Transverse cross­sectional conventional ultrasound image of a spinous process with articular process landmarks in view

Because bone exhibits very high reflectivity and absorption, very little remaining energy penetrates the bone surface. This effect gives rise to “acoustic shadowing” which appears in the ultrasound image as an anechoic, dark region of the image directly below the bone surface. Acoustic shadowing is most clearly visualized in the transverse cross-sectional image beneath the spinous process and is indicated by the white circle.