No abstract

No abstract

Rapid progress in digital X ray detectors has elimi nated the role of reference detector in X ray screen-film sets. More than 30 companies are involved in the devel opment of X ray image detectors capable of replacing X ray screen-film sets. A number of digital X ray detectors have been developed. These detectors provided a basis for commercial digital X ray apparatuses [4,5].Most manufacturers of digital X ray image detectors use one of two technologies: flat panel or CCD matrices. The flat panel technology falls into two classes: matrices of thin film field transistors coated with amorphous seleni um, and matrices of silicon photodiodes (aSi:H) coupled to cesium-iodine CsI(Tl) or gadolinium Gd 2 O 2 S screens.Matrices of amorphous selenium have some disad vantages compared to matrices of cesium iodine screen. The X ray image conversion in the matrix is based on photoconduction properties of selenium. Detector sensi tivity is determined by the X ray extinction coefficient of the selenium layer, neutralized surface charge per absorbed energy unit, and efficiency of hidden image commutation. With otherwise equal thickness, absorption in the amorphous selenium plate is lower than in needle screen CsI(Tl). The absorption abruptly declines upon increasing the quantum energy [5]. The potential relief is formed by only 15% of X ray induced electrons. The coefficient of light gathering to photodiodes in the CsI(Tl) screen is 0.3. The resulting quantum efficiency of selenium detector is lower than the quantum efficiency of screen detector. This difference increases upon increasing the X ray tube voltage. The selenium layer is charged at high voltage (kV), which is incompatible with modern microelectronics. Selenium detectors are not considered further in this work.Neither detectors with memory photostimulated X ray screen (CR technology) nor detectors with linear scanning sensors are considered in this work. The tech nologies of universal digital detectors (potential or presently used) for roentgenography and roentgenoscopy with frequency 2 60 frames per sec are considered.Flat panels with needle cesium-iodine screen, matrices of silicon photodiodes, and CCD matrix detec tors were comparatively assessed.The transition from a detector based on screen-objective-CCD components to a monolith sys tem is shown in Fig. 1. It follows from Fig. 1 that the screen detector based on one matrix video camera and flat panel are limiting cases of multimatrix detectors. Direct contact between screen and detector excludes optical ele ments, thereby increasing efficiency of light gathering in the photodetector, decreasing photodetector weight and dimensions but imposing a number of technological prob lems. Let us consider the main technological problems.Optical elements can be excluded by increasing matrix size to screen size. The working field size 430 × 430 mm requires a matrix of photodiodes or other pho todetectors of equal size. This technological problem is rather complicated. A full format matrix of photodetec tors is composed of sm...

No abstract

No abstract