THERMOGRAPHY.

Abstract

Thermography is a form of radiography portraying body-surface temperatures, and thermograms are photographic reproductions of infrared rays spontaneously emanating from the body. Apparatus is now available by which infrared rays can be converted to the visible portion of the electromagnetic spectrum. It is thus possible to expose ordinary Polaroid film, and thermograms so obtained are composed of various shades of gray, the lighter tones indicating the hotter areas and the darker tones, colder regions of the body's surface. Reading thermograms is not unlike interpreting roentgenograms, so that the art comes naturally to the roentgenologist. Because differences in gray tones are expressions of temperature differences, greater accuracy of interpretation is achieved by the use of a densitometric comparator (1). Finally, comparison of roentgenograms and thermograms often proves mutually helpful in formulating a diagnosis so that both disciplines can be expeditiously managed by the roentgenologist. In contrast to a single temperature recording with the mercury thermometer, thermography yields a quantitative thermal map of the entire outer surface of the body. Since skin temperatures largely depend upon the heat generated by the vascular sources in and beneath the surface and by the heat conducted to it from underlying tissues or organs, diagnostic information can be gained easily from the thermogram (2). Much thought has been given to the relationship which exists between body surface temperatures and underlying disease states (3, 5–9, 13). Stoll and Hardy compared many methods of making quantitative measurements of the temperature of living skin and agreed that non-contact devices are definitely superior to those touching the skin (10). All objects above absolute temperature emit infrared energy and those with temperatures at or near that of the body emit this energy in a broad band of wave lengths having a maximum intensity close to 10 μ. Emissivity for a given surface depends upon the chemical nature of the substance and the physical condition of its surface; it is a complex quantity which may vary widely over narrow intervals of the spectrum. Emissivity approaches zero for highly polished metallic surfaces and unity for totally absorbing surfaces having no transmission or reflection, referred to as “black bodies.” Hardy concluded from extensive studies that the human skin behaved like a black body and this is assumed in all of our work (11). While skin pigmentation plays a major role, including emissivity in the visible spectrum, it does not exert any influence in the infrared region where the skin emits this energy. Thus meaningful skin temperature measurements may readily be made in this region regardless of race, suntan, or any skin blemishes.