Ultrasound physics and sonoanatomy for regional anaesthesia

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Target exams

Red flags

• The needle tip must be visualised at all times. Advancing a needle whose tip is not seen risks intraneural, intravascular or other injury.
• An expanding hypoechoic mass within a nerve during injection is an intraneural injection — stop injecting immediately to avoid permanent nerve injury.
• In the out-of-plane approach the bright dot may be the shaft, not the tip; the tip can advance unseen beyond the beam. Track the tip by hydrolocation or the walk-off technique.
• Anisotropy makes a nerve vanish when the beam is angled. Keep the probe perpendicular so the target is at its brightest before injecting.
• A nerve, artery and vein can look alike on ultrasound; apply colour Doppler to identify vessels and prevent intravascular injection and local-anaesthetic systemic toxicity.
• Choose the probe frequency for the depth of the target: high frequency for superficial resolution, low frequency for deep penetration. The wrong choice either loses resolution or fails to reach the nerve.
• Bone produces a bright surface with a dark acoustic shadow; structures deep to bone are invisible to ultrasound.