How to Read Musculoskeletal MRI: A Practical Guide

First - Understand MRI Signal Basics

1. SHOULDER - Rotator Cuff & Labrum

How to approach the shoulder MRI

• Look at sagittal oblique images for rotator cuff thickness and continuity
• Look at coronal oblique for full-thickness tears and subacromial space
• Look at axial images for the labrum (anterior and posterior), biceps tendon, and subscapularis

Rotator Cuff Tears

• Tendinopathy - intermediate T2 signal within the tendon (thickened, disorganized collagen)
• Partial thickness tear - high T2 signal not spanning full thickness (articular surface tears are most common)
• Full thickness tear - fluid signal traversing the tendon from articular to bursal surface; creates an abnormal communication between the glenohumeral joint and subacromial bursa
• Massive tear - involves 2+ tendons or measures ≥5 cm; associated with tendon retraction and fatty muscle atrophy

Labral Tears - Bankart Lesion

• MRI finding: fluid/contrast signal extending between the glenoid and the labrum (primary sign)
• The labrum may be displaced, frayed, or absent at the 3-6 o'clock position (anterior-inferior quadrant)
• Best seen on axial images, especially on MR arthrography (gadolinium injected into joint)

2. KNEE - Ligaments & Menisci

How to approach the knee MRI

• Sagittal is your workhorse: ACL, PCL, meniscal posterior horns
• Coronal: MCL, LCL complex, meniscal bodies, tibial plateau
• Axial: patellofemoral compartment, popliteal structures

Anterior Cruciate Ligament (ACL)

• Complete tear: discontinuity of fibers + increased signal within the torn stump + laxity or horizontal orientation of the remaining fibers
• Partial tear: signal change and laxity with some fibers still intact

• "Kissing contusions" - bone marrow edema (bright T2) at the posterolateral tibial plateau + lateral femoral condyle = classic pivot shift injury pattern
• Anterior tibial translation ≥7 mm relative to the femur
• PCL buckling
• Segond fracture - tiny avulsion off lateral tibial rim (highly specific for ACL tear; subtle on MRI, better on X-ray)

Posterior Cruciate Ligament (PCL)

• Torn by "dashboard injury" (posterior force on bent knee)
• Partial middle-third tears are more common than complete tears
• Increased T2 signal + laxity = sprain; complete disruption = complete tear

Medial Collateral Ligament (MCL)

• Tears show fluid signal within or around the ligament
• Grade 1: edema around ligament (periligamentous fluid), ligament intact
• Grade 2: partial tear - signal within the substance
• Grade 3: complete tear - disrupted fibers with fluid gap

Menisci

• Grade 1-2: intrasubstance signal that does NOT touch the articular surface = degeneration (not a true tear)
• Grade 3: high signal extending to an articular surface = TEAR

3. SPINE - Cord, Disc, and Fractures

How to approach the spine MRI

• Sagittal T1: see anatomy, vertebral body height, marrow signal
• Sagittal T2: disc hydration (bright = healthy), cord signal, nerve root compression
• Axial T2/GRE: neural foramina, disc herniation level, cord cross-section

Spinal Cord Injury

Disc Herniation and Nerve Root Compression

• Disc bulge: broad, diffuse extension beyond vertebral endplates
• Disc protrusion: focal herniation, base wider than the dome
• Disc extrusion: herniated material extends beyond disc space
• Sequestered fragment (free fragment): disc material separated from parent disc - can migrate up/down canal

Osteoporotic/Insufficiency Vertebral Fractures

1. Dating fractures - acute fractures show diffuse low T1 + high T2 marrow edema; chronic fractures have normal marrow signal
2. Differentiating benign vs. malignant collapse - benign osteoporotic fractures preserve posterior element signal and show a fluid sign; malignant (metastatic) fractures replace all marrow, involve posterior elements, and show an epidural mass

• Wedge: anterior height loss, common at thoracolumbar junction (T12-L1)
• Crush: >40% height loss (grade 3 = severe)
• Fractures above T7 = suspect metastasis, not osteoporosis

4. ANKLE - Ligament Tears

Lateral ankle ligaments (most commonly injured)

• Normal ligament: dark band
• Acute tear: wavy, high-signal, thickened, or absent ligament
• Chronic tear: thickened, irregular, heterogeneous
• Bone marrow edema adjacent to the tear site (look for bright T2 in lateral talus/fibula)

Osteochondral Lesion of the Talus (OLT)

• Subchondral bone marrow edema (bright T2) in the talar dome
• A dark fracture line beneath the articular surface
• Fluid "undercutting" the fragment = unstable lesion requiring surgery

5. Quick Reference - "Red Flags" on MSK MRI

Key Practical Tips for MRI Reading

1. Always use multiple planes - a structure visible in only one plane may be artifact
2. Compare signal to known references - disc fluid (T2 bright), cortical bone (T2 dark), fat (T1 bright before fat-sat)
3. Follow the structure - trace a ligament or tendon through all slices in its best plane
4. Correlate with clinical history - mechanism of injury predicts which structures to scrutinize first
5. MR arthrography (gadolinium injected into joint) dramatically improves sensitivity for partial labral tears and partial cuff tears - particularly useful for shoulder and hip
6. Fat saturation is your friend - it eliminates the bright fat signal and makes edema/fluid POP

1. MRI signal basics - T1 vs T2/fat-sat and what each looks like for fluid, fat, bone, and soft tissue
2. Shoulder - Rotator cuff tears (partial vs full-thickness), Bankart labral lesions with actual MRI arthrogram image, SLAP tears, Hill-Sachs defect
3. Knee ligaments & menisci - Complete ACL tear with image, pivot-shift contusion pattern, PCL tears, MCL grading, all meniscal tear types (horizontal, bucket handle, ghost meniscus, flipped meniscus) with images
4. Spine - Cord edema vs hemorrhage (prognostic difference), disc herniation types, osteoporotic vs malignant vertebral collapse on MRI
5. Ankle - ATFL/CFL lateral ligament tears, osteochondral lesions of the talus
6. Red flags cheat sheet - 9 key signs to always check and what they mean