[Basics of ultrasound-guided nerve block]

The ultrasound waves (> 20 kHz) are high-frequency sound waves that are not audible to the human ear. As the ultrasound waves move through body tissues of different acoustic impedances, they are attenuated, reflected, or scattered. Reflected waves are transformed back into an electrical signal that is processed by the ultrasound machine to generate an image on the screen. In practice, an ultrasound probe emits and receives ultrasound waves, functioning both as a speaker and a microphone. The recent advance of ultrasonography has provided close-sectional images of the body in real time. Ultrasound imaging has been useful to visualize neural anatomical structures and the surrounding structures, and navigate the needle toward the target nerves. This review introduces the theories and practices of ultrasonography for the peripheral nerve blocks.