Fig. 1

MRI of the brachial plexus at 3T (Magnetom Verio, Siemens, Erlangen, Germany), the arm adducted: a axial T2w images showed the accessory subclavius posticus muscle (sp) on the left side, attached to the superior border of the scapula and the medial aspect of the first rib (fr). No regular subclavius muscle was found. b Sagittal T1w images showed the subclavius posticus muscle coursing over the brachial plexus (white arrow) and the subclavian artery (sa) and vein (sv), narrowing the costoclavicular space (cl = clavicle). There was only a little fat tissue left between this muscle, the neurovascular structures, the first rib and the anterior scalene muscle (as). No regular subclavius muscle was found, c for comparison a normal appearing costoclavicular space with a regular, thin subclavius muscle (sm) of a control subject.