Ultrasound-Guided Regional Anesthesiology
Michael D. Herrick
Brian D. Sites
KEY POINTS
1. Ultrasound technology allows the intimate imaging of nerves, needle, and surrounding structures.
2. Ultrasound images are generated when ultrasound waves reflect off of structures and return to the transducer. The degree to which ultrasound is reflected determines how bright and white the image appears on the display screen.
3. In order to optimize image quality and distinguish neural from non-neural structures, the anesthesiologist should understand machine controls and the underlying basic principles of ultrasound physics.
4. Ultrasound artifacts are common and their recognition is important for maximizing quality and safety of ultrasound-guided regional anesthesia.
THE USE OF ULTRASOUND TO FACILITATE NERVE LOCALIZATION has revolutionized the practice of regional anesthesia. To optimize the benefits of ultrasonography and minimize potential harm, the anesthesiologist should understand several important physical principles of sound energy. Such an understanding should translate into optimization of image quality as well as the appreciation of artifacts and technical limitations.
I. Terminology
It is crucial to understand several ultrasound terms and techniques to appreciate why ultrasound physics is important for generating optimal images in the performance of ultrasound-guided regional anesthesiology.
A. Glossary
1. Hyperechoic. Structures that strongly reflect sound waves back to the transducer and appear white on the ultrasound screen.
2. Hypoechoic. Structures that weakly reflect sound waves and appear less white on the ultrasound screen.
3. Anechoic. Structures that do not reflect sound waves and appear dark on the ultrasound screen.
4. Attenuation. The weakening of ultrasound waves as they are transmitted to greater depth.
B. Technique
1. In-plane. Insertion of the needle in relation to transducer so that the entire needle can be imaged in its long axis as it enters the body and approaches the target structure (Fig. 3.1). The advantage of this approach is that the entire needle and tip can be visualized and directed away from various non-neural structures such as blood vessels and pleura.
2. Out-of-plane. Insertion of the needle in relation to the transducer so that a short-axis (cross-sectional) view of the needle is generated (Fig. 3.1). Only a portion of the needle is visualized with this technique. The out-of-plane technique has the advantage that it mimics
conventional approaches to performing nerve blocks and minimizes the amount of tissue that the needle must transgress prior to reaching the target structures.
conventional approaches to performing nerve blocks and minimizes the amount of tissue that the needle must transgress prior to reaching the target structures.
 FIGURE 3.1 A: The in–plane approach for needle insertion. B: The corresponding ultrasound needle image for the in-plane approach. C: The out-of-plane approach for needle insertion. D: The corresponding ultrasound needle image for the out-of-plane approach. (Adapted from Sites BD, Brull R, Chan VW, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. Part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med 2007;32:415. Copyright 2007 by Lippincott Williams & Wilkins.) |
 FIGURE 3.2 Various approaches to visualizing the nerve and the needle. A: Nerve in cross-section (short-axis) and needle in-plane. B: Nerve in short-axis, needle “out-of-plane.” C: Nerve in longitudinal axis (long-axis) view, needle in-plane. D: Nerve in long-axis, needle out-of-plane. (Adapted from Mulroy MF, Bernards CM, McDonald SB, et al. A Practical Approach to Regional Anesthesia. 4th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2009. Copyright 2009 by Lippincott Williams & Wilkins.) |
3. Short-axis nerve imaging. In this approach, the nerve is visualized in cross-section (Fig. 3.2). This is the most common approach used to visualize nerves.
4. Long-axis nerve imaging. In this approach, the nerve is visualized longitudinally (Fig. 3.2).
CLINICAL PEARL
Axis refers to how the nerve is imaged with the transducer, while the plane technique refers to how the needle is inserted in relation to the image.
II. Ultrasound physics
Prior to adjusting the ultrasound machine interface to optimize image quality, it is helpful to understand how ultrasound waves are generated, how they interact with different tissues in the body, and how an image is generated from the return of ultrasound waves to the transducer.
A. Ultrasound wave generation
An ultrasound wave is generated when an electrical voltage is applied to piezoelectric crystals that are located inside the transducer. When this current is applied to the crystals, it causes them to vibrate and create sound waves that are longitudinally transmitted from the transducer to the patient through a conductive gel (1,2). Each longitudinal sound wave is characterized by compressions (high pressure) and rarefactions (low pressure). Wavelength is simply the distance between pressure peaks and frequency is the number of pressure peaks per second (Fig. 3.3). Period is the time it takes to complete a single cycle of a peak and a trough. The speed of sound in soft tissue is believed to be the constant value of 1,540 m/s (v). Frequency and wavelength are inversely related and together are proportionally related to the speed of sound:
1,540 m/s ˜ frequency × wavelength
 FIGURE 3.3 Depiction of an ultrasound wave. Wavelength is the distance between pressure peaks, and period is the time between pressure peaks. Frequency is the number of pressure peaks per second. In this figure, there are two pressures peaks per second so the frequency would be 2 Hz. |
B. Interaction with tissues. The ultrasound waves enter the body and are either transmitted, reflected, scattered, or refracted as they encounter body tissues (Fig. 3.4) (3).
1. Transmission. When ultrasound waves encounter a boundary between two different types of tissue, part of that energy is transmitted through the boundary to interact with deeper
tissues. Attenuation at deeper depths is caused by reflection, scatter, and absorption. This concept is important to understand when a transducer or transducer setting is selected for different types of blocks. High-frequency transducers are ideal for imaging superficial structures (e.g., interscalene brachial plexus) because they facilitate the best detail and tissue distinction. In ultrasound terminology, high-frequency ultrasound provides better axial and lateral resolution. However, as a result of attenuation, high-frequency ultrasound is limited in its ability to reach deeper structures (e.g., lumbar plexus; Fig. 3.5) (1,2).
tissues. Attenuation at deeper depths is caused by reflection, scatter, and absorption. This concept is important to understand when a transducer or transducer setting is selected for different types of blocks. High-frequency transducers are ideal for imaging superficial structures (e.g., interscalene brachial plexus) because they facilitate the best detail and tissue distinction. In ultrasound terminology, high-frequency ultrasound provides better axial and lateral resolution. However, as a result of attenuation, high-frequency ultrasound is limited in its ability to reach deeper structures (e.g., lumbar plexus; Fig. 3.5) (1,2).
 FIGURE 3.4 The many responses that an ultrasound wave produces when traveling through tissue. (A) Scatter reflection: the ultrasound wave is deflected in several random directions both to and away from the probe. Scattering occurs with small or irregular objects. (B) Transmission: the ultrasound wave continues through the tissue away from the probe. (C) Refraction: when an ultrasound wave contacts the interface between two media with different propagation velocities, the ultrasound wave is refracted (bent) depending upon the differences in velocities. (D) Specular reflection: reflection from a large, smooth object (such as the needle) which returns the ultrasound wave toward the probe when it is perpendicular to the ultrasound beam. (Adapted from Sites BD, Brull R, Chan VW, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. Part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med 2007;32:413. Copyright 2007 by Lippincott Williams & Wilkins.) |
CLINICAL PEARL
Most ultrasound machines allow the operator to adjust the emitting ultrasound frequency. Try different settings to see which frequency optimizes your image.
2. Reflection. Reflection is the key event that results in the generation of an ultrasound image. Reflection occurs when sound waves encounter a boundary between two tissues that have different acoustic impedances. Acoustic impedance is the tendency of a tissue to resist the passage of ultrasound. Interfaces that have large differences in their acoustic impedances (e.g., fluid-soft tissue interfaces) reflect most of the ultrasound wave back to the transducer. For a given interface, reflection is maximized when the ultrasound beam is perpendicular to the target. A spectral reflector is a large smooth structure (e.g., a block needle) that acts like a mirror and causes organized sound to be reflected back to the transducer and produces a hyperechoic structure.
3. Scatter. Most tissues have a rough service and are called diffuse reflectors. They cause the ultrasound beam to scatter in different directions including some that travel back to the transducer and contribute to image generation. Higher frequency transducers cause more scatter compared to their lower frequency counterparts (1,2).
4. Refraction. Refraction occurs when an ultrasound wave passes through an interface between two tissues that have slightly different acoustic impedances and changes its trajectory. More refraction occurs when the angle of the ultrasound beam is not perpendicular to the target structures (2).
 FIGURE 3.5 Attenuation is estimated as the attenuation coefficient × frequency × path length. Notice the lower frequency wave (2.5 MHz) has less attenuation at a given distance when compared with the 10 MHz wave. Thus, the 2.5 MHz wave is able to penetrate the tissue more effectively than the 10 MHz wave. (Adapted from Sites BD, Brull R, Chan VW, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. Part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med 2007;32:416. Copyright 2007 by Lippincott Williams & Wilkins.) |
CLINICAL PEARL
Try to image structures using angles of insonation that approach the perpendicular in order to maximize reflection and minimize refraction.
C. Image generation
After serving as ultrasound wave generators, the piezoelectric crystals switch modes and become image receivers that capture sound waves that reflect off structures and return to the transducer. The sound waves cause the crystals to vibrate and generate electrical energy that is transmitted to a receiver that processes and cleans the image. The image that is generated is a two-dimensional image that displays structure on a grayscale continuum. Structures that strongly reflect the sound waves back to the transducer appear hyperechoic, structures that weakly reflect the sound waves appear hypoechoic, and structures that do not reflect sound waves appear anechoic. The receiver performs numerous functions that sharpen the image.
Amplification is the ability of the receiver to brighten the entire image by amplifying the signals equally. Gain is adjusted by turning the gain control dial up or down. Another function of the receiver that can be adjusted is the time gain compensation (TGC) dials. As discussed above, images attenuate at greater depth. TGC adjustment allows deeper structures to be brightened more than more superficial structures, so similar tissues can appear the same color even when situated at different depths (Fig. 3.6).
Amplification is the ability of the receiver to brighten the entire image by amplifying the signals equally. Gain is adjusted by turning the gain control dial up or down. Another function of the receiver that can be adjusted is the time gain compensation (TGC) dials. As discussed above, images attenuate at greater depth. TGC adjustment allows deeper structures to be brightened more than more superficial structures, so similar tissues can appear the same color even when situated at different depths (Fig. 3.6).
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