Objectives
On completion of this chapter, you should be able to:
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Identify the normal anatomic location and function of the tendon, ligament, muscle, nerve, and bursa
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Know the advantages and disadvantages of sonographic artifacts in musculoskeletal imaging
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Summarize the basic sonographic examinations of the shoulder, wrist, knee, ankle, and foot
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Distinguish normal anatomy from common pathologic conditions
In the early 1990s, a radiologist asked me to try to image a torn suprapatellar tendon. It was difficult to image the torn tendon because of technologic limitations and our inexperience in musculoskeletal ultrasound imaging. Musculoskeletal imaging is now gaining in popularity in the United States, following in the wake of magnetic resonance imaging (MRI). However, ultrasound of the musculoskeletal system has been widely used outside of the United States.
Many things have changed since then, both in the delivery of medical care and in the production of sonographic images. The decrease in medical reimbursements has forced the development of less expensive modalities to complement or replace computed tomography (CT) or MRI. Ultrasound equipment manufacturers have also continued to improve and refine technology, and this has resulted in improved soft tissue imaging. The 5- or 7-MHz transducer commonly used in the 1990s is hardly acceptable for scanning superficial structures today. Current transducers create images with frequencies as high as 17 MHz.
This chapter is intended to provide a solid foundation for basic musculoskeletal ultrasound. Imaging of the muscular system is not limited to the muscles themselves, but also includes the tendons, nerves, ligaments, and bursa. Other areas of musculoskeletal imaging include the joints, pediatric imaging, bone, skin, many disease processes, foreign bodies, and postoperative scanning. Add the joint-specific scanning of shoulder, knee, ankle, elbow, and wrist, and you begin to understand that musculoskeletal ultrasound imaging is a significant area that we have just begun to explore.
Anatomy of the musculoskeletal system
Normal anatomy
Skeletal muscle contains long organized units called muscle fibers. The characteristic long fibers are under voluntary control, allowing us to contract a muscle and move a joint. The blood vessels, lymphatics, and nerves follow the fibrous partitions between the bundles of muscle.
Several different types of muscles are present in the human body. Muscles have fibers that run parallel to the bone, have a fan shape, or form a pennate pattern. These feather-like muscle patterns run oblique to the long axis of the muscle and are unipennate, bipennate, multipennate, or circumpennate. Think of a feather and how the fibers grow from a central section. Half of this feather is unipennate, whereas the whole feather is bipennate. A multipennate muscle is a division of several feather-like sections in one muscle, and the circumpennate is the convergence of fibers to a central tendon ( Figure 24-1 , A ). The deltoid muscle is an example of a unipennate muscle; it has feather-like fascicles with a unipennate, bipennate, or multipennate attachment ( Figure 24-1 , B ). The gastrocnemius muscle in the calf is a bipennate muscle in which the fibers have a central origin ( Figure 24-1 , C ). The large, flat muscles of the external oblique or the trapezius attach with a large, flat aponeurosis ( Figure 24-1 , D ).
Attachment of the muscle occurs at the proximal and distal portions of the bundle. This attachment, a collection of tough collagenous fibers, is a tendon. These attachments may be cordlike or flat sheets called aponeuroses. This type of attachment occurs in flat muscles, such as the rectus abdominis in the abdomen. The elastic tendon consists of collagen fibers that enable it to stretch and flex around structures. This avascular structure heals slowly and has a whitish appearance. Because of the lack of vascularity, tendons heal slowly; this is why an injury can incapacitate a patient.
Tendons occur with or without a synovial sheath. This tubular sac surrounding a tendon has two layers. Fluid separates the two layers of the sheath and is found in the shoulder, hand, wrist, and ankle. This sheath plays an important role in imaging these structures with sonography. The biceps tendon of the shoulder is one example of a tendon with a synovial sheath. Other tendons, such as the Achilles and patellar, lack this sheath and have a surrounding fat layer or loose connective tissue, which makes this type of tendon more difficult to image with sonography.
The support and strength of a joint are due in part to the ligaments. These short bands of tough fibers connect bones to other bones. This type of connective tissue is especially important in the knees, ankles, and shoulders.
The saclike structure surrounding joints and tendons that contains a viscous fluid is the bursa. This potential space provides an area for synovial fluid to aid in the reduction of friction between two musculoskeletal structures, such as tendon and bone or ligament and bone. For example, two of the knee joints that have such a bursa are the patellofemoral and femorotibial joints. The suprapatellar pouch has a continuous connection with the joint cavity but is often referred to as a bursa. The knee joint itself has nine bursae—three located anterior and six on the popliteal side of the joint.
Nerves are the conduits for impulses to and from the muscles and the central nervous system. Muscle action is under the control of the muscle system with the nerves in contact with the muscle through motor end plates. Elements of the nerves include the nerve fibers, arranged into bundles (fasciculi) and surrounded by dense insulating sheaths of myelin (forms the sheath of Schwann cells) and connective tissue.
Normal sonographic appearance
Imaging of the musculoskeletal system can be overwhelming because so many different muscles, attachments, ligaments, and tendons can be seen. All joints contain similar anatomic structures, and tendons and ligaments have the same sonographic imaging characteristics whether they are part of an ankle or a shoulder. Muscle attachments also have a similar sonographic appearance. The first step in sonographic imaging of any musculoskeletal structure is knowledge of the normal appearance.
Tendons.
MRI has been the modality of choice for physicians in the United States when diagnosing musculoskeletal problems. The advent of high-resolution ultrasound has challenged the superiority of MRI for imaging tendons, especially when the examination is performed by a skilled sonographer using high-quality equipment. The evolution of real-time ultrasound allows demonstration of the full range of motion of the tendon. The high resolution of modern transducers also allows imaging of the fine tendon fibers and comparison of normal versus abnormal using dual imaging techniques.
The tendon occurs in two forms: with and without a synovial sheath ( Figure 24-2 ). Wrapped around the tendon, the smooth inner layer of this tubular sac lies in close contact with the tendon. Between this inner layer and an outer layer, a small amount of thick mucinoid material helps facilitate movement. The biceps tendon is one example of a sheath-covered tendon that images well ( Figure 24-3 ). The thickness of this sheath measures only a couple of millimeters and is sonographically imaged as a hypoechoic halo surrounding the tendon. Inflammation of this sheath and tendon often aids in imaging and diagnosing problems with this tendon. Acute disease may reveal a sheath that is thicker than the contained tendon. Areas of high stress in the hand, wrist, and ankle also contain tendons with sheaths.
Paratenon, a loose areolar connective tissue, fills the fascial compartment of the tendon lacking a synovial sheath. The dense epitendineum, another layer of connective tissue, closely adjoins the tendon. The epitendineum images as an echogenic layer adjacent to the tendon. The lack of density differences in these interfaces makes the tendon somewhat difficult to image. Fortunately, many of the tendons without a synovial sheath are large and image relatively well. The accompanying bursa may also be abnormal, thus enhancing the tendon. Examples of this type of tendon include the Achilles, patellar, proximal gastrocnemius, and semimembranosus tendons ( Figure 24-4 ).
Interwoven and interconnected collagen fibers found in the tendon run in a parallel path. The numerous interfaces of the collagen fascicles provide a strong linear reflector that images well with ultrasound. The higher the frequency of the imaging transducer, the better these fibers image—a fact that underscores the need for a transducer of 7 MHz or greater. This normal fibrillar hypoechoic pattern and imaging detail become very important when diagnosing abnormalities.
Care must be taken when imaging the tendon because even a slight rotation off axis may produce an image that incorrectly suggests tendinitis. Both transverse and longitudinal planes help image the tendon, along with a side-by-side (dual) comparison of the contralateral side.
The tendon insertion site has its own sonographic characteristics. The joining of the tendon to the bone (enthesis) occurs with a narrow band of fibrocartilage. This avascular structure is approximately 1 cm long and images longitudinally as a triangular hypoechoic area in the distal tendon. Familiarity with the normal sonographic appearance is important because injury to this area of the tendon results in thickening of the insertion site ( Figure 24-5 ).
Ligaments.
Ligaments are thin, superficial structures, which makes them difficult to image. This superficial location requires the use of a higher-frequency transducer—10 MHz or greater—and possibly a stand-off pad to aid in imaging ligaments outside the joint. Critical to ligament identification is the equipment parameter adjustment. Adding too much gain to the image using overall gain or time gain compensation (TGC) results in loss of detail due to the strong bone reflections. Unlike imaging in other areas of the body, longitudinal imaging of the ligament is the only method used to image injuries. Transverse planes are of little help when imaging the ligament because they blend with the surrounding fat. The difficulty of imaging the ligament is helped by using a dual or side-by-side technique to compare normal and abnormal anatomy.
Many ligaments in the large joints of the body image well as hyperechoic straplike structures ( Figure 24-6 ). One exception is the cruciate ligament within the knee joint, which appears hypoechoic. The large joints include the hip, shoulder, ankle, wrist, and knee. Part of the difficulty associated with imaging the ligament is the lack of a contiguous structure, such as muscle, to aid in location. The dense fibers have a slightly less regular appearance and may help hold a tendon in place. Usually the ligament measures 2 to 3 mm thick and images as a hypoechoic band with a homogeneous appearance. These ligament structures are found close to both ends attaching to the bony cortex.
One ligament—the medial collateral ligament (MCL) or tibial collateral ligament, which connects the medial femoral condyle to the medial proximal tibia—deviates from the usual ligament appearance. This wide, smooth ligament is about 9 cm long and has deep and superficial portions. The external superficial portion consists of connective tissue appearing as a dense band that connects the medial femoral condyle to the proximal tibia. The deep layer connects the medial meniscus to the femur and tibia.
Sonographic imaging of the MCL reveals a three-layer structure. The superficial and deep layers have a hypoechoic-separating layer. Loose connective tissue forms the middle layer, which provides a potential space for bursas in some individuals ( Figure 24-7 ).
Muscle.
Discussions of muscle often include references to the origin and insertion of the muscle. The proximal portion of the muscle is considered the origin, whereas the insertion is the distal end. A muscle with two or more heads has an origin in more than one place on the bone. Most of us do not think in terms of origins and insertions, so for the purpose of this discussion, we will talk in terms of the location or attachment of the muscle.
To begin to learn the normal appearance of a muscle, it is easy to use the large quadriceps muscle located in the anterior thigh or the posterior calf muscles. Skeletal muscle imaged on a longitudinal plane appears homogeneous with multiple, fine parallel echoes ( Figure 24-8 ). Connective tissue surrounding the fiber bundles produces these echogenic bands. The main portions of the muscle fibers are hypoechoic and radiate toward a central tendon or aponeurosis ( Figure 24-9 ). The transverse plane discloses a less organized pattern of fine punctate echoes scattered through the muscle bundle. Encasing the muscle is a connective tissue fascia that has a bright echogenic appearance ( Figures 24-10 and 24-11 ). This fascia layer, although brighter than the sheathed muscle fibers, has less echogenicity than subcutaneous fat or tendons.
The muscle bundle contains nerves, fascia, tendons, fat, and fibrous connective tissue surrounding the muscle. The epimysium continues into the muscle, developing into the perimysium, which separates the bundles into muscle fibers. These hypoechoic structures, compared with the muscle fibers, help differentiate muscles. The sonographic appearance of muscle can be deceiving in some areas, such as the hand, because of the similarity of echo texture to a mass or tenosynovitis. Careful scanning and transducer rotation help image the pennate structure of the muscle, aiding in identification of a possible normal muscle variant.
Normal dynamics of the muscle images easily in real-time because contraction of the muscle increases muscle thickness and hypoechogenicity. In addition, echogenic connective tissue bands increase in obliquity. Sustained contraction of the muscle has the same sonographic appearance as muscle bundles found in the athletic patient. This decreased echogenic muscle, as a result of hypertrophy, is normal for this patient population. Compression of the muscle with the transducer condenses the tissue, resulting in an increase in muscle echogenicity.
Transducer orientation is another factor that influences muscle echogenicity. Ensuring a longitudinal and transverse plane with good contact helps negate the possibility of introducing artifactual information. It is helpful to scan the contralateral normal side to ensure technique or to ensure that normal variants do not result in a misdiagnosis.
Nerves.
The normal nerve has a hyperechoic appearance compared with muscle but is hypoechoic compared with tendons. The echogenicity depends on the surrounding structures and is not constant within the body. The longitudinal plane reveals a fibrillar pattern with parallel inner linear echoes similar to the tendon. In transverse imaging, the nerve fibers appear hypoechoic, with a hyperechoic perineurium surrounding each fiber. The collagenous epineurium —the outer layer of the nerve—appears as a hyperechoic layer ( Figures 24-12 and 24-13 ).
Differentiating nerves from tendons is a simple task when you contrast the two structures. Real-time imaging shows tendons that move when the corresponding joint or muscle contracts. The nerve will remain stable within the muscle tissue. Sonographic artifacts (anisotropy) are not as evident on the nerve as they are on the tendon, and nerves are imaged best with a transducer of 10 MHz or higher. Power Doppler is especially helpful because vessels accompany the nerves. Table 24-1 lists the nerves that are identifiable with sonography.
| Lower Limb | Location |
| Sciatic | Posterior thigh lateral to the hamstring muscle |
| Popliteal | Popliteal fossa superficial to the popliteal artery and vein |
| Upper Limb | Location |
| Suprascapular | Deep to the trapezius to the infraspinatus fossa |
| Median | Medial to the biceps tendon and brachial artery, elbow, right side of the carpal tunnel |
| Radial | Between the brachioradialis and brachialis muscles |
| Ulnar | Median epicondyle of the elbow, medial to the ulnar artery in Guyon’s canal |
Bursa.
The small sac between two moving surfaces, usually tendon and bone, is the bursa. These fluid-filled cavities facilitate the movement of tendons or muscles over bony projections. The minute amount of viscous fluid contained within the bursa helps reduce friction between moving parts of the joint ( Figures 24-14 and 24-15 ). The major bursa of the body is the subacromial-subdeltoid bursa, found in the shoulder, covering the deep surface of the deltoid muscle.
Two types of bursas are found in the body: communicating and noncommunicating. This categorization helps explain the relationship of the bursa to the joint space. One communicating bursa sonographers often see is Baker’s cyst, which is located in the medial popliteal fossa. This bursa, located between the semimembranosus and medial gastrocnemius tendons, has a connecting neck to the bursa contained within the knee joint. Usually, sonographers image bursas that do not communicate with the joint space. An example of a superficial noncommunicating type of bursa is the prepatellar bursa.
In the normal patient, the bursae are difficult to image because they often blend in with surrounding tissue and fat. The thin film of viscous fluid found within the bursa contributes to the hypoechoic appearance of the structure because the walls are too thin to image. These potential spaces will appear on ultrasound in the presence of an inflammatory process caused by fluid accumulation. Any bursa larger than 2 mm is enlarged and needs to be compared with the normal contralateral side.
Table 24-2 summarizes the normal sonographic appearance of tendons, ligaments, nerves, muscle, and bursae.
| Anatomy | General Sonographic Appearance | Longitudinal Appearance | Transverse Appearance |
|---|---|---|---|
| Tendon | Hyperechoic linear structure Dynamic with movement of corresponding joint/muscle | Cordlike | Oval, round, or cuboid |
| Ligament | Isoechoic, weakly hyperechoic | Striated structures connecting bone to bone | Difficult to image on the transverse plane |
| Nerve | Hypoechoic to tendons Hyperechoic to muscle Cannot be mobilized with movement Posterior enhancement lacking | Cordlike tubular structure | Hypoechoic with fascicles |
| Muscle | Muscular bundles—hypoechoic Perimysium, epimysium, fascia, fat plane—hyperechoic | Parallel echogenic linear appearance within hypoechoic muscle tissue; may appear featherlike depending on the type of muscle imaged | Punctate echogenic areas within the hypoechoic muscle |
| Bursa | Thin, hypoechoic structure that merges with the surrounding fat | Thin linear hypoechoic structure adjacent to a tendon | Normal bursae are difficult to image on the transverse plane |
Artifacts
Sonographers and sonologists have the daily challenge of separating artifacts from useful image information. All equipment manufacturers program in some basic assumptions about the interaction between tissue and sound: that the speed of sound is 1540 m/sec, that the area imaged is within the central beam, and that sound travels out and back in a straight line. Artifacts occur when these basic assumptions are not met, and something is created that is not real, is erroneously positioned, has improper brightness, or is absent from the image. Many manufacturers have developed technology to reduce and often eliminate some types of artifacts caused by compound and harmonic imaging.
Musculoskeletal imaging displays the same gamut of artifacts seen in other areas of the body. The superficial nature of musculoskeletal structures, often anterior to the highly reflective bone, causes artifacts to become more of a problem. Some artifacts aid in identifying pathologic conditions and structures; however, others hinder and even mimic disease.
Several artifact types—anisotropy, reverberation, time-of-flight artifact, and refractile shadowing—are important in musculoskeletal ultrasound. Understanding how artifacts occur and how to correct images increases both diagnostic confidence and image accuracy.
Anisotropy
The anisotropic phenomenon is one that occurs not only in sonography but also in other professions, such as astronomy, geology, and chemistry. Anisotropy occurs when the sound beam misses the transducer on the return because of the curve of the structure ( Figure 24-16 ). The angle and direction of the reflected beam depend on the angle of incidence.
The reflection coefficient is a function of the angle and becomes a problem when the reflected beam misses the receiver. The nonperpendicular tissue interfaces return echoes at an angle that does not return to the transmitting transducer, creating an imaging challenge. This results in differing image properties depending on the angle of incidence.
The loss of definition of the curved upper pole of the right kidney is one example of this artifact. The muscle, ligaments, and nerves also image as an anisotropic reflector because of the plane they occupy, with tendons having the most pronounced anisotropy in musculoskeletal imaging. This loss of image requires a heel-to-toe rocking of the transducer to create the optimal 90-degree angle ( Figures 24-17 and 24-18 ).
Reverberation
A reflective surface reverberates sound and may be beneficial or detrimental. We often experience this phenomenon without realizing its impact. Our senses use reverberation as a clue to the location of structures in a room through reflection of sound back to our ears. Any acoustic environment, such as an auditorium, relies on reverberation to transmit sound.
The same is true of sound transmitted into the body. The initial sound beam transmits and returns. Multiple delayed reflections from strong tissue boundaries, such as bone, result in a linear artifact that decreases in intensity with depth. This collection of reflected sound is superimposed over the primary signal, often adding distracting information to the image ( Figure 24-19 ).
