INFLUENCE OF X-RAY-ENERGY SPECTRUM, CONTRASTING DETAIL AND DETECTOR ON THE SIGNAL-TO-NOISE RATIO (SNR) AND DETECTIVE QUANTUM EFFICIENCY (DQE) IN PROJECTION RADIOGRAPHY

A lower limit to patient irradiation in diagnostic radiology is set by the fundamental stochastics of the energy imparted to the image receptor (quantum noise). Image quality is investigated here and expressed in terms of the signal-to-noise ratio due to quantum noise. The Monte Carlo method is used to calculate signal-to-noise ratios (SNR(DELTA-S)) and detective quantum efficiencies (DQE(DELTA-S)) in imaging thin contrasting details of air, fat, bone and iodine within a water phantom using x-ray spectra (40-140 Kv) and detectors of CsI, BaFCl and Gd2O2S. The atomic composition of the contrasting detail influences considerably the values of SNR(DELTA-S) due to the different modulations of the energy spectra of primary photons passing beside and through the contrasting detail. By matching the absorption edges of the contrasting detail and the detector, a partially absorbing detector may be more efficient (yield higher SNR(DELTA-S)) than a totally absorbing one; this is demonstrated for the case of detecting an iodine detail using a CsI detector. The degradation of SNR(DELTA-S) and DQE(DELTA-S) due to scatter is larger when the detector is operated in the photon counting compared to in the energy integrating mode and for partially absorbing compared to totally absorbing detectors.