Experimental Evaluation of a High Resolution CMOS Digital Imaging Detector Coupled to Structured CsI Scintillators for Medical Imaging Applications

Abstract

The purpose of the present study was to investigate the fundamental imaging performance of CsI scintillating screens specially treated for a CMOS digital imaging sensor, in terms of the Modulation Transfer Function (MTF), Normalized Noise Power Spectrum (NNPS) and the Detective Quantum Efficiency (DQE) in the general radiography energy range. The CMOS sensor was coupled to two columnar CsI:Tl scintillator screens obtained from the same manufacturer with thicknesses of 140 and 170 μm , respectively, which were placed in direct contact with the photodiode array. A CMOS photodiode array was used as an optical photon detector. The MTF was measured using the slanted-edge method while NNPS was determined by 2D Fourier transforming of uniformly exposed images. Both parameters were assessed by irradiation under the RQA-5 beam quality (IEC 62220-1). The DQE was assessed from the measured MTF, NNPS and the direct entrance surface air-Kerma (ESAK) obtained from X-ray spectra measurement with a portable CdTe detector. The detector response function was linear for the exposure range under investigation. The MTF of the present system was found higher than previously published MTF data for a 48 μm CMOS sensor, in the low and medium frequency ranges. DQE was found comparable, while the NNPS appeared to be higher in the 0–10 cycles/mm frequency range. The imaging performance of CsI:Tl scintillating screens in combination to the high resolution CMOS sensor, that was investigated in the present study, could be used in digital imaging systems in order to reduce exposure to patients.