Image quality determination of a novel digital detector for X-ray imaging and cone-beam computed tomography applications

The demand for adequate image quality with low radiation doses for patients has greatly increased. This is especially true in the case of position verification in radiotherapy which requires a high number of images per patient. This study presents a physical characterisation of a new clinical detector named “Lassena (CsI)” based on a thick layer of structured thallium activated caesium iodide and complementary metal-oxide semiconductor technology with active pixel sensor architecture for general X-ray imaging and cone-beam computed tomography (CBCT) applications. We made a critical appraisal of its performance for the first time and determined its signal transfer property (STP) and its detective quantum efficiency (DQE) by acquiring the pre-sampling modulation transfer function (pMTF) and normalised noise power spectrum (NNPS) in addition to the dark current calculation. The investigation was conducted with the application of three X-ray beam qualities: (50 kV (RQA3), 70 kV (RQA5) and 90 kV (RQA7)) in compliance with the International Electrotechnical Commission (IEC 62220-1(2003)) standard. The STP was found to be linear with the coefficient of determination (R2) more than 0.9995 in all cases. The spatial resolution and NNPS results led to acceptable DQE values at all energies; in particular the DQE values at 0.5 line pairs per mm (DQE(0.5)) which were 0.46 for RQA3, 0.52-0.56 for RQA5 and 0.55-0.59 for RQA7. Lastly, the dark current was 2.51 pA/cm2 for a 50μm pixel pitch. For CBCT applications, Lassena (CsI) showed very promising results.