Development of a beam hardening filter and characterization of the spatial resolution for a Cone Beam CT imaging system

摘要

Generally, Cone Beam CT imaging systems utilize polychromatic x-ray beams since they are easier to generate and their output power is larger than those from monochromatic beams. However, polychromatic beams have main drawbacks in medical imaging. For example, the relative high content of low energy photons induced two main phenomena that degrade the image quality. First, most of the low energy photons are absorbed within the soft tissue hardening the beam. This beam hardening deteriorates the image quality by decreasing the uniformity. Second, due to the polychromatic nature of the beam and beam hardening, the image is degraded by high contrast objects. In order to solve these problems, a filter was developed and implemented in a Flat Panel Detector-based cone Beam CT. First, the energy spectrum was acquired and computer simulations were done to evaluate and to develop the best suitable filter to harden the x-ray beam. Second, experiments were performed to evaluate the improvements on image quality based on the reduction of artifacts due to high contrast objects on two objects, namely a bone with titanium screws and a hand phantom. Lastly, the spatial resolution was evaluated to investigate the effects of this filter. The results demonstrate that the new developed filter improves the image quality in this area. The high contrast artifacts were also reduced as the images from the bone phantom with the metal implant illustrates. Evidently, the use of this filter to harden to beam has increased the image quality when metal implants are present.