The echo is the part of the transmitted ultrasound that is reflected back to the receiving transducer crystal.

See also Sonographic Features.

Anechoic areas are regions without internal echoes (echo free). Cysts or fluid filled regions are anechoic and may demonstrate enhanced echoes posterior to the fluid.

Echogenic is a tissue that structures are capable of producing echoes when they are in the path of a sound beam.

See also Echogenicity, and Isoechogenic.

Echography (also called sonography) allows visualizing deep structures of the body by recording the reflections (echo) of ultrasound waves directed into the tissues. A medical diagnostic sonogram (echogram), as in echocardiography and echoencephalography, utilizes a frequency range of 1 to 10 MHz.

Solid regions have internal ultrasound echoes and are classified as echo poor, hypoechoic or hypoechogenic if there are few internal echoes. Hypoechoic structures appear dark in ultrasound imaging, more homogeneous structures are darker than heterogeneous.
Soft atherosclerotic plaque, liver adenoma or FNH appear with a nodular hypoechogenicity. As metastases close the blood vessels they infiltrate, tumor tissues become hypoechogenic after injection of contrast agent. Muscle appears relatively hypoechoic to tendon fibers, also articular hyaline cartilage appears hypoechoic.