NORMAL SONO-ANATOMY OF MSK TISSUES IS VERY IMPORTANT TO KNOW, AND MAKE ABOUT 90% OF THE WAY TO KNOW THE PATHOLOGY.
THANKS TO DR .AHMED . F. ABOGAMAL FOR HIS GREAT EFFORT
Tendons are characterized by their parallel course with respect to the skin surface and by a typical fibrillar echotexture that is clearly detectable in longitudinal scans.
The fibrillar echotexture is generated by parallel running fascicles of collagen fibres, which appear as closely positioned, fine hyperechoic lines with even finer anechoic thin lines in between, corresponding to inter-fascicular ground substance. Fig.6
On tendons with a synovial sheath, a subtle anechoic rim interposed between the sheath and the tendon margin can be detected.1
Fig.6: Tendon longitudinal: A: sonographic picture,T= tendon, F= fat pad, C= cartilage, I.P=Proximal phalanx, I.MC= metacarpal head, JS= joint space. B: Tissue specimen of the same picture, T= tendon, S= subcutaneous fat, B= bone.
Fig.7: Tendon transverse: A: sonographic picture,T= tendon, B= bone. B: Tissue specimen of the same picture, T= tendon,B= bone.
In transverse scans, tendons appear as round or oval structures, characterized by numerous closely joined dots that are homogeneously distributed and correspond to the intra-tendinous connective fibers.
The tendon’s sonographic characteristics in healthy subjects are fairly homogeneous and have limited intra- and inter-individual variability.1 Fig.7
They are made up of two components: the muscle fibers, which are long and cylindrical in structure, representing the cellular unit of muscle, and stromal connective tissue.
Individual muscle fibers are grouped together in bundles, which are commonly known as fascicles, and several fascicles join together to form an individual muscle (Fig. 8).
Thin connective tissue strands – the endomysium – separate the individual muscle fibers; a more substantial connective sheath with small vessels and nerve endings, the perimysium (also referred to as fibroadipose septa), envelops individual fascicles; a thick fibrous layer, the epimysium, surrounds the entire muscle. 2,3 , Fig.8.
Fig.8: Muscle fiber: cross-section (coated from van Holsbeeck and Introcaso, 2001).3
The fasciculi can be identified as separate structures on ultrasound.2, 3 They are best identified in the longitudinal plane as hypoechoic cylindrical structures, separated by the hyperechoic intervening connective tissue, the perimysium (fig 9,10).
Individual fibers and the endomysium are not individually discernible. The epimysium, fascia, and intermuscular fat are all thin, linear hyperechoic structures on ultrasound.
During contraction of a muscle, the fibers shorten, causing an apparent increase in muscle bulk. When the muscle is contracted, the fascicles have a thicker and more hypoechoic appearance.3
Fig.9: Muscle longitudinal: A: sonographic picture F = fascia, M = muscle. B: Tissue specimen of the same picture, F = fascia, M = muscle. (coated from van Holsbeeck and Introcaso, 2001).3
Fig.10: Muscle Transverse: A: sonographic picture F = fascia, M = muscle. B: Tissue specimen of the same picture, F = fascia, M = muscle. (coated from van Holsbeeck and Introcaso, 2001).3
Ligaments are fibrous structures appearing similar to tendons. However, they are less compact and are composed of a more diverse pattern of collagen bundles.4
Ligaments are usually well defined and easily visible on ultrasound examination. However, when they occur as a focal thickening of a joint capsule, they may not be distinguishable as distinct structures.
Ligaments display a hyperechoic, linear appearance on ultrasound and are optimally evaluated when they are stretched.5, Fig.11
Similar to tendons, ligaments appear fibrillar during longitudinal scanning and have a broom-end appearance on transverse scans.
However, ligaments can be distinguished from tendons by tracing them back to the bony structures to which they attach.4
Fig.11: Ultrasound image of medial collateral ligament reveals the superficial ligament (arrows) and meniscofemoral ligament (mf) and meniscotibial ligament (mt) attached to the meniscus (asterisk).
A peripheral nerve is a cordlike structure containing a large number of individual nerve fibers.
The nerve fibers are grouped together into bundles known as fascicles Fig.12. The fascicles are enclosed in a connective tissue sheath or membrane known as the epineurium.
Each fascicle is in turn covered by a sheath of connective tissue, the perineurium. The individual nerve fibers within the fascicle are also enclosed by a sheath of connective tissue, the endoneurium.
Extending inward from the epineurium is the interfascicular epineurium, which is thin septae adding further support to the nerve bundles and their vascular supply. 6, 7.
Fig.12: Peripheral nerve: cross section (coated from van Holsbeeck and Introcaso, 2001).3
Ultrasound, in the longitudinal axis of the nerve, demonstrates a fascicular pattern of uninterrupted hypoechoic bands with intervening linear interrupted hyperechoic bands Fig.13.
The hypoechoic bands represent the fasciculi and the hyperechoic bands the supporting interfascicular epineurium.
The epineurium is hyperechoic and of similar appearance to perineural fat and may not be separable on ultrasound.
In the axial study, the nerve is composed of fasciculi seen as multiple hypoechoic dots, which may be of varying size, intermingled in a hyperechoic background of the supporting connective tissue.2, 8–10
Fig.13: Ultrasound image of median nerve (A= long axis, B= short axis) shows hypoechoic nerve fascicles (arrowheads). t = flexor digitorum tendon, p = palmaris longus tendon, R = radius, L = lunate.
Ultrasound provides limited views of bones. It can display vivid anatomic details of the cortical surfaces of the superficial bone.5
Bones appear with well-defined, linear, and smooth hyperechoic borders. This hyperechoic appearance is caused by the high reflectivity of the acoustic interface.
Because nearly the entire sound beam is reflected, ultrasound is unable to image beyond the bone surface or that of other calcified structures, so the image beyond the interface appears black; this is referred to as posterior acoustic shadowing.4
Because of this phenomenon, Ultrasound can provide information only about the superficial portion of bones Fig.14.
Fig.14: US appearance of normal bone: surface echotexture. a Longitudinal 12–5 MHz US image obtained over the diaphysis of the radius demonstrates the bone surface as a continuous straight hyperechoic line (arrows) produced by a strong reflection of sound due to the marked difference in acoustic impedance of the soft tissues and bone. Reverberation artifact (arrowheads) projecting in the shadow beyond the bone can be seen.
Synovial joints are the most common joints examined with ultrasound. They are formed by articulating bone surfaces, fibrous capsules, and ligaments, and other intraarticular structures (ligaments, menisci, labra, and fat pads) 2, Fig.15.
Ultrasound examination of joint surfaces reveals a homogeneously smooth hypoechoic smooth linear band (the hyaline cartilage).11,12
The joint capsule appears as a hyperechoic line merging with the para-articular tissues.2 The deeper subchondral bone is a regular, continuous, bright, hyperechoic line.
Fig (15): Elbow joint anterior longitudinal veiw: A: sonographic picture, M= muscle, F= fat pad, CA= cartilage, R=Radius phalanx, H= humerus, JS= joint space. B: Tissue specimen of the same picture. (coated from van Holsbeeck and Introcaso, 2001).3
Cartilage can be divided into hyaline cartilage, white fibrocartilage, and elastic or yellow fibrocartilage.
The latter is present in only select regions, e.g., the auricle of the external ear.
Articular hyaline cartilage is of varying thickness, being thicker in points of greater stress and on convex rather than concave surfaces.
It provides a degree of elasticity and shock absorption, as well as helping to dissipate stress across a joint.6,7 On ultrasound, it has a smooth, well-defined surface and border and is uniformly hypoechoic. 3, Fig.17.
Fig.17: Articular hyaline cartilage: A: sonographic picture, S= Skin, C= cartilage, B=bone. B: Tissue specimen of the same picture (coated from van Holsbeeck and Introcaso, 2001).3
It’s a variable mixture of white fibrous tissue and cartilaginous tissue with a large component of collagen fibrils.
It provides elasticity and flexibility. The menisci of the knee, temporomandibular and sternoclavicular joints, the glenoid and hip labra, and the triangular fibrocartilage of the wrist are composed of fibrocartilage.
On ultrasound, fibrocartilage is hyperechoic with well-delineated borders. Because of its position within joints, it is not always fully accessible to a full ultrasound examination.3, Fig.18
Fig.18: Fibrocartilage. The posterior glenoid labrum (arrow) is demonstrated here as a well-defined, hyperechoic triangular structure between the articulating surfaces of the glenoid (G) and humerus (H).
Normal bursae in healthy subjects are not always easily visualized. They appear as a thin hypoechoic space delimited by echoic borders corresponding to the tissue–fluid interface.
Bursitis is characterized by an increase in the synovial fluid that usually appears as a sharply defined anechoic area.13, Fig.16.
Fig.16: Retrocalcaneal bursa: + bursa , t, Achilles tendon; c, calcaneal bone.
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