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Anechoic means being without an echo or appearing without echoes. The term is usually used in medicine when discussing images of body parts that appear black on ultrasound scans, meaning that they do not produce echoes. An example of an anechoic structure on ultrasound scanning would be the urinary bladder filled with urine. Ultrasound scanning is a procedure that uses high-frequency sound waves to produce images of internal body structures. The sound waves bounce off internal body parts and reflect back as an echo to a computer, which produces an image.
Structures that produce echoes on ultrasound appear as varying shades of gray. Specifically, body structures that are less reflective and produce a lower amount of echoes are known as hypoechoic and appear as darker shades of gray. Conversely, body structures that are more reflective and produce a higher amount of echoes are known as hyperechoic and appear as lighter shades of gray. The opposite of anechoic is echogenic which refers to the ability of a structure to produce echoes. Anechoic means the same thing as echo-free. Anechoic comes from the Greek word "an" meaning "without," the Greek word "echo" meaning sound, "and the Greek word "ikos" meaning "pertaining to." Put the words together and you have "pertaining to without sound."