Figure 50–1. Color power Doppler imaging. In this forearm sonogram the ulnar artery and ulnar nerve are seen in transverse (short-axis) view. The robust color power Doppler signal clearly distinguishes the ulnar artery from the adjacent ulnar nerve.

NERVE IDENTIFICATION WITH ULTRASOUND

Peripheral nerves have a fascicular echotexture (Figure 50–2). This means that when viewed on a transverse scan (short-axis view) peripheral nerves have a “honey-comb” appearance.⁷ When questions regarding nerve identification arise it is often useful to trace the known course and divisions of the nerve. Transverse scanning by sliding a broad linear transducer is the preferred method for following a nerve along its course. Long-axis slides are useful for panoramic views of the nerve course but are difficult and time-consuming to construct.⁸

Nerves can change shape along the nerve path depending on the surrounding tissues. Nerve paths also can be curved and even change with extremity motion. However, in the absence of major branching the cross-sectional area of nerves is relatively constant along the nerve path and can be used as a discriminating feature. Using these techniques monofascicular nerves as small as 1 mm in diameter have been imaged with high-resolution ultrasound.⁹

Peripheral nerves exhibit anisotropy.^{10, 11} This implies the amplitude of the received echoes will vary with the angle of insonation. Angle changes as small as 10 degrees away from perpendicular to the axis of the nerve will substantially reduce its echogenicity. Tendons are even more highly ordered than nerves, so anisotropic effects are seen with angle changes as small as 2 degrees. Transducer manipulation plays a major role in optimizing nerve imaging. Trained clinicians will naturally adjust the transducer to fill in the received echoes from the nerve (Figure 50–3).

Figure 50-2. Fascicular echotexture of peripheral nerves. In this sonogram obtained from the thigh the fasciculated echotextures of the common peroneal (lateral) and tibial (medial) nerves are seen.

Figure 50-3. Anisotropy of peripheral nerves. In these side-by- side sonograms the sciatic nerve is seen in transverse (short-axis) view. Tilting the transducer from neutral position (A) to approximately 10 degrees (B) markedly reduces the received echoes from the nerve.

Nerve motion can be revealed by dynamic ultrasound imaging.¹² With this method nerves are imaged with extremity movement. This best example of nerve motion is in the popliteal fossa. Foot movement will result in a characteristic “seesaw” pattern of nerve motion that can be used to identify the common peroneal, tibial, and common sciatic nerves.

BLOCK NEEDLE TIP VISIBILITY

A number of factors influence needle tip visibility,¹³ one of the most important of which is the angle of the block needle with respect to the ultrasound beam. Strong specular reflections occur from beam angles perpendicular to the needle, even when the needle is slightly out of the scan plane.¹⁴ Studies of needle tip echo have found a linear correlation between angle of incidence and mean brightness.¹⁵

One recent strategy to improve needle tip visibility is to use spatial compound imaging (beam steering to differing angles to produce overlapping scans that will form a composite image). Although spatial compounding has been found to reduce the effect of angle on needle tip echo when compared with single-line imaging, decreases in needle visibility still occur.¹⁶ The needle tip is best visualized when the bevel is oriented either directly toward or away from the transducer.^{17, 18} As expected, increasing needle diameter improves visualization of the needle tip echo.

Needle visibility is more difficult during deeper blocks for a number of reasons. At steep angles only backscatter from the needle is received by the transducer rather than a strong specular reflection. The beam also attenuates and widens past the focal zone. Visualization of the needle tip can be problematic, particularly within echo-rich tissues such as adipose tissue. It may be necessary to slowly advance or withdraw the needle (a moving echo confirms needle tip identification). An alternative is to slowly turn the needle to change the bevel orientation.