Ultrasonography is a relatively underused modality in evaluating small animal musculoskeletal tissues but with some practice it can provide a lot of useful information.

Equipment

High frequency linear array transducers (7.5-13 MHz) are essential to identify subtle changes, particularly in more superficial structures. It is preferable to clip the affected area and use large amounts of acoustic coupling gel. Standoff pads may be used to ensure better transducer contact with curved structures or to get the pathology within the focal zone of the transducer.

Examination technique

Always try to orientate the transducer perpendicular to structures being imaged to avoid artificially creating hypoechoic areas. Always image in 2 planes as pathology must be demonstrated in both. Dynamic studies involve motion of the structures under examination to look for adhesions or the ability to bring ruptured end points together. Smaller intramuscular or tendinous lesions may be better defined under tension. Avoid too much transducer pressure which may obliterate small lesions and veins. Apply compression to distinguish masses from an abscess or haematoma. Cellular fluids will swirl on ballottement. Use colour and power Doppler to assess vascularity of structures: an abscess will be avascular centrally whereas a malignant tumour will have blood flow; active inflammation will have blood flow present and if it surrounds a calcified area it is likely to be clinically significant calcification. Always examine the unaffected contralateral side for comparison.

Applicable ultrasonographic artifacts

Off incidence artifact (anisotropy)--an incident beam more than 3 degrees off the perpendicular results in the reflected beam not returning to the transducer making structures under examination more hypoechoic and potentially being interpreted as pathology.

Acoustic enhancement--non-cellular fluids attenuate less sound than surrounding structures resulting in a brighter image beyond the structure. Free fluid typically results in distal acoustic enhancement. Oedematous tissues have a similar but less marked effect.

Edge shadowing--anechoic lines deep to edges of structures are due to refraction of sound beam and anisotropy and is indicative of a tissue with a different composition to the surrounding structures.

Acoustic shadowing-- anechoic area deep to hyperechoic structure is seen with wood, bone and calcification due to absorption of all the sound by these tissues. Gas will result in reverberation artifact or dirty shadowing.

Comet tail--hyperechoic band seen distally to metal and glass.

Normal structures

Skin--laminar hyperechoic appearance and may be up to 3 mm thick.

Muscle--fibres are surrounded by endomysium, bundles by perimysium, muscle belly by epimysium and groups of muscles by fascia. The muscle fibres are hypoechoic with the surrounding structures being hyperechoic resulting in a striated appearance in longitudinal planes and speckled appearance in cross section.

Adipose tissue--similar to muscle but less striated and has no blood flow. Located mainly subcutaneously.

Tendon--striated alternating hypo- and hyperechoic structures with a thin hyperechoic peritenon. Where muscle goes over into tendon hypoechoic areas are seen. Insertion into bone may be slightly hypoechoic due to the presence of fibrocartilage. A small amount of anechoic fluid may be seen surrounding the tendon where tendon sheaths are present.

Ligaments--have a similar appearance to tendons and only the patellar tendon can be accurately evaluated as the remainder are too small and close to the skin.

Nerves--larger nerves are visible with high resolution transducers. On longitudinal imaging they have a feathered appearance and on cross section are more speckled due to the hypoechoic nerve being surrounded by more hyperechoic fat and fascia. Diminish slowly in size. Often associated with vessels.

Vessels--anechoic with veins being easily compressible and often having visible motile speckle resulting from the slow blood flow.

Bone--only the hyperechoic interface is seen and should be a smooth uninterrupted line.

Cartilage--In joints hyaline cartilage can be seen as an anechoic layer (high water content) with equal thickness superficial to the hyperechoic subchondral bone.

Menisci--homogenous triangular echogenic fibrocartilaginous structures within the stifle.

Pathology

Seroma--anechoic with some more echogenic septation. Often seen at surgical sites.

Cellulitis--multiple tissue layers may be swollen and oedematous with a hypoechoic pattern of "marbleisation".

Abscess--speckled echogenic appearance with possible hyperechoic capsule. May contain gas speckles. Is compressible and has no blood flow within it.

Foreign bodies--size and location can be determined. The foreign body is often surrounded by some anechoic fluid and then echogenic granulation tissue. Glass, wood splinters and thorns will not be seen with survey radiographs. Glass and metal result in comet tail artifacts and wood in acoustic shadowing.

Haematoma--in the early stages is very similar to an unencapsulated abscess and requires aspiration to determine cell content. With time red blood cells clot and serum separates into anechoic fluid structures.

Cyst-- anechoic thin walled structure with acoustic enhancement and edge shadowing.

Muscle atrophy-- increased echogenicity due to fatty infiltration and decreased diameter.

Muscle strain--Grade 1 has normal architecture but is more hypoechoic due to oedema. Perifascial fluid possible. Grade 2 has interrupted fibres and haematoma formation--best to image after 2-3 days when haematoma is more anechoic. Grade 3 is complete rupture with no normal muscle visible and separated ends often surrounded by anechoic fluid.

Crush injuries-- irregular muscle borders with several adjacent muscles involved. May result in myositis ossificans.

Acute compartment syndrome--elevated interstitial pressures due to muscle volume increase causing bulging fascial planes. Capillary perfusion is compromised especially centrally where the muscle is more hyperechoic.

Muscle masses--lipoma is the most common tumour and appears as a mass of fat with a very thin capsule. Liposarcoma tends to be more echogenic and infiltrative. Malignant tumours are usually more lobulated, may be centrally necrotic and will have 3 or more feeding vessels on Doppler. Benign lesions have poor circulation.

Full thickness tendon tear--absence of tendon with naked insertion point and possible avulsion fragments. Anechoic fluid may surround retracted tendon end which may also have edge shadowing.

Tendinitis (partial tendon tear)--anechoic or hypoechoic clefts within tendon with irregular borders and increased size. Tender on pressure and may have visible blood flow in inflammatory stage.

Tenosynovitis--tendon sheath effusion is seen. The sheath may become thickened and the fluid may contain echogenic debris. On cross section the tendon within the distended sheath appears as a target lesion.

Calcific tendinitis-- hyperechoic area with acoustic shadowing. If inflamed, have oedema and increased flow on Doppler. May be associated with tenosynovitis.

Bursitis--usually due to chronic trauma. May be anechoic or more echogenic.

Fractures--A break in the continuity of the cortex is readily apparent and is very useful in the skull where fractures are often not seen on survey radiographs. Underlying brain trauma may also be evaluated. Fracture healing and its complications may also be evaluated. Stress fractures, although rare in dogs, may show hyperechoic periosteal thickening with hypoechoic haemorrhage and cortical disruption.

Bone neoplasia-- periosteal reactions, cortical defects, tumour invasion of surrounding tissues may all be seen. Biopsies or fine needle aspirates can be guided ultrasonographically into affected tissues for sampling.

Bone cysts--cortex may be thin allowing visibility of internal anechoic content. Aneurismal bone cysts will show turbulence of blood flow and positive colour Doppler findings.

Osteomyelitis--in acute cases changes may be seen before becoming radiologically visible. A thin anechoic line adjacent to a slightly irregular cortex may be seen.

Nerve pathology--pressure from surrounding masses, synovitis or effusions may result in neuritis. Neural sheath tumours tend to be hypoechoic and the nerve can be seen entering and leaving the mass. May have associated muscle atrophy. Brachial plexus tumour is the most common condition diagnosed ultrasonographically.

Joint effusions--cannot distinguish haemarthrosis from a septic joint--both have a speckled echogenic appearance and need aspiration to confirm the aetiology. Proliferating synovium or synovitis may also be difficult to distinguish from the above but is non-compressible and will have active blood flow on Doppler.

Arthrosis--anechoic cartilage may show thinning and irregular osteophytes may be seen.

Specific structure pathology

Biceps region--Tendinitis and tenosynovitis may be seen. Tendon absence indicates avulsion or tendon luxation. Sheath distention and joint mice may be observed.

Supra- and infraspinatous tendons--calcific tendinitis often occurs here.

Stifle joint --cranial cruciate and patellar ligament as well as meniscal pathology may be evaluated.

Calcaneal tendon-- complete tear has retracted rounded ends with a haematoma (scars in chronic) of fat interposed. Dynamic evaluation may be done to evaluate the gap size.

Iliopsoas muscle and tendon-- hyperextension may result in lameness. The muscle is evaluated transabdominally and the tendon of insertion at the trochanter minor.

References

1.  Samii V F and Long GD. Musculoskeletal system. In: Nyland T G, Mattoon J S(eds):Small Animal Diagnostic Ultrasound. 2^nd Ed. W B Saunders Company, 2002; 267-284.

2.  Kirberger R M. Imaging artifacts in diagnostic ultrasound -A review. Veterinary Radiology & Ultrasound 1995;36:297-306.