Effects of x-ray spectrum on the NPS of a computed radiography system

The purpose of this work was to determine the effects of various incident x-ray beam spectra on the measured noise power spectrum (NPS) of a computed radiography (CR) image acquisition device. A CR phosphor was uniformly exposed to 1 mR with x-ray beams whose peak tube potentials were 70, 95, and 120 kVp that were filtered by various thicknesses of a "patient equivalent phantom" (PEP; 2% aluminum, 98% acrylic by thickness), aluminum, and copper. From the uniform exposure images, NPS curves were calculated and their integral values were computed. The integral noise values were found to vary substantially as a function of x-ray beam spectral content. A simple x-ray beam and filter model that accounted for the shape of the filtered x-ray spectra and the mass energy absorption coefficient of the storage phosphor verified the qualitative behavior of the integral NPS values corresponding to changes in the incident x-ray beam used. The x-ray beam and filter combinations of 70 kVp and 10.1 cm of PEP filtration and 120 kVp and 20.2 cm of PEP filtration were chosen as standard techniques for evaluating clinical imaging systems. These two combinations represent a relatively low, clinically relevant CR noise (integral NPS = 2.6x10(-6) mm) technique and a relatively high, clinically relevant CR noise (integral NPS 3.3x10(-6) mm) technique.