Which term describes tissue with no echoes on ultrasound?

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Echoes on ultrasound depend on how strongly a tissue reflects sound. Fluid-filled spaces have very few internal interfaces, so they reflect almost nothing and appear completely dark. This lack of echoes is described as anechoic, a characteristic example being a simple cyst or the fluid-filled lumen. You’ll often see anechoic structures show posterior acoustic enhancement, where the area behind the fluid looks brighter because the sound travels through the fluid with little attenuation. By contrast, hypoechoic means darker than surrounding tissue but not completely black, isoechoic means similar brightness to surrounding tissue, and hyperechoic means brighter than the surrounding tissue.