Peripheral nerves of the upper limb may become entrapped at various points during their anatomical course. While physical examination and nerve conduction studies are the mainstay of diagnosis, there are multiple imaging options, specifically ultrasound and magnetic resonance imaging (MRI), which offer important information about the potential cause and location of nerve entrapment that can help guide management. This article overviews the anatomical course of various upper limb nerves, including the long thoracic, spinal accessory, axillary, suprascapular, radial, median, ulnar, and musculocutaneous nerves, and describes the common locations and causes of entrapments for each of the nerves. Common ultrasound and MRI findings of nerve entrapments, direct or indirect, are described, and various examples of the more commonly observed cases of upper limb nerve entrapments are provided.

Cystic lesions of the humeral head are commonly encountered on routine shoulder magnetic resonance imaging (MRI). Differential diagnoses include degenerative lesions, calcific tendinitis with osseous involvement, perianchor cysts, abscesses and less often, tumours. Degenerative lesions, including subcortical and subchondral cysts, are the most commonly encountered. These may be associated with rotator cuff disease and degenerative joint diseases or considered part of ageing depending on the location of the cystic lesions. For instance, cysts of the bare area of the humeral head are considered benign age-related entities, whereas cysts of the greater or lesser tuberosity may herald rotator cuff disease. Infectious lesions, particularly osteomyelitis and intraosseous (Brodie’s) abscesses, are intramedullary in location and should be suspected in the context of clinical features such as fever and radiological features such as the penumbra sign. Perianchor cysts are postoperative lesions associated with the use of suture anchors in surgeries such as rotator cuff tear repairs. They generally self-resolve over 18 to 24 months. On MRI, the distribution, morphology, and signal characteristics can help point towards a specific diagnosis. The patient’s demographic, clinical presentation, and past surgical history can be discriminatory. Knowledge of different cystic lesions in the humeral head and underlying aetiology can be useful in helping the radiologist develop a more thorough search pattern for associated conditions. Determining the underlying cause of cysts can have important implications on management, such as when differentiating perianchor cysts from infection. This pictorial review outlines the differential diagnoses of humeral head cysts on MRI and provides a diagnostic approach for the radiologist.