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In recent years interest has been growing in the practice of regional anesthesia and, in particular, in peripheral nerve blocks for surgical anesthesia and postoperative analgesia. Peripheral nerve blocks have been found to be superior to general anesthesia1 because they provide effective analgesia with few side effects2 and can hasten patient recovery.3 Unfortunately, the practice of regional anesthesia does not enjoy widespread endorsement because of inconsistent success, varying from one anesthesiologist to another. Current methods of nerve localization (eg, paresthesia and nerve stimulation) are essentially “blind” procedures, since they both rely on indirect evidence of needle-to-nerve contact.4,5 Seeking nerves by trial and error and random needle movement can cause complications. Although uncommon, complications such as intravascular local anesthetic injection resulting in systemic toxicity, inadvertent spinal cord injury following interscalene block, pneumothorax following supraclavicular block, and nerve injury have all been reported.6,7

Imaging guidance for nerve localization holds the promise of improving block success and decreasing complications. Among imaging modalities currently available, ultrasonography seems to be the one most suitable for regional anesthesia. Perhaps the most significant advantage of ultrasound technology is the ability to provide anatomic examination of the area of interest in real time.8 Ultrasound imaging allows one to visualize neural structures (plexus and peripheral nerves) and the surrounding structures (eg, blood vessels and pleura), navigate the needle toward the target nerves, and visualize the pattern of local anesthetic spread.9

An ultrasound probe (transducer) has dual functions. It emits and receives sound waves, thus functioning both as a speaker and a microphone. As the name implies, ultrasound waves are high-frequency sound waves (≥20,000 cycles/sec, 20 kHz) that are not audible to the human ear. Ultrasound frequencies useful in clinical medicine are in the megahertz (MHz) range.10 When an electric current is applied to an array of piezoelectric crystals (quartz) within the ultrasound transducer, mechanical energy, in the form of vibration, is generated, resulting in ultrasound waves. As the ultrasound waves move through body tissues of different acoustic impedances, they are attenuated (lose amplitude with depth), reflected, or scattered. Waves reflected to the transducer are then transformed back into an electrical signal that is then processed by the ultrasound machine to generate an image on the screen.

Depending on the amount of wave returned, anatomic structures take on different degrees of echogenicity. Structures with high water content, such as blood vessels and cysts, appear hypoechoic (black or dark) because ultrasound waves are transmitted through the structures easily with little reflection. On the other hand, bone and tendons block ultrasound wave transmission, and the strong signal returned to the transducer gives these structures a hyperechoic appearance (bright, white) on the screen. Structures of intermediate density and acoustic impedance, such as the liver parenchyma or the thyroid gland, appear gray on the screen. Knowing the speed of sound in tissue (1540 m/sec on average) and the time of ...

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