Effect of phosphor persistence on image quality in digital x-ray scanning systems

摘要

Abstract: A scanned-beam digital mammography system in which the x-ray detector employs a phosphor coupled to a light sensitive CCD by a fiber optic taper is under development. The CCD is a narrow slot-shaped detector that is operated in time delay integration (TDI) mode to acquire a complete mammogram. The temporal response of the phosphor (persistence) can introduce blurring in the scan direction which is also dependent on the scanning velocity. A model based on a simple exponential decay of the luminescence was developed to predict the degradation due to phosphor persistence. The temporal response of several phosphor materials, including Gd$-2$/O$-2$/S:Tb, CsI:Tl, and BaFCl:Eu was measured under pulsed x-ray excitation. The luminescence lifetimes of the phosphors were obtained using an iterative deconvolution technique to correct for arbitrary x-ray pulse shapes. Using the luminescence decay model, the modulation transfer function (MTF) degradation due to phosphor persistence was predicted. The model was verified by comparing the predicted MTF for Gd$-2$/O$-2$/S:Tb with that measured experimentally in a prototype scanning system after correction for other sources of MTF degradation. The lifetime of Gd$-2$/O$-2$/S:Tb was found to be approximately 0.5 ms. Using the lifetime in the exponential decay model, the MTF was found to be markedly reduced at high scanning speeds. Other phosphors, such as CsI:Tl had much shorter lifetimes and would consequently cause less MTF degradation. Although Gd$-2$/O$-2$/S:Tb is widely used for film-screen mammography, its decay characteristics suggest that it may not be the best phosphor for a scanning digital system. Slower scan rates or alternative phosphors with shorter lifetimes are necessary to provide optimal image quality. !10