Soft tissue, musculoskeletal system, and miscellaneous targets

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Soft tissue, musculoskeletal system, and miscellaneous targets

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.13 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.24 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).

image

Figure 51-3 A, Longitudinal view of a paraumbilical hernia containing omentum (12-mm defect). B, Lateral/coronal long-axis scan of the distal forearm in a trauma patient. Note a comminuted distal radius fracture: four distinct segments of bone with mutual misalignment, with a hypoechoic area of a likely hematoma (note the extensor pollicis brevis tendon across the screen, parallel to the skin, and the general axis of the fractured bone). C, Visualization of the “zone phenomenon” in heterotopic ossification: an inner hypoechoic core is surrounded by hyperechoic mineralized islands (arrow) within the iliopsoas muscle adjacent to the hip joint. D, Heterogeneous groin mass with ill-defined borders (sarcoma). E, Cobblestone appearance of the subcutaneous tissue resulting from generalized interstitial edema in a heart failure patient. F, Cobblestone echotexture of the subcutaneous tissue in a region of the gastrocnemius muscle and depiction of unusually high flow (increased peak systolic Doppler velocity) in a local perforator (cellulitis). G, Zoom image of a subcutaneous hypoechoic abscess with hyperechoic punctiform material. H, hypoechoic knee effusion with echogenic material (arrow, septic arthritis). I, Hypoechoic metastatic arm lymph node in a patient with thyroid cancer. J, Reactive round- and oval-shaped groin lymph nodes. K, Oval-shaped groin lymph node with a Doppler-derived resistive index (RI) 5 0.51 (power Doppler mode), which was initially characterized as reactive, but eventually proved (biopsy) to be malignant (lymphoma) with cystic necrosis. L, Longitudinal and transverse planes of the thigh depicting a bullet casting an acoustic shadow (arrow) and the formation of “halo” (arrow), respectively. M, Longitudinal view of superficial lipoma (arrow) over the xiphoid process. N, Visualization of a totally fluid-filled sinus that appears hypoechoic, with its posterior and lateral walls delineated (sinusogram). (Images in A, courtesy Dr. K. Stefanidis; D, L, and M courtesy Dr. K. Shanbhogue; G, courtesy Dr. J. Poularas.)

Recently, ultrasound has been used to determine the type and extent of traumatic nerve injuries

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