Ultrasound Techniques for Regional Anesthesia

Richard T. Brull MD

Ultrasound uses high frequency sound waves (i.e. greater than 20,000 cycles per second) that are transmitted from a transducer probe and passed through tissues. Reflections of the sound waves are picked up by the transducer and transformed into images. Reflection occurs when the sound wave passes through tissue interfaces of varying acoustic density (i.e. acoustic mismatch). Tissues with large differences in their acoustic density produce return signals approaching 100% since sound wave transmission is halted and reflected. The outline of a structure is generally best delineated when the ultrasound beam encounters the structure at a perpendicular angle. Coupling fluid (jelly) must be used on the skin to improve transmission from the transducer to deeper structures. Images are formed based on structural echogenicity. Bright structures are termed hyperechoic and represent none or limited transmission of sound waves through the tissue. Dark structures are termed hypoechoic and represent increased transmission of sound waves through the tissue. In the transverse (cross-sectional) view, nerves appear as round- or oval- shaped hypoechoic structures with internal hyperechoic punctuations. High frequency transducer probes (i.e. >7 MHz) afford high resolution images of superficial structures (e.g. interscalene region), while lower frequency probes (i.e. <7 MHz) allow for deeper tissue penetration (e.g. infraclavicular region) at the expense of fine resolution.

The benefits of ultrasound for regional anesthesia include the inherent safety of real-time imaging of the needle (and/or catheter), nerve bundle, and surrounding vasculature. Deposition and spread of local anesthetic are also easily appreciated. Additionally, ultrasound is non-invasive with no radiation exposure, is more affordable compared to other imaging techniques, requires little preparation for immediate use, and is taught and learned with relative ease.

These technique descriptions are presented for educational purposes and are not intended to substitute for actual hands-on training.