Addressing CT metal artifacts using photon‐counting detectors and one‐step spectral CT image reconstruction

Abstract

The constrained one-step spectral CT image reconstruction (cOSS-CIR) algorithm with a nonconvex alternating direction method of multipliers optimizer is proposed for addressing computed tomography (CT) metal artifacts caused by beam hardening, noise, and photon starvation. The quantitative performance of cOSSCIR is investigated through a series of photon-counting CT simulations. Methods: cOSSCIR directly estimates basis material maps from photoncounting data using a physics-based forward model that accounts for beam hardening. The cOSSCIR optimization framework places constraints on the basis maps, which we hypothesize will stabilize the decomposition and reduce streaks caused by noise and photon starvation. Another advantage of cOSSCIR is that the spectral data need not be registered, so that a ray can be used even if some energy window measurements are unavailable. Photon-counting CT acquisitions of a virtual pelvic phantom with low-contrast soft tissue texture and bilateral hip prostheses were simulated. Bone and water basis maps were estimated using the cOSSCIR algorithm and combined to form a virtual monoenergetic image for the evaluation of metal artifacts. The cOSSCIR images were compared to a "two-step" decomposition approach that first estimated basis sinograms using a maximum likelihood algorithm and then reconstructed basis maps using an iterative total variation constrained least-squares optimization (MLE+TV min ). Images were also compared to a nonspectral TV min reconstruction of the total number of counts detected for each ray with and without normalized metal artifact reduction (NMAR) applied. The simulated metal density was increased to investigate the effects of increasing photon starvation. The quantitative error and standard deviation in regions of the phantom were compared across the investigated algorithms. The ability of cOSSCIR to reproduce the soft-tissue texture, while reducing metal artifacts, was quantitatively evaluated. Results: Noiseless simulations demonstrated the convergence of the cOSS-CIR and MLE+TV min algorithms to the correct basis maps in the presence of beam-hardening effects. When noise was simulated, cOSSCIR demonstrated a quantitative error of −1 HU, compared to 2 HU error for the MLE+TV min algorithm and −154 HU error for the nonspectral TV min +NMAR algorithm. For the cOSSCIR algorithm, the standard deviation in the central iodine region of interest was 20 HU, compared to 299 HU for the MLE+TV min algorithm, 41 HU for the MLE+TV min +Mask algorithm that excluded rays through metal, and 55 HU for the nonspectral TV min +NMAR algorithm. Increasing levels of photon starvation did not impact the bias or standard deviation of the cOSSCIR images. cOSSCIR was able to reproduce the soft-tissue texture when an appropriate regularization constraint value was selected.

Low-Dose CT Imaging of the Pelvis in Follow-up Examinations—Significant Dose Reduction and Impact of Tin Filtration

“…18 Photon-counting detector CT scanners mark the beginning of a new era in CT imaging. [45][46][47][48] In addition to reconstruction algorithms, [49][50][51] tin filtering has also been described as a valuable method for reducing metal artifacts in photon-counting detector CTs. 52 Therefore, tin filter technology could also play a key role in photon-counting detector CT.…”

Experimental dual-kV reconstructions of objects containing metal using the cOSSCIR algorithm

“…Enforcing convex constraints, such as total-variation, has been shown to be effective for undersampling problems, as may occur with the presence of metal, stabilizing the inversion. 9 For this study, anisotropic TV was used.…”

“…In the case of Photon-counting CT, this approach has been found to be particularly effective at addressing metal artifacts. [7][8][9][10] Recently, the extension of cOSSCIR to the dual-kV problem has been demonstrated in preliminary studies using idealized simulations and small experimental phantoms. In our experimental dual-kV studies, material quantification was investigated using registered and unregistered data without the presence of metal, and was found to be generally accurate, with grayscale errors no greater than 4 % observed in 50 keV virtual mono-energetic images.…”

Direct Multi-Material Reconstruction via Iterative Proximal Adaptive Descent for Spectral CT Imaging

“…Barber et al [ 25 ] applied a primal-dual prototype framework [ 26 ] to the material imaging of spectral CT. Very recently, they further proposed investigating the convergence theory of the non-convex alternating direction of multipliers method (NcADMM) [ 27 ] and conducted the reconstruction of the PCD system to reduce beam hardening and metal artifacts [ 28 ].…”