Image Receptors

Abstract

The X-ray photons that build up the radiation image-possibly after transmission through the anti-scatter grid-are partly absorbed by the sensor layer of the image receptor (e.g. see scintillator in Fig. 8.6). The probability of interaction or the quantum detection efficiency (QDE; see Sect. 9.6, Eq. 9.19) for photons of energy E is equal to (Yaffe and Rowlands 1997): Z ¼ 1 À e ÀmðEÞÁ:d (8.1) where m(E) is the linear attenuation coefficient of the sensor material and d its active thickness (see also Sect. 4.1). The main interaction process is the photoelectric effect because of the relatively high atomic number of most sensor materials. To get an effective quantum detection efficiency with respect to the tube voltage selected, Eq. 8.1 must be averaged over the relevant incident X-ray spectrum. The application of the total attenuation coefficient m(E) in Eq. 8.1 is based on the assumption that the highest possible noise equivalent quanta (NEQ) (see Sect. 9.6) can be obtained with an ideal quantum-counting device, which should therefore be taken as the reference detector (Tapiovaara and Wagner 1985; Zhao et al. 1997). When calculating the sensitivity or the voltage-response of image receptors, one should use, however, the energy absorption coefficient m en (E) in Eqs. 8.3 and 8.4, because the detector signal is proportional to the energy actually absorbed in the sensor materials used (Asai et al. 1998; Boone 2000; Stierstorfer and Spahn 1999). After absorption of the impinging photons in the detector material, the image information is transferred by fast photo-electrons through excitation and ionisation to other information carriers. In the intensifying screens of film-screen systems and in the scintillators of digital detectors, these are light quanta (Dick and Motz 1981), which are generated by luminescence; in some direct digital image receptors (e.g. amorphous selenium), these are electric charges (electron-hole pairs). The better the absorption of the X-ray photons in the detector material is, the more is made use of the image information given by their spatial intensity distribution and extracted to the following imaging chain. The more light quanta (in the intensifying screens or scintillators) or the more electric charges (in the direct digital detectors) are generated, the more sensitive is the whole imaging system. The detective quantum efficiency DQE (see IEC 2003 and Sect. 9.6) has been introduced as the physical quantity to describe the efficiency of this signal transformation. The DQE(n) characterises the overall signal and noise performance of imaging detectors dependent on the spatial frequency n. The DQE is of great importance for the dose which is needed for a radiographic image with good image quality. In the last years medical imaging has changed because of the development of new image detectors. Analogue systems such as film-screen combinations for radiographs and image intensifiers for fluoroscopy are being replaced to an increasing extent by digital image receptors. These detectors produce digital projection images by using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD and MOSFET technology, partially in combination with various phosphor materials (see Fig. 8.6) as image sensor. The so-called flat panel detectors-made from amorphous silicon in connection with a CsI(Tl)-scintillator as X-ray converter-are used nowadays also for pulsed fluoroscopy (see Sect. 8.4.2). In the next two chapters a short review of the meanwhile most important sensor materials for film-screen systems (CaWO 4 , Gd 2 O 2 S, see Sect. 8.1) as well as digital image and CT detectors (CsI(Tl), Si, Se, GaAs, Cd(Zn)Te, CdTe, see Sect. 8.2) is given and the characteristics which are responsible for their different energy response to X-radiation are discussed. Because of their higher atomic number Z semiconductor compounds such as GaAs, Cd(Zn)Te and CdTe (see Fig. 8.8) are of great interest for digital imaging (and partly for CT) detectors; they offer the possibility to improve the detection efficiency (e.g. compared to silicon), while minimising the required thickness of the detector's sensor material to almost completely absorb the impinging X-rays. The knowledge of the energy dependence of the image receptor's sensitivity is H. Aichinger et al., Radiation Exposure and Image Quality in X-Ray Diagnostic Radiology,