Optimizing an image produced by ultrasound is an essential skill for performance of an ultrasound-guided nerve block. Anatomically, peripheral nerves are often located in the vicinity of an artery or between muscle layers. The echo texture of normal peripheral nerves can have a hyperechoic, hypoechoic, or honeycomb pattern (Figure 27-1). Several scanning steps and techniques can be used to facilitate adequate nerve imaging, including the selection of sonographic modes, adjustment of function keys, needle visualization, and interpretation of image artifacts.
Architecture of peripheral nerves.
All in all, sonographic imaging modes used for ultrasound-guided regional anesthesia and medical diagnostics are conventional imaging, compound imaging, and tissue harmonic imaging (THI). The conventional imaging is generated by a single-element angle beam. The compound imaging is implemented by acquiring several (usually three to nine) overlapping frames from different frequencies or from different angles. The tissue harmonic imaging acquires the information from harmonic frequencies generated by ultrasound beam transmission through tissue, which improves tissue contrast by suppression of scattering signals. Compound imaging with the tissue harmonic imaging can provide images with better resolution, penetration, and interfaces and margin enhancement, compared with those obtained using conventional sonography. In Figure 27-2, compound imaging and conventional imaging was used to view the interscalene brachial plexus. There is a clear margin definition of two hypoechoic oval-shaped nerve structures in compound imaging. As an example, the contrast resolution between the anterior scalene muscle and the surrounding adipose tissue is enhanced in comparison with those made with conventional imaging techniques.
Examples of image quality typically obtained with conventional versus compound imaging.
Five function keys on an ultrasound machine are of crucial importance to achieve an optimal image during the performance of peripheral nerve imaging (Figure 27-3A).
Optimizing an ultrasound image using five key functional adjustments (A) and specific tips on adjusting the focus (B) and gain (C). Some ultrasound models are specifically optimized for regional anesthesia application and may not incorporate user-adjustable focus and/or time gain compensation (TGC).
Depth: The depth of the nerve is the first consideration when ultrasound-guided nerve block is performed. The depth at which peripheral nerves are positioned and therefore imaged greatly varies and also depends on a patient's habitus. An optimal depth setting is important for proper focusing properties during imaging. Table 27-1 describes the recommended initial depth settings for common peripheral nerves. The target nerve should be at the center of the ultrasound image to obtain the best resolution of ...