Selenium direct converter structure for static and dynamic x-ray detection in medical imaging applications

Digital x-ray imaging techniques of today require electronic detectors that can be applied to all modalities of medical imaging. This paper presents work showing that selenium, when used as a direct converter, can be competitive with present day scintillator technologies targeting mammographic, radiographic and fluoroscopic applications. In this work, supporting results are presented on the dark currents, lag and ghosting effects and also on the theoretical and experimental x-ray absorption and sensitivity of selenium layers. Measurements were carried out on suitably alloyed selenium layers where the electronic transport properties have been optimized. Measured values for dark currents were below 100pA/cm(2) at operating fields up to 20V/mu m. Experimental measurement of the intrinsic lag in selenium has shown it to be less than 0.5% after 30 milliseconds under a dose of 50mR at 55keV mean beam energy, which is very low compared with present day image intensifiers. Similar measurements on ghosting, using multiple radiographic pulses, indicate that the magnitude of the ghost image after a few seconds is around 2000 electrons, which is comparable to the electronic noise of most read out systems. Measured sensitivity of a 200 mu m selenium layer under a mammographic spectrum was around 230 pC/mR/cm(2) at an operating field of 20V/mu m, which is significantly higher than that reported for competing technologies. Sensitivity for 1000 mu m selenium was also measured with an 80kVp spectrum and 20mm Al filtration and was found to be around 3400 pC/mR/cm(2) which is in close agreement with theoretically calculated values. Theoretical estimations for MTF and DQE are also given to assess the potential imaging performance of a selenium-based detector for various applications.