Echogenicity is a measure of acoustic reflectance, i.e. the ability of a tissue to reflect an ultrasound wave. The source of echogenicity is impedance mismatching between tissues. Impedance can be thought of as resistance to flow of mass or energy from a pulsatile source (unlike resistance, which generally assumes a steady source of mass or energy). When two tissues have different acoustic impedance, some of the ultrasonic energy will be reflected backwards (much as arterial pressure waves are reflected backwards when vascular impedance changes). Echogenic structures appear bright on ultrasound (the higher the amplitude of the reflected wave, the brighter the pixel).
Echogenicity of Various Structures
- Bones: hyperechoic
- Tendons: hyperechoic
- Nerves: variable (hyperechoic in the upper extremity, hypoechoic in the lower extremity)
- Fat: hypoechoic
- Arteries and Veins: anechoic