Reading a shoulder MRI for rotator cuff damage comes down to one core skill: recognizing what healthy tendon looks like versus what a tear looks like on different image sequences. A normal rotator cuff tendon appears uniformly dark (low signal) on all MRI sequences. When a tear is present, you’ll see bright signal on fluid-sensitive sequences where the dark tendon should be. The location, depth, and extent of that bright signal tell you exactly what kind of tear you’re dealing with.
How Normal Tendons Appear on MRI
Healthy rotator cuff tendons show homogeneous low signal, meaning they look consistently dark on both T1-weighted and T2-weighted images. This is because tendons are made of tightly organized collagen fibers that don’t hold much water. On a shoulder MRI, you’re primarily evaluating four tendons: the supraspinatus (most commonly torn), infraspinatus, teres minor, and subscapularis.
The key sequences to focus on are T2 fat-suppressed (T2 FS) or STIR images, which make fluid appear bright white against dark tissue. These are your go-to images for spotting tears because any fluid filling a gap in the tendon will light up. T1-weighted images are better for evaluating overall anatomy and, importantly, for detecting fatty changes in the muscle belly that indicate a chronic tear.
The coronal oblique plane (slicing front to back along the line of the supraspinatus) is the most useful view for evaluating supraspinatus and infraspinatus tears. Sagittal images let you assess the front-to-back extent of the tear and check which tendons are involved. Axial (cross-sectional) images are best for examining the subscapularis tendon and the biceps tendon in its groove.
Tendinosis vs. Tear: Telling Them Apart
Before jumping to “tear,” it helps to understand what tendinosis looks like, since it’s a common finding that can mimic a tear to an untrained eye. In tendinosis, the tendon appears thickened and shows patchy, intermediate (grayish) signal within its substance on T2 images. This intermediate signal is noticeably less bright than the fluid-bright signal you see in an actual tear. Think of it as a dull gray glow versus a sharp white streak. Tendinosis represents degeneration, not a structural break in the tendon.
Identifying Partial-Thickness Tears
A partial-thickness tear involves only part of the tendon’s depth, meaning it doesn’t extend all the way through from top to bottom. On T2 fat-suppressed images, a partial tear shows up as a curvilinear or focal area of bright signal within the tendon that doesn’t span the full thickness.
These tears are classified by their location and depth. Articular-sided tears (on the joint surface of the tendon) are the most common type and appear as bright signal along the undersurface of the tendon on coronal images. Bursal-sided tears occur on the outer surface, closest to the subacromial bursa. Intratendinous tears sit within the substance of the tendon itself, sometimes called delamination tears.
Depth matters for treatment decisions. The Ellman grading system categorizes partial tears into three grades:
- Grade 1: Less than 3 mm deep
- Grade 2: 3 to 6 mm deep
- Grade 3: Greater than 6 mm deep
Since the supraspinatus tendon is roughly 10 to 12 mm thick, a Grade 3 tear involves more than half the tendon thickness and is more likely to progress to a full tear.
Conventional MRI detects articular-sided partial tears with about 78% sensitivity. MR arthrography, where contrast dye is injected into the joint before scanning, bumps that sensitivity up to 81% and is particularly useful when a partial tear is suspected but hard to see on standard images.
Identifying Full-Thickness Tears
A full-thickness tear extends from the articular (joint) side to the bursal (outer) side of the tendon. On T2 fat-suppressed coronal images, this appears as bright fluid signal spanning the entire thickness of the tendon, creating a clear gap. Comparing the coronal and sagittal views helps you map the overall shape and size of the defect.
Full-thickness tears are graded by their width and how far the torn tendon has pulled back (retracted) from its attachment:
- C1: Small, pinhole-sized full-thickness defect
- C2: Moderate tear, under 2 cm, involving one tendon with no retraction
- C3: Large tear with 3 to 4 cm of retraction
- C4: Massive tear involving two or more tendons with significant retraction and scarring
To assess retraction, follow the torn tendon edge on coronal images. In mild retraction (Patte grade I), the tendon stump is still near its attachment. In grade II, it has pulled back to the top of the humeral head. In grade III, the fibers have retracted all the way to the glenoid rim or beyond. Greater retraction generally makes surgical repair more difficult and is associated with worse outcomes.
Checking for Muscle Atrophy and Fatty Changes
When a rotator cuff tendon has been torn for a long time, the muscle it connects to begins to shrink and fill with fat. This is one of the most important things to evaluate because advanced fatty infiltration is largely irreversible, even after surgical repair. You assess this on sagittal T1-weighted images, where fat appears bright and muscle appears gray.
The Goutallier classification is the standard grading system:
- Stage 0: Normal muscle with no fat
- Stage 1: Some fatty streaks visible in the muscle
- Stage 2: Significant fat present, but still more muscle than fat
- Stage 3: Equal amounts of fat and muscle
- Stage 4: More fat than muscle
Stages 3 and 4 are associated with poorer surgical outcomes and reduced strength recovery. If you see a bright, marbled appearance replacing the normally gray muscle belly on T1 images, that’s fatty infiltration.
Secondary Signs That Support the Diagnosis
Beyond the tendon itself, several surrounding structures provide clues that confirm or raise suspicion for a rotator cuff tear.
Subacromial Bursa Fluid
A thin layer of fluid in the subacromial/subdeltoid bursa is normal, but it’s rarely thicker than 2 mm and tends to sit posteriorly. When the fluid layer exceeds 3 mm, extends medial to the acromioclavicular joint, or appears in the anterior portion of the bursa, it’s considered abnormal and often accompanies a full-thickness tear. Fluid can leak through a full-thickness defect from the joint into the bursa, so seeing excessive bursal fluid on T2 images is a reliable supporting sign.
Acromiohumeral Distance
The space between the top of the humeral head and the undersurface of the acromion normally measures about 7 to 8 mm on MRI. When a large or chronic rotator cuff tear is present, the humeral head migrates upward because the cuff muscles are no longer holding it down. On MRI, a distance of 6 mm or less suggests a large, chronic tear and decreases the likelihood of a successful surgical repair.
Biceps Tendon Position
Check the long head of the biceps tendon on axial images. It should sit in the bicipital groove at the front of the humerus. If it’s subluxed (partially slipped) or fully dislocated out of the groove, this is strongly associated with larger rotator cuff tears, particularly tears that involve the subscapularis tendon. A medially dislocated biceps tendon typically means the upper portion of the subscapularis is torn.
Avoiding the Magic Angle Pitfall
One of the most common traps when reading a shoulder MRI is the magic angle artifact. When collagen fibers in a tendon are oriented at approximately 55 degrees relative to the main magnetic field, they can produce artificially increased signal on short echo time sequences like T1-weighted and proton density images. This makes a perfectly healthy tendon look like it contains abnormal signal.
The supraspinatus tendon is particularly susceptible to this artifact where it curves over the humeral head, and the intra-articular portion of the biceps tendon is another common location. The key to avoiding a false positive: always confirm any suspicious signal on T2 fat-suppressed images. The magic angle artifact disappears on long echo time sequences (T2-weighted). If the bright signal you see on T1 images isn’t reproduced on T2, it’s almost certainly artifact, not a tear.
Putting It All Together
A systematic approach prevents you from missing findings. Start on coronal T2 fat-suppressed images and scroll through the supraspinatus and infraspinatus from front to back, looking for any bright signal within or replacing the dark tendon. Switch to sagittal images to determine which tendons are involved and measure the front-to-back extent of any defect. Use axial images to evaluate the subscapularis and biceps tendon. Then check T1 sagittal images for fatty infiltration in the muscle bellies. Finally, note the secondary signs: bursal fluid, acromiohumeral distance, and bone marrow edema at the greater tuberosity, which can indicate an acute injury.
Every tear should be described by its thickness (partial vs. full), location (which tendon or tendons), size, degree of retraction, and the condition of the remaining muscle. These details directly determine whether someone is a candidate for conservative treatment, arthroscopic repair, or more complex reconstruction.