A hypoechoic lesion is a lesion that appears dark on an ultrasound image, according to Veterinary Radiology. This can be normal, or it can be the result of the lesion reflecting back less of the ultrasound waves than the area of the body does normally.
In dermatology, ultrasounds assess skin lesions. The technology is especially useful for small lesions that are too small for assessment by other imaging technologies, such as MRIs, explains the National Institutes of Health. Most of the layers of skin are hyperechoic, save an area of connective tissue, the nails and the membrane that covers the muscles. Most skin tumors are also hypoechoic.