Noise in energy-discriminating photon-counting x-ray imaging detectors

Abstract

A generalized approach to describing transfer of signal and noise (MTF and NPS) through medical imaging systems has been developed over the past several years in which image-forming processes are represented in terms of serial and parallel cascades of amplified point processes. We use the techniques of both cascaded systems analysis and stochastic point process theory to develop fundamental limitations of system performance for single photon counting (SPC) x-ray imaging detectors to assist in the optimal design of new systems. Using this approach the mean signal and signal variance for a simple model of a hypothetical flat-panel x-ray imaging detector are calculated. It is shown that energy imprecision is ultimately determined by the number of secondary quanta collected by the detector. Successful designs will likely have small work-function values and/or high collection efficiencies as well as adaptive binning strategies to collect all secondary quanta liberated for each x-ray interaction. K-escape, responsible for Swank noise in traditional imging detectors, can result in increases of relative imprecision to 50% and will impose a severe limitation on the ability of these detectors to determine x-ray spectra and methods will have to be incorporated into new detector designs to overcome these limitations. With the exception of K-escape effects, Se-based and CsI-based detectors could potentially measure spectra with energy RMS imprecision of 3 - 10%.