X-ray tube potential, filtration, and detector considerations in dual-energy chest radiography

The effect of x-ray tube potential and prepatient and interdetector filtration in single exposure dual energy chest imaging has been studied employing a carefully benchmarked model. The analysis utilized published methodology. Noise in simulated lung and mediastinum fields of the aluminum (bone) and Lucite (soft tissue) images were studied at fixed entrance skin exposure (ESE) for commonly employed sandwich detector and sandwich imaging plate configurations. Our results indicate noise in the lung increases slowly with tube potential above 120 kVp, while noise in the mediastinum decreases rapidly. Also, at high tube potential (> or = 120 kVp) adding moderate amounts of prepatient K-edge filtration (approximately equal to 100 mg/cm2) while optimizing imaging conditions for the lung tends to decrease noise in the lungs by approximately equal to 30% while increasing noise in the mediastinum by a similar amount. Without K-edge prepatient filtration, noise in the lung is minimized with Cu interdetector filter weights near 400 mg/cm2. In the mediastinum noise is minimized with heavier interdetector filter and prepatient K-edge filter weights. Prepatient K-edge filter weights that minimize image noise in either field can increase the tube loading by factors ranging from 10 to 10(10). Systems designed with sandwich detectors using commercially available phosphors and coating weights can produce contrast-to-noise ratios (CNRs) as high as 50% of the theoretical limit (defined as an optimized system with a totally absorbing rear detector).