Experimental investigation of metal artifact reduction using one-step spectral CT reconstruction

The constrained one-step spectral CT image reconstruction algorithm (cOSSCIR) is a model-based approach that iteratively estimates basis material maps directly from a set of transmission measurements. By incorporating a model of the imaging system's x-ray beam and detector response, the method can be applied to both dual-kV and photon-counting problems. Since the polyenergetic composition of the x-ray beam is incorporated into the reconstruction, cOSSCIR has the potential to address beam hardening metal artifacts using accurate spectral estimation for both modalities. Furthermore, the use of convex constraints in the inversion procedure can be leveraged to address photon starvation caused by metal using a masking technique.

The purpose of this study was to experimentally investigate metal artifact reduction performance within a one-step framework for spectral CT reconstruction.

Dual-kV and photon-counting transmission measurements were taken through a step-wedge phantom and were used to form spectral models for an energy-integrating and photon-counting detector for our benchtop micro-CT imaging system. Then experiments were performed using an electron-density rod phantom with and without metal to evaluate the effect of spectral modeling and masking on the cOSSCIR reconstructions for both imaging modalities. Further experiments were performed using a biological specimen to demonstrate cOSSCIR and the artifact reduction techniques on a phantom with more complicated structure. Finally, cOSSCIR was demonstrated on head CT data with dental hardware obtained from a first-generation silicon PCCT clinical prototype.

The cOSSCIR reconstructions demonstrated a reduction in metal artifacts for both dual-kV and photon counting data. For a region between the metal inserts of the electron-density phantom, the mean absolute errors in the 60 keV dual-kV virtual mono-energetic images were 115 HU for FBP, compared to 87 HU and 41 HU using cOSSCIR and cOSSCIR with a mask, respectively. Likewise, the mean absolute errors in the 60 keV virtual mono-energetic images were 217 HU for FBP, 71 HU using cOSSCIR, and 22 HU using cOSSCIR with a mask in the 60 keV photon-counting virtual mono-energetic images. In a region adjacent to the metal, the dual-kV images had mean absolute errors of 100 HU for FBP, 53 HU for cOSSCIR, and 41 HU for cOSSCIR+Mask and the photon-counting images had errors of 74 HU for FBP, 27 HU for cOSSCIR, and 63 HU for cOSSCIR+Mask. The additional spectral information available from the photon-counting detector further reduced beam hardening and metal artifacts compared to the dual-kV images.

cOSSCIR reduced metal artifacts in both dual-kV and photon-counting x-ray CT. When combined with a masking technique, the cOSSCIR reconstructions demonstrated further artifact reduction, although masking introduced new artifacts in some cases. In general, our results show that the cOSSCIR photon-counting reconstructions compare favorably to dual-kV and filtered backprojection, and when using a mask, can fully resolve regions in between two metal inserts for the objects scanned in this study.