The Layering Phenomenon and Boundary Formation in Radiographs

Abstract

In a radiographic examination using a water-soluble contrast medium this may form a layer beneath a body fluid. Between the two liquids a zone consisting of a mixture of the two liquids then forms through diffusion. This diffusion layer produces some characteristic features in the radiographic image, an analysis of which was the purpose of the model experiments performed in this investigation. In this analysis of the layering phenomenon the radiographed objects were cylindrical tubes of methyl methacrylate partly filled with water. In some cases a rod was placed concentrically in the tube. Contrast medium was layered below the water. Radiographs were produced with the tube either vertical or inclined, and with either a horizontal or a vertical projection. In the image the layer of contrast medium was visualized as a light field, and the water layer as an overlying relatively dark field. The diffusion layer was visualized as a transitional zone—the diffusion field. Distinct boundaries and Mach lines observed in the bottom field were produced by the interface between the contrast medium and the solid wall where it was touched by the roentgen rays. These boundaries continued into the diffusion field where they gradually became less visible and eventually disappeared. The upper and lower boundaries of the diffusion field were diffuse and associated with dark and light Mach bands, respectively. The upper boundary appeared to be convex upwards. In the case of the inclined model and a vertical beam the diffusion field was elliptical, with a still more diffuse transition to the fields above and below than in the case of the vertical model and a horizontal beam. When layering of the contrast medium and a body fluid occurs the imaged distinct boundary of the layer of medium always represents the surface of a solid anatomic structure. A diffuse boundary, on the other hand, can be produced by the diffusion layer, itself.