A Systematic Approach to the Evaluation of Lumbosacral Plexus MRI: Indications, Protocol Anatomy, and Pathology

ABSTRACT Magnetic resonance imaging (MRI) of the lumbosacral plexus (LSP) is a critical tool for evaluating pelvic and lower extremity peripheral nerve disorders. The rarity of this exam type and the complexity of underlying anatomy can produce challenges for interpretation. This pictorial review outlines a structured approach for evaluating lumbosacral plexus MRI (LSP MRI), emphasizing key imaging anatomy, MRI protocols, and a spectrum of commonly encountered pathologies to aid radiologists in interpretating LSP MRI. Anatomic variants and age-related changes in nerve morphology can further complicate evaluation, necessitating a systematic approach. We discuss the appearance of neoplastic, traumatic, infectious/inflammatory, treatment-related, and compressive etiologies. A search pattern and reporting pearls are provided to facilitate efficient assessment of lumbosacral plexus MRI. ABBREVIATIONS: LSP=lumbosacral plexus; CIDP = chronic inflammatory demyelinating polyneuropathy; HASTE = Half-Fourier Acquisition Single-Shot Turbo Spin Echo; STIR: Short Tau Inversion Recovery; TSE = Turbo Spin Echo; VIBE = Volumetric Interpolated Breath-Hold Examination; 3D DW-PSIF = Three-Dimensional Diffusion-Weighted Reversed Fast Imaging with Steady-State Free Precession; FS = Fat Saturated; ISO = Isotropic; TRA: Transaxial; SPACE = Sampling Perfection with Application-optimized Contrasts using different flip angle Evolution; DWI =Diffusion-Weighted Imaging.