ROENTGENOGRAPHIC VISUALIZATION AND MEASUREMENT OF SKIN THICKNESS AND ITS DIAGNOSTIC APPLICATION IN ACROMEGALY.

Abstract

Radiographic examination of the skin has attracted little attention. In standard textbooks of radiology, the chapters on soft-tissue radiography are mainly concerned with fat, muscle, and calcification. Such lack of interest is at least partly due to easy access to the skin by established dermatologie methods. Nevertheless, the estimation of its thickness by palpation is understandably inaccurate, and in the available literature no radiographic technic of measurement has been described. Such a method would provide a simple, rapid, painless, and inexpensive means of measuring skin thickness and would enable accurate studies of changes as they relate to sex, aging, complexion, exposure to sunlight, and perhaps certain morbid conditions. The purpose of this paper is to describe some relatively simple methods for visualization and accurate measurement of skin thickness and to give some preliminary results on their application. Theoretical Development And Methods The epidermis and corium have approximately the same roentgen absorption, and together they appear in the roentgenogram as a zone of decreased photographic density (hereafter referred to as “skin”), delineated internally by the subcutaneous fat. Skin thickness varies considerably in different parts of the body, and the site of observation should therefore be strictly standardized. In this work, the radial aspect of the right forearm was chosen. Since the epidermal thickness of the forearm measures only about 40 to 80 microns (5), the roentgenographic method will largely determine the thickness of the corium layer. Inadequate visualization of the skin in standard roentgenograms seems to be due to: (a) the marked difference in roentgen absorption between the fat and the air on each side of the skin and (b) the rounded outline of the skin surface resulting in a photographic density gradient near the surfaces tangential to the roentgen beam. Roentgen absorption in various tissue layers of an extremity surrounded by air is schematically indicated in Figure 1A. The calculated absorption curve (a) shows that the darker appearance on film of the zone of subcutaneous fat is largely an optical illusion, mainly due to a small difference in roentgen absorption in a narrow zone at the skin-fat margin (S-F on the magnified portion of the absorption curve in the insert of Fig. 1A), but to a certain extent also caused by a contrast effect of the lighter muscle density on the other side of the subcutaneous fat. Densitometrie readings (d), indicated by A, S, F, and M in the same graph to designate photographic densities of air, skin, fat, and muscle, respectively, closely conformed to the calculated absorption curve, thus proving the existence of lower photographic density in the central zone of subcutaneous fat than in the skin, in the 40 to 45 kvp range used in this technic. In the same diagram the principal effects of two different roentgen exposures on blackening of the roentgen film are shown.