Issue 50

N. Martini et alii, Frattura ed Integrità Strutturale, 50 (2019) 471-480; DOI: 10.3221/IGF-ESIS.50.39 474 F 1        hc (4) Φ X was determined by using Eqs.(2,3), replacing 0  by Φ 0 and dividing Eq.(3) by the X-ray energy [9]. Eqs(2,4) may be expressed in the spatial frequency domain. Within this framework the detector quantum gain may be expressed by a gain transfer function (GTF), defined as follows [36,45]: 0 0 0     GTF( E ,ν,w ) ( E ,ν,w ) / (5) where v denotes spatial frequency, w coating thickness and Φ Λ (Ε 0 ,ν,w) is the spatial frequency-dependent emitted light photon fluence. Gain transfer function, can be expressed through the MTF [36,45]:   GTF( ν,w ) MTF( ν ,w ) DQG (6) where DQG is the detector quantum optical gain. In medical imaging, where fluorescent screens are used in combination with optical detectors (films in the past, photocathodes, photodiodes), the spectral matching between the emitted phosphor light and the optical detector sensitivity must be considered. This is because the degree of spectral matching affects the amount of light utilized to form the final image. Thus, Eq.(6) is reduced by a factor a S , expressing the fraction of emitted light that can be detected by the optical detector, which exhibits a specific spectral distribution of sensitivity. a S , can be calculated by Eq.(7) [5,45]:   P D S P S ( λ )S ( λ )dλ α S ( λ )dλ (7) where S P ( λ ) is the spectrum of the light emitted by the phosphor and S D ( λ ) is the spectral sensitivity of the optical detector coupled to the phosphor [5]. By considering a S , we may define the effective gain transfer function as follows [36,45]:    S eGTF( v ,w ) DQG MTF( v ,w ) α (8) R ESULTS AND DISCUSSION ig.1 shows the grain-size deposition per thickness of CaWO 4 , estimated from scanning electron microscope images. Qualitatively the mean particle size of CaWO 4 phosphor (6.02 m  ) was estimated from the SEM images using the ImageJ analysis software, as shown from the grain-size distribution [48,49]. The calculated screen thickness was equal to 118.9 μm estimated by profile measurements on the area depicted as inset in Fig.1, across the material coating [5,22]. Furthermore, in Fig.1 the energy dispersive X-ray (EDX) analysis of the material is demonstrated. It was found that CaWO 4 was dominantly present in the sample along with carbon (C) due to the carbon thread evaporation process. Normalized stoi- chiometric results, obtained by the SEM on the region of interest (ROI) of Fig.1, showed the following % weights of the elements in the mixture: Calcium (Ca) 5.77%, oxygen (O) 26.54%, tungsten (W) 29.94 and carbon due to the carbon thread evaporation process 37.75% [22]. The X-ray characteristic curves (output signal versus incident exposure) of CaWO 4 and (for comparison purposes) of a flexible fluorescent Gd 2 O 2 S:Tb sample (gold standard for imaging applications) of similar coating thickness (30.8 mg/cm 2 for Gd 2 O 2 S:Tb versus 36.26 mg/cm 2 for CaWO 4 ) are plotted in Fig.2. These coating thicknesses were calculated assuming densities of 7.3 g/cm 3 for Gd 2 O 2 S:Tb and 6.1 g/cm 3 for CaWO 4 with packing densities of 50% for both materials. Results for CaWO 4 and Gd 2 O 2 S:Tb show linear dependence between the output signal and exposure rate in the 4-299 mR/s range. The linear no-threshold fit gave correlation coefficient values R 2 of 0.9997 for CaWO 4 and 0.9953 for Gd 2 O 2 S:Tb, which are very close to unity and indicate that the screens have linear response in this energy range. Gd 2 O 2 S:Tb was found with clearly higher output signal values than those of CaWO 4 due to its higher absolute efficiency values [5]. Fig.3 shows all the oversampled ESFs (Fig.3a) used to create the average ESF and then the Fermi-fitted ESF (Fig.3b), as well as, the resulted LSF (Fig.3c), following the IEC 2015 protocol and the edge phantom. The edge test device consists of a 1 mm thick W edge plate (100×75 mm 2 ) fixed on a 3 mm thick lead plate. Images of the edge, placed at a slight angle in order to avoid aliasing effects, were obtained. Irradiation was performed at 70kVp and 50 mAs for the tube current and