Ultrasound for peripheral nerve blocks

INTRODUCTION — Ultrasound imaging is increasingly used to guide peripheral nerve blocks. Ultrasound guidance allows real-time visualization of nerves, surrounding structures, and the needle-tip to maximize block success and minimize complications. Unlike other imaging modalities (ie, magnetic resonance imaging and computed tomography), ultrasound equipment is portable and carries no risk of ionizing radiation.

This topic will discuss the basic principles of ultrasound imaging, the equipment used, and techniques for ultrasound guidance for peripheral nerve block. Ultrasound guidance for neuraxial anesthesia is discussed separately. (See "Ultrasound guidance for neuraxial anesthesia techniques".)

Ultrasound guidance for venous access, obstetric ultrasound, echocardiography, and other clinical applications for ultrasound are discussed separately. (See "Basic principles of ultrasound-guided venous access" and "Echocardiography essentials: Physics and instrumentation" and "Overview of ultrasound examination in obstetrics and gynecology".)

Techniques for specific peripheral nerve blocks and other issues common to all peripheral nerve blocks are also discussed separately. (See "Upper extremity nerve blocks: Techniques" and "Lower extremity nerve blocks: Techniques" and "Thoracic nerve block techniques" and "Overview of peripheral nerve blocks".)

DEFINITIONS — The following are definitions of commonly used ultrasound-related terms.

●Ultrasound – The term "ultrasound" refers to sound waves of a frequency greater than that which the human ear can detect; namely, frequencies greater than 20,000 cycles/second, or Hertz (Hz). Medical imaging requires sound waves of much higher frequency, between 1 and 20 million Hz (ie, between 1 and 20 megahertz [MHz]).

●Insonation – Insonation refers to the process of sending ultrasound waves through tissue.

●Sonoanatomy – The appearance of anatomic structures on ultrasound scans is referred to as sonoanatomy.

●Acoustic impedance – Acoustic impedance is the resistance of a tissue to the passage of sound. Sound waves are reflected at the interface of tissues with different acoustic impedances. As an example, bone reflects sound waves strongly because its acoustic impedance is very high compared with other adjacent tissues.

●Attenuation – Ultrasound attenuation is the decay of sound waves as they travel through tissue as a result of scattering and absorption. Attenuation is a function of distance and frequency of the ultrasound wave [1]. The frequency of the ultrasound wave is determined by the selected transducer. (See 'Transducers' below.)

●Resolution – Resolution is defined as the ability to distinguish between two nearby structures; spatial resolution determines the clarity of the ultrasound image. Resolution of ultrasound imaging includes several spatial components (ie, axial, lateral, and elevational) and a temporal component. Spatial resolution depends primarily on the frequency of insonation and is therefore affected by the selection of the ultrasound transducer. (See 'Transducers' below.)

●Echogenicity – Ultrasonography depends on the return, or echo, of sound waves from a tissue of interest. The returning echo's amplitude, and the resulting brightness on the display screen, is referred to as its echogenicity. Strong reflection of sound waves results in hyperechoic (ie, very bright) images. Bone, air, and fascia are examples of tissues that appear hyperechoic. Solid organs, muscle, and, in some cases, nerves produce grayish, hypoechoic images. Fluid- and blood-filled structures such as blood vessels appear black, or anechoic, because ultrasound moves through them easily. Deep structures can often appear hypoechoic because attenuation limits sound wave transmission, resulting in a weak echo.

●Doppler effect – The Doppler effect refers to the change in frequency of wave transmission observed when relative motion occurs between the source of wave transmission and the observer (eg, when blood flows through vessels). Doppler ultrasound can be used to characterize blood flow and to identify vascular structures during needle manipulation. (See 'Doppler modes' below.)

●Anisotropy – Anisotropy is the change in echogenicity that occurs with a change in the angle, or inclination, of the transducer. In general, when structures are imaged obliquely, the amplitude of returning echoes diminishes, and structures appear less echogenic. Anisotropy is most important when imaging tendons and ligaments, but other musculoskeletal structures such as nerves and muscles also exhibit anisotropy [2,3]. Anisotropy is the reason the clinician must make fine adjustments of the transducer position in order to optimize the image.

THE WAY ULTRASOUND WORKS — The ultrasound transducer emits pulses of ultrasound waves, or short bursts of vibrations, which are transmitted through tissue, reflected off of tissue interfaces, and returned as echoes to the transducer. The returning echo is transformed into an electrical signal, which is processed and displayed on a screen as an ultrasound image. The conversion of mechanical force, in this case, sound waves, to electrical energy is called the piezoelectric effect. (See 'Transducers' below.)

Ultrasound pulses in clinical imaging are usually comprised of two to four cycles, with a pulse length of 0.5 to 3 milliseconds (figure 1). Pulse length is not changed by the ultrasonographer. The pulse repetition period is the time between the onset of a pulse and the onset of the next one, and includes both the pulse length and the listening time, during which the ultrasound machine receives returning echoes. The pulse repetition period increases when the ultrasonographer increases the depth to which the signal is sent, since the listening time is increased to allow return of more distant echoes. Pulse repetition frequency is the reciprocal of the pulse repetition period, and is defined as the number of pulses that occur per unit time.

Ultrasound imaging uses echo ranging to create an estimate of the distance to an object. That is, the time it takes for an echo to return to the transducer is converted into a distance using an assumed value of 1540 meters/second for the speed of sound in soft tissue [4]. Attenuation of an ultrasound wave occurs as it travels through tissue. Adjusting the gain and time-gain compensation settings can help accommodate for differences in returning waves from different depths in order to display the entire image evenly (see 'Maximizing machine settings' below). The interaction between ultrasound waves and tissue is discussed separately. (See "Echocardiography essentials: Physics and instrumentation".)

ULTRASOUND EQUIPMENT

Ultrasound machines — For use in the operating room, a variety of portable handheld or cart-mounted ultrasound machines are available. Ultrasound machines include a pulse generator, transducer(s), a receiver that amplifies signals returning to the transducer, a display or monitor that shows the image, and computerized memory that stores images. Most ultrasound machines include options for compound imaging and color-flow Doppler, and allow manual or automatic adjustment of frequency, gain, focus, and depth of scanning.

Portable handheld ultrasound machines have been developed with high-quality imaging resolution. These devices have been a viable option for performing peripheral nerve blocks in remote patient care areas and resource-limited locations; with current capabilities they may be the only ultrasound device needed at some institutions. Most of these devices have features that are like conventional ultrasounds, such as color Doppler or M-mode, and some even utilize machine learning and computer-aided detection to give its user real-time feedback to help optimize sonoanatomy [5].

Ultrasound software — Ultrasound machines typically allow use of a variety of technologies to process and display ultrasound images.

Spatial compound imaging — Spatial compound imaging is a technology that sends multiple ultrasound beams at predetermined angles, typically within approximately 20 degrees from perpendicular; the received echoes are combined to form a single compound image (figure 2). Spatial compound imaging reduces angle-dependent artifacts such as posterior acoustic enhancement, refractile shadowing, and anisotropic effects [6]. (See 'Ultrasound artifacts' below.)

This technology can also improve visualization of nerves, tissue planes, and needle-tips, and can provide a larger imaging field for a given transducer footprint. Stray lines of sight (ie, those that travel out of the rectangular field of vision under the ultrasound transducer) can be used to create a trapezoidal, larger imaging field, which is helpful for initial needle-tip visualization.

Doppler modes — For peripheral nerve block, two modes of Doppler ultrasound imaging are used to identify blood vessels. Doppler mode is selected by pressing a button or turning a knob on the ultrasound machine to add this scanning mode to the basic grayscale scan.

●Traditional color Doppler – Traditional color Doppler is based on detecting shifts in the mean frequency of the sound wave. When scanning in traditional color Doppler mode, the color on the screen indicates directional motion; blood flowing away from the transducer appears blue, while blood flowing towards the transducer appears red, whether the blood vessel is an artery or a vein. The acronym "BART" ("blue away, red towards") is often used to describe this display convention.

●Power Doppler – Power Doppler ultrasound integrates the power spectrum of the Doppler signal to produce real-time, color-coded images of blood flow. The technique differs from conventional color Doppler in the way the Doppler signals are processed. The colors in the power Doppler image indicate only that blood flow is present, but contain no information on flow velocity or direction. This Doppler mode can help detect small, tortuous arteries that often accompany peripheral nerves. Power Doppler is more sensitive than traditional color Doppler. Because power Doppler has high motion sensitivity, the transducer must be kept very still while imaging in this mode to avoid artifact (image 1).

Emerging technologies — Ultrasound technology is changing rapidly, with innovations that show promise for enhancing practice.

●Artificial intelligence – Artificial intelligence using machine and deep learning algorithms have allowed some ultrasound systems to interpret ultrasound images and to identify and label blood vessels, tendons, bony landmarks, and nerves. This technology may be helpful when training for image acquisition and interpretation [7-9].

●Computer-aided detection (CAD) – Some systems utilize CAD to help the end user adjust the transducer to find the optimal image and trajectory of needle placement, which may be particularly useful to facilitate learning for trainees and novices in using ultrasound for regional anesthesia [10]. This technology appears promising, however overall utility has not yet been demonstrated.

●Ultrasound elastography – Ultrasound elastography is a technique that measures tissue stiffness in response to an applied mechanical force (eg, compression or sheer force). This technique may prove useful for distinguishing between nerves and fascia, tendons, and other tissues, based on the differences in stiffness of these tissues [11].

●Ultra-high frequency (UHF) – UHF transducers can produce frequencies up to 70 megahertz (MHz) compared with traditional transducers that range from 2 to 20 MHz. UHF is capable of spatial resolution up to 30 micrometers and can help distinguish individual nerve fascicles which may be useful for smaller nerve imaging and shallow blocks [12,13].

Transducers — Ultrasound transducers contain piezoelectric crystals or ceramic elements that generate and receive ultrasound waves. The quality of the ultrasound image is determined primarily by the selection of a transducer with the optimal characteristics (ie, frequency, shape, and size) for the intended application. As an example, a linear transducer with a small footprint and high frequency would be used for a shallow nerve block (eg, axillary block), where structures of interest are typically 2 to 3 cm in depth from the skin. In contrast, a low-frequency, curvilinear transducer would be used for a deeper block (eg, lumbar plexus block) to view structures that are typically 4 to 8 cm deep.

Transducers are capable of producing a range of ultrasound frequencies (eg, 2 to 5 MHz, 5 to 10 MHz). They are available with a linear array of crystals on a flat surface or a curvilinear array on a rounded surface (picture 1).

●Frequency – In general, high-frequency sound waves produce the highest-resolution images but do not penetrate deeply into tissue. Lower-frequency transducers penetrate deeper but provide lower-resolution images.

The ideal transducer for a given nerve block is usually one with the highest frequency that allows adequate depth penetration. In general, the depth of penetration (in centimeters) equals 60/f, where f is the transducer center frequency (ie, the mean of the range of frequencies) in MHz. Therefore, the expected penetration from a 10 MHz center frequency transducer is 6 cm [14].

●Array shape – Linear array transducers display the image in a rectangular shape, mimicking the long, slender appearance of these transducers. They generally have a higher scan line density and produce a higher-quality image than a curvilinear array of similar frequency.

Curvilinear transducers provide a broad field of view and provide a sector-shaped image. These transducers generally use a lower frequency range and are useful for scanning deeper structures.

Needles — The block needle-tip should be visualized throughout the nerve block procedure to avoid inadvertent tissue damage or vascular puncture and inaccurate injection of local anesthetic (LA). Metal block needles are hyperechoic and appear white on the ultrasound image; they can be difficult to visualize in bright adipose tissue. Larger needles (ie, 17 gauge Touhy) are easier to visualize and to direct and are therefore useful for deeper blocks, when the angle of insertion is steep. (See 'Visualizing the needle' below.)

A number of block needles have been developed to increase their echogenicity and visibility during ultrasound-guided nerve block (image 2). Most of these technologies are based on texturing the surface of the block needle-tip to reflect sound waves back to the transducer regardless of the angle of insonation (ie, retro-reflection technology). In addition, machine-based technologies have been developed for needle enhancement (image 3). Whereas older machine-based technologies were limited to enhancing the entire needle echo, newer technology incorporating an ultrasound sensor near the needle tip has been developed to help locate the needle tip on the ultrasound display [15]. Although this newer technology may help improve needle control and tip visualization, currently there are limited studies on its clinical efficacy and safety.

Novel technologies are being developed to improve needle visualization (eg, magnetic needle guidance [16]), but clinical experience with their use is limited.

ULTRASOUND TECHNIQUE — Peripheral blocks are performed with real-time ultrasound (ie, scanning during needle placement and injection).

Preparation for scanning

Ergonomics — Ultrasound scanning should be performed with the clinician and the patient positioned to minimize strain and muscle fatigue, and to facilitate the movements required for successful block placement [17].

●Table height – The table should be positioned such that the clinician can sit or stand straight without needing to lean or stretch.

●Hand support – Both the hand holding the transducer and the hand holding the needle should be supported for stability and to avoid fatigue. The transducer should be held close to the face of the device, with the arm or hand resting on the patient's body.

●Operator body orientation – When possible, the clinician should stand or sit directly behind the transducer with a line of sight along the needle path.

●Monitor position – The ultrasound machine should be positioned in front of the clinician to allow a comfortable direct line of sight.

Time out — A preprocedure time out should be performed, adhering to a checklist that includes confirmation of patient identifiers, allergies, planned surgical procedure including laterality, surgical and anesthesia consents with site marked if applicable, and coagulation status [18-20].

Gel — Ultrasound waves do not travel well through air; coupling gel, which is placed on the patient's skin, removes air between the transducer and the skin. For peripheral nerve block, gel must be placed on the transducer inside the sterile sleeve and on the patient's skin.

Gel also permits the transducer to slide without resistance along the skin.

Scout scan — A scout scan (ie, a preliminary scan without sterile technique, also referred to as prescanning) may be useful in some cases. As an example, for larger patients or those with difficult anatomy, a scout scan can help determine whether a supraclavicular or infraclavicular brachial plexus block would be preferred.

Sterile technique — Peripheral nerve block must be performed using sterile technique and universal precautions. All providers present, the clinician, and the patient should wear a surgical cap.

●The clinician should:

•Remove jewelry, including watches and rings

•Wear a mask covering mouth and nose

•Wash hands prior to the procedure

•Wear sterile gloves

•Wear eye protection

●The patient's skin should be widely cleaned with an individual antiseptic packet of chlorhexidine, preferably with alcohol, allowing adequate time for the solution to dry, according to the package insert.

●After placing coupling gel on the transducer face, a sterile plastic sleeve should be placed over the transducer and its cord, and secured with an elastic band. The plastic must be smoothly applied on the transducer, without trapped air bubbles.

●Sterile gel should be applied to the skin at the block site.

Needle orientation — The block needle can be inserted either in plane (along the axis of the ultrasound beam and transducer) or out of plane (picture 2 and picture 3).

●In plane – Using the in-plane approach, the needle is inserted parallel to the long side of the transducer and is visualized in long-axis so the full needle is visualized. This permits the tip of the needle to be visualized at all times. The main challenge of the in-plane technique is to keep the entire needle-tip and shaft within the plane of imaging, rather than only part of the shaft (a partial lineup) (image 4). (See 'Visualizing the needle' below.)

●Out of plane – With the out-of-plane view of the needle, the needle is inserted perpendicular to the transducer. A cross-section of the needle appears as a small dot, which can be difficult to identify. It can be challenging to maintain a view of the needle throughout the entire procedure, and it is not always possible to distinguish the tip from other portions of the needle. The out-of-plane technique is often used for deeper blocks, where it is more difficult to visualize the entire length of the needle to the target nerve.

Ultrasound artifacts — Several artifacts can occur during scanning and can interfere with the quality of the image. However, characteristic artifacts can also be useful clinical indicators. As examples, a comet tail artifact can be used to estimate lung water and the hyoid bone can often be identified by its acoustic shadow. Common ultrasound artifacts include the following:

●Acoustic shadowing – Acoustic shadowing refers to the creation of a black, anechoic region behind an intensely reflective substance. Bone and air reflect and absorb ultrasound waves, thereby creating acoustic shadows that can obscure deeper structures (image 5 and image 6). Heavily calcified intravascular plaque can also produce acoustic shadowing (image 7).

●Posterior acoustic enhancement – Acoustic enhancement (also referred to as increased through-transmission) occurs deep to anechoic and hypoechoic fluids that absorb less sound than the surrounding tissue [21,22]. This enhancement can be observed deep to blood vessels or injected local anesthetic (LA). Acoustic enhancement is often observed deep to the second part of the axillary artery during infraclavicular block, in the axilla, deep to the axillary vein (image 8), or deep to the femoral artery during femoral nerve block (image 9). This artifact is important because it can resemble nerve echotexture.

●Refractile shadowing – Refractile or lateral-edge shadowing is a form of acoustic shadowing that occurs deep to the edges of blood vessels or, less commonly, from the sides of tendons or nerves (image 10).

●Reverberation – Reverberation occurs when the ultrasound beam bounces back and forth between reflective surfaces before returning to the transducer, resulting in discrete bands of echoes. Comet tail artifacts and needle reverberation are common forms of reverberation artifacts (image 11). (See 'Optimizing the image' below.)

●Speed of sound – Speed of sound artifacts can be observed during regional blocks [23]. The speed of sound in soft tissue is approximately 1540 m/second. Variations in the speed of sound within tissue are possible, depending on the mechanical properties of the tissue types [24]. The bayonet artifact is an example of a speed of sound artifact; an artifactual discrete needle bend can occur at the transition between tissues with different speeds of sound (image 12). This apparent bend remains in the same anatomic location when the needle is advanced. Bayonet artifacts are often observed during lateral popliteal blocks, in which the needle approaches nearly perpendicular to the sound wave, crossing a transition from muscle to subcutaneous adipose tissue.

Image acquisition — We follow a sequence of steps when performing an ultrasound-guided nerve block, including finding the nerve or relevant anatomic space, optimizing the image, and visualizing and maintaining the view of the needle. The following section describes placement and manipulation of the transducer and the steps used for image acquisition.

Transducer manipulation — The ultrasound transducer is positioned and moved in a variety of ways during scanning.

●Transducer orientation – Interpretation of the images on the ultrasound screen depends on an understanding of the position of the ultrasound beam relative to anatomic structures, and orientation of the transducer relative to the screen.

•Short versus long axis – Structures of interest can be imaged either in the short axis (cross section) or the long axis. For many peripheral nerve blocks, the transducer is positioned so that the target nerves are viewed in short axis. Orientation is individualized for interfascial plane blocks (eg, transversus abdominis plane block) and compartment blocks (eg, thoracic paravertebral block).

•Medial versus lateral – Ultrasound transducers typically have a palpable notch or ridge on one side that is used to help orient the transducer to the ultrasound screen. The screen shows a dot (usually blue) that corresponds to the indicator on the transducer. Correct orientation can be confirmed by touching the end of the transducer and watching for motion in the corresponding corner of the image on the screen.

●Stabilization – Both the hand holding the transducer and the hand holding the needle should be supported for stability. (See 'Ergonomics' above.)

●Transducer motions – During scanning, the transducer is manipulated to change the direction and orientation of the ultrasound beam (figure 3).

•Sliding – The transducer is moved across the skin along the course of a nerve, maintaining a short-axis view.

•Compression – Compressing soft tissue with the transducer may improve imaging and is used to confirm venous structures (ie, veins compress easily, while arteries do not).

•Tilting – The transducer is tilted side to side around its long axis. Tilting can change the echogenicity of the nerve.

•Rotation – Rotating the transducer clockwise or counter-clockwise is used to produce a true short- or long-axis view of a nerve and can be used to align the ultrasound beam with the needle.

•Rocking – Rocking the transducer by tilting it in plane may be used to improve visibility of the needle or anatomic structures.

Finding the nerve or space — For many peripheral nerve blocks, the goal is to visualize individual nerves and to then inject LA near those nerves. Peripheral nerves often exhibit polyfascicular echotexture, meaning a large number of hypoechoic fascicles can be identified with surrounding hyperechoic connective tissue ("honeycomb" appearance) (image 13 and image 14) [25]. Central nerves (such as the cervical ventral rami for interscalene block) can have a monofascicular or oligofascicular appearance on ultrasound scans, meaning only one or a few hypoechoic fascicles can be identified. What appear to be nerve fascicles are actually groups of nerve fascicles because the very fine layers of collagen that separate individual nerve fibers are usually too thin to be visualized with ultrasound [26,27].

For some peripheral nerve blocks, the nerves themselves are not visualized or targeted; rather, we visualize an anatomic space known to contain the nerves or structures near the nerves. This type of technique is used for compartment blocks and interfascial plane blocks. As an example, for a transversus abdominis plane block, the needle-tip and LA injection are placed in the fascial plane between the internal oblique and transversus abdominis muscles, where thoracic intercostal nerves run. (See "Transversus abdominis plane (TAP) blocks procedure guide".)

Optimizing the image — Once the intended target is found, the image can be optimized by manipulating the transducer, minimizing artifact, and adjusting machine settings.

Manipulating the transducer — The various transducer motions can be used to refine the image and identify structures during scanning. Examples include the following:

●Sliding can be used to confirm the nerve course or verify local anesthetic tracking after injection.

●Compression may be used for some peripheral nerve blocks (eg, axillary block), to identify blood vessels and distinguish arteries from veins, or to move blood vessels out of the needle path (image 8).

●Tilting the transducer during scanning can enhance the echogenicity by using the anisotropic properties of the intended nerve (eg, popliteal block).

●Rocking the transducer may help to increase the visualized space for needle insertion (eg, during infraclavicular block).

Minimizing artifact — Some ultrasound artifacts can be prevented, reduced, or eliminated to improve image quality. As examples, coupling gel reduces contact artifact, and spatial compound imaging technology reduces angle-dependent artifacts (eg, posterior acoustic enhancement, refractile shadowing, and anisotropic effects). (See 'Spatial compound imaging' above.)

Air creates acoustic shadowing that can obscure deeper structures. Therefore, injection of air should be avoided during LA infiltration prior to nerve block, and the block needle and connected tubing should be flushed with LA prior to needle insertion into the skin.

Maximizing machine settings — Ultrasound imaging technology has been increasingly automated with preset machine settings that reduce, but do not eliminate, the need for fine-tuning. Newer machines feature touchscreens rather than knobs, although the subject of image optimization is still referred to as "knobology." The following parameters should be adjusted to optimize the ultrasound image:

●Depth – Set depth so that target structures are in the center of the field of imaging according to the initial range of depth settings appropriate for the planned nerve block (eg, 2 to 11 cm, depending on the block) and the patient's body habitus. Adjust as necessary based on the image.

●Focus – The focus setting narrows the ultrasound beam at the desired depth for an area of interest, which results in greater lateral resolution at that depth. Set the focus to mid-field or deeper. Avoid setting the focus too shallow, which would result in a wider beam and lower resolution in the deeper imaging field.

●Gain – The gain controls the amplification of all returning echoes displayed on the image, at all depths, and thus controls the brightness of the image. Set the receiver gain to optimize the overall information content of the image. Needle-tip visibility is usually better with low receiver gain because identifying a bright needle-tip is easier with a dark background. Time-gain compensation sliders are used to selectively amplify echoes that return to the transducer at a certain time and are used to vary the gain in different parts of the image.

Visualizing the needle — A basic principle of ultrasound-guided nerve block is that the needle-tip should be visualized throughout the procedure. The needle should not be advanced without identifying the tip. The most important factors influencing needle-tip visibility are the needle's gauge and angle of insertion [28]. Needle bevel orientation is also important to needle tip visibility [29]. The bevel should be oriented to face the transducer in order to enhance needle-tip imaging.

We follow a sequence of steps to identify and maintain visualization of the block needle.

For an in-plane approach (image 4):

●Look at the needle and transducer prior to insertion to align their axes.

●Identify the target structure and tilt the transducer to maximize nerve brightness.

●Maintain the tilt and slide the transducer towards the needle until the needle comes into view.

●Rotate the transducer until the full needle is in view, if necessary.

For an out-of-plane approach:

●Look at the needle and transducer prior to insertion, making sure the needle will be inserted at the midpoint of the transducer, with its axis perpendicular to the axis of the transducer.

●Identify the target structure and tilt the transducer to maximize nerve brightness.

●Insert the needle until the tip crosses the plane of imaging as an echogenic (white) dot.

●Slide the transducer along the axis of the needle, following the tip as the needle is advanced.

Differences in important outcomes have been difficult to demonstrate when comparing in-plane versus out-of-plane approaches to regional blocks. Therefore, the overall quality of the guidance is more important than the approach per se. Accurate identification of the needle-tip, nerve borders, and surrounding anatomic structures is critical to the efficacy and safety of ultrasound-guided interventions.

Injection of fluid (usually normal saline) can help to confirm needle tip location, a technique known as hydrolocation [30]. Hydrodissection refers to the injection of fluid through the block needle to separate tissue planes, in order to maneuver the block needle to the desired target. As an example, during an axillary nerve block, hydrodissection can be used to open the plane between the axillary artery and terminal branches of the brachial plexus.

Injection of local anesthetic — There are several sonographic signs of regional block success. In general, local anesthetic (LA) should track around and along peripheral nerves and their branches. During injection, specific sonographic signs of success are associated with some nerve blocks (eg, a U-shaped distribution deep to the axillary artery for infraclavicular block, or anterior displacement of the pleura with injection for thoracic paravertebral block). The distribution of LA can be confirmed after injection by sliding the transducer along course of the nerve or anatomic space (image 14 and image 13).

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Local and regional anesthesia".)

SUMMARY AND RECOMMENDATIONS

●Utility of ultrasound imaging for peripheral nerve block – Ultrasound imaging for peripheral nerve block allows real-time visualization of nerves, surrounding structures, the block needle-tip, and local anesthetic (LA) injection to maximize block success and minimize complications. (See 'Image acquisition' above.)

●Equipment

•Ultrasound machines include technologies that improve image quality, allow identification of blood vessels, and automate image optimization. (See 'Ultrasound machines' above and 'Optimizing the image' above.)

•Optimal imaging depends on selection of an appropriate ultrasound transducer. The ideal transducer for a given nerve block is usually one with the highest frequency that allows adequate depth penetration. In general, the depth of penetration (in centimeters) equals 60/f, where f is the transducer center frequency in megahertz (MHz) (picture 1). (See 'Transducers' above.)

-High-frequency transducers produce the highest-resolution images but do not penetrate deeply into tissues.

-Lower-frequency transducers penetrate deeper but produce lower-resolution images.

•A number of block needles have been developed to increase their echogenicity and visibility with ultrasound. Machine-based technology for needle enhancement is also available (image 3). (See 'Needles' above.)

●Steps for ultrasound guidance – We follow a sequence of steps for ultrasound guidance for nerve block, as follows:

•Find the nerve or related structure. (See 'Finding the nerve or space' above.)

•Optimize the image. (See 'Optimizing the image' above.)

•Visualize the needle-tip throughout the procedure. (See 'Visualizing the needle' above.)

•Successful deposition of LA around and along nerves or within anatomic spaces can be confirmed by visualization of spread of LA after injection. (See 'Injection of local anesthetic' above.)

●Dealing with artifacts – A variety of artifacts can occur during ultrasound scanning. Some artifacts can be prevented, reduced, or eliminated to improve image quality, while others can be useful clinical indicators. (See 'Ultrasound artifacts' above.)