Figure 19b. Tumor involvement of the brachial plexus. (a) Coronal T1-weighted spin-echo MR image of the brachial plexus shows slightly thickened upper, middle, and lower trunks (arrowhead) lying cephalad to the subclavian artery (A) and vein (V). (b) Coronal contrast-enhanced fat-suppressed MR image shows enhancement and thickening (arrows), findings that suggest extensive tumor involvement of the brachial plexus by a Pancoast tumor.