**3.1 MRI protocol**

Patients should be supine in a comfortable position with the arm to be imaged in supine position as well. Images should include from the distal humeral metaphysis up to the radial tuberosity and this area should be imaged in axial, coronal and sagittal planes. Sequences should include non-fat saturated T1, PD, and fat-saturated T2WI; gradient echo (GRE) may also be included depending on the pathology suspected [10].

### **3.2 Ultrasound examination technique**

Ultrasound of the elbow usually focuses on the area of clinical interest, nonetheless, the anterior, lateral, medial and posterior compartments should all be evaluated. A high frequency linear transducer of 12–17 mHZ is preferred. To evaluate the anterior compartment of the elbow, which includes the distal biceps tendon, it should be extended with a supine forearm. Evaluation should include transverse and longitudinal planes from 5 cm proximal and distal to the joint. The lateral elbow compartment, which includes the common extensor tendon, is evaluated with the arm placed in internal rotation and elbow joint in flexion. The medial compartment includes the common flexor tendons, which is evaluated sonographically by extending the forearm in forceful external rotation. Lastly, the posterior elbow, which contains the distal triceps tendon, is evaluated by placing the elbow in 90° flexion with the arm internally rotated [11].

### **3.3 Common extensor tendon**

The common extensor tendon attaches to the humeral lateral epicondyle uniting the individual tendons of the extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi and the extensor carpi ulnaris. Normally the common extensor tendon is a band of low signal intensity on both T1WI and T2WI, seen superficial to the radial collateral ligament complex and the tendon should show complete fibers at its insertion in the lateral epicondyle.

A common cause for elbow pain is lateral epicondylitis, also known as tennis elbow. In these cases, the tendon may appear thickened with increased intermediate signal intensity on T1WI and T2WI. Abnormal fluid signal intensity may be seen traversing the tendon fibers in partial tendon tears, most common in the extensor carpi radialis brevis tendon [12] (**Figure 2**). If there is a fluid signal intensity gap with discontinuity of the tendon fibers, a full thickness tear is present. Avulsion injuries may be present when there is associated bone marrow edema at the tendinous insertion site. The US evaluation of lateral epicondylitis shows a heterogeneous tendon with focal hypoechoic areas.
