Issue 50

N. Martini et alii, Frattura ed Integrità Strutturale, 50 (2019) 471-480; DOI: 10.3221/IGF-ESIS.50.39 473 CMOS sensor The CaWO 4 scintillating screen was manually coupled to an optical readout device including a CMOS Remote RadEye HR photodiode pixel array [39]. The CMOS photodiode array consists of 1200x1600 pixels with 22.5 μm pixel spacing. This sensor was initially manufactured for non-destructive testing (NDT)/industrial inspection applications especially for tight or difficult to reach spaces. However, due it its unique resolution properties can be also used in medical imaging applications. Thus, it would be of interest to integrate a scintillating material that came to the spotlight once again, with this state-of- the-art sensor in order to further exploit their imaging characteristics. The CaWO 4 screen was directly overlaid onto the active area of the CMOS and irradiated with a BMI General Medical Merate tube having rotating Tungsten anode and in- herent filtration equivalent to 2 mm Al at 70 kV (RQA-5) X-ray beam quality and source to detector distance of 156 cm [21]. Modulation Transfer Function (MTF) The modulation transfer function (MTF) was measured by irradiating a PTW Freiburg tungsten edge test device, following the procedures described in the IEC standard [11,32,33]. The updated IEC 62220-1-1:2015 [11,33,40] standard describes certain modifications, such as the method for the determination of the modulation transfer function (MTF) in which the final MTF can be now obtained only through averaging of the oversampled edge spread function (ESF) [33,41-43]. The average of all oversampled ESFs was then fitted with a modified Fermi-Dirac distribution function as follows [11,21]: ( x a )/ b c Fermi(x)= +d e +1        (1) The values of fitting parameters are a=7200, b=15, c=65500 and d=0. The fitted ESF was differentiated to obtain the line spread function (LSF) and Fourier transformed to finally obtain the MTF [21,44]. Luminescence efficiency measurements The efficiency (output signal) of a scintillator to emit light, upon X-ray irradiation is experimentally determined by measuring the emitted light energy flux λ   and the exposure rate ( X  ) using a calibrated dosimeter. In this study, the Piranha P100B RTI was used. The light flux measurements were performed using a light integration sphere (Oriel 70451), coupled to a photomultiplier (PMT) (EMI 9798B) and connected to a Cary 401 vibrating reed electrometer [5,9]. The circular CaWO 4 sample was also exposed to X-rays on the BMI General Medical Merate tube, with energies ranging from 50 to 125 kVp. An additional 20 mm Al filtration was introduced in the beam to simulate beam quality alternation by a human body [45,46]. X-ray luminescence efficiency (XLE) The X-ray luminescence efficiency (XLE) is a unitless measure of the fraction of incident energy converted into emitted light energy, i.e. the ratio of the emitted light energy flux over the incident X-ray energy flux ( η ψ = Ψ Λ / Ψ 0 ). XLE was determined [9] by converting the measured X-ray exposure data into X-ray energy flux ( Ψ 0 ) [9], as follows: 0 ˆ    where ˆ  is de- fined as the X-ray energy flux per exposure rate, estimated according to Eq.(2) [5,36]:   0 0 0        ( E )dE ˆ ( E ) X / ( E ) dE (2) where 1 0 / ( ) ( ( ) / ) ( / ) en air A X E E W e       (3) is the factor converting energy flux into exposure rate, ( μ en / ρ ) air the X-ray mass energy absorption coefficient of air, at energy E , and ( W A / e ) is the average energy per unit of charge required to produce an electron-ion pair in air. ( W A / e ) and ( μ en / ρ ) air were obtained from tabulated data [47]. Detector quantum optical gain (DQG) Detector quantum optical gain (DQG) is the ratio of the light photon flux ( Φ Λ ) over the X-ray photon flux ( Φ X ) and expresses the emission efficiency in terms of quantum gain (number of emitted light photons per incident X-ray). Using this quantity, the emitted light photon fluence can be expressed in terms of experimentally determined quantities (absolute efficiency, exposure, mean light wavelength), by using Eq.(4) [5]: