Abstract
BackgroundMetal artifacts appearing as streaks and shadows often compromise readability of computed tomography (CT) images. Particularly in a dental CT in which high resolution imaging is crucial for precise preparation of dental implants or orthodontic devices, reduction of metal artifacts is very important. However, metal artifact reduction algorithms developed for a general medical CT may not work well in a dental CT since teeth themselves also have high attenuation coefficients.MethodsTo reduce metal artifacts in dental CT images, we made prior images by weighted summation of two images: one, a streak-reduced image reconstructed from the metal-region-modified projection data, and the other a metal-free image reconstructed from the original projection data followed by metal region deletion. To make the streak-reduced image, we precisely segmented the metal region based on adaptive local thresholding, and then, we modified the metal region on the projection data using linear interpolation. We made forward projection of the prior image to make the prior projection data. We replaced the pixel values at the metal region in the original projection data with the ones taken from the prior projection data, and then, we finally reconstructed images from the replaced projection data. To validate the proposed method, we made computational simulations and also we made experiments on teeth phantoms using a micro-CT. We compared the results with the ones obtained by the fusion prior-based metal artifact reduction (FP-MAR) method.ResultsIn the simulation studies using a bilateral prostheses phantom and a dental phantom, the proposed method showed a performance similar to the FP-MAR method in terms of the edge profile and the structural similarity index when an optimal global threshold was chosen for the FP-MAR method. In the imaging studies of teeth phantoms, the proposed method showed a better performance than the FP-MAR method in reducing the streak artifacts without introducing any contrast anomaly.ConclusionsThe simulation and experimental imaging studies suggest that the proposed method can be used for reducing metal artifacts in dental CT images.
Highlights
Metal artifacts appearing as streaks and shadows often compromise readability of computed tomography (CT) images
In a dental CT, wrong metal segmentation is a big concern since teeth have x-ray attenuation coefficients that are not very different from those of metallic objects [14]
The metal artifact reduction algorithm Segmentation of metal trace in the projection images Due to higher attenuation coefficients of metallic dental devices than those of biological tissues and teeth, metallic devices make high contrast in every projection image acquired by a 3D dental CT
Summary
Metal artifacts appearing as streaks and shadows often compromise readability of computed tomography (CT) images. Metal artifact reduction algorithms developed for a general medical CT may not work well in a dental CT since teeth themselves have high attenuation coefficients. High attenuation of the x-ray beam in the metallic objects induces signal saturation in the projection data, beam hardening, photon scattering, and photon starvation, all of which contribute to producing metal artifacts. To reduce the metal artifacts effectively, generating prior images, that can provide missing projection data at the metal trace, is critical. In a dental CT, wrong metal segmentation is a big concern since teeth have x-ray attenuation coefficients that are not very different from those of metallic objects [14]. A metal artifact reduction method has been introduced with consideration of the beam hardening effect of a polychromatic x-ray beam [24]
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