Optimizing an ultrasound image is an essential skill during ultrasound-guided nerve blockade. Anatomically, a peripheral nerve is always located in the vicinity of an artery between fascial layers. The echotexture of normal nerve shows a hyperechoic, hypoechoic, or honeycomb pattern (Figure 29–1).1,2 There are several scanning steps to obtain adequate nerve imaging, including the selection of sonographic modes, adjustment of function keys, needle visualization, and interpretation of image artifacts.
Echotexture of peripheral nerves. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)
Common sonographic imaging modes used for medical diagnostics, such as, conventional imaging, compound imaging, and tissue harmonic imaging (THI) can all be utilized in imaging of peripheral nerves. Conventional imaging is generated from a single-element angle beam at a primary frequency designated by the transducer. Compound imaging is implemented by acquiring several (usually three to nine) overlapping frames from different frequencies or from different angles.3 THI acquires the information from harmonic frequencies generated by ultrasound beam transmission through tissue. Harmonic frequencies are multiples of the primary frequency. THI improves axial resolution and boundary detection by suppression of scattering signals from tissue interfaces, especially for obese patients.
Currently, THI has been set as the default mode by many, if not most, US manufacturers. Compound imaging with THI can provide images with better resolution, penetration, and interfaces and margin enhancement compared with conventional sonography. In Figure 29–2, both compound imaging and conventional imaging were employed to visualize an interscalene brachial plexus. There is clear margin definition of two hypoechoic oval-shaped nerve structures in compound imaging; the contrast resolution between anterior scalene muscle and surrounding adipose tissue is increased in comparison with conventional imaging.
Examples of image quality typically obtained with conventional versus compound imaging. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)
Five function keys on an ultrasound machine are of crucial importance to achieve an optimal image during the performance of peripheral nerve imaging (Figure 29–3).4
Optimizing an ultrasound image using five key functional adjustments and specific tips on adjusting the focus and gain. Some ultrasound models are specifically optimized for regional anesthesia application and may not incorporate user-adjustable focus or time-gain compensation (TGC). (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)