Dual-energy technique for digital flat-panel detectors without x-ray tube voltage switching

Dual-energy imaging increases the possibility of pulmonary nodule detection by reducing the bone structure noise. The major problem of the dual-energy acquisition process with digital flat-panel detectors is the interval of time between low-energy (LE) exposure and high-energy (HE) exposure. Due to misregistration between LE and HE images, motion artifacts pollute the subtracted image. This paper presents a new acquisition approach for dual-energy imaging developed in order to reduce this inter-exposure time. The idea is to keep the tube voltage constant and to just switch a filter in front of the imaged object and thus to modulate the outgoing x-ray spectrum. The first part of this study presents how to optimize system parameters for the new acquisition protocol: source voltage, dynamic filtration before the patient, exposure time for LE and HE acquisition. The tube load is kept constant to focus the optimization study on the dose and the exposure time. A noise quality factor (NQF) and a spectral quality factor (SQF) are used as criteria for optimization. The new approach system is then compared to the state-of-the-art system with voltage switching between low and high energy. A filtering algorithm of dual energy acquisitions enabling a significant noise reduction is presented. Performance between its combination with the new acquisition protocol and the reference one are compared. For a limited noise quality factor, three times faster acquisition time is obtained using the new system. Noise reduction techniques improve the image SNR by 61% in the new system and only 32% in the reference system, without taking into account the impact of better registration on the dual-energy image quality.