PSI - Issue 33

Dionysios Linardatos et al. / Procedia Structural Integrity 33 (2021) 304–311 Linardatos / Structural Integrity Procedia 00 (2021) 000 – 000

305

2

1. Introduction Scintillating materials are used in various applications, including medical imaging (Kandarakis, 2016), at harsh environments, non-destructive testing (NDT), high energy physics, homeland security, etc. (Koukou et al., 2017a, 2017b). They are coupled to optical sensors such as complementary metal oxide semiconductors (CMOS) and charge coupled devices (CCD) in almost all these imaging field applications, as well as in X-ray imaging (Bohndiek et al., 2008; Koukou et al., 2015, 2017b; Martini et al., 2019; Michail et al., 2016a, 2018a; Saatsakis et al., 2020a, 2020b). CMOS based detectors provide high resolution, high frame rate imaging and, in association with scintillating screens, have been increasingly investigated for medical imaging applications (Anastasiou et al., 2020; Endrizzi et al., 2013; Konstantinidis et al., 2012; Zhao et al., 2015a, 2015b; Zhao and Kanicki, 2014). Frequently used scintillating materials in this context include Gd 2 O 2 S:Tb, CsI:Tl, Gd 2 O 2 S:Eu, Gd 2 O 2 S:Pr,Ce,F (Bertolini et al., 2012; Dobbins III and Godfrey, 2003; Kalyvas et al., 2015; Michail, 2015; Michail et al., 2010, 2018b). Some of these are suitable as well for application in non-destructive evaluation methods, since their radiation hardness is retained at acceptable levels after the absorption of radiation doses up to a few hundred rads (Woo and Kim, 2012; Zhao et al., 2004). In the present study, a new NDT CMOS detector was evaluated under general radiography conditions. The investigation of the information content of this device was achieved through the estimation of the information capacity (IC), as defined in Shannon’s mathematical communication th eory (Shannon, 1948). Information capacity has been considered as an overall image quality metric that encompasses the effects of sharpness, contrast and noise (Papalazarou et al., 2008; Wagner et al., 1979). It provides quantitative estimation of the image information content, associating objective image quality metrics (i.e., spatial frequency dependent detector signal to noise ratio - SNR) to perceptual image quality for task-oriented medical imaging applications (Michail et al., 2014; Papalazarou et al., 2008). In order to estimate IC, the noise equivalent quanta (NEQ) were experimentally assessed. The experimental method was based on the guidelines published by the International Electrotechnical Commission (IEC), which has standardized the methodology for digital detectors (Martini et al., 2018; Michail et al., 2016b). A state-of-the-art 20 μm pixel pitch CMOS sensor coupled to a Gd 2 O 2 S:Tb scintillator screen was studied. In addition, the results of the present work have been compared with those of a previously studied similar CMOS sensor, coupled to Gd 2 O 2 S:Eu, Gd 2 O 2 S:Pr,Ce,F, CsI:Tl and Gd 2 O 2 S:Tb phosphor screens (Michail et al., 2011a; Michail, 2015; Michail et al., 2015; Seferis et al., 2013). 2. Materials and Methods 2.1. CMOS sensor The detector under investigation consists of a Carestream Min-R 2190 scintillator screen (gadolinium oxysulfide activated with terbium – Gd 2 O 2 S:Tb) of thickness 85 μm and coating weight 33.91 mg/cm 2 , coupled to an optical readout device CMOS Remote RadEye HR photodiode pixel array (Cho et al., 2008; Teledyne, 2021). The CMOS array consists of 1650 x 1246 active pixels (active pixel sensor – APS) at 20 μm pitch. Experiments were carried out in the X-ray radiography energy range, the RQA-5 beam quality (70 kVp), according to the IEC 62220-1-1:2015 standard (International Electrotechnical Commission, 2015; Martini et al., 2019). The Del Medical Eureka radiographic system (Harrison, NY, USA) was used for the experiments. It features a rotating tungsten (W) anode with a focal spot size of 0.6 mm (“small” foc al spot size selected) and an inherent filtration equivalent to 3 mm aluminum (Al). In order to obtain the RQA-5 beam quality, another 21 mm of Al filtration (type 1100, purity 99%) were added to the tube ’s output window. With this configuration, a half value layer (HVL) of 6.8 mm was attained. The source to detector distance (SDD) was 156 cm. 2.2. X-ray spectra measurement A portable Amptek XR-100T X-ray spectrometer (XR-100T-CdTe, Amptek, Bedford, MA, USA), based on a cadmium telluride (CdTe) crystal solid-state detector was used for direct diagnostic X-ray spectra measurements (Martini et al., 2015, 2017; Michail et al., 2011a; Sotiropoulou et al., 2015). X-ray spectra were corrected by the