Optimal Beam Quality in Chest Radiography Using CsI-flat Panel Detector for Detection of Pulmonary Nodules

Objectives: Optimal beam quality for detection of pulmonary nodules in digital chest radiography using CsI-flat panel detector (FPD) was investigated in consideration of image quality and patient dose. Methods: The human chest phantom with inserted imitated nodules (diameter: 10 mm, CT value: +30 Hounsfield unit (HU), –375 HU, –620 HU) was used for the measurement of contrast-to-noise ratio (CNR) of imitated nodules by twenty beams arranged by five tube voltages and four filters. Results: The CNR varies with X-ray tube voltage and added filter. CNR correlates weakly to the tube voltage, fairly to the effective energy in second-order polynomial and strongly to the quality index (effective energy divided X-ray tube voltage). In order to improve the CNR, the effective energy and the quality index are kept about 50 keV and more than 0.5, respectively, using an 80–100 kV beam with a copper filter. Conclusion: A 90 kV (2.5 mm Al inherent filtration) beam with a 0.15 mm copper filter and a 90 kV or 100 kV (2.5 mm Al inherent filtration) beam with a 0.2 mm copper filter are appropriate for chest radiography using CsI-FPD.