The Physiological Basis for Clinical Thermography and the
Detection of Infrared Radiation from the Human Body
Peter Leando PhD.
The emissivity of human skin is almost 100% (close to black body emissivity) so the human is an ideal subject for thermographic imaging.
All thermographic images (thermograms) of the human body depend on the sympathetic control of skin blood-flow.
Only the dermal blood flow changes explain the heat seen on the surface of the body. The heat of a muscle, joint, or a bone is not conducted to the dermal tissues and cannot influence the dermal temperature recorded by thermography. Conduction of heat from the deeper portions of the body to the surface does not occur or create changes in the surface temperature. The major basis of clinical thermography is the correlation of temperature recordings with various conditions from disease and injury as it relates to autonomic function.
Temperature change associated with disease has been known for centuries. The development of instrumentation to produce thermal
maps of large surfaces has provided a new approach to the study of body heat in health and disease.
The microcirculation of the dermis can be recorded by thermography, and this depth of temperature recording is approximately 5
mm, which is the dermal depth in most areas of the body.
There are major influences on this dermal temperature other than neurogenic control. When performing thermography, each of the influences must be evaluated clinically to ascertain the cause of any asymmetry that might occur. The major vascular areas that maybe responsible for the asymmetry are arterial perfusion, venous drainage, and lymphatic content. The skin, in disease and injury, as well as its surface condition, will also be an influence.
From a neuromusculoskeletal viewpoint and in regard to sympathetic hyperdysfunction, the above conditions must be ruled out clinically. Thus, palpation of peripheral radial, ulnar, or dorsal pedal pulses can grossly suggest normal arterial perfusion. Review of the venous circulation in regard to the emptying of the extremity likewise can be clinically correlated. Edematous areas from accumulated extra vascular fluid of lymph tissue are generally cold due to the displacement of the space for the dermal microcirculation.
If the surface of the skin is altered by the use of oils or creams or other types of embalments, it may reduce the emissivity of the skin. Dysplasia of psoriasis, scleroderma, scars and injury will also influence the ability of a body to take on and give off the thermal infrared energy. The human body is almost perfect in its emissivity and is considered as able to perform as a black body. That is, it is able to radiate all of the temperature that is present at any time.
When conditions of the vascular arterial, venous, and lymph, and the skin conditions are normal, then the major control and regulation of the dermal circulation is neurovascular. The neurovascular control of the skin is the sympathetic nervous system. The components of this sympathetic response, however, are both adrenogenic and cholinergic.
The adrenogenic portion produces vasoconstriction and the cholinergic portion produces vasodilation in the cutaneous microcirculation. Regulation of this cutaneous circulation is from the embryonic anterior neural ridge, which produces the sympathetic paravertebral ganglion.
The postganglionic fibres connect with each and every organ of the body as well as to the skin. When the 31 pairs of peripheral nerves are established, their content is anywhere from 8 to 35 percent unmyelinated fibres, which are of the sympathetic afferent or efferent function. These fibres are responsible for the control of this dermal microcirculation. Each of the spinal nerves has a dorsal and ventral division, and the cutaneous territory of a given peripheral nerve is ascribed by these divisions with their subsequent distal divisions. The ventral division of a peripheral nerve having a lateral division, as well as an anterior division, with its subsequent control over a designated territory. Skin territorial blood flow control is byway of these peripheral nerve conduits carrying the unmyelinated postganglionic fibers to the neurovascular receptors in the skin.
First published© Australian Thermology Association J. 1998.