Cone-beam computed tomography with a flat-panel imager: Initial performance characterization

The development and performance of a system for x-ray cone-beam computed to mography (CBCT) using an indirect-detection flat-panel imager (FPI) is pres ented. Developed as a bench-top prototype for initial investigation of FPI- based CBCT for bone and soft-tissue localization in radiotherapy, the syste m provides fully three-dimensional volumetric image data from projections a cquired during a single rotation. The system employs a 512x512 active matri x of a-Si:H thin-film transistors and photodiodes in combination with a lum inescent phosphor. Tomographic imaging performance is quantified in terms o f response uniformity, response linearity, voxel noise, noise-power spectru m (NPS), and modulation transfer function (MTF), each in comparison to the performance measured on a conventional CT scanner. For the geometry employe d and the objects considered, response is uniform to within 2% and linear w ithin 1%. Voxel noise, at a level of similar to 20 HU, is comparable to the conventional CT scanner. NPS and MTF results highlight the frequency-depen dent transfer characteristics, confirming that the CBCT system can provide high spatial resolution and does not suffer greatly from additive noise lev els. For larger objects and/or low exposures, additive noise levels must be reduced to maintain high performance. Imaging studies of a low-contrast ph antom and a small animal (a euthanized rat) qualitatively demonstrate excel lent soft-tissue visibility and high spatial resolution. Image quality appe ars comparable or superior to that of the conventional scanner, These quant itative and qualitative results clearly demonstrate the potential of CBCT s ystems based upon flat-panel imagers. Advances in FPI technology (e.g., imp roved x-ray converters and enhanced electronics) are anticipated to allow h igh-performance FPI-based CBCT for medical imaging. General and specific re quirements of kilovoltage CBCT systems are discussed, and the applicability of FPI-based CBCT systems to tomographic localization and image-guidance f or radiotherapy is considered. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)01306-7].