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 during axillary brachial plexus block.12 In 1994, Stephan Kapral and colleagues systematically explored brachial plexus with B-mode ultrasound. Since that time, multiple teams worldwide have worked tirelessly to define and improve the application of ultrasound imaging in regional anesthesia.13 Ultrasound-guided nerve blockade is currently used routinely in the practice of regional anesthesia in many centers worldwide.
A: Early application of Doppler ultrasound by LaGrange to perform supraclavicular brachial block. B: Relationship of the brachial plexus of nerves and the subclavian artery.
Here is a summary of ultrasound quick facts:
1880: Pierre and Jacques Curie discovered the piezoelectric effect in crystals.
1915: Ultrasound was used by the navy for detecting submarines.
1920s: Paul Langevin discovered that high-power ultrasound can generate heat in osseous tissues and disrupt animal tissues.
1942: The Dussik brothers described ultrasound use as a diagnostic tool.
1950s: Ultrasound was used to treat patients with Ménière disease, Parkinson disease, and rheumatic arthritis.
1965: The real-time B-scan was developed and was introduced in obstetrics.
1978: La Grange published the first case series of ultrasound application for placement of needles for nerve blocks.
1989: Ting and Sivagnanaratnam used ultrasonography to demonstrate the anatomy of the axilla and to observe the spread of local anesthetics during axillary block.
1994: Steven Kapral and colleagues explored brachial plexus blockade using B-mode ultrasound.
Sound travels as a mechanical longitudinal wave in which back-and-forth particle motion is parallel to the direction of wave travel. Ultrasound is high-frequency sound and refers to mechanical vibrations above 20 kHz. Human ears can hear sounds with frequencies between 20 Hz and 20 kHz. Elephants can generate and detect sound with frequencies less than 20 Hz for long-distance communication; bats and dolphins produce sounds in the range of 20 to 100 kHz for precise navigation (Figures 28–2A and 28–2B). Ultrasound frequencies commonly used for medical diagnosis are between 2 and 15 MHz. However, sounds with frequencies above 100 kHz do not occur naturally; only human-developed devices can both generate and detect these frequencies, or ultrasounds.
A: Elephants can generate and detect the sound of frequencies less than 20 Hz for long-distance communication. B: Bats and dolphins produce sounds in the range of 20–100 kHz for navigation and spatial orientation.
Ultrasound waves can be generated by material with a piezoelectric effect. The piezoelectric effect is a phenomenon exhibited by the generation of an electric charge in response to a mechanical force (squeeze or stretch) applied on certain materials. Conversely, mechanical deformation can be produced when an electric field is applied to such material, also known as the piezoelectric effect (Figure 28–3). Both natural and human-made materials, including quartz crystals and ceramic materials, can demonstrate piezoelectric properties. Recently, lead zirconate titanate has been used as piezoelectric material for medical imaging. Lead-free piezoelectric materials are also under development. Individual piezoelectric materials produce a small amount of energy. However, by stacking piezoelectric elements into layers in a transducer, the transducer can convert electric energy into mechanical oscillations more efficiently. These mechanical oscillations are then converted into electric energy.
The piezoelectric effect. Mechanical deformation and consequent oscillation caused by an electrical field applied to certain material can produce a sound of high frequency.
Period is the time for a sound wave to complete one cycle; the period unit of measure is the microsecond (µs).
Wavelength is the length of space over which one cycle occurs; it is equal to the travel distance from the beginning to the end of one cycle.
Frequency is the number of cycles repeated per second and measured in hertz (Hz).
Acoustic velocity is the speed at which a sound wave travels through a medium. It is equal to the frequency times the wavelength. Speed c is determined by the density ρ and stiffness κ of the medium (c = (κ/ρ)1/2). Density is the concentration of a medium. Stiffness is the resistance of a material to compression. Propagation speed increases if the stiffness is increased or the density is decreased. The average propagation speed in soft tissues is 1540 m/s (ranges from 1400 to 1640 m/s). However, ultrasound cannot penetrate lung or bone tissues.
Acoustic impedance z is the degree of difficulty demonstrated by a sound wave being transmitted through a medium; it is equal to density ρ multiplied by acoustic velocity c (z = ρc). It increases if the propagation speed or the density of the medium is increased.
Attenuation coefficient is the parameter used to estimate the decrement of ultrasound amplitude in certain media as a function of ultrasound frequency. The attenuation coefficient increases with increasing frequency; therefore, a practical consequence of attenuation is that the penetration decreases as frequency increases (Figure 28–4).
The ultrasound amplitude decreases in certain media as a function of ultrasound frequency, a phenomenon known as the attenuation coefficient. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
Ultrasound waves have a self-focusing effect, which refers to the natural narrowing of the ultrasound beam at a certain travel distance in the ultrasonic field. It is a transition level between near field and far field. The beam width at the transition level is equal to half the diameter of the transducer. At the distance of two times the near-field length, the beam width reaches the transducer diameter. The self-focusing effect amplifies ultrasound signals by increasing acoustic pressure.
In ultrasound imaging, there are two aspects of spatial resolution: axial and lateral. Axial resolution is the minimum separation of above-below planes along the beam axis. It is determined by spatial pulse length, which is equal to the product of wavelength and the number of cycles within a pulse. It can be presented in the following formula:
Axial resolution = Wavelength λ × Number of cycles per pulse n ÷ 2
The number of cycles within a pulse is determined by the damping characteristics of the transducer. The number of cycles within a pulse is usually set between 2 and 4 by the manufacturer of the ultrasound machines. As an example, if a 2-MHz ultrasound transducer is theoretically used to do the scanning, the axial resolution would be between 0.8 and 1.6 mm, making it impossible to visualize a 21-gauge needle. For a constant acoustic velocity, higher-frequency ultrasound can detect smaller objects and provide an image with better resolution. The axial resolution of current ultrasound systems is between 0.05 and 0.5 mm. Figure 28–5 shows images at different resolutions when a 0.5-mm diameter object is visualized with three different frequency settings.
Ultrasound frequency affects the resolution of the imaged object. Resolution can be improved by increasing frequency and reducing the beam width by focusing. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
Lateral resolution is another parameter of sharpness to describe the minimum side-by-side distance between two objects. It is determined by both ultrasound frequency and beam width. The higher frequencies have narrower focus and provide better axial and lateral resolution. Lateral resolution can also be improved by adjusting focus to reduce the beam width.
Temporal resolution is also important for observing a moving object such as blood vessels and heart. Like a movie or cartoon video, the human eye requires that the image be updated at a rate of approximately 25 times a second or higher for an ultrasound image to appear continuous. However, imaging resolution will be compromised by increasing the frame rate. Optimizing the ratio of resolution to the frame rate is essential for providing the best possible image.
INTERACTIONS OF ULTRASOUND WITH TISSUES
As the ultrasound wave travels through tissues, it is subject to a number of interactions. The most important features are as follows:
When ultrasound encounters boundaries between different media, part of the ultrasound is reflected and the other part is transmitted. The reflected and transmitted directions are given by the reflection angle θr and transmission angle θt, respectively (Figure 28–6).
The interaction of ultrasound waves through the media in which they travel is complex. When ultrasound encounters boundaries between different media, part of the ultrasound is reflected and part is transmitted. The reflected and transmitted directions depend on the respective angles of reflection and transmission. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
Refection of sound waves is similar to optical refection. Some of its energy is sent back into the medium from which it came. In a true reflection, the reflection angle θr must equal the incidence angle θi. The strength of the reflection from an interface is variable and depends on the difference of impedances between two affinitive media and the incident angle at the boundary. If the media impedances are equal, there is no reflection (no echo). If there is a significant difference between media impedances, there will be nearly complete reflection. For example, an interface between soft tissues and either lung or bone involves a considerable change in acoustic impedance and creates strong echoes. This reflection intensity is also highly angle dependent. In practical terms, it means that the ultrasound transducer must be placed perpendicular to the target nerve to visualize it clearly. A change in sound direction when crossing the boundary between two media is called refraction. If the propagation speed through the second medium is slower than that through the first medium, the refraction angle is smaller than the incident angle. Refraction can cause the artifact that occurs beneath large vessels on the image.
During ultrasound scanning, a coupling medium must be used between the transducer and the skin to displace air from the transducer-skin interface. A variety of gels and oils are applied for this purpose. Moreover, they can act as lubricants, making a smooth scanning performance possible. Most scanned interfaces are somewhat irregular and curved. If the boundary dimensions are significantly less than the wavelength or not smooth, the reflected waves will be diffused.
Scattering is the redirection of sound in any directions by rough surfaces or by heterogeneous media (Figure 28–7). Normally, scattering intensity is much less than mirror-like reflection intensities and is relatively independent of the direction of the incident sound wave; therefore, the visualization of the target nerve is not significantly influenced by other nearby scattering.
Scattering is the redirection of ultrasound in any direction caused by rough surfaces or by heterogeneous media. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
Absorption is defined as the direct conversion of the sound energy into heat. In other words, ultrasound scanning generates heat in the tissue. Higher frequencies are absorbed at a greater rate than lower frequencies. However, a higher scanning frequency gives better axial resolution. If the ultrasound penetration is not sufficient to visualize the structures of interest, a lower frequency is selected to increase the penetration. The use of longer wavelengths (lower frequency) results in lower resolution because the resolution of ultrasound imaging is proportional to the wavelength of the imaging wave. Frequencies between 6 and 12 MHz typically yield adequate resolution for imaging in peripheral nerve blockade, whereas frequencies between 2 and 5 MHz are usually needed for imaging of neuraxial structures. Frequencies of less than 2 MHz or higher than 15 MHz are rarely used because of insufficient resolution or the insufficient penetration depth in most clinical applications.
The A-mode is the oldest ultrasound technique and was invented in 1930.14 The transducer sends a single pulse of ultrasound into the medium. Consequently, a one-dimensional simplest ultrasound image is created on which a series of vertical peaks is generated after ultrasound beams encounter the boundary of the different tissue. The distance between the echoed spikes (Figure 28–8) can be calculated by dividing the speed of ultrasound in the tissue (1540 m/s) by half the elapsed time, but it provides little information on the spatial relationship of imaged structures. Therefore, A-mode ultrasound is not applicable in regional anesthesia.
The A-mode of ultrasound consists of a one-dimensional ultrasound image displayed as a series of vertical peaks corresponding to the depth of structures the ultrasound encounters in different tissues. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
The B-mode is a two-dimensional (2D) image of the area that is simultaneously scanned by a linear array of 100–300 piezoelectric elements rather than a single one as in A-mode (Figure 28–9). The amplitude of the echo from a series of A-scans is converted into dots of different brightness in B-mode imaging. The horizontal and vertical directions represent real distances in tissue, whereas the intensity of the grayscale indicates echo strength (Figure 28–10). B-mode can provide an image of a cross section through the area of interest, and it is the primary mode currently used in regional anesthesia.
The B-mode transducer incorporates numeric piezoelectric elements that are electrically connected in parallel.
An example of B-mode imaging. The horizontal and vertical directions represent distances and tissues, whereas the intensity of the grayscale indicates echo strength. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
The Doppler effect is based on the work of Austrian physicist Johann Christian Doppler.15 The term describes a change in the frequency or wavelength of a sound wave resulting from relative motion between the sound source and the sound receiver. In other words, at a stationary position, the sound frequency is constant. If the sound source moves toward the sound receiver, the sound waves have to be squeezed, and a higher-pitch sound occurs (positive Doppler shift); if the sound source moves away from the receiver, the sound waves have to be stretched, and the received sound has a lower pitch (negative Doppler shift) (Figure 28–11). The magnitude of Doppler shift depends on the incident angle between the directions of emitted ultrasound beam and moving reflectors. With a 90° angle there is no Doppler shift. If the angle is 0° or 180°, the largest Doppler shift can be detected. In medical settings, the Doppler shifts usually fall in the audible range.
The Doppler effect. When a sound source moves away from the receiver, the received sound has a lower pitch and vice versa. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
Color Doppler produces a color-coded map of Doppler shifts superimposed onto a B-mode ultrasound image. Blood flow direction depends on whether the motion is toward or away from the transducer. Selected by convention, red and blue colors provide information about the direction and velocity of the blood flow. According to the color map (color bar) in the upper left-hand corner of the figure (Figure 28–12), the red color on the top of the bar denotes the flow coming toward the ultrasound probe, and the blue color on the bottom of the bar indicates the flow away from the probe.
Color Doppler produces a color-coded map of Doppler shapes superimposed onto a B-mode ultrasound image. Selected by convention, red and blue colors provide information about the direction and velocity of the blood flow. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
In ultrasound-guided peripheral nerve blocks, color Doppler mode is used to detect the presence and nature of the blood vessels (artery vs. vein) in the area of interest. When the direction of the ultrasound beam changes, the color of the arterial flow switches from blue to red, or vice versa, depending on the convention used (Figures 28–13, 28–14A, 28–14B, and 28–14C). Power Doppler is up to five times more sensitive in detecting blood flow than color Doppler, and it is less dependent on the scanning angle. Thus, power Doppler can be used to identify the smaller blood vessels more reliably. The drawback is that power Doppler does not provide any information on the direction and speed of blood flow (Figure 28–15).
Color Doppler mode is used to detect the direction of the blood vessel.
A: Carotid artery displays red color when the blood flows toward the transducer. B: Carotid artery displays ambiguous color at a 90° Doppler angle; the equal waveform can be seen on both sides of the baseline. C: Carotid artery displays blue color when the blood flows away from the transducer.
Although the power Doppler may be useful in identifying smaller blood vessels, the drawback is that it does not provide information on the direction and speed of blood flow. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
A single beam in an ultrasound scan can be used to produce a picture with a motion signal, where movement of a structure such as a heart valve can be depicted in a wave-like manner. M-mode is used extensively in cardiac and fetal cardiac imaging; however, its present use in regional anesthesia is negligible (Figure 28–16).
M-mode consists of a single beam used to produce an image with a motion signal. Movement of a structure can be depicted in a wavelike matter. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
Ultrasound machines convert the echoes received by the transducer into visible dots, which form the anatomic image on an ultrasound screen. The brightness of each dot corresponds to the echo strength, producing what is known as a grayscale image.
Two types of scan transducers are used in regional anesthesia: linear and curved. A linear transducer can produce parallel scan lines and a rectangular display, called a linear scan, whereas a curved transducer yields a curvilinear scan and an arc-shaped image (Figures 28–17A and 28–17B). In clinical scanning, even a very thin layer of air between the transducer and skin may reflect virtually all the ultrasound, hindering any penetration into the tissue. Therefore, a coupling medium, usually an aqueous gel, is applied between surfaces of the transducer and skin to eliminate the air layer. The ultrasound machines currently used in regional anesthesia provide a 2D image, or “slice.” Machines capable of producing three-dimensional (3D) images have recently been developed. Theoretically, 3D imaging should help in understanding the relationship of anatomic structures and spread of local anesthetics. There are three major types of 3D ultrasound imaging: (1) Freehand 3D is based on a set of 2D cross-sectional ultrasound images acquired from a sonographer sweeping the transducer over a region of interest (Figures 28–18A and 28–18B). (2) Volume 3D provides 3D volumetric images using a dedicated 3D transducer. The transducer elements automatically sweep through the region of interest during the scanning; the sonographer is not required to perform hand motions (Figure 28–18C). (3) Real-time 3D takes multiple images at different angles, allowing the sonographer to see the 3D model moving in real time. However, typical spatial resolution of 3D imaging is about 0.34–0.5 mm. At present, 3D imaging systems still lack the resolution and simplicity of 2D images, so their practical use in regional anesthesia is limited.
A: Rectangular scan field given by linear transducer. B: Arc-shaped scan field given by curved transducer. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
A: Freehand 3D imaging. A linear transducer produces parallel scan lines and a rectangular display; linear scan. B: Freehand 3D imaging. A curved “phase array” transducer results in a curvilinear scan and an arch-shaped image. C: Fetal face viewed by volume 3D imaging. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
The echoes exhibit a steady decline in amplitude with increasing depth. This occurs for two reasons: First, each successive reflection removes a certain amount of energy from the pulse, decreasing the generation of later echoes. Second, tissue absorbs ultrasound, so there is a steady loss of energy as the ultrasound pulse travels through the tissues. This can be corrected by manipulating time-gain compensation (TGC) and compression functions. Gain is the ratio of output to input electric power; it controls the brightness of the image. The gain is usually measured in decibels (dB). Increasing the gain amplifies not only the returning signals, but also the background noise within the system in the same manner. TGC is time-dependent amplification. TGC function can be used to increase the amplitude of incoming signals from various tissue depths.
The layout of the TGC controls varies from one machine to another. A popular design is a set of slider knobs. Each knob in the slider set controls the gain for a specific depth, which allows for a well-balanced gain scale on the image (Figures 28–19A, 28–19B, and 28–19C).
A, B, and C: The effect of the time-gain compensation settings. Time-gain compensation is a function that allows time- (depth) dependent amplification of signals returning from different depths. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
Amplification is the conversion of the small voltages received from the transducer into larger ones that are suitable for further processing and storage. There are two amplification processes considered to increase the magnitude of ultrasound echoes: linear and nonlinear amplification. Currently, the ultrasonic imaging system with linear amplifiers is commonly used in medical diagnostic applications. However, strength of echoes attenuates exponentially as the distance between the transducer and the reflector increases. Ultrasonic imaging instruments equipped with logarithmic amplifiers can display echo signals with a wider dynamic range than a linear amplifier and remarkably improve the sensitivity for a small magnitude of echoes on the screen.
Dynamic range is the range of amplitudes from largest to the smallest echo signals that an ultrasound system can detect. The wider/higher dynamic range presents a larger number of grayscale levels, and it creates a softer image; the image with a narrower/lower dynamic range appears with more contrast (Figures 28–20A and 28–20B). Dynamic range less than 50 dB or greater than 100 dB is probably too low or too high in terms of visualization of peripheral nerve. Compression is the process of decreasing the differences between the smallest and largest echo-voltage amplitudes; the optimal compression is between 2 and 4 for a maximal scale equal to 6.
A: A softer image provided by a higher dynamic range. B: An image with more contrast provided by a lower dynamic range.
As previously discussed, it is common to use electronic means to narrow the width of the beam at some depth and achieve a focusing effect similar to that obtained using a convex lens (Figure 28–21). There are two types of focusing: annular and linear. These are illustrated in Figures 28–22A and 28–22B, respectively.
A demonstration of focusing effect. An electronic means can be used to narrow the width of the beam at a specific depth, resulting in the focusing effect and greater resolution at a chosen depth. (Adapted with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc; 2011.)
A: Annular focusing is electronic focusing from all directions in the scan plane given by an annular transducer that contains several ring elements arranged concentrically. B: Linear focusing is electronic focusing applied along both lateral sides in the scan plane.
Adjusting focus improves the spatial resolution on the plane of interest because the beam width is converged. However, the reduction in beam width at the selected depth is achieved at the expense of degradation in beam width at other depths, resulting in poorer images below the focal zone.
The mechanisms of action by which an ultrasound application could produce a biologic effect can be conceptually categorized into two aspects: heating and mechanical. In reality, these two effects are rarely separable except for extracorporeal lithotripsy, the therapeutic application of mechanical bioeffects alone. The generation of heat increases as ultrasound intensity or frequency is increased. For similar exposure conditions, the expected temperature increase in bone is significantly greater than in soft tissues. In in vivo experiments, high-intensity ultrasound (usually > 2 W/cm2) is used to evaluate harmful biological effect; it is 5 to 20 times larger than therapeutic intensities (0.08–0.5 W/cm2) and 8 to 100 times larger than diagnostic intensities (color flow mode 0.25 W/cm2, B-mode scan 0.02 W/cm2). Reports in animal models (mice and rats) suggest that application of ultrasound may result in a number of undesired effects, such as fetal weight reduction, postpartum mortality, fetal abnormalities, tissue lesions, hind limb paralysis, blood flow stasis, and tumor regression. Other reported undesired effects in mice are abnormalities in B-cell development and ovulatory response and teratogenicity.16,17
In general, adult tissues are more tolerant of rising temperature than fetal and neonatal tissues. A modern ultrasound machine displays two standard indices: thermal and mechanical. The thermal index (TI) is defined as the transducer acoustic output power divided by the estimated power required to raise tissue temperature by 1°C. The mechanical index (MI) is equal to the peak rarefactional pressure divided by the square root of the center frequency of the pulse bandwidth. TI and MI indicate the relative likelihood of thermal and mechanical hazard in vivo, respectively. Either TI or MI greater than 1.0 is hazardous.18,19
Biologic effect due to ultrasound also depends on tissue exposure time. The researchers usually use pregnant mice to expose to ultrasound with a minimum intensity of 1 W/cm2 for 60 to 420 minutes to evaluate the time-dependent adverse events that happen in rodent fetuses.20 Fortunately, ultrasound-guided nerve block requires the use of only low TI and MI values on the patient for a short period of time. Based on in vitro and in vivo experimental study results to date, there is no evidence that the use of diagnostic ultrasound in routine clinical practice is associated with any biologic risks.
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Optimizing an Ultrasound Image
Optimizing an ultrasound image is an essential skill during ultrasound-guided nerve blockade. Anatomically, a peripheral nerve is always located in the vicinity of an artery between fascial layers. The echotexture of normal nerve shows a hyperechoic, hypoechoic, or honeycomb pattern (Figure 29–1).1,2 There are several scanning steps to obtain adequate nerve imaging, including the selection of sonographic modes, adjustment of function keys, needle visualization, and interpretation of image artifacts.
Echotexture of peripheral nerves. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)
Common sonographic imaging modes used for medical diagnostics, such as, conventional imaging, compound imaging, and tissue harmonic imaging (THI) can all be utilized in imaging of peripheral nerves. Conventional imaging is generated from a single-element angle beam at a primary frequency designated by the transducer. Compound imaging is implemented by acquiring several (usually three to nine) overlapping frames from different frequencies or from different angles.3 THI acquires the information from harmonic frequencies generated by ultrasound beam transmission through tissue. Harmonic frequencies are multiples of the primary frequency. THI improves axial resolution and boundary detection by suppression of scattering signals from tissue interfaces, especially for obese patients.
Currently, THI has been set as the default mode by many, if not most, US manufacturers. Compound imaging with THI can provide images with better resolution, penetration, and interfaces and margin enhancement compared with conventional sonography. In Figure 29–2, both compound imaging and conventional imaging were employed to visualize an interscalene brachial plexus. There is clear margin definition of two hypoechoic oval-shaped nerve structures in compound imaging; the contrast resolution between anterior scalene muscle and surrounding adipose tissue is increased in comparison with conventional imaging.
Examples of image quality typically obtained with conventional versus compound imaging. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)
Five function keys on an ultrasound machine are of crucial importance to achieve an optimal image during the performance of peripheral nerve imaging (Figure 29–3).4
Optimizing an ultrasound image using five key functional adjustments and specific tips on adjusting the focus and gain. Some ultrasound models are specifically optimized for regional anesthesia application and may not incorporate user-adjustable focus or time-gain compensation (TGC). (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)
Depth: The depth of the nerve is the first consideration when an ultrasound-guided nerve block is performed. Peripheral nerve branches have a great variation of depth, which depends on patients’ habitus; the optimal depth setting can provide good focusing properties for imaging. Table 29–1 recommends the initial depth and frequency settings for common peripheral nerve blockades. The target nerve should be at the center of ultrasound imaging because it not only has the best resolution of the nerve but also reveals the other anatomical structures in the vicinity of the nerve. For example, ultrasound imaging during supraclavicular or infraclavicular brachial plexus blockade must require that the first rib and pleura are observed simultaneously to avoid lung puncture with the needle.
Frequency: The ultrasound transducer with the optimal frequency range should be selected to best visualize the target nerves. Ultrasound energy is absorbed gradually by the transmitted tissue; the higher the frequency of ultrasound, the more rapid the absorption and the less distance propagation. Therefore, a low-frequency transducer is used to scan structures at a deeper location; unfortunately, this is at the expense of reduced image resolution. In some particular cases, like lumbar plexus block, a lower-frequency transducer with a Doppler setting is useful for identifying the vasculature close to the lumbar plexus in obese patients.
Focusing: Lateral resolution can be improved by choosing a higher frequency as well as by focusing the ultrasound beam. In clinical practice, the focus is adjusted at the level of the target nerve; the best image quality for a given nerve is obtained by choosing an appropriate frequency transducer and the focal zone (Figure 29–4A). Furthermore, when possible, selecting no more than two focus zones yields better image because multiple focal zones can slow the frame rate and decrease the temporal resolution.
Gain: Screen brightness can be adjusted manually by two function buttons—gain and time-gain compensation (TGC)—on ultrasound machines that have TGC built in. Excessive or inadequate gain can cause blurring of tissue boundaries and loss of information. Optimal gain for scanning peripheral nerves is typically the gain at which the best contrast is obtained between the muscles and the adjacent connective tissue. This is because muscles are well-vascularized tissue invested with connective tissue fibers, whereas the echo texture of connective tissue is similar to that of nerves. In addition, increasing gain below the focus works well with the TGC control to visualize both the target nerve and the structures below it. Figure 29–4B shows the same section with both correct and incorrect gain and TGC settings. TGC sliders aligned in a curve can lead to a desirable image with appropriate gain.
Doppler: In regional anesthesia, Doppler ultrasound is used to detect vascular structures or the location of the spread of the local anesthetic injection. Doppler velocity scale is best set between 15 and 35 cm/s to reduce aliasing of color Doppler imaging and artifacts of color (Figure 29–5). Of note, power Doppler is more sensitive for detecting blood flow than color Doppler. The gate size is another common setting when color Doppler is used. It should be as small as possible to overlay the area of interest. An appropriate small gate not only can exclude distractive signals from adjacent tissues but also can improve temporal resolution by increasing the frame rate.
Table 29–1.Suggested optimal imaging depth and frequency for common peripheral nerve blocks. ||Download (.pdf) Table 29–1. Suggested optimal imaging depth and frequency for common peripheral nerve blocks.
|Field Depth (cm) ||Frequency (MHz) ||Peripheral Blockades |
|<2.0 ||12–15 ||Wrist, ankle block |
|2.0–3.0 ||10–12 ||Interscalene, supraclavicular, axillary brachial plexus block |
|3.0–4.0 ||10–12 ||Femoral nerve block, TAP block |
|4.0–7.0 ||5–10 ||Infraclavicular, popliteal, subgluteal sciatic nerve blocks |
|7.0–10.0 ||5–10 ||Pudendal, gluteal sciatic nerve, lumbar plexus block |
|>10.0 ||3–5 ||Anterior approach to sciatic nerve, celiac ganglion block |
A: Focusing narrows ultrasound beam width to improve the lateral resolution and sensitivity. Shown are three examples of focusing when imaging the sciatic nerve: below the nerve, at the level of the nerve, and superficial to the nerve. B: Optimal and incorrect gain and TGC settings. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)
Color Doppler aliasing occurs when the velocity scale for color Doppler is set too low.
Two needle insertion techniques with relevance to the needle-transducer relationship are commonly used in ultrasound-guided nerve block: the in-plane and out-of-plane techniques (Figure 29–6). An in-plane technique means the needle is placed in the plane of the ultrasound beam; as a result, the needle shaft and the tip can be observed in the longitudinal view in real time as the needle is advanced toward the target nerve. When the needle path is not seen on the image, the needle advancement should be paused; tilting, sliding or rotating the transducer can bring the ultrasound beam into alignment with the needle. In addition, a subtle, fast needle shake and or injection of a small amount of injectate may help depict the needle location.
In-plane and out-of-plane needle insertion and the appearance in a corresponding ultrasound image.
The out-of-plane technique involves needle insertion perpendicular or any other angle to the transducer to the transducer. The needle shaft is imaged in a cross-sectional plane and often can be identified as a bright dot in the image. Visualization of the tip of the needle, however, requires higher degree of skill. The method used to visualize the tip of the needle is as follows: Once a bright dot (shaft) is seen in the image, the needle can be shaken slightly or the transducer can be tilted toward the direction of needle insertion simultaneously until the dot disappears. Shaking the needle helps differentiate the echo as emanating from the needle or from the surrounding tissue. The last capture of the hyperechoic dot is its tip. A small amount of injectate can be used to confirm the location of the needle tip. Whenever injectate is used to visualize the needle tip, attention must be paid to avoid resistance (pressure) to injection because when the needle-nerve interface is not well seen, there is a risk for the needle to be against the nerve or to inject intrafascicularly.5
If the needle trajectory is lost visually, the operator should stop advancing the needle and then tilt the transducer to visualize the needle.
When the spread of the local anesthetic is not seen during the injection process, the operator should stop the injection, tilt the transducer, and inject a tiny amount of local anesthetic (or air) to locate the needle tip and spread of injectate.
Continuous peripheral nerve blocks (CPNBs) have become a common practice; however, the visualization of the catheter tip can be challenging. Direct visualization of the catheter tip can be obtained when the catheter is introduced at a short distance from the needle tip (eg, 2 cm past the needle tip) (Figure 29–7). However, when the catheter is inserted 3–5 cm past the needle tip, the needle, nerve, and catheter are never in the same plane of the ultrasound beam, therefore becoming challenging to image. There are two ways to confirm the catheter tip: (1) The operator can tilt or slightly slide the transducer to see a “bright dot,” which is the transverse view of the catheter. The position of the catheter tip can be detected by observing the spread of 1–2 mL injectate through the catheter, and the use of color Doppler may help visualize the spread more significantly (Figures 29–8A and 29–8B). (2) In some cases, the bright dot may not be obviously visualized or ensured; the operator has to slide the transducer within a certain distance away from the needle tip, with the distance based on the length of catheter threaded past the needle tip. Injection of 0.5 mL air can be beneficial to ascertain the position of the catheter tip with a sharp echoic contrast on the ultrasound image (Figures 29–9A and 29–9B). The obvious drawback is that injection of air may degrade the image for other purposes.
The catheter tip can be directly seen just beneath femoral nerve.
A: The position of catheter tip can be estimated by observing the spread of injectate. B: Doppler can be used to confirm the location of the spread.
A: The location of the catheter tip cannot be visualized before a small amount of air is injected. B: The discernable brightness indicates the location of the catheter tip when 0.3–0.5 mL air is injected.
Ultrasound artifacts occur commonly and, in fact, are an intrinsic part of ultrasound imaging. By definition, an ultrasound artifact is any image aberration that does not represent the correct anatomic structures. Most artifacts are undesirable, and operators must learn how to recognize them during nerve blockade. The five artifacts most commonly seen in regional anesthesia practice (Figure 29–10) are the following6,7,8:
Five common ultrasound artifacts during ultrasound-guided peripheral nerve block. (Reproduced with permission from Hadzic A: Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd ed. New York: McGraw-Hill, Inc.; 2011.)
Shadowing is a significant attenuation of ultrasound signal deep to tissues and structures that absorb or reflect most of the ultrasound waves, as bones, calcifications or air. This is manifested by a weak or absent echo area which appears as a shadow on the imaging behind a bright, hyperechoic interface. Acoustic shadowing has a favorable diagnostic value for detection of calcified lesions, such as gallstones, scar tissue, and the like. However, the shadowing may interfere with nerve visualization in regional anesthesia. Changing the scanning plane to find the best acoustic window is the best strategy to avoid shadowing when necessary.
Enhancement manifests as overly intense echogenicity behind an object (a fluid-filled structure, such as a vessel or cyst) that is less attenuating than the surrounding soft tissues. Enhancement occurs when the echo signals are overamplified in brightness disproportional to the echo strength at the same depth. Scanning from different angles or from different planes may help to decrease shadowing/enhancement artifacts and to visualize the target nerve; using automatic TGC may make enhancement artifact less apparent as well.
Reverberation displays in the form of parallel, equally spaced bright linear echoes behind the reflectors in the near field of the image. The multiple echoes occur when the ultrasound beam bounces repeatedly between the interfaces of the transducer and a strong reflector, especially when these two interfaces are parallel to each other. It may be attenuated or eliminated when the scanning direction is changed slightly or the ultrasound frequency is decreased.
Mirror image artifact results from an object located on one side of a highly reflective linear boundary that acts like an acoustic “mirror,” appearing on the other side as well. The transducer receives both direct echoes from the object and indirect echoes from the mirror (Figure 29–11). Both virtual and artifactual images have an equal distance to the mirror from opposite directions. The duplicated artifactual image is always less bright and deeper than the real image because indirect echoes transmit a longer distance and attenuate more wave energy. Changing the scanning direction may decrease the artifact.
Mirror image artifact: Transducer receives both direct echoes from the object (1) and indirect echoes from the “mirror” (2).
Velocity error is the displacement of the interface, which is caused by the difference of actual velocity of ultrasound in human soft tissue, compared with the calibrated speed, which is assumed to be a constant velocity of 1540 m/s set by the ultrasound system. Consequently, a reflector is displaced toward the transducer by a significant error in distance calculations. The inherent artifact in the process of scanning cannot be completely eliminated in all cases by manipulating ultrasound devices or changing the settings. However, recognizing and understanding ultrasound artifacts help the operator avoid misinterpretation of images.
An acronym, SCANNING, can be used by operators to prepare for scanning:
Gather supplies: All equipment necessary for ultrasound scanning should be prepared. Equipment may differ slightly depending on the area to be scanned; however, some necessary equipment includes the following:
Nerve block kit, nerve stimulator
Sterile work trolley
Local anesthetic drawn up and labeled
Whenever possible, connect the ultrasound machine to the power outlet to prevent the machine from powering down during a procedure. Although many point-of-care ultrasound machines are equipped with batteries, these run out of power during the most importunate part of the procedure.
Comfortable patient position: The patient should be positioned in such a way that the patient, the anesthesiologist, the ultrasound machine, and the sterile block tray are all arranged ergonomically to allow for time-efficient performance of the procedure.
The ultrasound machine should be set up on the opposite side of the patient from the operator with the screen at the operator’s eye level.
The block tray should be positioned close enough to the operator can easily reach for needle, gel, and other supplies without interference with the scanning procedure.
Ambiance set room settings: Adjust the lights in the room to view the ultrasound machine and procedural site adequately.
Dim lighting optimizes visualization of the image on the screen; more lighting may be needed for the procedural site.
Adjust the room light settings to allow for proper lighting to both areas, as well as for safe monitoring of the patient.
Name of patient, procedure, and site of procedure: Before performing a scan take a “time-out” to ensure patient information is correct, the operation being done is confirmed, and the side in which the procedure is being done is validated. The New York School of Regional Anesthesia (NYSORA) team uses the acronym ECT for the time-out procedure: E for equipment for patient monitoring and needle-nerve monitoring; C for the patient’s consent for the procedure; and T for the time for the time-out to identify the patient and ensure correct laterality. Checking that patient information is entered into the ultrasound machine and matches the information on the patient’s wristband not only confirms identity but also allows for images to be saved during the scanning process for documentation.
Select transducer: Select the transducer that best fits the scheduled procedure. A linear transducer is best for scanning superficial anatomic structures; a curved (phased-array) transducer displays a sector image and is typically better for deeper-positioned structures. A hockey stick ultrasound transducer is an ideal choice for vascular access or a superficial block with limited space, such as an ankle block.
Disinfection: Disinfect the patient’s skin using a disinfectant solution to reduce the risk of contamination and infection.
Orient transducer and apply gel: The operator should orient the transducer to match the medial-lateral orientation of the patient. This is conventionally not done by radiologists/sonographers, but it is useful for intervention-oriented regional anesthesia procedures.
Touch one edge of the transducer to orient the side of the transducer so the medial-lateral orientation on the patient corresponds to that on the screen.
A sufficient amount of gel is applied to either the transducer or the patient’s skin to allow for transmission of the ultrasound. A copious amount of disinfectant solution can be used instead of gel in many instances.
Insufficient quality of gel will decrease reflection-absorption rates and may result in unclear/blurry images on the ultrasound image being displayed.
Place the transducer on the patient’s skin and adjust the ultrasound machine settings:
The gain should be adjusted with the general gain setting or by using TGC.
The depth is adjusted to optimize imaging of the structures of interest.
Where available, focus point level.
Scanning mode can be switched to assist in the recognition of the structures as necessary. Power Doppler can help depict blood vessels; color mode can distinguish between arteries and veins.
I: Echotexture of peripheral nerves: Correlation between US and histologic findings and criteria to differentiate tendons. Radiology 1995;197(1):291–296.
WH: Sonography of peripheral nerve pathology. AJR Am J Roentgenol 2004;182(1):123–129.
H: Multiangle compound imaging. Ultrason Imaging 1998;20:81–102.
BB: Ultrasound Scanning: Principles and Protocols, 3rd ed. Saunders Elsevier, 2009.
A: Opening injection pressure consistently detects needle-nerve contact during ultrasound-guided interscalene brachial plexus block. Anesthesiology 2014;120(5):1246–1253.
et al: Artifacts and pitfall errors associated with ultrasound -guided regional anesthesia. Part 1: Understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med 2007;32(5):412–418.
et al: Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. Part II: A pictorial approach to understanding and avoidance. Reg Anesth Pain Med 2007;32(5):419–433.
E Jr, Boone
JM. Ultrasound image quality and artifacts. In The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2012, pp 560–567.
Introduction to Ultrasound-Guided Regional Anesthesia
Ultrasonography (US) as a means to guide peripheral nerve blockade (PNB) was first explored by anesthesiologists at the University of Vienna in the mid-1990s.1 Although radiologists had made use of ultrasound technology to guide needles for biopsy, the application of this imaging modality for PNB was novel at that time.2 The utility of ultrasound to facilitate a range regional anesthesia techniques including brachial plexus and femoral blocks was demonstrated.1,3 A decade later, colleagues from the University of Toronto, Canada, began to embrace this technology, further demonstrating its utility and describing in detail the sonoanatomy of the brachial plexus.4 A number of advances in technology took place in the meantime, including smaller and more mobile ultrasound platforms, improved resolution, and needle recognition software, all cumulatively leading to increased bedside utility of ultrasound by anesthesiologists.5
ADVANTAGES OF ULTRASOUND GUIDANCE
The previously used surface anatomy-based techniques, such as nerve stimulation, palpation of landmarks, fascial “clicks,” paresthesias, and transarterial approaches, did not allow for the monitoring of the disposition of the local anesthetic injectate. Ultrasound guidance, however, offers a number of important practical advantages for nerve blockade. Ultrasound allows visualization of the anatomy of the region of interest. This allows more informed guidance for the needle pathway to the target while avoiding structures that might be damaged by the needle.6 Ultrasound also allows visualization of the needle tip as it is passed through the tissues, confirming alignment with the intended path, again reducing the likelihood of inadvertent needle trauma to unintended structures. Perhaps most important, real-time ultrasound imaging permits continual visualization of local anesthetic solution delivery to ensure proper distribution, with the potential for adjustment of the needle tip position as necessary to optimize local anesthetic distribution.7 Introduction of ultrasound guidance in regional anesthesia has led to refinement of many nerve block techniques, expanded use of PNB, and greater acceptance by surgical colleagues and patients.
ULTRASOUND AND SONOANATOMY
Ultrasound-guided PNB may be broken down into two fundamental aspects: imaging structures in the plane of section, including the target nerve, and guiding the needle. Understanding and recognition of three-dimensional anatomic structures on a two-dimensional image requires training in the technology and sonoanatomy pattern recognition (Table 30–1). As anatomic recognition remains essential to placing blocks, even with real-time visual guidance, specialty society guidelines for training residents and fellows continue to stress the importance of anatomical dissection and gross anatomy training as an inherent component of learning ultrasound-guided regional anesthesia (UGRA).8 In a study conducted over a 1-month regional anesthesia rotation, residents demonstrated markedly improved recognition of relevant structures at the sites of several different PNBs, using ultrasound imaging.9 In an evaluation of ultrasound-guided interscalene block instruction, residents demonstrated increasing efficiency of sonoanatomy recognition as their experience over the course of the rotation increased.10
Table 30–1.Optimizing sonoanatomy visualization. ||Download (.pdf) Table 30–1. Optimizing sonoanatomy visualization.
Choose appropriate transducer/frequency
Understand underlying anatomic relationships
Apply varying degree of pressure with transducer
Align transducer with underlying nerve target
Rotate transducer to fine-tune image
Tilt the transducer to optimize image
More innovative methods of training have shown promise as well.11 Integrating an anatomic program into the software of a bedside ultrasound machine has been shown to improve scores on a written test of anatomy.12 After exposure to a multimedia anatomy presentation, residents and community anesthesiologists demonstrated increased knowledge of ultrasound anatomy on a posttest, although they were not able to improve scores on a practical examination of sonoanatomy on live models.13 However, the optimal link between anatomic knowledge and recognition of two-dimensional anatomic patterns on ultrasound has not yet been adequately explored.
Certain basic tenets of optimizing an ultrasound image are applicable to all nerve blocks. For instance, sonography requires an understanding of mechanics and ergonomics. Novices are subject to errors such as probe fatigue, reversing probe orientation, and inadequate equipment preparation.14 To optimize the ultrasound image, the mnemonic PART (pressure, alignment, rotation, tilting) has been recommended.8 Pressure is necessary to minimize the distance to the target and compress underlying subcutaneous adipose tissues. Alignment refers to placing the transducer in a position over the extremity (or trunk) at which the underlying nerve is expected to be in the field of view. Rotation allows fine-tuning of the view of the target structure. Tilting helps to bring the face of the probe into a perpendicular arrangement with the underlying target to maximize the number of returning echoes and thus provide the best image (Figure 30–1). In-depth discussion on optimizing ultrasound imaging is discussed in Chapter 29.
Fine adjustment of the probe tilt is necessary to optimize echo return from the target structure and enhance image resolution (yellow arrowheads indicate sciatic nerve at the popliteal fossa).
To optimize the ultrasound image, the mnemonic PART has been recommended: pressure, alignment, rotation, tilting.
Recognition and understanding of sonoanatomy requires knowledge of the underlying three-dimensional anatomy.
Optimum visualization of the target nerve requires appropriate transducer pressure, alignment with nerve, and rotation and tilting of the probe to fine-tune the image.
OPTIMIZING NERVE AND NEEDLE IMAGING WITH ULTRASOUND-CLINICAL SCENARIOS
Nerve imaging may be performed in either short-axis (probe face perpendicular to axis of nerve) or long-axis (probe face parallel to axis of nerve) position (Figure 30–2). It is frequently easier to recognize the round, often-hyperechoic, neural element with short-axis imaging, especially for a beginner. Because most nerve blocks are conducted in the extremities, this orientation results in a transducer position that is transverse, across the long axis of the arm or leg. In general, understanding the course of the nerves, based on knowledge of gross anatomy, allows one to align and rotate the transducer perpendicular to the course of the nerve subsequently adjusting the tilt as described previously to optimize the image.
The median nerve. A: Cross-section (target structure out of plane to ultrasound beam; yellow arrowhead). B: Longitudinal section (target structure in plane to ultrasound beam; red arrowheads).
Once the nerve and surrounding anatomy are identified, a needle path may be chosen so that it is imaged either in plane (needle parallel to long axis of probe) or out of plane (needle perpendicular to long axis of probe) to the ultrasound beam. While neither method has been shown superior for either block success or patient safety, the preferred approach may vary with anatomic or technical considerations. However, with in-plane imaging, it is possible to maintain an image of the entire needle, including the tip, although it can be challenging to keep the needle entirely in the viewing plane of the transducer.15 This method is especially beneficial during instruction, as the supervisor has continual visualization of the needle tip as it is advanced through the tissues. During out-of-plane imaging, the observer is able to see only the cross section of the needle, which appears as a small hyperechoic dot, at any plane along its entire length, so that distinguishing the tip from the shaft is much more difficult.15
Guiding the needle tip to the target while maintaining the entire needle in the plane of imaging, however, can be challenging (Table 30–2). Appropriate adjustment of the bed height and ergonomic placement of the ultrasound so that operator’s eyes can easily and rapidly shift from the image to the field (Figure 30–3), where needle alignment with the long axis of the probe can be ensured, is beneficial. It is surprisingly easy for the transducer to wander away from the plane of the needle while one’s vision is fixed on the ultrasound screen. This is more likely if the operator has the probe and needle aligned perpendicular to his or her own axis of viewing, as opposed to aligning the needle and probe with the viewing axis.
Table 30–2.Optimizing needle imaging with ultrasound. ||Download (.pdf) Table 30–2. Optimizing needle imaging with ultrasound.
Utilize a shallow angle of approach, if possible
“Heel” the transducer to make the face more parallel to the needle
Rotate the transducer to ensure the entire needle is seen
Tilt the transducer as necessary
Choose an “echogenic” needle
Apply needle recognition software, if available
“Hydrolocation” may help ascertain needle tip location
Ergonomic positioning for bed height and ultrasound position.
In a study of novice medical students learning the basics of UGRA, Speer et al found that the subjects required less time to locate the target, and were better able to keep the needle visualized in plane on the ultrasound image, when eyes, needle, probe, and viewing screen were aligned.16 Needle guides may also permit improved imaging of the needle during approach to the target, although more work has been done in vascular access.17 One disadvantage of needle guides is that they restrict needle motion to one plane, which may not be always desirable.
Nerves in short axis have an appearance that is to some extent determined by their proximity to the neuraxis. Although in most areas nerves are round, they may appear fusiform, such as the musculocutaneous nerve in the proximal arm,18 or oval-shaped, such as the sciatic nerve in the infragluteal region.19 In close association with the spine, nerves and nerve roots are comprised primarily of neural tissue, with minimal connective tissue.20 Because neural tissue appears hypoechoic on ultrasound imaging, while the connective tissue between fascicles is hyperechoic, nerves near the neuraxis appear as dark nodules.21 As nerves course peripherally, the number of fascicles increases, although they diminish in size, while the amount of connective tissue also increases.22 These changes lead to an increasingly complex “honeycomb” appearance on ultrasound in short-axis viewing21 (Figure 30–4). Unfortunately, because of the technology limitations of the current ultrasound machines, the number and arrangement of fascicles within a peripheral nerve may not be accurately portrayed.23
A: Proximal nerve appearance in the interscalene groove (yellow arrows indicate nerve roots) with little echogenic connective tissue. B: More distal in the supraclavicular fossa (red arrows indicate brachial plexus trunks) with “honeycomb” appearance.
While different tissues have characteristic appearances on ultrasound, nerve may not be easily be distinguished from tendon when both are viewed in short axis. However, using knowledge of anatomy, the operator can follow the course of the structure caudad-cephalad to determine the nature of the structure imaged. The tendons will eventually disappear into the muscle of origin or insert into bones. A good example is the median nerve at the wrist, where it is difficult to discern the neural structure from the many tendons in the carpal tunnel, versus at the midforearm, where the nerve is much more visually distinct, as it is situated between two layers of muscle, with no surrounding tendons24 (Figure 30–5).
A: The median nerve at the wrist among many tendons within the carpal tunnel. B: The median nerve more proximal in the forearm surrounded by muscle.
An important aspect of preparing for a block is to obtain the preferred imaging plane while planning the route for needle path. The operator should make certain that no vulnerable structures are in the projected course, such as a blood vessel, the pleura, or sensitive structures such as periosteum. This process is referred to as a “preblock scan,” which can contribute to patient safety and block success.6 In addition to two-dimensional imaging, the color Doppler setting should be utilized to identify small vessels, which may readily be confused with nerve structures (particularly roots) when viewed in short axis25 (Figure 30–6).
The supraclavicular brachial plexus with surrounding vasculature. The subclavian artery is indicated by the multicolor area, with the transverse cervical artery indicated by the red area.
To maintain the view of the needle tip and shaft, several techniques can be used. The more parallel the needle is to the face of the probe, the more echoes are transmitted back to the transducer, resulting in a superior image. This can be accomplished by gently indenting the skin at the needle insertion site or by moving the insertion site further away from the probe, resulting in a less-acute angle of insertion (Figure 30–7). The limitation of this approach is that a longer needle may be required, and more tissue is traversed en route to the target.26
Needle insertion directly beside the ultrasound probe may result in difficult visualization. Insertion at a distance from the probe permits a shallower approach, allowing for stronger echo return and better visualization of the needle (green arrow) although traversing a longer tissue path.
Another technique, referred to as heeling, involves pressing in on the edge of the transducer opposite the side of needle insertion, which results in a more parallel alignment of the probe face with the needle. In addition, the needle itself may be structurally altered to increase its echogenicity; commercially available versions of these “echogenic needles” usually have been etched on the surface of the shaft with crosshatches to create a greater degree of scatter of the ultrasound beam.27
As noted, needle guides may be utilized to improve needle imaging, though at the cost of constraint of movement.28 Laser guidance systems have also been created to improve alignment, with some success.29 One novel, alternative method of targeted needle placement and local anesthetic delivery utilizes a GPS guidance system, which may be especially useful when imaging is made difficult by steep needle angles.30 Proprietary software for needle localization at steep angles makes use of spatial compound imaging, which combines images of different angles of insonation. This results in enhanced needle imaging with both standard and echogenic block needles.31 Finally, localization of the needle tip may be accomplished with “hydrolocation,” in which small volumes of either dextrose solution or local anesthetic are injected to visualize spread within tissues, which typically reveals the position of the needle tip.26
Several different techniques are useful to maintain visualization of the needle with ultrasound imaging, including use of a shallow angle of approach, “heeling” the transducer, commercially available echogenic needles, and physical measures such as rotation and tilting of the transducer.
In addition, hydrolocation with a small injection of fluid can be utilized to facilitate localization of the needle in difficult situations.
The needle should be advanced with continuous visualization to avoid injury to anatomic structures.
A preblock scan, including use of the color Doppler function, helps plan the course of the needle.
Passage of the needle tip through fascial planes that abut a nerve should be conducted in a tangential fashion to avoid impaling the nerve when the fascia “releases” the needle.
SAFE NEEDLE GUIDANCE WITH ULTRASOUND
In advancing the needle tip toward the targeted nerve with in-plane imaging, one should be cautious and deliberate, attempting to maintain the needle in plane at all times (Table 30–3). The in-plane needle tip is characterized by a double-echo return generated from the beveled surface. Ultrasound is reflected from both the superficial and the deep walls of the needle, resulting in a step appearance that can be distinguished from the single return of the needle shaft. A subtle sliding motion of the ultrasound probe can aid in confirming the location of the tip as the beam walks up and down the needle shaft.
Table 30–3.Safety tips during ultrasound-guided nerve blocks. ||Download (.pdf) Table 30–3. Safety tips during ultrasound-guided nerve blocks.
Perform a “preblock scan” to ascertain anatomy
Utilize the color Doppler setting to identify blood vessels
Do not advance needle if tip is not localized
“Hydrodissection” can be utilized to delineate anatomy
When pushing through fascia toward a nerve, approach tangentially
Pass through fascia slowly, awaiting a “pop” or sudden release
Reoptimize image of needle tip after passing through fascia
When in doubt about needle-nerve interface, gently move needle to ascertain that the nerve does not move with it (indicating that tip is embedded within epineurium)
Commonly, fascial planes will be encountered that resist advance of the needle. These tough layers of connective tissue may be seen to “tent” as the tip pushes against them, suddenly giving way and snapping back to their original position. This abrupt change may have two consequences: First, the needle may advance quickly and inadvertently beyond the intent of the operator (unless this is anticipated); second, the needle may move out of plane. At this point, the forward motion of the needle should be stopped until the in-plane image is once again optimized. It is common for such fascial planes to lie just superficial or adjacent to the nerve target, as at the interscalene groove, the axillary neurovascular bundle, or the femoral nerve.22,32 This motion may actually result in the needle thrusting forward and encountering the nerve if the sudden give of the fascial plane is not anticipated. For this reason, it is recommended to approach nerves tangentially, projecting the advance of the needle so that its tip will lie adjacent to the nerve, but not aiming for its center.33
The resistance encountered by these tough facial planes may also inadvertently redirect a needle when approached at a shallow angle. Temporarily steepening the needle angle may permit an easier and more controlled passage. Unfortunately, ultrasound guidance does not always produce clear images that allow one to distinguish the nerve tissue from surrounding tissue. In such situations, as the needle is advanced, “hydrodissection” (deliberate injection of fluid into tissue planes) can be utilized to separate structures, allowing better clarity in imaging, with either dextrose or local anesthetic solution. In addition, the behavior of tissues can be observed as the needle is advanced to help localize the needle tip in relation to neural tissue.
While it was once held that contacting a nerve with a needle tip would likely result in paresthesia, and, indeed, this was considered an appropriate nerve localization technique, we now know that paresthesia is not consistently elicited with needle-nerve contact. This emphasizes the need to accurately localize the needle tip with ultrasound imaging as well as using additional monitoring during PNBs to detect hazardous needle-nerve relationships, such as nerve stimulation and injection pressure monitoring.34,35
Deposition of local anesthetic solution should be optimized by taking advantage of fascial planes or sheaths that can contain or channel the drug around the nerve and longitudinally along its course.
For nerves without such local fascial containment, the solution should be injected in a circumferential manner to hasten onset of the block.
USE OF PERIPHERAL NERVE STIMULATION WITH ULTRASOUND
The peripheral nerve stimulator (PNS) has been a standard tool in nerve localization during PNBs for several decades, with a high degree of success and low complication rate.36 However, the widespread adoption of ultrasound imaging has called into question its ongoing role in PNB. Over a decade ago, Perlas et al evaluated the sensitivity of upper extremity peripheral nerves to peripheral nerve stimulation during ultrasound imaging of needle to nerve contact.37 The authors reported that, despite visualizing the needle tip indenting the surface of the nerve, with the stimulator set to deliver a current of 0.5 mA or less, no motor stimulation occurred over 25% of the time.
Several studies, with a variety of different blocks, have been performed to assess the utility of this localization tool in association with UGRA. Whether for supraclavicular block,38 for axillary block,39 or for femoral block40 authors have shown that addition of the nerve stimulator as a nerve localization tool during ultrasound-guided PNB was not contributory to success. Moreover, Robards et al found that absence of a motor response to PNS between 0.2 and 0.5 mA during popliteal block did not always exclude placement of the needle within the nerve, and that stimulation might actually lead to unnecessary manipulation of the needle into the nerve.41
However, the stimulator may be useful as an adjunct to UGRA for reasons other than ensuring block efficacy. Because it has been well established that a threshold of nerve stimulation lower than 0.2 mA indicates high likelihood of needle tip placement within the nerve, the stimulator may be employed during UGRA as a safety monitor.34 The nerve stimulator is particularly necessary during US-guided block of the deep nerves, or when the ultrasound image is less precise than desired. In this setting, an evoked motor response could warn against intrafascicular injection of local anesthetic.
Moreover, in some circumstances, it may be desirable to identify different nerves with more precision, as during axillary block, for which the PNS serves to delineate the nerves by their specific motor response to electrical stimulation. There may be, in some anatomic locations, neural structures that can be challenging to identify by visualization alone, whether they are the target of blockade42 or one simply wishes to avoid them with the needle43; in these cases, a PNS may be invaluable to provide this identification.
Finally, there are nerves that do not lend themselves readily to ultrasound visualization, primarily because of depth or osseous interference with ultrasound transmission. The most common example of this is the posterior approach to the lumbar plexus, in which ultrasound can be used to identify local osseous structures to guide the block, but for which the PNS remains a valuable tool for guidance of the needle tip into proximity with the nerves of the plexus.44
Taken overall, a plethora of data indicate that routine use of a nerve stimulator during ultrasound-guided nerve blocks yields clinically relevant safety information that can influence the clinical decision making and positively affect patient safety. However, the primary purpose of the suggested routine use of nerve stimulation with UGRA is for safety monitoring, rather nerve localization (Figure 30–8). In this capacity, the nerve stimulator can be simply set at 0.5 mA (0.1 ms), 2 Hz, without changing the current intensity throughout the procedure. While the motor response is not sought, occurrence of the motor response should necessitate cessation of the needle advancement and slight withdrawal of the needle as a motor response at this current delivery setting almost always indicates needle-nerve contact or intraneural needle placement.45,46
Algorithm: The primary purpose of the suggested routine use of nerve stimulation with UGRA is for the purpose of safety monitoring, rather than nerve localization.
OPTIMIZING THE DELIVERY OF LOCAL ANESTHETIC NEAR THE TARGET NERVE
After accurate needle placement near the target nerve, and after ascertaining that aspiration is negative for intravascular needle placement, injection of the local anesthetic is made in the tissue plane that contains the nerve(s) to be anesthetized (Table 30–4). Brull et al evaluated the long-held notion that the local anesthetic solution should be directed in a circumferential manner around the visible nerve, with change of needle position if necessary, in comparison to simply allowing the solution to accumulate along one aspect of the nerve with one needle position.47 They found that the resultant block set up 33% more rapidly with the former than with the latter. While the creation of a “donut” or “halo” around the nerve may be suggested as a general recommendation, some nerves, by virtue of their anatomical situation, may not require such deliberate circumferential placement. This is typically dictated by the location and configuration of overlying or surrounding fascial planes, such as in the interscalene groove48 and at the femoral triangle.32 Optimal delivery of local anesthetic around each nerve is described in subsequent specific UGRA chapters.
Table 30–4.Optimizing local anesthetic deposition. ||Download (.pdf) Table 30–4. Optimizing local anesthetic deposition.
Inject local anesthetic solution in small aliquots
Observe for pain or high pressure during injection
Make certain that spread of fluid is observed at needle tip during injection
Aspirate between injections
Be aware of intervening fascial planes that may sequester or channel the solution
Avoid deposition of local anesthetic into muscle
For solitary nerves in extremities, seek to create a “donut” or “halo” around nerve
For nerves within a fascial enclosure, seek to “fill” the fascial confines with solutiona
Real-time imaging of local anesthetic injection permits assessment of correct disposition of the fluid. The injection phase should be carried out with small aliquots of local anesthetic (3–5 mL), with a short period allowed to elapse between each, to allow evidence of any symptoms of local anesthetic systemic toxicity (LAST) to be manifest before continuing to administer the drug, as recommended by the American Society of Regional Anesthesia and Pain Medicine (ASRA) guidelines.49 In addition, delivery of each aliquot should be preceded by aspiration and should progress with attention to opening injection pressures or complaints of pain or paresthesia in the distribution of the target nerve.
While ultrasound has been shown to produce a lower likelihood of intravascular needle placement,50 intravascular injection with LAST may still occur.51 It is thus imperative to be aware of the location of vessels, which have a distending pressure so low that ordinary pressure at the body surface with a transducer obliterates their lumen entirely.4 Therefore, it is helpful to screen for the presence of vessels using color Doppler during the preblock scan. However, small vessels may be missed, and Doppler function deteriorates at greater depths. Thus, it is imperative to observe the ultrasound image throughout the injection for evidence of tissue spread by the local anesthetic solution at the tip of the needle. Failure to visualize such spread suggests that the tip of the needle either is out of plane or is in the lumen of a vessel.
Errant needle placement has been described both into vessels52 and into nerves.53,54 Moayeri et al in a cadaver-based study, have shown that ultrasound imaging is sensitive to injection into peripheral nerve, with as little as 0.5 mL causing visible evidence of nerve distension.55 Such visualization allows immediate withdrawal of the needle, which may reduce the chance for nerve injury, compared to injection of a large volume of local anesthetic.
Ultrasonography has revolutionized the field of regional anesthesia. The effective application of this technology requires understanding of two-dimensional anatomy, optimal imaging of the nerves and anatomical structures, accurate real-time needle guidance, and precise local anesthetic delivery. The combination of these elements ensures that the most benefit can be derived from this powerful imaging modality, ensuring high nerve block success and improved patient safety, particularly with regard to LAST. Individual ultrasound-guided blocks are discussed in detail in Chapters 31 through 35.
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