How MRI Visualizes Tendons
MRI visualizes tendons by detecting their characteristically low signal intensity on most sequences, with signal changes occurring when tendons are injured, degenerated, or oriented at specific angles to the magnetic field. 1
Normal Tendon Appearance on MRI
- Tendons typically demonstrate low signal intensity on most MRI sequences due to their dense collagen structure with parallel molecular alignment 1, 2
- On T1-weighted, proton density, and T2-weighted sequences, normal tendons appear as dark (hypointense) structures against surrounding tissues 1
- MRI provides excellent soft tissue contrast and anatomic overview, making it particularly valuable for tendon evaluation 2
Tendon Pathology Visualization
- Tendinosis (tendon degeneration) appears as increased signal intensity within the tendon on T1-weighted imaging, often with slight signal increase on T2-weighted sequences 1
- Partial-thickness tears show focal areas of increased signal intensity that don't extend through the entire tendon 1
- Full-thickness tears demonstrate increased signal intensity extending from the inferior to superior surface of the tendon on all imaging sequences 1
- MRI can detect associated findings such as tendon retraction, muscle atrophy, and fatty infiltration that help guide treatment decisions 1
Technical Considerations
Magic Angle Phenomenon
- When tendons are oriented at approximately 55° relative to the main magnetic field (B₀), they can show artificially increased signal intensity on short TE sequences 3
- This magic angle effect occurs over a wide angular range (approximately 40°) and can be seen on T1-weighted, FSE, PD, and sometimes even T2-weighted, STIR, and DWI sequences 3
- This phenomenon can potentially mimic tendon pathology if not recognized 3
Optimal Sequences
- Conventional non-arthrographic MRI can accurately identify full-thickness tendon tears with high sensitivity and specificity 1
- MR arthrography (injection of contrast into a joint) may be helpful when distinguishing between full-thickness and partial-thickness tears 1
- For specific tendons like the distal biceps, specialized positioning techniques such as the flexion-abduction-supination (FABS) view can improve visualization 1
Clinical Applications
- MRI has high sensitivity (>90%) for diagnosing tendon tears in various anatomic locations 1
- For rotator cuff tendons, MRI can accurately identify both full and partial thickness tears 1
- For ankle tendons, MRI sensitivities for tendon tears range from 83.9% to 92.4% with specificities from 74.5% to 100% 1
- For elbow tendons, MRI has sensitivity of 92.4% and specificity of 100% for distal biceps tendon ruptures 1
Advantages and Limitations
- MRI is considered the imaging gold standard for tendon evaluation due to its excellent soft tissue contrast and ability to display large anatomic regions 2
- MRI can detect both tendon abnormalities and associated pathology in surrounding structures 1
- Limitations include cost, availability, and potential artifacts from the magic angle phenomenon 2, 3
- MRI is not commonly used for image-guided aspiration or drainage procedures 1
Comparison with Other Modalities
- Ultrasound offers higher resolution than MRI and allows dynamic evaluation of tendons, but is more operator-dependent 2
- CT arthrography may be more reliable than MR arthrography for certain applications like talar cartilaginous lesions 1
- For some tendons like the peroneal tendons, ultrasound may have higher sensitivity (100%) and accuracy (90%) compared to MRI 1