Characterization of an IGZO flat-panel detector for image-guided CBCT radiotherapy

Flat panel detectors based on Indium Gallium Zinc Oxide (IGZO) thin-film transistor (TFT) for x-ray imaging have the potential to overcome limitations of those based on a-Si:H TFT in terms of electron-hole mobility, infill factor, dimension and spatial resolution with no excessive impact on the cost. This work aims at characterizing a large area IGZO flat-panel detector in terms of response, lag and spatial resolution for spectra and exposure condition tipically adopted in cone-beam CT scanners used for image guided radiotherapy. The detector response curve was evaluated by relating the pixel value to the air kerma incident on the detector surface and the noise evaluated as the standard deviation of the pixel values over a selected region of interest. Continuous exposure acquisitions permitted to quantify any lag characterizing the detector and the spatial resolution was evaluated in terms of modulation transfer function (MTF). The detector reported a linear response for incident air kerma up to 45 mu Gy, featuring a noise reduction between 32% and 45% passing from 1 x 1 to 2 x 2 pixel binning mode. Continuous acquisition showed a detector signal increase of no more than 1% over 2 s. The MTF curves showed no dependence on the spectra and the limiting spatial resolution for the MTF at 10% resulted 3.1 mm(-1) for 1 x 1 pixel binning and of 2.3 mm-1 for 2 x 2 pixel binning.