Modes of Ultrasound

Doppler Ultrasound

Doppler US (based on the Doppler principle) detects a shift in frequency between the emitted US waves and their echoes.29–³² It is used to detect and measure blood flow, and the major reflector for this purpose is red blood cells. Several modes are available:

Color Doppler measures and color codes the direction and magnitude of the mean Doppler frequency shifts that occur in moving red blood cell and superimposes a color depiction of these data on the gray-scale image (Fig. 42-2, A).

Power color Doppler depicts the amplitude, or power, of the Doppler signals (see Fig. 42-2, B). This allows better sensitivity for visualization of small vessels, but at the expense of directional information.

Pulsed Doppler allows a sampling volume (or gate) to be positioned in a vessel visualized on the gray-scale image and displays a spectrum of the full range of blood velocities within the gate plotted as a function of time (see Fig. 42-2, C ).

FIGURE 42-2 Doppler ultrasound. A, Color Doppler. B, Power Doppler. C, Pulsed Doppler.

The Ultrasound Machine

US machines are either cart-based or portable systems. Irrespective of their shape or size, US machines are made up of the following components: a monitor (where the clinical images are displayed), the US unit (where the signals are processed), the control panel (with the knobs and controls), one or more transducers, and a data storage device.33–³⁵ For an anesthesiologist, the first encounter with an US machine can be quite intimidating. The wide array of knobs and controls that are available may be confusing. However, several controls are common in most US machines, and a clear understanding of their function (“knobology” ) is essential for optimal imaging.

Gain

The gain control adjusts the amplification of the returning acoustic signals and is used to optimize the US image (Fig. 42-3). Reduced gain produces a dark image (see Fig. 42-3, A) and detail is masked. In contrast, too much gain produces a white image and detail is saturated (see Fig. 42-3, B). In some US machines there are separate controls for overall gain and gain for the near and far fields. “Auto-gain,” by which the US machine automatically adjusts the gain, is also available in some machines.

FIGURE 42-3 Transverse sonogram of the forearm demonstrating reduced gain (A), excess gain (B), and optimal gain (C).

Ultrasound Transducers

The transducer functions both as a transmitter and a receiver of the ultrasound signal.^33–35 Three types of transducers are currently used (Fig. 42-4): (1) in a linear-array transducer, the piezoelectric crystals are arranged in a linear fashion and sequentially fired to produce parallel beams of ultrasound in sequence, creating a field of view that is rectangular and as wide as the footprint of the transducer (see Fig. 42-4, A); (2) a curved linear-array transducer has a curved surface, creating a field of view that is wider than the footprint of the probe (see Fig. 42-4, B), but at the cost of reduced lateral resolution in the far field as the scan lines diverge; (3) a phased-array transducer has a small footprint, but the ultrasound beam is steered electronically to produce a sufficiently wide far field of view. The ultrasound beam diverges from virtually the same point in the transducer (see Fig. 42-4, C ). Phased-array transducers are routinely used for transthoracic echocardiography.³⁴ The footprints of these transducers are small enough to fit between the ribs and still produce a wide far field of view to image the heart. US transducers serve either a single frequency or a range of frequencies (broadband). For example, a transducer with the notation HFL38/13-6 indicates that it is a high-frequency broadband (13-6 MHz) linear transducer with a 38-mm footprint. Note that the nomenclature used for transducers varies among manufacturers of US devices.

FIGURE 42-4 Schematic diagram illustrating the different types of ultrasound transducers. Note how the ultrasound beam is emitted from each of these transducers.

Axis of Scan

In diagnostic ultrasonography, scans are performed in the transverse, longitudinal (sagittal), oblique, or coronal axis. During a transverse (axial) scan the transducer is oriented at right angles to the target, producing a cross-sectional display of the structures (Fig. 42-5, A). During a longitudinal scan, the transducer is oriented parallel to and along the long axis of the target (e.g., a blood vessel or nerve) (see Fig. 42-5, B). During USGRA procedures, US scans are most commonly performed in the transverse axis. In this axis, the nerves, the adjoining structures, and the circumferential spread of the local anesthetic are easily visualized.

FIGURE 42-5 Axis of scan. A, Transverse scan. B, Longitudinal scan. CA, Carotid artery; IJV, internal jugular vein; SCM, sternocleidomastoid muscle; THY, thyroid.

Probe and Image Orientation

The US image must be properly oriented to accurately identify the anatomic relations of the various structures on the monitor. To facilitate this, all US probes have an orientation marker, which is usually represented by a groove or a ridge on one side of the transducer and corresponds to a green dot (or a logo) on the monitor. By convention, the orientation marker on the transducer is directed cephalad when performing a longitudinal scan and directed toward the right side of the patient when performing a transverse scan. This way the orientation marker on the left upper corner of the monitor always represents the cephalad end during a longitudinal scan or the right side of the patient during a transverse scan. The top of the display monitor therefore represents superficial structures and the bottom of the monitor the deep structures.

Echogenicity

Certain terms are frequently used to describe the sonographic appearance of musculoskeletal structures (Fig. 42-6):

FIGURE 42-6 Schematic diagram to demonstrate the relative echogenicity of various tissues.

Axis of Intervention

The plane of US imaging is only 1 mm thick (Fig. 42-7); for a needle to be visible during US imaging it must lie within this narrow plane of imaging. During USGRA procedures, the block needle is inserted either outside of the plane (out-of-plane approach) (Fig. 42-8, A) or within the plane of the US beam (in-plane approach) (see Fig. 42-8, B). In the out-of-plane approach, the needle is inserted in the short axis and is initially outside the plane of imaging and therefore not visible. It becomes visible only when the needle crosses the plane of imaging and is seen as an echogenic dot on the monitor (see Fig. 42-8, A). It is important to note that this echogenic dot may be just the cross-sectional image of the shaft of the needle as it passes through the plane of the US beam and thus may not represent the tip of the needle. In the in-plane approach, the needle is inserted along the long axis of the transducer in the plane of imaging and therefore both the shaft and tip of the needle are visible on the monitor.

FIGURE 42-7 The plane of ultrasound imaging. Note that the ultrasound beam is only 1 mm thick and for a needle to be visible during an ultrasound-guided intervention it must lie within this plane of imaging.

FIGURE 42-8 Axis of intervention. Out-of-plane (A) and in-plane (B) techniques.

Both approaches are commonly used, and no data have shown that one is better than the other. Proponents of the out-of-plane approach^13,14,36 have had great success with this method and claim that it causes less needle-related trauma and pain because the needle is advanced through a shorter distance to the target. However, critics of the short-axis approach express concerns that the inability to reliably visualize the needle and to use tissue movement as a surrogate marker to locate the needle tip during a procedure can lead to complications. The needle is better visualized in the in-plane approach,^37,38 but this requires good hand and eye coordination and reverberation artifacts from the shaft of the needle can be problematic. Moreover, there are claims that the in-plane approach also causes more discomfort in awake patients because longer needle insertion paths are required.^13,14,36

Needle Visibility

The ability to visualize the needle during a US-guided procedure is critical for precision, safety, and success. However, this is often limited by the dispersion of the reflected US signals away from the transducer. Several factors have been identified that can influence needle visibility. The shaft of the needle is better visualized in the long axis than in the short axis, and its visibility decreases linearly with steep angles of insertion and smaller needle diameters. The needle tip is better visualized when it is inserted in the long axis for shallow angle of insertion (less than 30 degrees) and in the short axis when the angle of insertion is steep (greater than 60 degrees). This is also true when the needle is inserted with its bevel facing the US transducer. To overcome the effect of angle on needle visibility, some high-end US machines allow the operator to steer the US beam toward the needle during steep needle insertions. However, this requires experience and decreases in needle visibility can still occur. Needle visibility is also enhanced in the presence of a medium-sized guidewire. Priming a needle with saline or air, insulating it, or inserting a stylet before insertion does not improve visibility.39–⁴¹

We think that the anesthesiologist’s skill in aligning the needle along the plane of imaging is by far the most important variable influencing needle visibility because minor deviations of even a few millimeters from this plane will result in inability to visualize the needle. Even with experience, needle tip visibility is a problem when performing blocks at a depth in areas that are rich in fatty tissue. Under such circumstances, gently jiggling (rapid in-and-out movement) the needle and observing tissue movement or performing a test injection of saline or 5% dextrose (1 to 2 mL) and observing tissue distention can help locate the position of the needle tip. Five percent dextrose is preferred for the latter when nerve stimulation is used because it does not increase the electric current required to elicit a motor response.⁴²

Anisotropy

Anisotropy, or angular dependence, is a term used to describe the change in echogenicity of a structure with a change in the angle of insonation of the incident US beam (Fig. 42-9).⁴³ It is frequently observed during scanning of nerves, muscles, and tendons. This occurs because the amplitude of the echoes returning to the transducer varies with the angle of insonation. Nerves are best visualized when the incident beam is at right angles (see Fig. 42-9, A); small changes in the angle away from the perpendicular can significantly reduce their echogenicity (see Fig. 42-9, B). Therefore, during USGRA procedures, the transducer should be tilted, from side to side, to minimize anisotropy and optimize visualization of the nerve.⁴⁴ Although poorly understood, different nerves also exhibit differences in anisotropy, which may be related to the internal architecture of the nerve.

FIGURE 42-9 Anisotropy. Note how a small change in angle of the ultrasound beam from the neutral position (A) has affected the visibility of the median nerve (white arrow, B) in the forearm.

Nerves

On a transverse scan, nerves appear round, oval, triangular, lip shaped, or even flat.^45,46 Nerves also assume different shapes along their course depending on the surrounding structures. The echogenicity of nerves also varies and depends on the nerve and area scanned. They are generally hyperechoic and stand out in the background of the hypoechoic muscles (Fig. 42-10, A), but they can also appear hypoechoic with a hyperechoic rim (see Fig. 42-10, B). They also have been described to have a fascicular or honeycomb appearance (i.e., echogenic structures with internal punctate, echo-poor spaces) (see Fig. 42-10, C ). On longitudinal scan, the appearance of peripheral nerves has been likened to a “tram track”; that is, parallel hyperechoic lines are seen against a background of echo-poor space (see Fig. 42-10, D). Nerve motion also can be demonstrated on dynamic US imaging.

FIGURE 42-10 Ultrasound appearance of peripheral nerves. A, Transverse sonogram of the sciatic nerve in the thigh. B, Transverse sonogram of the brachial plexus in the interscalene groove. C, Transverse sonogram of the median nerve in the forearm. D, Longitudinal sonogram of the sciatic nerve in the thigh.

Tendons

Tendons appear to have numerous fine parallel hyperechoic lines separated by fine hypoechoic lines (fibrillar pattern) on long-axis scans.⁴⁶ Compared with nerves, tendons have more hyperechoic lines and move more than adjacent nerves when the corresponding muscle is contracted or passively stretched.

Muscle

Muscle fibers are hypoechoic, but the connective tissue structure enveloping the entire muscle (epimysium) is hyperechoic.^47,48 The perimysium that envelops individual muscle fascicles is also hyperechoic. Muscle fibers converge to become tendons or aponeurosis.

Fat

Fat lobules appear as round to oval hypoechoic nodules that are separated by fine hyperechoic septa. Fat tends to be superficially distributed (subcutaneous fat), slightly compressible, and similar on transverse and longitudinal scans.

Bone

Bone reflects most of the US energy. Therefore it appears bright and has a hyperechoic edge on US imaging, with a large anechoic shadow (acoustic shadow) distal to it (Fig. 42-11).

FIGURE 42-11 Longitudinal sonogram of the intercostal space demonstrating the ultrasound appearances of bone and pleura.

Fascia

Fascia, peritoneum, and aponeurosis appear as thin hyperechoic layers on US imaging.

Blood Vessels

Arteries are identified by their intrinsic pulsatility, are not compressible, and have anechoic lumens. Veins are not pulsatile, are compressible, and have anechoic lumens. Color Doppler or power Doppler modes can also be used to demonstrate blood flow and differentiate arteries from veins (see Fig. 42-2).

Pleura

The pleura appear as a hyperechoic line on US imaging (see Fig. 42-11).^49–51 During scanning of the intercostal space, the pleural line is located slightly below the hyperechoic ribs. “Comet-tail” artifacts may be present as a series of vertical lines arising from the pleura. On real-time imaging, lung sliding movement between the parietal and visceral pleura can be discerned from movement of the comet-tail artifacts (“lung sliding sign”).

Tissue Harmonic Imaging

The term harmonic refers to frequencies that are integral multiples of the frequency of the transmitted pulse (which is also called the fundamental frequency or first harmonic). The second harmonic has a frequency of twice the fundamental frequency. Harmonics are generated in the tissues by the nonlinear propagation of sound. Tissue harmonic imaging (THI) is a technique in which the harmonic signals reflected from tissue interfaces are selectively displayed.52–⁵⁴ This results in reduced image artifacts, haze, and clutter and improved contrast resolution (Fig. 42-12).

FIGURE 42-12 Tissue harmonic imaging (THI). Sagittal sonogram of the infraclavicular fossa. A, Conventional scan. B, Conventional scan with THI.

Compound Imaging

US imaging depends on the reflection of the US from tissue interfaces. Not all tissues are good reflectors, and certain structures also cause scattering of the US signals. Unlike reflected signals, scattered signals radiate in all directions. As a result, only a small amount of energy is reflected back to the transducer. The scattering of the US signal results in speckle artifacts, also described as noise, which reduces image resolution and makes the US image appear grainy. Compound imaging is a technique used to improve resolution by reducing the contrast-to-noise ratio (speckle).⁵⁵ The US beam from the transducer is electronically steered, and the same structure is imaged from several different angles. The returning echoes are then processed with simultaneous filtering of the artifacts in real time, producing a composite image that has reduced noise or speckle and improved definition (Fig. 42-13).

FIGURE 42-13 Compound imaging. Transverse sonogram of the axilla. A, Conventional scan. B, Conventional scan with compound imaging.

Panoramic Imaging

B-mode (2D) ultrasonography has a limited field of view and allows visualization of only a small portion of any large structure. Panoramic imaging, as the name implies, is a technique used to extend the field of view so that larger structures and their surrounding tissues can be visualized together.⁵⁶ During a panoramic scan, the operator slowly slides the US transducer across an area of interest. During this motion, multiple images are acquired from many different transducer positions across the area of interest. The registered image data are accumulated in a large buffer and then combined to form the composite panoramic image (Fig. 42-14). Although useful for annotation, documentation, teaching, and research, it is rarely used in children during USGRA procedures.

FIGURE 42-14 Panoramic imaging. Transverse panoramic scan of the forearm. FCU, Flexor carpi ulnaris; FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis; FPL, flexor pollicis longus.

Artifacts

US artifacts are structures that are visible in the US image that do not correlate with any anatomic structure.⁵⁷ The US machine makes the following assumptions when generating an image:

When a deviation from any of these assumptions occurs, the US machine is unable to determine it. This results in the display of an echo that has no relation with the interface that actually produced the echo; that is, an artifact is produced. Some artifacts are undesirable and interfere with interpretation, whereas others help identify certain structures. It is essential to recognize them to avoid misinterpretation. Therefore, when a structure appears abnormal on ultrasonography, it must be examined in two planes to avoid making a wrong interpretation. Real anatomic structures are visible in both planes of imaging, whereas artifacts are visible in only one plane.

Scanning Routine

Being able to consistently produce high-quality images of the area scanned is vital for safety and success during any USGRA procedure. Without optimal images, it is not possible to accurately identify musculoskeletal structures or perform interventions with precision. We have found that following a “scanning routine” or a set of simple steps, which is repeatable, is essential for optimal imaging; the routine that we follow is outlined in Table 42-2. Although the suggested routine may appear complicated at first, with repetition these steps are gradually internalized. Attaching a card with the scanning routine to the US machine facilitates easy recall.

TABLE 42-2 Scanning Routine

General Considerations in Children

Preparations for an ultrasound-guided nerve block should begin during the preoperative visit by adequately explaining the technique, its benefits and risks, and, more importantly, the possibility of a failed block to the parents. In the event of failure, a contingency plan to quickly convert to general anesthesia or another form of postoperative analgesia must always be in place. In children, most regional anesthetic procedures are performed while the child is anesthetized. However, in a cooperative child or under special circumstances, such as in a child with difficult airway or a child predisposed to malignant hyperthermia, it is possible to perform the block after light sedation. We find that it is easy to explain the procedure to children who are older than 8 years of age. Some of them may even express a wish to stay awake and observe the US images during the block. Eutectic mixture of local anesthetic (EMLA) cream applied an hour before the procedure to the skin over the area where the block needle and the intravenous catheter are to be inserted helps reduce needle-related pain. Parental presence during the nerve block may also be helpful. In older children, allowing the child to listen to favorite music through a personal stereo or watch a video are useful distraction techniques that make the whole experience a more pleasant one for the child. We have connected a DVD player to our US machine, and this is used to play movies or cartoons through the monitor during the surgical procedure (E-Fig 42-1).

E-FIGURE 42-1 The monitor of the ultrasound machine is being used to display a cartoon from a DVD player while a child is undergoing surgery under regional anesthesia.

Before any USGRA procedure, intravenous access is established, standard monitoring is applied, and equipment and drugs appropriate for the child are prepared. Aseptic precautions are maintained, and the skin over the needle puncture site is prepared with antiseptic solution in the usual fashion. The US probe is prepared by covering the footprint with a sterile transparent dressing. It is important to avoid trapping any air between the transparent dressing and the footprint. This is done by gently stretching the transparent dressing before applying it on the footprint. The transducer and cable are then covered using a sterile plastic cover. We use the same plastic cover that our surgeons use to cover their laparoscopic camera.

Tips and Tricks for Success

Certain steps are common to all US-guided procedures, and, if followed, they may increase success. The lights in the room must be dimmed to avoid any glare or reflection from the US monitor. The operator must assume a comfortable position (Fig. 42-15). For upper extremity blocks, the operator sits at the ipsilateral head end of the child and the US machine is placed directly in front. For lower extremity blocks, such as femoral nerve block, the operator stands on the ipsilateral side of the child and the US machine is placed on the opposite side. For lower extremity or central neuraxial blocks in the lateral position, the operator sits behind the child and the US machine is placed in front, with the monitor in the line of view of the operator. Because of the small muscle bulk in young children the nerves are relatively superficial and can most frequently be easily visualized using high-frequency linear transducers. The exact choice of transducer depends on the area scanned, but a high-frequency linear transducer with a small footprint (13-6 MHz, 25-mm footprint) is particularly suited for young children. The 15-6–MHz broadband linear-array transducer, which has recently become available, is also useful for most blocks in young children. In older children, a 10-7–MHz broadband linear-array transducer, which allows greater flexibility with the depth of scan, is adequate for most procedures. Low-frequency (5-2 MHz) curved-array transducers are rarely used in children but are useful for imaging deeper structures such as the lumbar plexus and sciatic nerve in older children.

FIGURE 42-15 Position of the child, anesthesiologist, and ultrasound machine during a ultrasound-guided regional anesthesia procedure.

To improve dexterity, hold the transducer with the nondominant hand and perform interventions with the dominant hand. Holding the transducer steady for even short periods can be quite testing. We have found that gently resting the hand that is holding the transducer on the child during a procedure helps to keep the transducer steady (see Fig. 42-15). It is important to maintain light contact between the transducer and the skin because excessive pressure in a child will cause the veins to collapse or distort the anatomy of the area of interest. Always apply liberal amounts of US gel to maintain adequate acoustic coupling between the skin and the transducer because even small amounts of air trapped between the two can result in artifacts. We use sterile US gel from single-use sachets for all US-guided peripheral nerve blocks. At any given time during an US-guided intervention either the transducer or the needle must be moved. It is impossible to maintain the needle within the plane of imaging if both are moving, a common error by novices. This results in an inability to visualize the needle. If the needle is not visible in the US image, a good strategy is to keep the needle steady and manipulate the transducer (slide, tilt, or rotate) until the needle becomes visible on the monitor. Thereafter the transducer should be held steady and the needle should be gently advanced to the target nerve, maintaining it in the imaging plane. When the angle of insertion of the needle is steep (greater than 60 degrees), it is preferable to introduce the needle in the short axis using the out-of-plane technique. However, if the in-plane approach is used for all US-guided interventions, as in our case, inserting the needle a few centimeters away from the edge of the transducer may improve needle visibility by decreasing the angle between the needle and the imaging plane.

Injecting air into the area of the intervention must be avoided at all cost because air bubbles in the field of imaging will degrade the US image. We routinely introduce the needle into the subcutaneous tissue and then purge it with saline or the local anesthetic to remove any air from the shaft of the needle, extension, and syringe system before proceeding with the block. An assistant aids with the injection. When the needle tip is close to the target nerve, the assistant gently aspirates to exclude unintended intravascular placement. The assistant must avoid generating excessive negative pressure because small blood vessels are prone to collapse. A short length of extension tubing attached between the needle and the local anesthetic syringe allows the operator to hold the needle steady while the assistant performs the injection.⁶¹ We routinely perform a test injection with 1 to 2 mL of saline or 5% dextrose (when nerve stimulation is also used) and visualize the distribution of the injectate in real time before injecting the local anesthetic. Failure to visualize the injectate in the US image indicates that the needle is not in the plane of imaging or it is intravascular until proven otherwise. No further injection should be made until the needle is repositioned and the distribution of the injectate is confirmed.

Ancillary Equipment

Other than the US machine, equipment required for US-guided nerve block procedures in children is relatively simple. We are not aware of any needle that has been specifically designed for US-guided interventions in children but do believe that such equipment will be available in the future. In our experience, most single-shot peripheral nerve blocks can be performed with a standard short-bevel needle designed for regional anesthesia. In older children, we have found the 22-gauge Tuohy needle (B. Braun Medical Inc., Bethlehem, Pa.) to be a very useful alternative. The tip of the needle is relatively blunt, and its design (when introduced with the bevel facing upward) also allows it to be easily identified on US imaging. Moreover, graduations on the shaft of the needle provide useful cues about the depth to which the needle has been inserted. Insulated needles used for peripheral nerve stimulation (Stimuplex A, 50 or 100 mm, B. Braun Melsungen AG, Melsungen, Germany) are also suited for US-guided peripheral nerve blocks in children. Indwelling catheters also can be placed under US guidance; however, compared with single-shot techniques, experience with US-guided catheter placement for continuous peripheral nerve blocks in children is still limited. A standard epidural kit or an insulated continuous peripheral nerve block kit of an appropriate size can be used for catheter placement.

Upper Extremity Blocks