Tracking and targeting the tumors are simultaneous processes in the image-guided radiotherapy (IGRT); this is expected to boost the efficiency, the reliability, and the safety in the treatment. Varian Medical Systems (VMS) has already produced and installed the first IGRT machine; the device comprises the VMS Clinac equipped with the On-Board Imager (OBI) component. Cone-beam CT (CBCT) imaging, one of the options of the OBI machine, aims at high-quality volumetric reconstruction. A number of calibrations are needed in order to operate our CT-imaging machines properly; they ensure that the machine components are properly aligned, the mechanical distortions are small, and yield important output that is used in the reconstruction of the actual scan data. The geometrical calibration is achieved by using a needle phantom. In order to increase the dynamic range of our imager (hence, to obtain reliable information simultaneously in the high- and the low-attenuation areas of the irradiated object), VMS has developed a dual-gain mode. Next on our agenda is the suppression of (ring, streak, and beam-hardening) artefacts in our reconstructed images and the further development of our detectors in order to remove patterns relating to lag and ghosting effects.
The dynamic range of many flat panel imaging systems are fundamentally limited by the dynamic range of the charge amplifier and readout signal processing. We developed two new flat panel readout methods that achieve extended dynamic range by changing the read out charge amplifier feedback capacitance dynamically and on a real-time basis. In one method, the feedback capacitor is selected automatically by a level sensing circuit, pixel-by-pixel, based on its exposure level. Alternatively, capacitor selection is driven externally, such that each pixel is read out two (or more) times, each time with increased feedback capacitance. Both methods allow the acquisition of X-ray image data with a dynamic range approaching the fundamental limits of flat panel pixels. Data with an equivalent bit depth of better than 16 bits are made available for further image processing. Successful implementation of these methods requires careful matching of selectable capacitor values and switching thresholds, with the imager noise and sensitivity characteristics, to insure X-ray quantum limited operation over the whole extended dynamic range. Successful implementation also depends on the use of new calibration methods and image reconstruction algorithms, to insure artifact free rebuilding of linear image data by the downstream image processing systems.
The multiple gain ranging flat panel readout method extends the utility of flat panel imagers and paves the way to new flat panel applications, such as cone beam CT. We believe that this method will provide a valuable extension to the clinical application of flat panel imagers.