Applied Radiology

Although ultrasonography (US) has gained acceptance in differentiatingbetween cystic and solid masses in various locations in the body, theavailability of MRI has shifted clinicians' interest in the United Statestoward that modality to evaluate the soft-tissue components of themusculoskeletal system. However, the unique real-time capability of US, whichpermits examination during movement and allows guidance of biopsy needles,combined with the outstanding resolution of state-of-the-art, high-frequencytransducers and advances in color Doppler imaging, makes US a powerful tool forevaluating soft-tissue masses, from the skin to the surface of the bones. 1-4Although a multimodality approach involving conventional radiography, CT,and/or MRI is often needed for further characterization of soft-tissue masses,in some circumstances, US in combination with physical examination andUS-guided needle biopsy can provide a cost-effective shortcut to a finaldiagnosis. This article will review the advantages of sonography in theevaluation of soft-tissue masses.

Dr. Fornage is with the Department of DiagnosticRadiology,The University of Texas M. D. Anderson Cancer Center, Houston, TX. Heis also a member of the Editorial Board of this journal.

Technical considerations

Instrumentation -- High-frequency (7- to 15-MHz)transducers are needed for the evaluation of very superficial masses. However,such transducers usually have a narrow (about 4 cm wide) field of view andlimited penetration. Therefore, probes of lower frequency (5 MHz or even 3.5MHz) are required whenever a wider field of view and greater ultrasound beampenetration are needed, e.g., to visualize a deeply located lesion or toencompass a large mass. Dedicated scanners for the examination of the skin thatemploy 20-MHz transducers can be used to evaluate the subcutaneous tissues(e.g., the median nerve in the carpal tunnel or flexor tendons of the fingers)at a depth of up to 1 cm. Recently, new software (SieScape ® , Siemens,Issaquah, WA) that extends the field of view up to 60 cm has become available,offering the long-forgotten wider field of view reminiscent of the early daysof B-mode compound scanning; this wide field of view is important inmusculoskeletal US (figures 1 and 2). 5 In addition, three-dimensional (3D) USis becoming increasingly available and popular and there is no doubt thatmusculoskeletal US will benefit from 3D rendering and reconstructed sonogramsalong planes that would not be available otherwise. Promising new techniquesdestined to improve the resolution of gray-scale US include tissue harmonicimaging and compound electronic scanning. Elastography, which provideselasticity mapping of the soft tissues as well as conventional B-modesonographic images, is still in the research stage. 6 Assessment of the degreeof vascularity of a mass with color and power Doppler imaging and analysis offlow patterns are routine steps in the US evaluation of any mass, and masses inthe musculoskeletal system are no exception. Refinements in the resolution andsensitivity of Doppler equipment combined with 3D imaging, injection ofintravenous ultrasound contrast agents, and flow mapping quantificationtechniques are expected to enhance the display of the vascularization of agiven mass, which should enable more accurate differentiation between benignand malignant masses.

Technique of examination

The US examination should begin with a detailed history and a limitedphysical examination targeted to the symptomatic region. The standard US studyincludes longitudinal and transverse sonograms and measurement of the threelargest perpendicular diameters of lesions, to allow calculation of theirvolume. A reference for normal anatomy is readily provided by examining thecontralateral anatomic region, assuming that the pathologic process is notbilateral. An accurate correlation between the palpation and sonographicfindings can be achieved with the use of a standoff pad, by sliding the fingersof the palpating hand between the pad and the skin while maintaining thetransducer over the region of interest with the other hand. 1

Dynamic maneuvers, such as flexion or extension of the adjacent muscles, mayalso yield valuable information about the relationship between the mass andadjacent structures, and thereby identify the anatomic structure from which itis derived.

US-guided interventional procedures- Because of itsresolution and real-time capability, US is the best imaging modality to guideinterventional procedures involving any soft-tissue lesion that can bedemonstrated by US. 2 Fluid collections can be aspirated with fine (20- or22-gauge) needles, although the presence of thick material such as pus mayrequire the use of a larger needle. Percutaneous catheter drainage of abscessesis readily performed with US guidance, 7 as is diagnostic aspiration of smalljoint effusions. US-guided intraarticular injection of contrast medium or drugssuch as steroids or antibiotics is also feasible and can be done with greataccuracy and safety. This has proved to be particularly valuable in themanagement of hip effusions in children 8-10 and tenosynovitis. US-guidedaspiration of ganglion cysts with injection of corticosteroids and localanesthetic has been reported, although the long-term efficacy of this treatmentremains to be determined. 11 Aspiration of calcific deposits in the rotatorcuff under US guidance has also been reported with satisfactory results. 12

A tissue diagnosis of solid soft-tissue tumors, particularly soft-tissuesarcomas, is best achieved with the use of large-core needles and automaticbiopsy devices. Various automatic "biopsy guns" are available,including those with Tru-Cut-type and full-cylinder cutting needles (figure 3).To obtain adequate cores, cutting needles of 14 to 18 gauge should be used.Some devices offer an adjustable needle throw; the longer the throw, the betterthe core. Several cores are usually obtained in different areas of the tumor.For masses that contain areas of necrosis, US is used to guide the needle intothe most solid-appearing areas. Color Doppler imaging helps identify and avoidlarge vessels in the tumor or in the planned pathway of the biopsy needle. Incontrast to the diagnosis of a primary sarcoma, the diagnosis of a localrecurrence of a soft-tissue sarcoma does not require core biopsy and is easilyestablished through fine-needle aspiration.

If needed, US can be used to localize nonpalpable masses (e.g., recurrencesof soft-tissue sarcomas) either preoperatively or even intraoperatively, and exvivo US examination of the fresh specimen can confirm the successful excisionof a lesion.

Nontumoral masses

US can diagnose a number of conditions that present as a soft-tissue massbut that are not true neoplasms. Such conditions may result from trauma,inflammatory processes, or cystic changes.

Trauma-related masses-- These masses includehematomas, muscular ruptures, muscular hernias, fibrous scars, myositisossificans, rhabdomyolysis, and inflammatory reactions around retained foreignbodies. 4, 13

The sonographic appearances of soft-tissue hematomas vary considerably anddepend on the tissue in which the hematoma develops and the hematoma'sage. Fresh hematomas in the subcutaneous fat usually appear as areas ofincreased echogenicity. Organized hematomas may appear as complex masses withechogenic clots, and chronic hematomas may be completely anechoic.

Major muscular tears can result in palpable masses. This is rarely adiagnostic problem because the clinical context and the physical examinationare usually straight-forward. At the site of the tear, the discontinuity in theechogenic striae representing the perimysium is the direct sonographic sign ofa tear, while an indirect sign of rupture is the presence of an associatedhematoma, whose echogenicity may sometimes be misleading. 14 The "clapper-in-the-bell" sign on sonograms consists of a large retracted rupturedmuscle "floating" in the surrounding hematoma. Another classicsono-graphic appearance of muscular tear is the "tennis leg," whichis characterized by the detachment of the distal fibers of the gastrocnemiusmedialis from the common aponeurosis with the soleus muscle (figure 4).

Muscle hernias bulge through a weakened or ruptured fascia or aponeurosis.Real-time US under contraction of the involved muscle readily demonstrates thatthe palpable mass is actually made of normal muscle tissue and that no furtherimaging is needed.

Myositis ossificans may complicate a hematoma developed after a trauma.Changes in the soft tissues are visible on the sonograms before calcificationcan be seen on plain films. 15-17 The calcification is usually visible onradiographs starting about 4 weeks after the trauma. Large characteristiccalcifications appear on sonograms as bright linear echoes with acousticshadowing. The presence of sheets of echogenic material representing lamellarcalcification has also been described as an early sonographic finding inmyositis ossificans. Heterotopic ossification can also develop in a surgicalscar.

Rhabdomyolysis, the necrosis of muscle fibers with the release of muscularenzymes and myoglobin into the bloodstream may develop when muscle fiberssuffer from severe ischemia. The best-known causes of hypoxia/ anoxia ofmuscles are crush syndrome, drug toxicity, drug abuse, and, in athletes,exertion such as observed in marathon runners. Rhabdomyolysis may be unifocalor multifocal. Sono-graphically, early lesions appear as patchy areas of mixedechogenicity, usually with ill-defined margins. 18 Later, focal areas ofnecrosis may appear as well-defined hypoechoic lesions or even fluid-filledcollections. However, MRI seems to be more sensitive and accurate than US indiagnosing this condition. 19

Retained foreign bodies- usually fragments of wood, glass, ormetal- are often found in the extremities, particularly the hand and foot.US has the significant advantage over plain radiographs of visualizingradiotransparent foreign bodies such as wooden splinters (figure 5). 20 Allforeign bodies appear as echogenic foci. Depending on their nature, acousticshadowing, comettail artifact (reverberation echoes), both, or neither willalso be present. 21 A surrounding ill-defined hypoechoic area suggests thepresence of inflammatory changes, and a fluid collection suggests abscessformation. In addition to confirming the presence of the foreign body, US is aninvaluable tool for its preoperative (or intraoperative) 3D localization,resulting in shorter operative time. 22-25

Inflammatory masses-- It is sometimes difficult forthe clinician to distinguish between cellulitis and abscess based solely on thephysical examination. In cellulitis, US shows thickening and increasedechogenicity of the subcutaneous fat with obliteration of the interface betweenthe echogenic fat and dermis. Color (power) Doppler imaging often shows diffusehypervascularity throughout the area of inflammation. 26 In contrast, abscessesin soft tissues are anechoic or complex, uni- or multiloculated masses withthick, irregular wall (figure 6). When present, a gas collection is easilyidentified through the associated "ring-down" artifact. Color Dopplerstudy demonstrates increased vascularity around and in the wall of the mass.

Muscular abscesses and pyomyositis are common in tropical climates but canalso develop in diabetics and in immunosuppressed patients, e.g., leukemicpatients receiving chemotherapy. 27-29 These muscular infections deserveimmediate attention because they can extend rapidly. Sonographically, there isfocal hypoechoic swelling of a muscle that rapidly transforms into a hypoechoicor complex abscess mass with echogenic debris, pus, and occasionally gas(figure 7). Adjacent bones should be examined to rule out osteomyelitis. 30 USis the best technique with which to guide percutaneous diagnostic aspirationand drainage of soft-tissue abscesses. 7

On rare occasions, a palpable mass can result from tendonitis of asuperficial tendon, such as the patellar or Achilles tendon. US readilyconfirms that the palpable mass is in fact the swollen, hypoechoic, inflamedtendon. 31 Intratendinous calcifications are common in chronic tendonitis.

More frequently, a superficial mass in the distal extremities will be theresult of tenosynovitis. US will easily confirm that the mass is indeed a fluidcollection in the tendon sheath in cases of acute tenosynovitis, whereas incases of chronic tenosynovitis, US will show a prominent, hypoechoic thickeningof the synovium, with or without fluid. 32-34 Power Doppler imaging maydemonstrate some associated hypervascularity in the sheath. 35 In a patientwith rheumatoid arthritis, any new lump along the course of the tendons in thedistal extremities should be viewed as a probable area of rheumatoidtenosynovitis. US will show the markedly hypoechoic pannus, sometimes withhypervascularity on color Doppler imaging and may show early signs of partialtendon rupture. 36 A recently developed, soft, tender mass in the elbow, knee,or heel may result from bursitis. In chronic bursitis, the echogenicity of thebursa often becomes mixed, with echogenic debris and occasionallycalcifications. Color Doppler imaging will show increased vascularity in thebursa's thickened wall (Figure 8). The most commonly affected bursae arethe subdeltoid, olecranal, patellar, and calcaneal bursae.

If needed, evaluation of the response of the above listed inflammatoryconditions to conservative treatment can be done cost-effectively with US.

Cysts-- Historically, US has been the best modalityto confirm that a mass is cystic. In fact, one of the first applications ofB-mode US was the diagnosis of Baker's cysts. 37

Baker's cysts: Popliteal cysts, also known as Baker's cysts, aredue to an abnormal distention of the gastrocnemio- semimembranosus bursa, whichfrequently communicates with the knee joint through a slitshaped opening at theposteromedial aspect of the joint capsule. Rheumatoid arthritis is a commonpredisposing factor. 38 Small popliteal cysts may be asymptomatic andnonpalpable, while ruptured cysts are well known for causing sharp pain andswelling of the calf that raise concerns about deep venous thrombosis.

A typical popliteal cyst appears on sonograms as a well-defined, anechoic,fluid-filled collection. 4 Large cysts extend downward, dissecting into thecalf. On transverse scans, Baker's cysts are seen to surround the medialaspect of the origin of the gastrocnemius medialis muscle in a horseshoe manner(figure 9). Internal echoes representing fibrinous strands or debris andsynovial thickening are usually seen in inflamed or infected cysts, althoughsome echoes can also be present in uncomplicated cysts. In patients withrheumatoid arthritis, popliteal cysts may be completely filled with hypoechoicpannus and mimic solid masses. 39

A ruptured Baker's cyst may be difficult to diagnose as most of thefluid has already leaked into the calf and resorbed, leaving only anill-defined hypoechoic residual area. US can still show subcutaneous fluiddissecting distally into the lower calf, often as low as the ankle (figure 10).2 Rarely, a fluid collection has accumulated within the calf, and it isessential to demonstrate the connection between that mass and the residualamount of fluid in the popliteal fossa.

Ganglion cysts: Ganglion cysts usually arise from the wrist joint, withwhich they communicate; they also are found adjacent to tendon sheaths in thehand and foot. 40-42 Ganglion cysts are generally sonolucent, with thininternal septa being present on occasion (figure 11). Low-level internal echoescan be seen in chronic or inflamed ganglion cysts.

Meniscal cysts: Meniscal cysts arise from the menisci (more often from thelateral one) and are seen in the periarticular soft tissues of the knee; theirpathophysiology remains controversial. Meniscal cysts often have a hypoechoicappearance, probably because of the combination of loculation and septa and thelack of a large amount of fluid. The key finding is the connection between themass and the lateral wall of the meniscus. 43,44 Although US can often suggestthe diagnosis, MRI is by far superior to US in demonstrating the frequentlyassociated meniscal lesions.

Hydatid cysts: In countries where hydatid cysts are endemic, hydatid cystsmay be found in the soft tissues of the extremities. 45 They have the same widespectrum of sonographic appearances as do visceral hydatid cysts, ranging fromthe classic multivesicular pattern to the misleading predominantly solidpattern, which can mimic a soft-tissue tumor.

Synovial proliferations-- Whereas joint effusions arerarely a diagnostic difficulty for sonography, 46 proliferative diseases of thesynovium that present clinically as soft-tissue masses can be misleading; thesediseases include osteochondromatosis, pigmented villonodular synovitis,rheumatoid arthritis, and changes associated with hemophilic arthritis. 47-49On sonograms, these lesions appear as hypoechoic masses. The possibility of asynovial mass should be included in the differential diagnosis whenever asoft-tissue mass is located adjacent to a joint space.

Miscellaneous masses-- Subcutaneous rheumatoidnodules, characterized by a central area of fibrinoid necrosis, occur in 20% ofpatients with rheumatoid arthritis. Rheumatoid nodules in soft tissues appearas elongated hypoechoic masses. 36,50 They can also be found within thetendons, especially in the distal extremities. 25,36

In patients with hypercholesterolemia, US demonstrates the intratendinousxanthomas as hypoechoic masses and it is an ideal modality with which tomonitor the effect of therapy on the Achilles tendon's thickness andechotexture. 51,52 In patients with gout, intratendinous tophi appear markedlyechogenic with acoustic shadowing. Giant cell tumors of tendon sheathsrepresent a circumscribed form of tenosynovitis related to pigmentedvillonodular synovitis. They involve preferentially the flexor surfaces of thefingers in middle- aged women. Sonographically, these tumors appear ashypoechoic, sometimes lobulated masses. 24

In palmar fibromatosis (Dupuytren's contracture), US shows a hypoechoicmass in the subcutaneous tissues of the palm, associated with skin retractionand usually located immediately over the course of the flexor tendons of thefourth and/or fifth fingers. Real-time examination shows that the tendons slidesmoothly in their sheaths, at least in the early stages of the disease. Plantarfibromatosis, the counterpart of palmar fibromatosis in the foot, also appearsas an ill-defined, elongated, hypoechoic mass in the subcutaneous tissues,superficial to the echogenic plantar fascia. 53 Color Doppler imaging may showsignificant vascularity.

Soft-tissue tumors must be evaluated for their number, location, shape,size, margin regularity, echogenicity, echotexture, presence of diagnosticartifacts (e.g., shadowing or sound through-transmission), vascularity on colorDoppler mapping, and deformability during contraction/relaxation of themuscle(s) involved and application of pressure with the transducer. US isaccurate in the detection of soft-tissue tumors, with a very high negativepredictive value; 54,55 but sonographic characterization of solid massesremains limited, and US-guided needle biopsy is often needed.

Benign tumors-- Lipomas and hemangiomas are the mostcommon benign tumors found in the superficial soft tissues.

Lipomas: On sonograms, lipomas are elongated, with their greatest diameterparallel to the skin and an average length/anteroposterior diameter ratio ofabout 3:1. 56 In a study of subcutaneous lipomas, two thirds showed ahomogeneous echotexture. Sixty percent were well defined, with the remaindershowing ill-defined margins blending into the surrounding tissues. Twenty-ninepercent of the lipomas were hyperechoic (figure 12), 22% were isoechoic, 29%were hypoechoic, and 20% showed a mixed pattern. 56 Not rarely, lipomasdemonstrate internal linear echoes oriented along the lesion's longestaxis, giving a striated appearance. When an elongated isoechoic or hyperechoicmass is seen in the subcutaneous tissues, a lipoma should be considered,especially if the lesion is soft and compressible. Low-kilovoltage radiographsor CT scans can be used to confirm the radiolucent fatty tumor, although thesetechniques will not delineate the radiolucent lipoma if it is small,nonencapsulated, and embedded in the subcutaneous fat.

Hemangiomas: Sonographically, hemangiomas range from markedly hypoechoic tohyperechoic and from homogeneous to multiloculated. 57 Intramuscularhemangiomas may be circumscribed or infiltrate into the adjacent muscle, inwhich case they may be very difficult to delineate. A clue to the diagnosis isthe demonstration of phleboliths, which typically appear as echogenic foci withacoustic shadowing. 58 Another finding that is very suggestive of hemangioma isthe easy and marked compressibility of the lesion, with disappearance of itshypoechoic components, which represent venous lakes inside the lesion. MRI issuperior to US in demonstrating the extent of hemangiomas that are poorlydefined, especially the infiltrating intramuscular hemangiomas. Color Dopplerfindings in hemangiomas vary greatly, and although high vessel density has beenreported in hemangiomas in children, 59 not uncommonly are no Doppler signalsdetectable within a lesion.

Nerve sheath tumors-- The largest peripheral nervesin the extremities appear sonographically as echogenic tubular structures whenscanned at 7.5 or 10 MHz. On longitudinal sonograms, the echotexture isfibrillar, similar to that of tendons, albeit slightly less echogenic. 60,61 Ontransverse scans, nerves exhibit a round or oval cross section with a coarselydotted pattern. When a very-high-frequency (13-MHz or higher) transducer isused, the echogenic septa separating the hypoechoic bundles of nerve fibersappear finer than with a 7.5-MHz transducer, and the hypoechoic background(composed of the nerve fibers) becomes more conspicuous.

The diagnosis of a nerve sheath tumor can be made with confidence when ahypoechoic soft-tissue mass is seen to connect at both ends to a normal nerve(figure 13). Schwannomas are usually better circumscribed than areneurofibromas, tend to be eccentric in relation to the nerve axis, may containcharacteristic internal cystic cavities, and show good soundthrough-transmission. 62 Neurofibromas, on the other hand, are often elongatedalong the nerve axis and lobulated. However, there is a significant overlapbetween the sonographic appearances of these two types of nerve sheath tumors.Dynamic examination during active or passive flexion/extension maneuversconfirms the lack of (or very limited) longitudinal mobility of a nerve sheathtumor compared to the adjacent muscles and tendons. The only significantmobility of these tumors is in the transverse direction. Color Doppler imagingoften shows increased internal vascularity.

US-guided needle biopsy of nerve sheath tumors is sometimes attempted. Theinsertion of the needle into the tumor may trigger a sharp, excruciating pain,which requires the procedure to be halted but indirectly confirms the neuralorigin of the tumor.

Other nerve-related masses that can be visualized by US include traumaticneuromas (including stump neuromas), neurilemmitis and perineural abscesses inleprosy, 63 intraneural ganglia, and Morton's neuromas. 64-66

Other benign soft-tissue tumors-- Intramuscularmyxomas are rare benign mesenchymal neoplasms that appear on sonograms aswell-demarcated, markedly hypoechoic masses that may contain fluid-filledclefts or cystic areas. 67 Desmoid tumors are fibromatous lesions arising frommuscular aponeuroses and often appear on sonograms as ill-defined, hypoechoicmasses. Large desmoid tumors are often associated with significant shadowingthat may obscure them completely; MRI is the modality of choice in such cases.US is used to confirm and localize nonpalpable recurrences of desmoid tumorspreoperatively. Lymphangiomas are often poorly defined on sonograms, and MRIgives better results than US. Granular cell tumors are firm, small, benigntumors that can be found virtually anywhere in the soft tissues. On sonograms,they are hypoechoic, with irregular margins, and some are associated withmarked acoustic shadowing suggesting malignancy. Glomus tumors have beenreported in various locations in the body but are commonly found in thefingers. Glomus tumors are well-circumscribed and markedly hypoechoic. US isvery helpful in the diagnosis and preoperative localization of these elusivelesions. 68

Malignant tumors-- The vast majority of primarymalignant tumors that develop in soft tissues are soft-tissue sarcomas.

Soft-tissue sarcomas: Soft-tissue sarcomas develop preferentially in theextremities. In adults, the two most frequent subtypes are malignant fibroushistiocytoma and liposarcoma. Soft-tissue sarcomas usually metastasizehematogenously to the lungs; lymphatic spread is extremely rare.

Soft-tissue sarcomas are hypoechoic, often relatively well-circumscribed,lobulated masses; the exception is some liposarcomas, which are echogenic.Areas of necrosis and calcifications can be seen in sarcomas. Theextended-field-of-view technology is useful to encompass and measure very largelesions (figure 14). Increased, chaotic vascularity is often visible on colorDoppler imaging, although some tumors may remain totally flow-negative, evenwhen examined with the most sensitive power Doppler equipment.

Because US cannot deliver global and reproducible pictures in a systematic,operator-independent manner, it cannot compete with MRI for preoperativestaging of soft-tissue sarcomas.

Local recurrences of soft-tissue sarcoma are quite common after surgicalexcision of the primary tumor. Recurrences usually appear as small, round oroval, hypoechoic masses. 69,70 US has been reported to be as accurate as MRI inthe detection and diagnosis of such recurrences. 70 The use of color Dopplerimaging can assist in differentiating a postoperative collection containinglow-level echoes (with no vessels) from a markedly hypoechoic, solid recurrentor residual tumor; the presence of even the faintest internal vascularity rulesout the former diagnosis and boosts the level of suspicion for the latter one(figure 15). 71 A significant advantage of US over MRI is the capability ofguiding needle biopsy of any small suspicious mass. When a nonpalpablerecurrence has been diagnosed, US can also be used effectively for thepreoperative or even intraoperative localization of the lesion. 72

Other malignancies: Metastases to soft tissues from primary malignanciesother than melanoma are rare and usually derive from carcinomas of the lung, ofthe gastrointestinal tract, and from renal cell carcinomas (figure 16). Whensmall, soft-tissue metastases tend to be round. Calcifications are often seenin metastases from primary tumors of the gastrointestinal tract. Metastasesfrom cutaneous melanoma to the subcutaneous tissues are quite common. Suchmetastases are markedly hypoechoic on gray-scale US and typically hypervascularon color Doppler imaging (figures 17 and 18). US-guided fine-needle aspirationbiopsy readily yields cytologic confirmation. 73

In a patient with known leukemia or lymphoma, any new focal hypoechoic massin the soft tissues should undergo fine-needle aspiration. In lymphoma, thelesions are hypervascular with a characteristic absence of distortion of thevessels.

Skin masses

US of the skin is rarely performed by radiologists. With the use ofcommercially available dedicated scanners operating at 20 or 30 MHz, skinlesions as thin as 0.2 mm can be demonstrated. Although very-high-frequency UScannot differentiate benign from malignant skin masses, 74 it has proved usefulin measuring accurately and noninvasively the maximum thickness of lesions,which is the single most important prognostic factor in patients with melanoma(figure 19).

Bone-related masses

Sometimes, a mass thought to have arisen from the soft tissues is in fact abone lesion, and US may be the first imaging test to detect a bone lesion thatdeforms or interrupts the surface of the bony cortex (figures 20 and 21).Occasionally, bone fractures are visualized by US. This is advantageous fordiagnosing fractures involving nonossified epiphyses in children 75 and fordiagnosing fractures in adults in areas difficult to assess with plainradiographs, such as the shoulder and the ribs (figure 22). 76,77

An advantage of US over CT and MRI in assessing bone-related masses is thatUS can be performed even in the presence of metallic orthopedic fixationdevices. Periprosthetic fluid collections are easily demonstrated with US, 78and US can even visualize migrated fixation devices. 79 In bone tumors, US canvisualize both the soft-tissue component and the hypoechoic intraosseouscomponent if the overlying bone cortex is destroyed (figures 20 and 21). 80,81US-guided needle biopsy of the tumor can then be performed easily. 82-85

Conclusion

Provided the examination is done by a well-trained operator using state-of-the-art equipment, US delivers valuable information in the evaluation ofsoft-tissue masses:

• the presence of a true lesion can be confirmed or ruled out with a highnegative predictive value;
• US can show whether the mass is cystic or solid;
• US can guide interventional procedures, such as percutaneous needle biopsyor drainage;
• US localization of nonpalpable masses can be done pre- or intraoperatively;
• lesions that are treated conservatively can be monitored with US;
• US can be used to follow up patients with a history of soft-tissue sarcomasand detect recurrences early.

Limitations of musculoskeletal US include the incomplete evaluation ofjoints and areas of complex anatomy, the inability to stage tumorspreoperatively, its operator dependence, the restricted field of view ofsonograms, and the fact that many clinicians are uncomfortable with sonograms.However, in many situations involving soft tissues-especially the mostsuperficial tissues, which benefit from the remarkable resolution ofvery-high-frequency transducers-US can provide information similar to thatobtained with MRI at a fraction of the cost. For superficial soft-tissuemasses, US should be used as the first-line examination technique, with MRIbeing reserved for use as a problem-solving tool when US is inconclusive.

REFERENCES

1. Fornage BD: Ultrasonography of Muscles and Tendons. ExaminationTechnique and Atlas of Normal Anatomy of the Extremities. New York,Springer-Verlag, 1988.

2. Fornage BD: Ultrasound of the Extremities [in French]. Paris,Vigot, 1991.