Physics of Ultrasound
Ultrasound application allows for noninvasive visualization of tissue structures. Real-time ultrasound images are integrated images resulting from reflection of organ surfaces and scattering within heterogeneous tissues. Ultrasound scanning is an interactive procedure involving the operator, patient, and ultrasound instruments. Although the physics behind ultrasound generation, propagation, detection, and transformation into practical information is rather complex, its clinical application is much simpler. Because ultrasound imaging has improved tremendously over last decade, it can provide anesthesiologists opportunity to directly visualize target nerve and relevant anatomical structures. Ultrasound-guided nerve block is a critical growth area for new applications of ultrasound technology and become an essential part of regional anesthesia. Understanding the basic ultrasound physics presented in this chapter will be helpful for anesthesiologists to appropriately select the transducer, to set the ultrasound system, and then to obtain a pleasing imaging.
In 1880, French physicists Pierre Curie and his elder brother, Paul-Jacques Curie, discovered the piezoelectric effect in certain crystals.1 Paul Langevin, a student of Pierre Curie, developed piezoelectric materials, which can generate and receive mechanical vibrations with high frequency (therefore ultrasound).2 During World War I, ultrasound was introduced in the navy as a means to detect enemy submarines.3 In the medical field, however, ultrasound was initially used for therapeutic rather than diagnostic purposes. In the late 1920s, Paul Langevin discovered that high-power ultrasound could generate heat in bone and disrupt animal tissues.4 As a result, throughout the early 1950s ultrasound was used to treat patients with Ménière disease, Parkinson disease, and rheumatic arthritis.5
Diagnostic applications of ultrasound began through the collaboration of physicians and sonar (sound navigation ranging) engineers. In 1942, Karl Dussik, a neuropsychiatrist, and his brother, Friederich Dussik, a physicist, described ultrasound as a medical diagnostic tool to visualize neoplastic tissues in the brain and the cerebral ventricles.6,7 However, limitations of ultrasound instrumentation at the time prevented further development of clinical applications until the mid-1960s. The real-time B-scanner was developed in 1965 and was first introduced in obstetrics.8,9 In 1976, the first ultrasound machines coupled with Doppler measurements were commercially available.10
With regard to regional anesthesia, as early as 1978, La Grange and his colleagues were the first anesthesiologists to publish a case series report of ultrasound application for peripheral nerve blockade.11 They simply used a Doppler transducer to locate the subclavian artery and performed supraclavicular brachial plexus block in 61 patients (Figures 28–1A and 28–1B). Reportedly, Doppler guidance led to a high block success rate (98%) and absence of complications such as pneumothorax, phrenic nerve palsy, hematoma, convulsion, recurrent laryngeal nerve block, and spinal anesthesia. In 1989, Ting and Sivagnanaratnam reported the use of B-mode ultrasonography to demonstrate the anatomy of the axilla and to observe the spread of local anesthetics ...