Physical measures of image quality in photostimulable phosphor radiographic systems

The purpose of this study was to critically assess and compare the intrinsic resolution, noise; and signal-to-noise transfer characteristics of two modern digital photostimulable phosphor (PSP) radiographic systems. Two commercial PSP systems (FCR-9501/FCR-9501-HQ, Fugi Medical Systems and KESPR-400, Eastman Kodak Company) were evaluated by identical methodologies. Measurements were made at three beam qualities (70, 115, and 150 kVp) using standard-resolution (ST-Va and GP-25) and high-resolution (HD) screens. The presampled modulation transfer functions (MTF) of the systems were measured using an edge method. The noise power spectra (NPS) were determined by 2D Fourier analysis of uniformly exposed radiographs. The frequency-dependent detective quantum efficiencies (DQE) were obtained from the MTF and NPS measures and the input signal-to-noise ratio, determined by a computational model for x-ray spectrum which was verified against exposure and half-value-layer measurements. The physical performance of the systems were very similar; a DQE(0) of 0.24 and a spatial frequency of 2.5 cycles/mm at 0.2 MTF were estimated for both systems at 115 kVp using a pixel size of 0.1 mm and standard-resolution screens. The HD screen provided improved MTF (50% increase in the frequency at 0.2 MTF) with an adverse effect on noise (50% decrease in DQE(0)) as compared to the GP-25 screen using the same pixel size and beam quality.