The Appearance of Liquid Surfaces and Layers in Routine Radiographs

Abstract

Image features in the radiograph produced by deformation of a liquid surface by surface tension and by the density gradient in a diffusion layer may present unexpected difficulty of interpretation. Such features have been analysed in model experiments, which have been reported earlier. The aim of the present investigation was to examine the occurrence and the clinical implications of corresponding phenomena in routine radiographs. In the human body liquid surfaces and diffusion layers can occur only in cavities, both normal and abnormal. A liquid surface tends to extend up a cavity wall to form a meniscoid or, if the cavity is small enough, a discoid. The liquid surface continues further up the wall as a liquid film. The shape of the meniscoid and the discoid varies with the shape and inclination of the wall. Most of the image features of interest are produced by rays that are tangential to a horizontal surface, a meniscoid, a discoid or a concave wall, any of which is visualized as an internal boundary with a light Mach line. When the wall is convex towards the cavity the meniscoid is saddle-shaped and an external boundary with a dark Mach line is produced. The horizontal part of a liquid surface can be touched only if it is at the same level as the focus of the roentgen tube. A liquid surface at any other level can be touched in its meniscoid only by rays that are not horizontal. It is reproduced as an internal boundary, slightly concave upwards; above this boundary the rest of the liquid surface is reproduced as a wedge field. In the case of a discoid all the rays of a horizontal beam produce an internal boundary, concave upwards. When a water-soluble contrast medium is layered below a body fluid 3 layers can be distinguished. The top layer consists of the body fluid, the bottom layer of the contrast medium and the intervening one of a mixture of the 2 liquids, the diffusion layer. In the image the 3 layers produce 3 corresponding fields, a dark top field and a light bottom field, separated by a field in which the density changes continuously from one extreme to the other. The interface between the top layer and an organ wall does not produce any boundary in the image, because of the similarity in their attenuation. When the part of an organ wall in contact with the contrast medium is concave and is touched by the rays, the medium is demarcated at the wall by a distinct external boundary with a dark Mach line. The perceptibility of this boundary decreases as it continues up through the diffusion layer. When the beam is vertical and the organ wall is inclined, this distinct boundary changes abruptly into a diffuse one, which no longer represents the wall but the transition between the medium and the body fluid. If the wall of an organ is surrounded by a fat capsule the outer bounding surface of the wall, which is convex outwards, produces an external boundary, which is parallel to the distinct boundary between the contrast medium and the inner surface of the wall. When the boundary of the medium becomes diffuse it no longer follows the outer boundary, and then no longer represents the interface to the wall's internal surface. A diffuse boundary then conceivably does not represent an anatomic structure, and the thickness of the wall can no longer be perceived. Structures in the bottom of the diffusion layer consisting of a substance of lower attenuation than the medium—contrast defects—are demarcated by an internal boundary with a light Mach line when the interface is convex to the medium.

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