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

James Gordon Mainprize; Martin Joel Yaffe

doi:10.1117/12.237847

11 April 1996 Effect of phosphor persistence on image quality in digital x-ray scanning systems

James Gordon Mainprize, Martin Joel Yaffe

Proceedings Volume 2708, Medical Imaging 1996: Physics of Medical Imaging; (1996) https://doi.org/10.1117/12.237847
Event: Medical Imaging 1996, 1996, Newport Beach, CA, United States

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 Gd₂O₂S: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 Gd₂O₂S:Tb with that measured experimentally in a prototype scanning system after correction for other sources of MTF degradation. The lifetime of Gd₂O₂S: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 Gd₂O₂S: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.

Citation Download Citation

James Gordon Mainprize and Martin Joel Yaffe "Effect of phosphor persistence on image quality in digital x-ray scanning systems", Proc. SPIE 2708, Medical Imaging 1996: Physics of Medical Imaging, (11 April 1996); https://doi.org/10.1117/12.237847

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