QUANTUM-NOISE IN DIGITAL X-RAY IMAGE DETECTORS WITH OPTICALLY COUPLEDSCINTILLATORS

Digital x-ray imaging detectors designed for soft x-rays (1 to 50 keV) are significant for medical mammography, dental radiography, microrad iography, and microtomography. Detector designs involve either direct absorption of x-rays in solid state devices or thin scintillator scree ns optically coupled to solid state sensors. Well designed scintillato r systems produce 10 or more electrons per detected x-ray and, used wi th charge coupled devices (CCD), detect 100,000 x-rays per pixel befor e saturation. However, if the scintillator is directly coupled to the detector, radiation can penetrate to the semiconductor detector with a small number of events producing large charge and noise. We have inve stigated the degradation of image noise by these direct absorption eve nts using numerical models for a laboratory detector system consisting of a 60 mu m CsI scintillator optically coupled to a scientific CCD. Monte Carlo methods were used to estimate the charge deposition signal and noise for both the CsI and the semiconductor. Without a fiber opt ic coupler, direct absorptions dominate the signal and increase the si gnal variance by a factor of about 30 at energies above 10 keV. With a 3 mm fiber optic coupler, no significant degradation is observed for input energies below 45 keV.