The study of peripheral nerves (PN) is a challenge because of the orientation, size, and also because of the fact that these pathologies are not well known. The diagnosis of PN damage was based exclusively on clinical examination and electrophysiologic testing until few years ago. MRI is the examination of choice for studying peripheral nerves; recently, magnetic resonance neurography has been added as a part of clinical routine. However, the analysis of PN after surgical treatment remains difficult. This clinical case of a sarcoma of the right brachial plexus illustrates how hybrid PET-MRI in postsurgical evaluation will clearly improve the exact delimitation of residual metabolically active tumor after surgery.