Imaging performance of a high resolution CMOS sensor under Mammographic and Radiographic conditions

Abstract

The fundamental imaging performance in terms of Modulation Transfer Function (MTF), Noise Power Spectrum (NPS) and Detective Quantum Efficiency (DQE) was investigated for a high resolution CMOS based imaging sensor. The latter consists of a 33.91 mg/cm2 Gd 2 O 2 S:Tb scintillator screen, placed in direct contact with a CMOS photodiode array, featuring up to 1200x1600 pixels with a pixel pitch of 22.5 um. The MTF was measured using a slanted-edge method to avoid aliasing while the NNPS was determined by two-dimensional (2D) Fourier Transforming of uniformly exposed images. Both measurements were performed under the representative radiation quality settings, RQA-5 (70kVp Digital-Radiography) and RQA-M2 (28kVp Digital-Mammography) recommended by the IEC Reports 62220-1 and 62220-1-2 respectively. The DQE was assessed from the measured MTF and NPS. The ESAK values ranged between 11-87uGy for RQA-M2 and between 6-40uGy for RQA-5. Additionally the output efficiency of the detector and its signal transfer characteristics were assessed via an analytical model, within the framework of the linear cascaded systems (LCS) theory. It was found that the detector response function was linear for the dose ranges under investigation. Additionally our results showed that for the same RQA quality the output efficiency as well as the measured and analytically predicted MTF, were not significantly affected by ESAK. This could be of importance if further reducing X-ray exposures without image quality degradation is required. MTF and DQE retains high values in the whole spatial frequency range, while NNPS appeared with essentially white noise.