Adjective
hypoechoic (comparative more hypoechoic, superlative most hypoechoic)
Of low echogenicity.
Tendon fibers appear hyperechoic (bright) when the transducer is perpendicular to the tendon, but can appear hypoechoic (darker) when the transducer is angled obliquely. Source: Internet
The IUD is surrounded by a hypoechoic (dark) foreign-body granuloma Perforation: Very rarely, the IUD can move through the wall of the uterus. Source: Internet