous silicon panels, are being used in image guided radiotherapy. Using kilo-voltage X-ray sources panel transducers are providing cone beam volume reconstruction. Theoretical considerations show that even sub-degree skew can result in the wandering of projection points across many detector elements to produce serious blurring of the cone beam reconstruction away from the imaging isocentre. The importance of identifying and correcting transducer skew is highlighted and shown to be of potentially greater importance than mechanical flex. Radiographic imaging of a suspended radio-opaque test object is described for direct characterisation of skew. The results of skew correction by image processing are presented. The use of ´auto-focusing´ grey level change detection algorithms in cone beam reconstruction images, following programmed incremental angular transformation of the projection profiles of a ´Rando´ anthropomorphic phantom, is shown to have the potential for automating skew correction.