Development of K-edge spectral tomography using XPAD3 composite pixels

Hybrid pixel cameras are new devices in which photon counting replaces charge integration used in standard radiography systems. This novel approach involves several advantages, in particular the absence of dark noise integration and the ability to set an energy threshold on each pixel of the detected photons. This ability is of uppermost importance for the development of K-edge imaging approaches, which exploit spectral information on the counted photons. We investigate the capability to perform K-edge imaging by using the hybrid pixel camera XPAD3 with 500,000 pixels of (130 × 130) μm² integrated in the micro-CT demonstrator PIXSCAN II. K-edge images are obtained from three sets of projections acquired from the same subject under three different energy threshold conditions around the K-shell binding energy of a contrast agent. To minimize the time dependent changes of the biological samples between the three acquisitions, we have developed a composite pixel protocol allowing simultaneous acquisitions of the projections for three different energy thresholds. This methodology reduces the acquisition duration by a factor three at the expense of a corresponding loss of spatial resolution. We nevertheless demonstrate the validity of the method on different phantoms and finally on mice.