Image quality in diagnostic x-ray detectors is limited by statistical properties governing how, and where, x-ray energy is deposited in a detector. This in turn depends on the physics of underlying x-ray interactions, and the development of theoretical models of x-ray interaction physics is therefore a critical step in optimal detector design and assessment. While cascaded-systems analyses are often used to describe image signal and noise in many systems, it has always been assumed there is only a single element (single Z) with which all x rays interact even though most commonly used and promising candidates are compound materials. In addition, coherent and incoherent scattering and their effects on image quality are usually ignored but may be important in some situa- tion such as in low-Z atoms with high x-ray energies. We present a theoretical model of energy deposition within a double-Z x-ray detector material that addresses the nature of energy absorption following photoelectric and incoherent interactions and the effects of coherent scatter prior to energy deposition by photoelectric interactions. A cascaded systems approach is used to describe the transfer of signal and noise in terms of the modulation transfer function (MTF), Wiener noise power spectrum (NPS), and detective quantum efficiency (DQE). The model is validated by comparing Monte Carlo simulation results with CsI and PbI2 double-Z materials. Excellent agreement is obtained for each metric over the entire diagnostic energy range up to 10 cycles/mm. It is shown that in all cases tested, a combination of two single-Z models weighted by the atomic density of each atom type gives equivalent results to the more comprehensive double-Z model within a few percent. This result suggests the simpler model is adequate and may be preferred for the optimal design of conventional radiography detectors and the estimation of x-ray imaging performance of novel photoconductor materials.

Date Published: 19 March 2013
PDF: 11 pages
Proc. SPIE 8668, Medical Imaging 2013: Physics of Medical Imaging, 86682L (19 March 2013); doi: 10.1117/12.2007999

Published in SPIE Proceedings Vol. 8668:
Medical Imaging 2013: Physics of Medical Imaging
Robert M. Nishikawa; Bruce R. Whiting; Christoph Hoeschen, Editor(s)