Physiologic Imaging Techniques - The Merck Veterinary Manual

Physiologic Imaging Techniques

These techniques provide images that reflect physiologic processes. Unlike anatomic imaging, which reflects structure, physiologic imaging techniques assess metabolism or circulation. Thermography and scintigraphy allow examination of the entire horse. When combined with a thorough clinical examination, these methods are useful for identifying injuries that may otherwise go undetected.

Thermography is the pictorial representation of the surface temperature of an object. It is a noninvasive technique that measures emitted heat and is useful for detecting inflammatory changes that may contribute to lameness. Relative blood flow dictates the thermal pattern; normal thermal patterns can be predicted based on vascularity and surface contour. Skin overlying muscle is also subject to temperature increase during muscle activity. Circulation is invariably altered in injured or diseased tissues. Thermographically, the “hot spot” associated with the localized inflammation generally is seen in the skin directly overlying the injury. However, diseased tissues may have a reduced blood supply due to swelling, vessel thrombosis, or tissue infarction. With such lesions, the area of decreased heat is usually surrounded by increased thermal emissions, probably due to shunting of blood.

During scintigraphy, polyphosphonate radiopharmaceuticals are given IV. Their distribution is then measured by a gamma camera. The polyphosphonates bind rapidly to exposed hydroxyapatite crystal, generally in areas where bone is actively remodelling. Because inflammation causes an increase in blood flow, capillary permeability, and extracellular fluid volume, inflamed tissues accumulate high levels of radiopharmaceutical during the soft-tissue phase of scintigraphy, allowing evaluation of soft-tissue injuries. During the bone phase, the radiopharmaceutical accumulates in areas of increased remodelling or vascularity. Because injured bone is remodelled more rapidly, scintigraphy is useful for detecting lesions in bone and ligaments, particularly in identifying enthesopathy (damage to the insertions of tendons and ligaments on bone).