ASHOT E. SARGSYAN, GLYKERIA PETROCHEILOU, SERAFIM NANAS, ARIEL L. SHILOH, HEIDI LEE FRANKEL and DIMITRIOS KARAKITSOS

Overview

Besides encompassing specific focused techniques, the holistic approach (HOLA) ultrasound concept, introduced in Chapter 1, promotes generic scanning of any body part (head-to-toe ultrasound imaging) as modulated by current clinical indications. Any ultrasound view obtained through the skin contains some information about soft tissues. While serving as an imaging window and as anatomic reference structures in focused techniques (e.g., the chest wall in lung scanning), soft tissues per se are often a primary target (e.g., in extremity crush injury). Therefore mastery of fine anatomy is essential for the HOLA-level ultrasound operator, both in terms of tissue type (e.g., fascia, tendon, peritoneum), and in terms of named structures (e.g., basilic vein, gracilis muscle, median nerve). This chapter reviews nonspecific (generic) soft tissue and musculoskeletal (MSK) imaging and miscellaneous intensive care unit (ICU)-relevant HOLA targets.

Soft tissue and skeletal scanning may be indicated in many clinical situations, including the following:

Equipment and technique

Any modern multipurpose system with a high-frequency (7-15 MHz) transducer is appropriate for most superficial targets. Lower frequencies (2-5 MHz) are used for deeper tissues in large subjects or large body parts (e.g., the thigh). More than one transducer can be used on the same region of interest (ROI) to exploit advantages of each. Some experts recommend broadband microconvex transducers (usually 5-8 MHz) for most soft tissue targets because they offer a good balance of resolution and penetration in a wide-angle view through a small footprint. With any transducer, both B-mode and color Doppler mode are often used. Although elastography, an emerging method to map elasticity of tissues, currently has limited use in the ICU, every superficial tissue scanning procedure includes elements of “visual elastography” (see Pathology section later).

Commercially available gel pads or a thick layer of gel (probe “floating” technique) are used for imaging the most superficial tissues or lesions, especially when an adequately high-frequency probe is not available.

Normal patterns

The uppermost layer of the image is a thin hyperechoic line that corresponds to the skin. Subcutaneous adipose tissue underneath is relatively hypoechoic, with linear echoes (septa). Fascia is a brightly echogenic line (perceived as a contiguous layer during scanning). Muscles appear as hypoechoic structures with organized echogenic fibroadipose septa between fasciculi; the septa merge as the transducer is moved toward a tendon (Figure 51 E-1). Tendons appear hyperechoic and fibrillar when scanning along their course and granular in cross sections; however, even a small deviation from the 90-degree orientation of the ultrasound beams relative to the fibers results in the loss of the echogenic pattern, mimicking a tear or defect. This feature is known as anisotropy (Figure 51-1)—dependence of the imaging pattern on the beam orientation. In tendons, the smooth type-1 collagen fiber bundles are good reflectors (direction dependent) but poor scatterers (direction independent). Peripheral nerves also have a fascicular appearance, but with less anisotropy, and are surrounded by loose connective tissue (see Figure 51-1). Vessels appear as anechoic (black) stripes across the image when the scanning plane aligns with their course. In transverse orientation, arteries are circular and may visibly pulsate. Veins have a near-circular appearance with less prominent walls, easily compress with transducer pressure, but fail to collapse entirely in case of thrombosis (see Figure 51-1). The bone surface is normally depicted as a brightly echogenic line with acoustic shadowing. The joint space is identified by a V-shaped discontinuity in the cortical line from adjacent bones, usually representing fibrocartilaginous tissue (e.g., a meniscus or a labrum). Random collagen fiber orientation in these structures determines their high, nonanisotropic echogenicity.^1–3 Hyaline cartilage, on the other hand, looks like a thin hypoechoic rim over the bone (see Figure 51-1).

Pathology: High-level considerations

Ultrasound imaging allows observing the instantaneous anatomy of body parts with reasonably consistent normal patterns for each type of tissue, each area, and each named structure. The ability of the operator to recognize the normal anatomy and any deviations from the norm grows fast with scanning experience. Most physicians learn quickly to correlate the real-time, two-dimensional (tomographic) visual information with their knowledge of topographic anatomy and to interpret the data in the clinical context. The following high-level criteria are used to a varying extent in each scanning procedure to discriminate between the normal and pathologic topography and structure:

The shape and the response of a structure to transducer pressure are two interconnected criteria that are important for the following reason: elastic structures with fluid content (gas, liquid, loose tissue) assume a more spheric shape and become less compressible as the pressure within that space (compartment) increases. Experienced operators consider shape and use compression intuitively as part of the scanning scrutiny of both anatomic and pathologic structures.

Extraneous structures include cysts, abscesses, hematomas and pseudoaneurysms, hernias, tumors, enlarged lymph nodes (most of which do not stand out if normal), and foreign bodies (including devices), among others.

Pathology: Specific types

Hematomas are collections of extravascular blood caused by common and iatrogenic trauma, coagulation disorders, necrosis, and other mechanisms. They usually appear as poorly demarcated but structurally differing areas within or between anatomic structures, often distorting them (mass effect). A hematoma shows serial changes over time (Figure 51-2). Fresh blood initially looks anechoic and then becomes finely echogenic when clotted. After clot liquefaction, hematomas regain anechoic or hypoechoic appearance with diffuse internal echoes that may form a layer by sedimentation. Most hematomas resolve, but some organize into a chronic hyperechoic mass with echogenic inclusions or concentric layers from repeated bleeds.^2–4 Color Doppler ultrasound helps demonstrate deviations of adjacent vessels; it is also helpful in distinguishing between a hematoma and a pathologic tissue.

Ultrasound imaging is valuable in musculotendinous injuries as part of the physical examination in trauma. Acute muscle contusion and hemorrhage appear hyperechoic, whereas later stages of injury exhibit mixed patterns. Partial or complete muscle tears with or without retractions are usually obvious (see Figure 51-2). Intramuscular hematomas may later evolve into seromas or intramuscular cysts (anechoic fluid collections) that may require aspiration or surgical drainage. Heterogeneous, grainy muscular patterns with a hypoechoic appearance of septa and fasciae are described in malignant hyperthermia. Hypoechoic muscular swelling with architectural disorganization may be observed in traumatic rhabdomyolysis.

In crush injuries, ultrasound imaging can assist in critical decision-making and have significance for the extremities and be lifesaving because a buildup of pressure within the fascial compartment disrupts tissue perfusion (compartment syndrome) with dire consequences, unless fasciotomy is emergently performed. An advanced HOLA protocol in extremity crush injuries would therefore include assessment of (1) the shape and structure of all the fascial compartments (looking for outward convexity of the normally flat fascial partitions between compartments), fractures, tears, hematomas, and hypoechoic areas of potential fluid collection or necrosis; (2) color and pulsed wave Doppler monitoring of the vessels within the compartment and the main artery feeding the compartment; (3) renal imaging (monitoring size/volume, renal arterial spectral Doppler, and parenchymal differentiation); and (4) search for free abdominal fluid if the thigh and pelvis areas are involved.

Partial and complete tendon tears can be identified by dynamic ultrasound evaluation assessing the tendon’s integrity through the respective range of motion. Hypoechoic-to-anechoic intratendinal regions indicate partial tearing. Full-thickness tears may appear as tendon discontinuity with or without retraction and acoustic shadowing, whereas defects may be filled by hematomas or fat pad herniation (see Figure 51-2). An anechoic fluid cuff around a thickened, hyperechoic tendon with increased vascularity suggests tenosynovitis. Tendinopathy may appear as an intratendinous, hypoechoic patchy or hyperechoic calcific area with increased vascularity and loss of anisotropy and fibrillar pattern (see Figure 51-2). Ganglion cysts arise from the joint capsule or tendon sheaths and appear anechoic. The Baker cyst, first described by Dr. William Baker in 1877, is the most common mass in the popliteal fossa, which represents a fluid-distended gastrocnemio–semimembranosus bursa (see Figure 51-2).³

A cortical discontinuity with step-off deformity is the diagnostic hallmark of a bone fracture that is best seen in a plane perpendicular to the fracture line (Figure 51-3). Intensivists rarely deal with fracture diagnosis because patients arrive with a full set of radiographic imagery or after orthopedic surgery. Ultrasound imaging is relevant for monitoring adjacent soft tissue damage, such as hematomas or joint effusions (hemarthrosis). Monitoring can detect fracture nonunion and other complications (e.g., osteomyelitis) because ultrasound imaging depicts callus formation earlier than radiography, particularly in open injuries.^2,3 Heterotopic ossification (HO) is usually observed in periarticular muscles of the limbs, mainly in neurologic states (spinal cord injury, hemiplegia after stroke, or traumatic brain injury) or after burns, trauma, and joint arthroplasty, and it can severely affect the joint’s range of motion. Early clinical features (pain, swelling, erythema, warmth) may resemble skin infection, deep vein thrombosis (DVT), or acute arthritis. Mineralization is detected by ultrasound earlier than in radiography as an inner hypoechoic core surrounded by a peripheral hyperechoic band of mineralized islands (“zone phenomenon”) with increased vascularity. In cases when calcifications have not yet occurred, HO may be confused with sarcomas (see Figure 51-3).

Recently, ultrasound has been used to determine the type and extent of traumatic nerve injuries. In cases of nerve entrapment, associated changes in nerve contour and echotexture and causative extrinsic abnormalities may be depicted.⁵ The role of ultrasound in guiding regional anesthesia techniques is well established (see Chapter 53).

Ultrasonographic evaluation of infections in the ICU is another important application. Local spread of infection (e.g., involvement of muscles, joints, etc.), thrombophlebitis, causative factors (e.g., foreign bodies), or occult abscesses can be detected. Cellulitis is a diffuse infection of skin and subcutaneous tissue that may progress to abscess formation if left untreated. Initially, swelling with increased echogenicity of the involved area is depicted. Progressively, fluid-filled regions of hypoechoic edema create a cobblestone pattern. Detecting hypervascularity may be helpful to differentiate cellulitis from other causes of interstitial edema (e.g., congestive heart failure), which may also appear as anechoic, fluid-filled zones dissociating the connective septa (see Figure 51-3). Subcutaneous fat necrosis resulting from panniculitis or trauma resembles an ill-defined hyperechoic mass containing hypoechoic areas related to infarcted fat. Necrotizing fasciitis is a rapidly progressing and life-threatening infection involving deep fascia and muscle (usually located in the limbs, abdominal wall, or perineum). Thickened fascia with cloudy fluid collection (>4 mm in depth) and muscle swelling indicate the diagnosis, whereas subcutaneous gas can be identified as “dirty””acoustic shadowing with a reverberation artifact.^2,6

Abscesses may exhibit a wide range of ultrasonographic patterns but are usually surrounded by an irregular rim that represents edematous soft tissue or cellulitis. An abscess cavity typically appears hypoechoic, although debris, septa, or bacterial gas can sometimes result in an isoechoic or hyperechoic appearance, making recognition difficult (see Figure 51-3). The increased through transmission effect and the movement of purulent material (induced with gentle transducer pressure) confirm the fluid nature. Color Doppler may depict hypervascularity within the abscess wall and in surrounding tissues while showing no Doppler signal within the abscess cavity. Abscesses and associated edema in skeletal muscle layers indicate pyomyositis (usually found in immunocompromised patients). Hypoechoic fluid collections adjacent to bone cortex (subperiosteal location in children), along with cortical irregularity, raise suspicion of osteomyelitis. Septic tenosynovitis may be encountered after penetrating injury and often shows fluid around the tendon and flow on color Doppler. Septic arthritis may be suggested by the presence of irregular internal echoes in a joint effusion (see Figure 51-3).^2,6

Normal and reactive lymph nodes are usually oval shaped (short-to-long–axis ratio < 0.5); hypoechoic at the periphery compared with adjacent muscles, with echogenic medulla and hilum; and usually show low-resistive Doppler waveforms from the feeding artery (see Figure 51-3). Features favoring malignancy include larger size, rounded shape, eccentric versus concentric cortex thickening, heterogeneous echotexture with loss of identifiable medulla/hilum, presence of microcalcifications or cystic necrosis, and ill-defined borders. Malignant lymph nodes usually have high-resistance Doppler waveforms (resistive index [RI] > 0.8, pulsatility index [PI] > 1.5), but low-resistance flow patterns may be encountered in case of necrotic changes or advanced neovascularization (see Figure 51-3). The differential diagnosis of lymph node pathology (e.g., lymphoma, metastases) therefore requires an ultrasound-guided fine-needle aspiration (FNA) or excisional biopsy.^2,4

Retained foreign bodies in soft tissues may lead to severe infections and disability. Ultrasound imaging detects even radiolucent foreign bodies (e.g., wood, plastic) and guides their removal. Most materials appear hyperechoic. Posterior acoustic shadowing or reverberation and comet-tail artifacts may be observed, depending on the size, shape, and structure; over time, edema, pus, or granulation tissue create a hypoechoic “halo” around the foreign body, enhancing its identification (see Figure 51-3).⁷ When searching for superficial foreign bodies in soft tissues of hand or foot, it is practical to scan the area immersed in saline bath to improve acoustical transmission and avoid pressure on the sensitive area.

Ultrasound can identify soft tissue tumors and assist in guided biopsy. Differentiating benign from malignant lesions may be facilitated by assessing size, border echogenicity, structure, adjacent tissues, vascularity, and Doppler-derived RI. Superficial lipomas usually do not require additional imaging or biopsy when demonstrating characteristic features (spindle shaped in cross sections; ovoid in the plane parallel to the skin; well-defined, elastic-compressible, with linear striations). Invasive tumors are typically hypoechoic heterogeneous masses with ill-defined borders; adjacent malignant lymph nodes may also be present (see Figure 51-3).⁴

Miscellaneous scanning targets

The HOLA ultrasound examination approach has a general head-to-toe sequence (see Chapter 1) and includes various tissue types and specific targets as modulated by the clinical circumstances. The examination may begin with facial structures, to assess soft tissues and facial bones as indicated. Eyelid, eye, and orbit scanning is discussed in Chapter 6. Ultrasound imaging is a valuable screening tool in intubated patients with maxillary bacterial sinusitis, which could be an occult cause of fever, nosocomial pneumonia, and meningitis.^8,9 Radiography is unreliable in recumbent patients, whereas the costs and effort associated with computed tomography (CT) or magnetic resonance imaging (MRI) are not necessarily justified. The maxillary sinuses are scanned in sagittal and transverse planes just below the orbital rim (see Chapter 1). Normal air-filled sinuses elicit a reverberation artifact with parallel, evenly spaced echo lines. Totally fluid-filled sinuses allow a brightly echogenic line of the posterior wall and occasionally the two lateral walls to be seen, producing a pattern known as the “sinusogram” (see Figure 51-3). In partially fluid-filled sinuses, scanning with the patient in a more upright position results in periodic resonance artifact (as in a normal sinus) in the upper half (above the air-fluid level) and a posterior wall line in the lower, fluid-filled part of the image.^8,9 Differentiation between tissue hypertrophy (significant mucosal thickening, nasal polyps) and fluid-filled sinus is achievable, but mucoid cysts or blood resulting from facial trauma (e.g., sinus wall fractures) are not easily differentiated. Other facial targets of interest are the parotid and submandibular glands. Parotitis is a rare complication of mechanical ventilation; the gland is easily identified in the preauricular area as hypoechoic and heterogeneous tissue with increased vascularity. Infection affecting the submandibular gland can also be ultrasonographically investigated. The mutual proximity of these two glands allows their side-by-side comparison; the pathologic gland is almost always relatively hypoechoic (darker). This side-by-side comparison of echogenicity is also highly useful in other anatomic sites such as testicular imaging.

Neck scanning zones are imaged in midsagittal and lateral planes, suprasternal views, as well as supraclavicular and infraclavicular approaches (Figure 51 E-2). Neurovascular structures, thyroid gland, extrathoracic trachea, lymph nodes, and soft tissues can be assessed. Ultrasound is useful in tracheal imaging and airway management. The longitudinal midline view shows tracheal rings as small well-aligned structures with a “string of beads” appearance (Figure 51 E-3). Beneath the cartilages, visualization of a brightly echogenic line and periodic resonance artifact correspond to the tracheal lumen. Additional oblique views may illustrate the entire tracheal anatomic configuration (see Figure 51 E-3). Preintubation scanning measures tracheal diameters (applied mainly in children), and aids in evaluating tracheal compression (e.g., diving goiter, lymph node) or stenosis (e.g., pharyngeal or laryngeal pathology). Landmarks (e.g., cricoid cartilage) that may be difficult to palpate (e.g., in obese patients, etc.) are depicted along with adjacent structures, such as the thyroid gland (e.g., thyroid isthmus, goiter, etc.) or vascular structures (e.g., aberrant brachiocephalic artery, innominate vein). This helps choose the tracheostomy and/or cricothyroidotomy site (see Chapter 52). Midline transverse planes can be used to depict the anterior jugular veins and tributaries of the venous jugular arch that remain major sources of hemorrhage when performing percutaneous tracheostomy in the ICU (see Figure 51 E-3). Postintubation scanning depicts the endotracheal tube and/or its cuff’s position in the midtrachea (suprasternal view). The tube is depicted as parallel echogenic lines with acoustic shadowing (see Figure 51 E-3), whereas the air-filled cuff is recognized by its curved contour and associated comet-tail artifacts (filling the cuff with saline improves visualization). Correct tube placement is also confirmed by its absence in the esophagus, presence of bilateral diaphragmatic motion (B-mode and M-mode) and bilateral lung sliding (pleural line movement) at the lung–chest wall boundary (see Chapters 1 and 19). Right mainstem bronchus intubation can be determined by the lack of motion of the left hemidiaphragm and a nonsliding left lung. Bilateral absence of lung sliding and paradoxic diaphragm motion during inspiration would indicate “esophageal intubation.”¹⁰ The tube will then be clearly seen in the esophagus and best approached in a transverse plane from the left side.

Thyroid gland ultrasound is a routine, highly revealing procedure and, in the ICU setting, is invaluable for screening for autoimmune thyroiditis, which is highly prevalent and underdiagnosed. The typical pattern is irregular, small, and hypoechoic and requires a certain level of experience to recognize. A normal pattern allows ruling out the condition, whereas positive or indeterminate results warrant laboratory assessment. The occasional detection of parathyroid tumors (homogeneously hypoechoic to the overlying thyroid gland) in acute hypercalcemia aids in prompt surgical intervention. CT is the preferred imaging modality for examining the retropharyngeal space; however, branchial cleft cysts, abscesses or hematomas can be evaluated by ultrasound. Supraclavicular and infraclavicular views (see Figure 51 E-2), which are extended laterally toward the axilla, may be used to evaluate tissues and structures, or plan and perform guided procedures (e.g., cannulation of central veins).

Scanning is extended to the upper limbs by using generic scanning approaches (applying longitudinal and transverse views bilaterally). Upper limb exploration uses MSK landmarks, such as shoulder, elbow, and wrist joints (Figures 51 E-4 and 51 E-5).¹ Anterior and lateral dynamic shoulder views (Figure 51 E-6) are applied to detect the biceps tendon, tendons of the rotator cuff, and adjacent muscles. Scanning can then proceed to the arm to examine the brachial artery and vein (see Figures 51 E-4 and 51 E-7). The brachial artery can be followed distally to the anterior elbow, where it lies next to the median nerve. Anterior, lateral, and posterior views (see Figure 51 E-7) may assess the elbow and adjacent structures (e.g., effusion, tenosynovitis, muscle and tendon tears, etc.). Next, forearm exploration effectively reaches the wrist and the hand. By sweeping the transducer medially to examine the Guyon tunnel, the pisiform is detected adjacent to the ulnar artery, whereas lateral wrist exploration is facilitated by identifying the radial styloid adjacent to the radial artery (Figure 51 E-8). The latter view may evaluate the first wrist compartment (e.g., de Quervain disease). Moving the transducer distally from the level of the Lister tubercle, the scaphoid, the scapholunate ligament, and the fourth compartment of extensor tendons can be evaluated, whereas the median nerve is identified within the carpal tunnel, where it can be entrapped (see Figure 51 E-8). In the ICU, hand scanning has limited applications; however, it may be performed to evaluate traumatic injuries or other abnormalities (Figure 51 E-9 and see Figure 51 E-5). The veins of the upper extremity are often used for vascular access and may be evaluated for signs of phlebitis and/or thrombosis as clinically indicated.

Surface ultrasound imaging of the thoracic and abdominal wall may reveal comet-tail vertical lines that start at a level external to the ribs and hide underlying structures. This finding signals parietal subcutaneous emphysema, a possible sign of pneumomediastinum or esophageal rupture. A focal region of abdominal tenderness and swelling may be due to a rectus sheath hematoma (e.g., bleeding disorders, postprocedural). Ultrasound delineates the entity within the abdominal wall and may guide its percutaneous drainage. Other abdominal wall abnormalities include hernias, lipomas, tumors, pseudoaneurysms, or dilatated superficial veins (Figure 51 E-10).^2,4 A recanalized left umbilical vein is possible to follow to the teres ligament from the umbilical area in many patients with portal hypertension.

Scanning is extended to the lower limbs via the groin (Figure 51 E-11). Several abnormalities may be encountered in the inguinal area, such as DVT, pseudoaneurysms, vascular bypass grafts, inguinal and femoral hernias, reactive and metastatic lymph nodes, lymphomas, abscesses, hematomas, tumors, and so forth (Figure 51 E-12). The exploration of lower limbs is facilitated by using MSK landmarks, such as hip, knee, and ankle joints. A low-frequency transducer (convex) is used when scanning the hip joint. By rotating the transducer between 45 to 60 degrees, and translating slightly forward, a longitudinal scan of the femoral neck can be obtained, and the femoral head can be identified Figure 51 E-13). Examination of deep veins to exclude DVT is a required examination in many intensive care units (see Chapter 9). Assessing other abnormalities that can mimic DVT (Baker cyst, hematomas, cellulitis, lymph nodes, heterotopic ossification, etc.) is equally important (see Figure 51 E-10). Dynamic MSK scanning in the lower limbs can be facilitated by a two-person technique as previously described. Scanning starts from the hip joint and continues caudally. The anterior and posterior aspects of the thigh are investigated (e.g., muscles, bones, neurovascular structures). The sciatic nerve can be depicted as it courses the posterior thigh (Figure 51 E-14). A microconvex, or even a convex probe, may be chosen for this area because of depth penetration challenges and the large structures that a linear probe may fail to adequately show. By sweeping the transducer toward the knee (anterior plane), the quadriceps tendon “anchoring” at the patella is depicted (see Figure 51 E-14). Anterior and lateral views (see Figures 51 E-14 and 51 E-15) may be used to evaluate the knee (e.g., effusions); posterior views access the popliteal fossa to evaluate major neurovascular structures (see Figure 51 E-15) or depict other abnormalities (e.g., popliteal cysts, etc.). As scanning advances distally toward the ankle, muscles (e.g., gastrocnemius muscle) and vessels (anterior tibial and peroneal veins) are accordingly explored. In supine ICU patients, midline longitudinal planes over the ankle dorsum identify the anterior recess of the tibiotalar joint. By sweeping the transducer medially and laterally, the talar dome is seen in 60% to 70% of cases (Figure 51 E-16).¹ Lateral ankle planes evaluate the anterior talofibular ligament; medial ankle scanning is performed by rotating the foot slightly laterally, thus allowing visualization of medial malleolus and posterior tibial vessels (see Figure 51 E-16). Dorsal foot scanning offers a view of the small and tortuous dorsalis pedis artery, which may be used for placing an arterial line. Finally, the Achilles tendon, the most commonly injured ankle tendon, should be examined along its full course by using both longitudinal and transverse planes; using two or three operators and lateral positions may permit dynamic scanning (passive dorsal and plantar flexion to distinguish partial from complete Achilles tendon tears (see Figure 51-2).

The HOLA concept facilitates the evaluation of various abnormalities in the upper and lower limbs, including the limited use of the common MSK landmarks. With experience, this approach enhances the ability of the operator to perform various applied techniques, such as ultrasound-guided cannulations of arteries and veins and peripheral nerve blocks. Once the operator becomes skillful in identifying all structures in the ROI, specific targets such as nerves or vessels are found quickly and confidently. Of note, this is not only easier but may prove to be safer too, as the operator learns to concurrently rule out other potential changes in the ROI. Discovering unsuspected DVT, for example, alters therapy and facilitates urgent interventions (e.g., placement of an inferior vena cava filter).

Pearls and highlights

References