Artifacts In Magnetic Resonance Imaging and Computed Tomography Caused By Dental Materials

BACKGROUND: Artifacts caused by dental restorations, such as dental crowns, dental fillings and orthodontic appliances, are a common problem in MRI and CT scans of the head and neck. The aim of this in-vitro study was to identify and evaluate the artifacts produced by different dental restoration materials in CT and MRI images. METHODS: Test samples of 44 materials (Metal and Non-Metal) commonly used in dental restorations were fabricated and embedded with reference specimens in gelatin moulds. MRI imaging of 1.5T and CT scan were performed on the samples and evaluated in two dimensions. Artifact size and distortions were measured using a digital image analysis software. RESULTS: In MRI, 13 out of 44 materials produced artifacts, while in CT 41 out of 44 materials showed artifacts. Artifacts produced in both MRI and CT images were categorized according to the size of the artifact. SIGNIFICANCE: Metal based restoration materials had strong influence on CT and less artifacts in MRI images. Rare earth elements such as Ytterbium trifluoride found in composites caused artifacts in both MRI and CT. Recognizing these findings would help dental materials manufacturers and developers to produce materials which can cause less artifacts in MRI and CT images

A new conceptual framework for the erosion of fine sediments from a gravel matrix based on experimental analysis

An experimental non-intrusive methodology is proposed to estimate the spatially averaged erosion rate of fine sediment from a gravel bed. The experiments performed in a laboratory flume show a progressive slow-down of the erosion rate as the level of fine sediment becomes deeper within the gravel matrix, until a maximum depth of erosion is reached. Two original relations are proposed for the maximum cleanout depth and the erosion rate, based on a dimensional analysis applied to the experimental results. The proposed erosion rate relation modifies the original Van Rijn formula for uniform bed, introducing a damping function below the gravel crest. Both the evolution of the erosion rate with depth and the maximum depth of erosion can be defined as simple functions of the general characteristics of the flow and the fine and coarse fractions of the sediment. Our approach will lead to improved estimates of the conditions under which fine sediments that have infilled gravel beds are re-entrained. This will help inform strategies aimed at restoring degraded river systems and mitigating the undesired side effects of activities such as sediment flushing which can result in colmation

A mass-conservative semi-implicit volume of fluid method for the Navier–Stokes equations with high order semi-Lagrangian advection scheme

This paper deals with the development of a semi-implicit numerical method for the Navier–Stokes equations using the non-linear volumes of fluid (VOF) approach and a semi-Lagrangian scheme for the discretization of the advection contribution based on a high order reconstruction of the velocity field. The VOF approach guarantees high flexibility and is able to reproduce several phenomena that appear in real scenarios such as free surface flows, pressurized channels and jets. The discrete velocity field from the momentum conservation law is formally inserted into the discrete continuity equation, hence yielding a mildly non-linear system for the unknown hydraulic head which can be solved through a nested Newton-type algorithm. The computation of the non-linear convective diffusion contribution is then based on a high order reconstruction of the velocity field, which is furthermore constrained to exactly recover the original pointwise values of the numerical solution. As a consequence, the mass conservation is fully preserved while providing information about the main velocity field and its high order moments, later employed in the computation of the Lagrangian trajectories needed for the discretization of the convective and diffusive terms. Furthermore, the bottom friction and the tangential stresses can be directly computed from the high order velocity reconstruction. The method is derived in a general form with the only requirement to be structured in the z− direction, so that it applies to the three- and the two-dimensional cases with unstructured grids in the horizontal space. Convergence studies are carried out to demonstrate the accuracy of the reconstruction operator. Finally, the numerical scheme is validated against several benchmarks that include 2Dxz, 2Dxy and 3D non-hydrostatic flows with complex geometry in order to show the flexibility of the proposed algorithm, including a real-world application