Musculoskeletal (MSK) ultrasound has grown rapidly as a specialty, but many general sonographers receive limited MSK training. Whether your department performs dedicated MSK exams or you're occasionally asked to image a tendon or joint effusion, this guide covers the essentials.

Why MSK Ultrasound?

MSK ultrasound advantages over MRI for musculoskeletal imaging:

Real-time dynamic scanning (assess tendons during motion)
Lower cost and no magnet contraindications
Immediate guidance for injections and aspirations
Patient comfort (no enclosed scanner)
Excellent for superficial structures

Limitations: operator-dependent, limited for deep structures and bone marrow, worse for cartilage than MRI.

Equipment

Probe: High-frequency linear, 10–18 MHz for most MSK work. The higher the frequency, the better the resolution for superficial structures (tendons, nerves just under the skin). For larger/deeper muscles or hips, drop to 7–9 MHz.

Key settings:

Focus: Place at the level of the structure of interest. For tendons, this is usually 1–3 cm depth.
Harmonic imaging: Usually off for MSK — at high frequencies, harmonic imaging can reduce axial resolution. Test it in your machine.
Gain: Tendons should look echogenic (bright), not washed out. Muscle should show echogenic fibrillary lines on a hypoechoic background.
Compound imaging / SonoCT: Helps reduce artifact for most applications — keep on.

Normal Tissue Appearances

Tendon: Echogenic, fibrillar (parallel bright lines), anisotropic (see below). Tight bundle of fibers running in one direction.

Muscle: Hypoechoic bulk with echogenic lines (perimysium/septa running obliquely). In cross-section, looks like a "starry night" pattern.

Ligament: Similar to tendon but often shorter, thicker, and connects bone to bone. Echogenic, compact.

Nerve: Fascicular pattern — hypoechoic fascicles within an echogenic epineurium. In cross-section: "honeycomb" appearance.

Bursa: Normally a thin anechoic line (< 2 mm). Inflamed bursae enlarge and fill with fluid.

Cartilage: Anechoic (hyaline cartilage) or echogenic (fibrocartilage like menisci). Hyaline cartilage lines joint surfaces as a thin anechoic band overlying the echogenic bone cortex.

Bone cortex: Hyperechoic line with posterior shadowing. Normal cortex is smooth and continuous.

Anisotropy — The Critical MSK Pitfall

Anisotropy is the most important MSK ultrasound artifact to understand. Tendons appear bright (echogenic) only when the sound beam hits them at 90°. When the beam strikes at any other angle, the tendon appears hypoechoic (dark) — mimicking pathology.

How to avoid false positives:

Keep the probe exactly perpendicular to the tendon surface at all times
Heel-toe the probe to adjust the angle as the tendon curves
If you see a dark area in a tendon, change your angle first before calling it pathology

This is why scanning a tendon at the heel of the foot, where it curves, requires constant probe adjustment. An area that looks like tendinosis may simply be the probe losing perpendicularity.

Common Exam Types

Rotator Cuff Evaluation

The most commonly performed MSK exam. Evaluates four muscles: supraspinatus, infraspinatus, subscapularis, teres minor.

Supraspinatus (most clinically important):

Patient seated, arm in internal rotation ("hand in back pocket" position)
Probe on the anterior shoulder, transverse and longitudinal to the tendon
Normal: echogenic, uniform, fibrillar
Pathology: hypoechoic tears (partial or full-thickness), calcific tendinosis (echogenic foci with or without shadowing)

Full-thickness tear: Complete discontinuity of the tendon with fluid or echogenic material filling the defect. Joint effusion may be present.

Partial-thickness tear: Focal hypoechoic defect that doesn't extend completely through the tendon. Bursal-side, articular-side, or intratendinous.

Achilles Tendon

Posterior ankle, probe sagittal and transverse to tendon
Normal tendon: 4–7 mm AP diameter (thin, echogenic, fibrillar)
Tendinosis: fusiform thickening, loss of fibrillar pattern, hypoechoic
Partial tear: focal hypoechoic defect
Complete rupture: tendon discontinuity, gap filled with hypoechoic fluid, wavy tendon ends
Dynamic assessment: dorsiflex/plantarflex during scanning to assess tendon integrity

Plantar Fascia

Probe on plantar surface of heel, sagittal
Normal thickness: ≤ 4 mm at insertion on calcaneus
Plantar fasciitis: thickening > 4 mm, hypoechoic, with or without calcification at insertion
Check for associated calcaneal enthesophyte (spur) — echogenic cortical irregularity

Carpal Tunnel

Palmar wrist, transverse plane
Measure median nerve cross-sectional area (CSA) by tracing the nerve contour in transverse
Normal CSA: < 10 mm² (some sources: < 12 mm²)
Carpal tunnel syndrome: nerve enlargement proximal to the flexor retinaculum, hypoechoic, loss of fascicular pattern
Compare to the contralateral side if abnormality is uncertain

Joint Effusion

Probe over the joint in the plane that shows the joint recess
Normal: thin anechoic line in the joint space (< 2–3 mm depending on joint)
Effusion: anechoic or complex fluid in joint recess
Synovitis: hypoechoic, proliferative tissue within the joint — does not compress like fluid, shows internal vascularity on color Doppler

Measuring Tendon Thickness

Standard measurements:

Achilles tendon: AP diameter in sagittal, 2 cm above the calcaneal insertion
Patellar tendon: AP diameter in sagittal, at the patellar insertion
Plantar fascia: Thickness at the calcaneal insertion in sagittal

Always compare to the contralateral side — asymmetry is often more meaningful than absolute values.

Image Documentation

For any MSK exam, document:

Long axis (sagittal/longitudinal to the structure)
Short axis (transverse/cross-section of the structure)
Any abnormality in two planes with measurements
Color Doppler of pathological areas (tendinosis, synovitis, masses)
Normal comparison side if asymmetry noted

Scope of Practice Considerations

MSK ultrasound interpretation varies by institution. Some practices have sonographers perform the scan with a written report by a radiologist or MSK specialist. Others have physicians (orthopedists, rheumatologists, sports medicine) performing and interpreting point-of-care MSK ultrasound at the bedside.

Know your department's protocol and scope. Document what you see; leave interpretation to the reading provider.

SonoBuddy's Measurements section includes reference values for common MSK structures — search "tendon" or "Achilles" to find relevant measurements quickly.

Musculoskeletal Ultrasound Basics for Sonographers