U L T R A S O U N DC L I N I C S

Ultrasound Clin 2 (2007) 569–576

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Introduction to MusculoskeletalUltrasoundJon A. Jacobson, MD

- Ultrasound equipment- Scanning technique- Normal sonographic anatomy- Artifacts

- Special ultrasound techniques- Musculoskeletal applications- Summary- References

Since the initial application of musculoskeletal addition, linear transducers are used in musculo-

ultrasound in evaluation of the rotator cuff in1977 [1], popularity of this imaging method has in-creased markedly as well as the number of acceptedapplications. Such advances primarily have beenlinked to improvements in technology, whichnow allow exquisite visualization of structures assmall as individual peripheral nerve fascicles [2].Essentially all soft tissue structures of the extremi-ties and their pathologic conditions can be visual-ized with proper equipment and technique.

Ultrasound equipment

Ultrasound units come in various sizes, whichrange from portable hand-held devices to moreconventional ultrasound units as seen in most hos-pitals or imaging centers. The most important con-sideration of any type of equipment is frequency ofthe transducer given in megahertz. The resolutionincreases as the frequency of the transducer in-creases, but this is at the expense of depth penetra-tion. To evaluate the shoulder or knee, typicallya 10 MHz or higher transducer is used, althoughlarger patients may require the use of a 7 MHz trans-ducer (Fig. 1). To evaluate the small nerve fasciclesof the distal extremities or the pulley system of thedigits, at least a 12 MHz transducer is optimal. In

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skeletal imaging to avoid anisotropy, where a ten-don appears artifactually hypoechoic when notimaged perpendicular to the ultrasound beam [3].Curved transducers may be used to increase thefield of view when imaging larger body parts, suchas the hip and thigh. Smaller machines may nothave as many applications, such as power Dopplerimaging, but this varies between various ultrasoundmachines.

Scanning technique

First a transducer is selected that optimally balancesthe highest resolution and proper depth penetra-tion, which depends on the body part and structureto be imaged. For example, a 10 to 12 MHz trans-ducer may be used of most extremity applications,with lower frequency transducers considered forthe hip and other joints in a patient who has a largebody habitus, and higher frequency transducers forthe most distal extremities (see Fig. 1). The trans-ducer then is placed on the subject with ampleultrasound acoustic gel. It is optimal to hold thetransducer near the imaging surface while anchor-ing the transducer to the patient with the small fin-ger or the heel of the hand [4]. This will allow oneto make fine adjustments in transducer position,

adiology, University of Michigan, 1500 East Medical

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Fig. 1. Transducer selection and image resolution. Ultrasound images longitudinal to the long head tendon ofbiceps brachii at 14 MHz (A) and 9 MHz (B). In this patient with a large body habitus, note improved resolutionof hyperechoic tendon fibers (arrows) and hypoechoic joint effusion (arrowhead) distending biceps tendonsheath with lower frequency transducer. Abbreviation: H, humerus.

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and control the amount of transducer pressureplaced on the patient. This technique is also helpfulto stabilize the transducer when imaging a curvedsurface or when performing dynamic imaging.The combination of transducer stabilization asdescribed previously and a thick layer of gel allowsone to float the transducer just above the skinsurface, which is helpful when evaluating for super-ficial abnormalities, such as foreign bodies. The re-sulting image then is optimized by adjusting thedepth of the image to bring the areas on interestinto view. If the ultrasound machine has adjustablefocal zones, these also should be moved to thedepth of interest to optimize resolution. The grayscale gain then is adjusted for brightness of theimage. When describing anatomic structures at ul-trasound, one refers to the imaging plane relativeto the structure itself, such as transverse and longi-tudinal, rather than the imaging plane relative tothe body.

Normal sonographic anatomy

An image is produced by a sound wave reflection,which depends on the acoustic impedance of the

Fig. 2. Normal flexor pollicis longus. Ultrasound images lolongus show normal hyperechoic fibrillar tendon (arrows

tissue and the angle of incidence. An interfacebetween tissues with large differences in acousticimpedance will reflect the sound wave, such asbetween soft tissues and bone. Similarly, a soundbeam that is perpendicular to the surface of anobject will produce a bright echo. The characteristicappearance of a structure at ultrasound is ap-preciated best when the structure is imaged perpen-dicular to the ultrasound beam. The ultrasoundappearance is described as hyperechoic (highecho), isoechoic (equal echo), hypoechoic (lowecho), or anechoic (no echo) relative to the adjacentsoft tissue structures. In addition, the sound beammay be brighter deep to a structure (called increasedthrough transmission) or may be absent deep toa structure (called shadowing). A repeated echodeep to a structure is called reverberation artifact[5]. A normal tendon appears hyperechoic with a fi-ber-like or fibrillar echotexture (Fig. 2) [6]. Normalmuscle tissue is predominately hypoechoic withinterspersed hyperechoic fibroadipose septationsor perimysium (Fig. 3A) [7]. These septationsconverge to an aponeurosis or tendon when themuscle is imaged longitudinally, while in the trans-verse plane, the septations produce a starry sky

ngitudinal (A) and transverse (B) to the flexor pollicis) surrounded by relatively hypoechoic muscle.

Fig. 3. Normal biceps brachii and brachialis. Ultrasound images longitudinal (A) and transverse (B) to the bicepsbrachii (Bi) and brachialis (Br) show hypoechoic muscle with interspersed hyperechoic fibroadipose septations(arrows), Abbreviations: H, humerus; arrowhead, dermis and epidermis.

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appearance (Fig. 3B). Ligaments appear hypere-choic with an echotexture that is relatively morecompact when compared to tendon (Fig. 4), ex-tending from one bone attachment site to another[8]. Normal bone cortex reflects the ultrasoundbeam, which produces a very bright, smooth, andcontinuous echo (see Fig. 3). There is predomi-nantly shadowing deep to the bone, althoughreverberation also may be seen when imaged per-pendicular to the sound beam. Peripheral nerveshave a fascicular appearance when imaged longitu-dinally, where the individual nerve fascicles appearhypoechoic surrounded by hyperechoic connectivetissue (Fig. 5A) [2]. When imaged transversely,a peripheral nerve has a honeycomb appearance(Fig. 5B). Subcutaneous fat is hypoechoic but maybe more echogenic with increasing amounts offibrous tissue. The epidermis and dermis appear hy-perechoic (see Fig. 3A). Most bursae about the bodyare collapsed and difficult to identify at ultrasound.Some bursae may have a tiny amount of hypoechoic

Fig. 4. Normal anterior talofibular ligament. Ultra-sound image longitudinal to the anterior talofibularligament (arrows) shows normal compact hypere-choic appearance. Abbreviations: F, fibula; T, talus.

fluid, separating the adjacent hyperechoic bursalwalls. Although evaluation of cartilage is limiteddo to depth and location between articulating osse-ous structures, fibrocartilage appears hyperechoic(Fig. 6), and hyaline cartilage appears hypoechoic(Fig. 7) [9].

Artifacts

One must be aware of artifacts, as they can both as-sist in image interpretation and be a source of error.One of the most important of these artifacts isanisotropy, which can cause a normal tendon or lig-ament to appear abnormally hypoechoic, simulat-ing pathology (Figs. 8–10). The characteristicsonographic appearance of a tendon or ligamentis appreciated when the sound beam is perpendicu-lar to the axis of the structure being imaged. If thisangle of incidence is as little as 2� from perpendic-ular, the sound beam does not reflect back to thetransducer completely, and the tendon or ligamentwill appear artifactually isoechoic to muscle, andeventually hypoechoic when this angle reaches 7�

[3]. During real-time evaluation of a tendon or lig-ament, the transducer continually is being reposi-tioned or angled so that the area of concern isperpendicular to the sound beam. If a segment oftendon or ligament remained hypoechoic in spiteof transducer angling or repositioning, then pathol-ogy should be suspected. Anisotropy also can beused to one’s advantage. One example is when a ten-don is being imaged transversely and is surroundedby fat, such as in the ankle. In this situation, thehyperechoic tendon may be difficult to identifysurrounded by hyperechoic fat. By angling thetransducer along the long axis of the tendon, thetendon will become hypoechoic due to anisotropy,

Fig. 5. Normal median nerve at wrist. Ultrasound images longitudinal (A) and transverse (B) to the median nerveshow hypoechoic nerve fascicles (arrows) surrounded by hyperechoic connective tissue. Abbreviations: R, radius;L, lunate; t, flexor tendons.

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while the surrounding fat remains hyperechoic(Fig. 11). This maneuver also can help differentiatethe median nerve (which does not show anisot-ropy) from the adjacent flexor tendons in the wrist.Angling the transducer additionally will make anintratendinous hyperechoic calcification more con-spicuous when the surrounding tendon becomeshypoechoic from anisotropy.

In addition to shadowing deep to calcification orbone (see Fig. 3A) and reverberation artifact deepto a smooth flat surface (see Fig. 6), the soft tissuesdeep to a fluid collection or compact mass mayshow posteriorly through transmission [5]. Anotherartifact includes beam width artifact, where poste-rior shadowing or reverberation is not seen, becausethe structure being imaged is very small comparedwith the sound beam width [5]. One also may seeshadowing at the edge of a torn tendon, becauseof refraction shadowing [10].

Special ultrasound techniques

There are several more specialized technical optionson many ultrasound machines. One such technique

Fig. 6. Normal medial knee. Ultrasound image in thecoronal plane over medial knee shows hyperechoic fi-brocartilage meniscus (m), and compact hyperechoicsuperficial layer of the medial collateral ligament(arrows). Abbreviations: F, femur; T, tibial, arrow-head, reverberation artifact from bone cortex.

that is incorporated into many transducers is spatialcompounding. With this technique, the ultrasoundimage is produced from several different insonationangles [11]. Averaging images from these multipleinsonation angles improves tissue plane definitionand reduces speckle noise, but at the expense ofdecreased temporal resolution causing real-timemotion blur. Spatial compound imaging also maydecrease the echogenicity of structures such astendons, and the image may have a blurred ap-pearance, depending on the amount of spatialcompounding.

Another technique available on some ultrasoundmachines allows steering of the ultrasound beam.This may assist in reducing anisotropy when imag-ing a tendon that is coursing oblique relative to thesound beam. Angling the sound beam can decreasethis relative obliquity and decrease anisotropy.

Most ultrasound machines have color Dopplerimaging as an option, which displays blood flowas either red or blue if the flow is toward or awayfrom the transducer, respectively (Fig. 12A). PowerDoppler imaging is similar in that it also displaysblood flow, but the color display is independent

Fig. 7. Normal hyaline cartilage of knee. Transverseultrasound image over trochlea of distal femur (F)shows normal hypoechoic hyaline cartilage (arrows).

Fig. 8. Anisotropy of supraspinatus tendon. Ultrasound images longitudinal to the distal supraspinatus (S) showartifactual hypoechoic distal tendon (arrow) (A), which is no longer present when this segment of tendon isimaged perpendicular to sound beam (B). Abbreviations: D, deltoid muscle; H, humerus; arrowhead, subacro-mial-subdeltoid bursa.

Fig. 9. Anisotropy of subscapularis tendon. Ultrasound images longitudinal to the distal subscapularis (S) showartifactual hypoechoic distal tendon (arrow) (A), which is no longer present when this segment of tendon isimaged perpendicular to sound beam (B). Abbreviations: L, lesser tuberosity; arrowhead, long head of bicepsbrachii tendon in bicipital groove.

Fig. 10. Anisotropy of anterior talofibular ligament. Ultrasound images longitudinal to the anterior talofibularligament (arrows) show artifactual hypoechogenicity (A), which is no longer present when the ligament is im-aged perpendicular to sound beam (B). Abbreviations: F, fibula; T, talus.

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Fig. 11. Anisotropy of ankle tendons. Ultrasound images transverse to tibialis posterior (t) and flexor digitorum(d) tendons show normal hyperechoic tendons (arrows) in (A), which become more conspicuous in (B) when thetransducer is angled along the longitudinal axis of the tendon, causing hypoechogenicity from anisotropy.

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of flow direction (Fig. 12B) [12]. Power Dopplerimaging is more sensitive compared with conven-tional color Doppler in demonstration of vascular-ity. The use of color and power Doppler imaging isimportant when differentiating between a solidmass and a cyst, as internal vascularity suggestssolid mass rather than cyst. In addition, color andpower Doppler imaging is used to detect synovitisand to determine the activity of the synovitis [13].It is important to understand that increased flowon color or power Doppler imaging does not alwaysindicate inflammation. Increased flow also may beseen with neovascularity related to tendinopathyor tumor.

Ultrasound machines also may have the optionof extended field-of-view. With this option, thetransducer is moved across an extremity, and theimage data are summated to produce the extendedfield-of-view image (Fig. 13). This technique ishelpful when measuring pathology that is largerthan the transducer [14]. Such situations includemeasurement of large tumors or measuring theamount of tendon retraction in the setting ofa full-thickness tear.

Fig. 12. Color and power Doppler imaging. Transverse im(A) and power Doppler (B) imaging shows blood flow. N

One additional option with some ultrasoundmachines is tissue harmonic imaging. With thistechnique, harmonic frequencies are used to pro-duce the ultrasound image [15]. Because theseharmonic frequencies are amplified rather than at-tenuated, penetration is improved; contrast resolu-tion is increased, and artifacts such as side-lobe andreverberation are reduced [16]. Tissue harmonicimaging may improve visibility of structures, espe-cially deeper structures such as the subscapularisin the shoulder.

Musculoskeletal applications

The most common application in musculoskeletalultrasound is evaluation of tendon and muscle ab-normalities. Ultrasound is effective in showing nor-mal tendon, diagnosing tendinosis and tendon tear,and diagnosing muscle tears [9]. Calcific tendinitisalso is demonstrated effectively with ultrasound,and ultrasound can be used to guide percutaneousaspiration of the calcification [17]. The rotatorcuff is the most common site of tendon evaluationwith ultrasound.

aging of the posterior tibial artery (arrow) with colorote adjacent veins.

Fig. 13. Extended field-of-view imaging. Longitudinalimaging of the forearm with extended field-of-viewshows the brachioradialis muscle (arrows). H,humerus.

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Another common application is the evaluationfor soft tissue infection, such as abscess and celluli-tis, and to diagnose joint effusions [18]. Ultrasoundalso may be used to percutaneously guide aspira-tion of any suspected abscess or infected jointeffusion. In the setting of rheumatoid arthritis,ultrasound may detect and quantify synovitis [19].

Because the resolution of ultrasound is greaterthan routine MR imaging, ultrasound is beingused to evaluate peripheral nerves for entrapmentdisorders, nerve trauma, and peripheral nervesheath tumors [20]. An entire extremity can be eval-uated in less time than what it would take with MRimaging, and direct correlation with symptoms andcontralateral comparison are both possible with ul-trasound. Dynamic imaging effectively diagnosesulnar nerve dislocation at the elbow, and relatedsnapping triceps syndrome [21].

The most common ligaments evaluated with ul-trasound are the ulnar collateral ligament of thefirst metacarpophalangeal joint for Gamekeeper’sthumb [22] and the ulnar collateral ligament ofthe elbow. In this latter situation, dynamic imagingcan evaluate for joint space widening during valgusstress across the elbow joint [23].

There are many other miscellaneous conditionsevaluated with ultrasound, such as bursitis, frac-tures, cysts, and masses. Dynamic imaging is helpfulfor diagnosing muscle hernias, often only presentduring muscle contraction [24]. The high-resolutioncapabilities of ultrasound also make it successful inthe evaluation of soft tissue foreign bodies.

Summary

The first step in effective use of musculoskeletal ul-trasound is proper transducer selection and techni-cal adjustments to optimize resolution. Scanningtechnique is also important so that small structuresand subtle pathology can be recognized. Normalstructures have characteristic appearances at ultra-sound. Knowledge of common ultrasound artifactshelps to identify many pathologic conditions.Specialized ultrasound techniques, such as beamsteering, extended field-of-view, color and powerDoppler imaging, and harmonic imaging can assist

diagnosing and characterizing many pathologicconditions.

References

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[17] del Cura JL, Torre I, Zabala R, et al. Sonographi-cally guided percutaneous needle lavage incalcific tendinitis of the shoulder: short- andlong-term results. AJR Am J Roentgenol 2007;189:W128.

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