Magnetic Resonance Imaging of the Shoulder

Anatomy

The rotator cuff is comprised of the tendons of four muscles that converge on the greater and lesser tuberosities of the humerus: the supraspinatus, infraspinatus, subscapularis, and teres minor ( Fig. 10.1 ). Of these, the supraspinatus is the one that most commonly causes clinically significant problems and is almost exclusively the one that is addressed surgically.

The supraspinatus tendon lies just superior to the scapula and inferior to the acromioclavicular joint and the acromion. In inserts onto the greater tuberosity of the humerus. One to two centimeters proximal to its insertion is a section of the tendon called the “critical zone.” This area is reported to have decreased vascularity and is therefore less likely to heal following trauma. It is also the area of the tendon that undergoes fibrillar and myxoid degeneration (also called tendinosis), presumably from aging and trauma, although this has not been proven. The critical zone of the supraspinatus tendon is where many rotator cuff tears occur, although the majority occur at the bone/tendon interface.

The glenoid labrum is a fibrocartilaginous ring that surrounds the periphery of the bony glenoid of the scapula. It serves as an attachment site for the capsule and broadens the base of the glenohumeral joint to allow increased stability. Tears of the glenoid labrum most commonly occur from, and result in, humeral head instability and dislocations.

Imaging Protocol

There are many variations in the imaging protocol that are all acceptable for showing normal and pathologic findings in the shoulder. The rotator cuff, that is, the supraspinatus tendon, is best seen on oblique coronal images that are aligned parallel to the supraspinatus muscle ( Fig. 10.2 ). T2-weighted sequences, or acceptable variations, are mandatory. A commonly used protocol is an oblique coronal fast spin-echo (FSE) T2-weighted sequence with fat suppression. The slice thickness should be no greater than 5 mm, and 3 mm is preferable. As with most joint imaging, a small field of view (16–20 cm) is recommended. A dedicated shoulder coil or a surface coil placed anteriorly over the shoulder is necessary, although no particular type of shoulder coil appears to be clearly superior.

The glenoid labrum is best seen on axial T2-weighted images. T1-weighted images do not give any additional information and can be eliminated. If a joint effusion is present, the labrum is easily identified. Without fluid in the joint, it can be more difficult to clearly see the labrum; therefore many radiologists will perform an MR arthrogram. Either saline alone, or saline mixed with a small amount of gadolinium (a ratio of 1:250 is recommended), can be injected into the joint followed by MR imaging. MR arthrography is a routine part of shoulder imaging in many centers.

The oblique sagittal sequence is one of the most useful and is performed in our protocol as a standard T1-weighted sequence without fat suppression (to identify fatty atrophy) and as an FSE T2-weighted sequence with fat suppression. This is very useful in identifying cuff tears, fluid collections about the shoulder, and muscle edema. With T2 weighting, fluid in the subacromial bursa can occasionally be seen to better advantage than on the oblique coronal images.

Rotator Cuff

The etiology of rotator cuff disease has for decades been thought to be due to impingement or wear and tear on the cuff due to entrapment from the acromion and A-C joint osteophytes. Coracoacromial arch decompression was one of the most common procedures for shoulder pain, whereby the coracoacromial ligament was cut, the anterolateral portion of the acromion was removed, and A-C joint osteophytes were resected. More recently, coracoacromial arch decompression has largely been abandoned, with most shoulder specialists agreeing that impingement is not a true entity and that intrinsic degeneration is the most likely source of most rotator cuff problems. Treating intrinsic degeneration requires debriding the abnormal tissue and repairing the cuff.

In examining the rotator cuff, the anterior-most oblique coronal images will show the critical zone of the supraspinatus tendon. A useful landmark for noting the supraspinatus tendon is the bicipital groove, which has the anterior-most fibers of the supraspinatus just lateral to the groove. This is where most cuff tears begin and can be easily overlooked if the shoulder is internally rotated, which is common ( Fig. 10.3 ).

The normal supraspinatus tendon is said to be uniformly low in signal on all pulse sequences. Unfortunately, this is not always the case. In fact, it usually has some intermediate to high signal in the critical zone, which has caused much confusion in the evolution of interpreting shoulder MR imaging exams. We imaged around 20 “normal” volunteers (residents and fellows) in the early days of learning shoulder MR imaging, and only found one or two that had uniform low signal throughout the critical zone. This was very distressing because the literature at that time said any high signal in the critical zone meant it was abnormal. We know now that there are many causes for intermediate to high signal on T1-weighted images in the normal shoulder. We no longer even obtain an oblique coronal T1-weighted sequence, as it does not add any additional information to the oblique coronal FSE T2-weighted sequence.

If signal in the critical zone is brighter on the T2-weighted images, it is abnormal and represents a partial tear if it is fluid bright. A partial tear can also be present if the cuff has focal thinning of the tendon ( Fig. 10.4 ).

Myxoid or fibrillar degeneration of the supraspinatus tendon are commonly found in autopsy specimens, which increases with age. The majority of asymptomatic shoulders in patients over the age of 50 are felt to have some tendon degeneration in the supraspinatus, which has been termed “tendinosis.” This is seen as intermediate to high signal in the critical zone on T1-weighted images that does not increase with T2 weighting. Some tendon degeneration (tendinosis) can be seen in asymptomatic shoulders of all ages; hence it needs to be correlated with the clinical picture. If the signal gets brighter on T2-weighted images, it must be considered pathologic—a partial tear. If intermediate signal in the cuff tendons is accompanied by fusiform or focal thickening, myxoid degeneration is present ( Fig. 10.5 ). Surgeons will debride this when it is prominent.

If disruption of the supraspinatus tendon can be seen, obviously a full-thickness tear is present ( Fig. 10.6 ). In these cases, fluid is invariably present in the subacromial bursa. Care should be taken to look for retraction of the supraspinatus muscle, as marked retraction will obviate some types of surgery.