Unlike mammography and ultrasound, thermography is a test of physiology. This means thermography looks for functional changes in breast tissue which may indicate abnormalities. Thermography accomplishes this through detection of infrared heat radiating from the surface of the breasts. Mammography and ultra sound , on the other hand, are tests of anatomy. When a tumor has grown to a size that is large enough and dense enough to block an x-ray beam (mammography) or sound wave (ultrasound), it produces an image that can be detected by a trained radiologist. Most cancers are detected by mammography when the diameter of the tumor has grown to the size of a dime and contain over 4 billion cells. Thus, thermography may detect abnormalities years before others methods would detect anything.