Imaging methods

When it comes to shoulder imaging, each method has its own set of advantages and limitations.

1. Radiography

It is a quick, inexpensive, and easily accessible approach for detecting fractures, arthritis, and shoulder dislocations. It has limits in terms of soft tissues, which are difficult to visualise [1].

The patient may stay upright, supine, or seated.

Patient position for each view are demonstrated and described in figure 1.

Fig 1: A- The x ray positioning for the AP view, B- the x ray positioning for the lateral view, C- the x ray positioning for the Y view. For the AP view: the patient is up against the detector, with the centre beam aimed at the glenohumeral joint. We can also do this incidence with the arm in neutral position or internal rotation for a better overview of the humeral head. The resulting image ought to incorporate the sternoclavicular joint medially, the upper third of the humerus inferiorly, and the skin margins on the superior and lateral sides [1]. For the lateral view: This view is taken from the lateral side, ensuring that the patient is in a true lateral position. The patient will raise the unaffected arm above their head. The centre ray and detector should be aimed at the affected arm's surgical neck [1]. For the Y view: The name of this projection comes from the shape of the scapula on this view. This projection is used for detecting shoulder dislocations, as the humeral head should be superimposed over the scapula. The patient sits in an oblique position, facing the detector, forming an angle of 40 to 60 degrees with the detector. The centre ray should be perpendicular to the glenohumeral joint, while the detector should be centred on this joint [1]. The resulting x ray is represented below the positioning.

2. CT

Is another x-ray-based imaging approach. It is the gold standard for detecting fractures, including those that cannot be seen with traditional radiography.

The positioning of the patients is demonstrated and described in figure 2.

Fig 2: Patient CT position. The affected arm should be in slight external rotation, with the palm facing up. The unaffected arm should be placed above head. If the patient has a metallic implant on the unaffected arm, we can ask him to lower that shoulder below the level of the affected shoulder, in order to reduce the metallic artifacts. The laser for the localiser should be places as shown in B and C images.

The scan should begin above the acromioclavicular joint and continue to the bottom of the scapula. The protocol should contain thin slices (at least 1.25mm) with a pitch of approximately 0.5 and the narrowest field of view possible (a FOV of 25 cm is preferred). After the image acquisition, we can use the data for the reconstructions; a sharp kernel paired with a bone window is needed for assessing the bony regions, and a normal kernel with soft tissues windows should be used for evaluating the soft tissues. Additionally, coronal and sagittal reformats can be obtained, as well as 3D reconstructions. The coronal reformat plan is parallel to the supraspinatus tendon while the sagittal is perpendicular to the supraspinatus tendon. The reconstruction planning is showed in figure 3  (being similar to the acquisition planes for the MRI exam) [1].

Fig 3: MRI planning starting from the localiser sequences.

3. MRI

This is the gold standard for evaluating soft tissues, and it does not employ radiations, making it suitable for recurrent usage. The downsides of this approach include a prolonged acquisition time, which can cause claustrophobia in the patient, and limits in imaging the cortical bone.

The patient is positioned similar to the CT position (figure 2), but with the unaffected arm by their side. A coil is used to increase the signal to the region. After that axial, sagittal and coronal views are obtained. The planning of these sequences uses the supraspinatus muscles as a landmark, coronal views being parallel to it, while sagittal views being perpendicular to it, as shown in figure 3. Our protocol includes fat saturated protein density sequences (PD FS) in all planes, T1 sequences in sagittal plane, short tau inversion recovery (STIR) sequences is sagittal plane and a T2 sequence in coronal plane. Slices should be 3mm thick with a 10% gap between them, with a FOV between 20 and 25 cm. The coverage should be like the one described at the CT section (above the AC joint to the base of scapula) [1].

4. Ultrasound

Is a fast and dynamic method of evaluating the rotator cuff and the superficial structures of the shoulder. It uses ultrasound waves, which are harmless, allowing for repeated uses. Bony structures can not de examined in detail the same applying for the deep structures because of the ultrasound physics [2].

A musculoskeletal or superficial tissues preset is used on the ultrasound machine, coupled with a linear probe, with a high frequency. The patient is seated, with the affected arm at the side or resting on the torso. The scanning protocol is described in figures 4-8 also demonstrating the patient’s position and the structures that can be visualised [1,2].

Fig 4: Ultrasound of the shoulder, highlighting the biceps tendon in his short axis. The position of the patient’s arm is also demonstrated, the palm should be in supine position , with the probe in the biceps grove.

Fig 5: Ultrasound of the shoulder, highlighting the biceps tendon in his long axis. The position is the same as in the short axis view, but the probe is shifted, being parallel to the humerus.

Fig 6: Ultrasound of the shoulder, highlighting the subscapularis tendon. The position is the same as the biceps tendon short view, but with the arm externally rotated so the subscapularis will slide in the view.

Fig 7: Ultrasound of the shoulder, highlighting the supraspinatus tendon. The patient position is the Crass position, with the arm against the lower back.

Fig 8: Ultrasound of the shoulder, highlighting the infraspinatus tendon. The patient’s arm is by their side, relaxed and internally rotated. Moving the transducer lower, reveals the teres minor tendon.

5.Arthrography

Shoulder arthrography can also be done in order to evaluate labral tears, rotator cuff tears, capsular pathology and intra-articular loose bodies. Classically it was done using fluoroscopy or plain x ray, nowadays being done using CT and MRI. An injection with contrast substance (iodate for CT and gadolinium for MRI) is done under ultrasound guidance, fluoroscopic guidance or CT guidance in the joint space [1].

A brief comparison of the imaging modalities can be found in table 1.

Table 1: Comparison of the imaging modalities used in evaluating shoulder.

Anatomy

Understanding the anatomical relationships between them is the first step towards understanding the imaging of this region.

Glenoid labrum is a fibrocartilaginous structure that runs along the margin of the glenoid cavity. It has a triangle cross section, with the base attached to the cavity's edges and the free edge narrow and sharp [3,4].

The labrum is best evaluated on MRI, tears in it’s structure being bright on T2 due to the fluid.

Bones

The bones that are encompassed in the shoulder are represented by the clavicle, scapula (shoulder blade), and the proximal part of the humerus. Their parts are being illustrated in

Fig 9: The bones of the shoulder and their most important landmarks. 3D View from anterior and posterior, Xray of AP view. 3D images are courtesy of Complete Anatomy.

On Xray, bones are radiopaque, cortical bone being more radiopaque while medullar bone being less radiopaque (being less dense). A similar aspect is also seen on CT scan, but we can make 3D reconstructions of the bones, especially useful for preoperative planning.

MRI is not ideal for bone imaging, but we can evaluate the bone marrow (bright both on T1 and T2) and bone marrow oedema (bright on T2, PD, STIR and dark on T1).

Ultrasounds do not penetrate bones, being limited to visualising only the periosteum and cortical bone.

Muscles

Rotator Cuff Muscles

This set of four muscles and their tendons stabilises the shoulder by retaining the humeral head in the glenoid cavity. There are four muscles: supraspinatus, infraspinatus, teres minor and subscapularis. They are presented and described in figure 10.

Fig 10: 3D model with anterior and posterior view of the shoulder muscles. Supraspinatus originates in the supraspinal fossa of the scapulae and inserts into the superior facet of the greater tubercle of the humerus. Its primary role is to abduct the humerus (particularly when commencing the abduction) and prevent it from slipping inferiorly [4]. The infraspinatus muscle originates in the infraspinal fossa of the scapulae and inserts onto the middle facet of the greater tubercle of the humerus. Its primary purpose is to externally rotate the humerus [4]. Teres minor originates in the infraspinal fossa of the scapulae and inserts on the inferior facet of the greater tubercle of the humerus. Its primary role is to aid external rotation and adduct the shoulder (albeit it cannot create significant adduction) [4]. Subscapularis originates on the anterior (costal) face of the scapula, in the subscapular fossa, and inserts into the lesser tubercle of the humerus. Its role is to internally rotate the humerus and adduct it [4]. Not shown on the model, the deltoid muscle "covers" the shoulder and consists of three parts: anterior, posterior, and intermediate [4]. The anterior part originates from the anterior edge and superior surface of the lateral third of the clavicle. The intermediate part originates from the superior surface of the acromion. The posterior part originates from the bottom lip of the spina scapulae. All the fibres converge on the antero-lateral face of the humerus, at the deltoid tuberosity, which is located between the top and middle thirds of the bone. The deltoid heads work together to produce shoulder abduction, but they also aid in shoulder flexion and extension. It can compensate for a lack of rotator cuff strength [4]. 3D images are courtesy of Complete Anatomy.

The muscles are also highlighted in axial, coronal, sagittal planes as viewed on MRI and CT. An sketch was also included.

Fig 13: Axial section of the shoulder on both MRI and CT compared to a human cadaveric specimen. The human specimen picture is courtesy of the Visible Human Project

Fig 14: Multiple coronal sections of the shoulder on both MRI and CT showcasing the muscles. The 3D image is courtesy of Complete Anatomy.

Fig 15: Sagittal section of the shoulder on both MRI and CT, showcasing the muscles.

Fig 16: Sketch of the shoulder anatomy. Reference: https://radiologyassistant.nl/musculoskeletal/shoulder/mri-anatomy .

Radiography does not directly view the rotator cuff muscles, but we can suspect a rotator cuff tear if the humeral head is migrated superiorly. Also, calcifications at the level of rotator cuff tendons can be seen on radiography.

CT is also not very sensitive in evaluating the rotator cuff muscles, but we can see fatty infiltration, muscle atrophy and calcifications.

MRI is the best imaging method for evaluating the muscles and tendons of the rotator cuff, including tears (bright on T2), tendinosis (intermediate T1 and T2 signal) and atrophy. Also, MRI can detect muscle oedema.

Ultrasound is also great at evaluating the rotator cuff muscles, while also allowing for a dynamic assessment. Partial tears appear as focal hypoechoic areas while full thickness tears are seen as discontinuity of the tendon.

Ligaments

The structures that connect the bony parts, providing stability of the shoulder are: glenohumeral ligament (that has a superior, middle and inferior part), coracohumeral ligament, transverse humeral ligament and coraco-acoromial ligament [4]. They are represented in figure 11.

Fig 11: 3D model with anterior and posterior view of the shoulder ligaments. 3D images are courtesy of Complete Anatomy.

They can be seen on x ray and CT if they are calcified. MRI can detect tears, fraying or thickening of these structures. Ultrasound can be helpful in evaluating the stability of joint and ligaments.

Bursae

Synovial bursae which are fluid-filled sacs are found around the joint capsule, help with shoulder mobility. The shoulder bursae are as follows:

• The subacromial and subdeltoid bursa are located between the joint capsule and the deltoid muscle.
• The subacromial bursa sits between the joint capsule and the acromion.
• The subcoracoid bursa sits between the joint capsule and the coracoid process.
• Coracobrachial bursa is found between the subscapularis muscle and the tendon of the coracobrachialis muscle.
• The subscapular bursa (subtendinous) is located between the joint capsule and the tendon of the subscapularis muscle [4].

They are also illustrated in figure 12.

Fig 12: 3D model with anterior view of the shoulder bursae. 3D images are courtesy of Complete Anatomy.

Radiography can show calcified bursae in chronic bursitis. CT in addition to the classic radiography can also demonstrate fluid collections. MRI is best for evaluating bursitis, the bursae being bright on T2 images. Ultrasound can show fluid effusions, and while doing an dynamic exam, we can detect impingement.