Investigation of the effect of tube voltage and imaging geometry on phase contrast imaging for a micro-CT system

Based upon a bench-top micro-CT system, propagation-based phase-contrast imaging has been investigated using insects and a thin plastic sheet. The system mainly includes a micro-focus source with focal spot size of 13–20 μm and a cooled X-ray CCD detector with pixel size of 24 μm. The edge-enhancement effect can be found clearly in the acquired images. With a 0.5 mm thickness plastic edge phantom, the effects of X-ray tube voltage and imaging geometry on the phase-contrast imaging were investigated, and quantitative index, edge-enhancement index (EEI), were also calculated. In our study, an interesting phenomenon was observed that the phase-contrast effect becomes more pronounced as the tube voltage increases from 20 kVp to 90 kVp. Further investigation indicates that smaller focal spot size resulting from the reduction of tube current at higher tube voltage, has caused the unexpected phenomenon. Inferred from our results, phase-contrast effect is insensitive to the tube voltage in the range of 20–90 kVp (widely used in medical diagnosis); however, it is sensitive to the focal spot size. In addition, for the investigation of the effect of imaging geometry, an optimal geometric magnification range of 2.5–4.5 is suggested to get a good phase-contrast imaging for a micro-CT system with source-to-detector distance of 720 mm.