Normal shoulder MRI
The shoulder joint is a joint that connects the upper limb to the axial skeleton. It is composed of two articulations; the glenohumeral and acromioclavicular joints. The glenohumeral joint is a synovial joint, formed by the glenoid fossa of the scapula and the head of the humerus, while the acromioclavicular joint connects the acromion and the lateral part of the clavicle. The shoulder joint is the most mobile joint of the human body, which comes at a cost of also being relatively unstable. Thus, it is one of the most frequently injured joints of the body.
The evaluation of the shoulder, and especially its soft tissue structures, is best done with an MRI. The MRI allows accurate assessment of any pathologic changes of the structures of the shoulder, including the glenoid labrum, the humeral head, the articular cartilage, and the rotator cuff. In order to recognize the pathology, it is essential to master normal shoulder MRI images, which we will cover in this article.
|MRI definition||Medical imaging technique used to examine the bones and soft tissue structures of the shoulder|
|Mechanism||Emission of magnetic fields that trigger the protons of tissues to produce a signal measured by the MRI and converted into a gray-scale image|
T1- weighted image: fats and bone marrow produce high signal (white); ligaments, cartilage, fluid produce low signal (black)
T2- weighted image: ligaments, cartilage and fluid produce high signal (white); bone marrow produces a low signal (black).
Proton density image: enhances tissues with high proton density (fatty bone marrow, hyaline cartilage, muscles)
Coronal oblique: slices parallel to the tendon of the supraspinatus muscle, going from posterior to the anterior shoulder.
Sagittal oblique: slices perpendicular to the supraspinatus muscle, going from head of humerus to the scapula.
Axial plane: makes cross-sections of the shoulder from top to bottom.
- MRI basics
- How to read a normal shoulder MRI
- Static stabilizers
- Dynamic stabilizers
- Deltoid muscle
The MRI is a sophisticated imaging technique that stands out with its superior ability to discriminate soft tissue structures. Simply put, the MRI triggers the protons in the tissues to produce a signal that is measured and transformed into a gray-scale image. Different tissues have different density of protons, hence the signal varies in intensity, allowing the MRI to discriminate one tissue from another. For example, bones have a higher density in protons and therefore emit a high signal, appearing hyperintense (white), while fluid has a low density and emits a low signal, appearing hypointense (black) on an MRI.
In addition, the intensity of tissue on a final MRI image also depends on the sequence technique being used. Most commonly used sequences for the shoulder are T1- and T2-weighted, as well as proton density (PD) images. We can switch between these modalities depending on the tissue we want to observe:
- T1-weighted images are useful to visualize the tissues that predominantly contain fat (bone marrow) since they produce a high T1 signal and appear white, which makes them easier to analyze. The tissues that contain more water (ligaments, cartilage, synovial fluid) produce a lower T1 signal and appear darker.
- T2-weighted images are useful when we want to evaluate tissues that contain more water, or we want to detect pathological changes that are followed by fluid accumulation. Therefore tissues such as ligaments, cartilage and fluids produce a high T2 signal and appear white in this sequence, while bone marrow produces a low T2 signal and appears black. This can be remembered by the mnemonic WW2, which stands for “water is white on T2”
- Proton density (PD) images are useful when we want to take a closer look at the bones, hyaline cartilage and muscles. PD images, as the name suggests, enhance structures with a higher proton density. Thus, fatty bone marrows and hyaline cartilage produce a high signal intensity and appear white or gray, muscles produce an intermediate signal and appear gray, whereas ligaments produce a low signal and appear black.
Another important property of the MRI is its ability to produce images in multiple planes, which allows us to visualize the shoulder from different angles. Usually, the images are taken in three planes; coronal oblique, sagittal oblique and axial plane.
- In the coronal oblique plane, slices are made parallel to the tendon of the supraspinatus muscle, starting from the posterior to the anterior aspect of the shoulder.
- In the sagittal oblique plane, slices are made parallel to the glenoid fossa and perpendicular to the supraspinatus muscle, starting from the humeral head to the scapula.
- In the axial plane, slices of the shoulder are made from top to bottom, starting from the acromion to the humeral shaft.
To get a deeper understanding of the fundamentals of the MRI, have a look at our article below to learn more information.
How to read a normal shoulder MRI
There are two main articulations in the shoulder region: the glenohumeral joint and the acromioclavicular joint. The glenohumeral joint is an articulation formed by the glenoid fossa of the scapula and the head of the humerus; while the acromioclavicular joint is formed by the acromion and clavicle. These joints are stabilized by soft tissue structures, that are divided into static stabilizers, which include the glenoid labrum, fibrous capsule, glenohumeral and coracohumeral ligaments; and dynamic stabilizers, which include the rotator cuff and surrounding muscles.
Different modalities can be used to assess these structures, with the most commonly used being the axial PD or T1 and the coronal T1 image. As an example, below is an overview of the shoulder on an axial PD image at the level of the glenoid cavity. In this modality, bones show as white, muscles as dark gray, and tendons and ligaments as black.
We will now proceed to give a more in depth overview of the individual bones as well as the surrounding soft tissue structures of the shoulder, to provide a simple guideline on how to evaluate these structures on the MRI.
When looking at the bones, we should evaluate their intensity, shape and contours and look out for any osteophytes or fractures. The MRI is also very useful to inspect the bone marrow to see if there are any pathologic changes such as neoplasms, marrow-packing diseases or infections (osteomyelitis).
The most prominent bony structure seen on the shoulder MRI is the humerus. A T1 coronal oblique image gives us a nice overview through the long axis of the humerus. On a slice through the center of the glenohumeral joint, we see the contours of the proximal shaft, the neck, and the head of the humerus. On the superior aspect of the humeral head, we can visualize the lesser tuberosity medially, and the greater tuberosity laterally.
On an axial T1 or PD image at the level of the superior portion of the glenohumeral joint, the head of the humerus appears as a round white high signal structure. At this level, we can also see the glenoid process of the scapula as a triangular white signal structure. The glenoid process contains a concave surface called the glenoid fossa that articulates with the head of the humerus on its inferomedial side. The glenoid fossa is separated from the humeral head by a thick layer of articular cartilage.
Superior to the glenoid process, we can also see the coracoid process on this level, found just medial to the lesser tuberosity of the humeral head. The coracoid process is an anterior bony extension arising from the anterolateral aspect of the scapula. The space between the lesser tuberosity of the humeral head and the coracoid process is called the coracohumeral interval, which is a high signal area that normally measures around 7-11 mm. Narrowing of the coracohumeral interval to <6 mm highly associated with anterior shoulder disorders such as rotator cuff tears.
Now let’s go back to the coronal image that cuts through the center of the glenohumeral joint. At this level, we can see the acromion, which is a posterolateral extension of the scapular spine. The acromion appears as an oval high signal structure found superiorly to the humeral head, separated from it by the supraspinatus muscle, which appears as a large rhomboid structure that has an intermediate (gray) signal. By scrolling anteriorly, we can follow the acromion to the point where it articulates with the lateral clavicle and forms the acromioclavicular joint. Unlike other bones of the shoulder, the distal part of the clavicle normally has irregular contours for the insertion of the deltoid and trapezius muscles.
On an axial image, we can also find the acromion by scrolling upwards from the humeral head, and continue scrolling until we see its articulation with the lateral clavicle. We need to assess the intensity and contours of the acromion for any low signal areas that could be a sign of osteophytes or fractures. However, it is important to note that in about 15% of people the acromion contains unfused ossification centers that are characterized by decreases in intensity on MRI. This is a normal variant called os acromiale and it should not be mistaken for a fracture.
Check out our study units below to solidify your knowledge of the glenohumeral and acromioclavicular joints with our video tutorials, quizzes and labeled diagrams!
After inspecting the bones, we can now focus on the surrounding soft tissue structures. We will first look at the static stabilizers, that include the glenoid labrum, fibrous capsule, glenohumeral and coracohumeral ligaments.
As we’ve seen, the glenohumeral joint is formed by the glenoid fossa of the scapula and the humeral head. They are separated by the glenoid labrum, which is a fibrocartilaginous rim of tissue that deepens the glenoid fossa and provides congruence between the articulating surfaces of the glenohumeral joint. The glenoid labrum is also important as it serves as a point of attachment for the long head of the biceps brachii tendon, the glenoid capsule and the glenohumeral ligaments.
The glenoid labrum is best seen in the axial plane, appearing on the anterior and posterior rim of the glenoid as two triangular-shaped low signal structures on all pulse sequences. We can see that the anterior labrum is usually larger than the posterior labrum. Upon assessing the glenoid labrum, we need to look out for any tears or detachments, which would be seen as a fluid signal extending between the labrum and the bony glenoid or as a truncation of the labrum.
Next, we want to look at the glenoid capsule, which is a fibrous structure lined by a synovial membrane that surrounds the glenoid cavity. It is seen as a black space between the humerus and scapula. We need to look out for any abnormalities of the glenoid capsule such as thickening or retraction, which would indicate an inflammatory process.
The superior, middle and inferior glenohumeral ligaments are thickenings of the glenoid capsule that attach onto the anteroinferior margin of the glenoid labrum and reinforce the capsular tissue.
- The superior glenohumeral ligament extends from the superior labrum to the lesser tuberosity of the humerus.
- The middle glenohumeral ligament extends from the anterior glenoid to the lower part of the lesser tuberosity.
- The inferior glenohumeral ligament extends from the inferior labrum to the anatomic neck of the humerus.
Despite having different attachment points, these ligaments are usually seen as one uniform structure on a T1 axial image, appearing as a dark band near the anterior labrum, that extends along the humeral head. If needed, we can use a sagittal image to visualize the glenohumeral ligaments separately.
The coracohumeral ligament is a triangular ligament that originates on the lateral horizontal portion of the coracoid process and fans out to insert in the lesser and greater tuberosities of the humerus, as well as the biceps sheath. The coracohumeral ligament acts to limit inferior translation and excessive external rotation of the humerus.
The coracohumeral ligament consists of a medial and lateral band.
- The medial band of the coracohumeral ligament blends with the fibers of the superior glenohumeral ligament complex that surrounds the medial and inferior aspect of the long head of biceps tendon before it inserts on the lesser tuberosity of the humerus.
- The lateral band of the coracohumeral ligament surrounds the superior and lateral aspect of the long head of biceps tendon before inserting on the greater tuberosity of the humerus.
Don’t be alarmed if you cannot see these ligaments on an axial image, as they are very challenging to visualize on MRI, even when intact. It is thought that the best way to assess the coracohumeral ligaments is by using an oblique sagittal image.
Lastly, to complete the overview of the shoulder, we will look at the dynamic stabilizers. These include the rotator cuff and the surrounding muscles.
The main dynamic stabilizer of the glenohumeral joint is the rotator cuff, which is a complex of muscles and tendons of the supraspinatus, infraspinatus, teres minor, and subscapularis, memorized by the mnemonic “rotator cuff SITS on the shoulder”. The function of the rotator cuff is to produce movement at the shoulder joint while keeping the head of humerus stable and centralized within the glenoid cavity.
The rotator cuff muscles originate on the scapula with their tendons converging towards their respective attachment sites. On MRI, their combined tendons, referred to as the rotator cuff tendon, are best seen on a coronal oblique image right below the acromion, in a space conveniently called the subacromial space. The rotator cuff tendon has a uniformly low signal on all sequences. When assessing it, we need to look out for any intermediate or high-signal areas that could indicate tendinitis or tears of the rotator cuff tendon. These can also be followed by fluid in the subacromial space or retraction of the rotator cuff tendon.
On a slice below the acromion in the axial plane, we can see the supraspinatus muscle, which appears as a large low signal rhomboid structure on T1. As we scroll further downwards, we can follow the muscle as it extends laterally into the supraspinatus tendon, which is seen as a low intensity structure that arches over the humeral head to attach on the greater tuberosity of the humerus. It is important that we assess if there are any tears in the supraspinatus tendon, since this tendon is the most frequently torn in the shoulder joint.
The long head of the biceps brachii originates from the supraglenoid tuberosity, the labrum and the coracoid process, then extends obliquely through the rotator interval, and makes a turn along the anterior surface of the humerus before it exits the shoulder joint between the lesser and greater tuberosities into the bicipital groove.
The biceps brachii is best seen on an axial PD image on a slice through the center of the glenohumeral joint. At this level, the tendon of the long head of biceps brachii is located in the bicipital groove, while the tendon of the short head is found at the tip of the coracoid process.
The deltoid muscle has a significant role as a shoulder stabilizer, and is generally regarded as a primary muscle acting on the glenohumeral joint during abduction, along with the supraspinatus muscle. The deltoid muscle can be visualized on a slice through the center of the glenohumeral joint, where it is seen overlying the anterior, lateral, and posterior aspect of the shoulder.
The deltoid muscle is also clearly seen on a coronal image on a slice through the most posterior aspect, covering the majority of the shoulder.