Thermographic Anatomy and Physiology
James Christiansen, PhD
Heat is continuously produced in the body as a by-product of metabolism. Three major factors determine the rate of heat production: basal metabolic rate, specific organ activity, and local muscular activity.
A person’s basal metabolic rate is largely a function of predetermined hypothalamic set points in the central nervous system (CNS) and hormonally controlled metabolism, influenced primarily by secretions from the hypo- thalamus and thyroid glands. Metabolic heat production remains constant throughout the thermoneutral range of environmental temperatures, principally through reflex changes in the body’s insulative and heat distribution mechanisms. These reflex vascular changes may alter the thermal profile as seen thermographically, but they have little importance in the interpretation of diagnostic thermography. Instead, diagnostic thermography is based for the most part on thermal asymmetries, right to left, and the relation of discrete surface areas to their surroundings. Although thermography has been used in an attempt to measure basal metabolic rate (1), it has little relevance for clinical diagnosis and will not be considered here.
The metabolic activity of individual organs may have a profound effect on
the overall temperature of the body. For example, metabolic heat production following a meal has been termed “food inducible thermogenesis” (2), and is related to increased metabolism during digestion. Disease processes within visceral organs may also stimulate metabolism. Bacterial and intrinsic pyrogens may produce fever, such that body temperature may increase dramatically. The heat generated by visceral metabolism is transferred directly to the blood passing through these organs.