Clinical Orthopedic Imaging High-Resolution 3T MR Neurography of the Lumbosacral Plexus Avneesh Chhabra, M.D.; Aaron Flammang, MBA, BS; John A. Carrino, M.D., M.P.H. Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, Baltimore, MD, USA 1A 1B 1C 1A–C Lumbosacral Plexus 3T MR Neurography evaluation: Isotropic multiplanar reformats from 3D T2 SPACE (1A–1C). Abstract The lumbosacral (LS) plexus is a series of trum of pathologies along with their ings and electrodiagnostic study results. nerve convergences and ramifications respective imaging findings using However, differentiation of LS plexopathy that provide motor and sensor innerva- 3 Tesla MR Neurography. from spine related abnormality and tion to pelvis and lower extremities. LS definition to type, location and extent plexopathy is a serious condition caused Introduction of pathology often remains a diagnostic by a variety of pathologies. Magnetic The lumbosacral (LS) plexus is com- challenge due to deep location of the Resonance Neurography is an important prised of an intricate architecture of nerves and variable regional muscle modality for evaluation of LS plexopathy nerves that supply the pelvis and lower innervation [1, 2]. With current high due to variable clinical presentations extremity. It can be subject to a variety resolution 3 Tesla (T) Magnetic Reso- and deep location leading to suboptimal of pathologies, which may result in LS nance Neurography (MRN) techniques, accessibility of the plexus to electrodiag- plexopathy, a clinical syndrome that diagnostic evaluation of large LS plexus nostic studies. This article describes the includes motor and sensory distur- branches, such as sciatic and femoral role of MR Neurography in evaluation of bances. The diagnosis of LS plexopathy nerves, as well as smaller segments, the LS plexus and illustrates the spec- has traditionally relied on clinical find- such as nerve roots convergences and 26 MAGNETOM Flash · 3/2011 · www.siemens.com/magnetom-world Orthopedic Imaging Clinical 1D 1E 1F 1G 1D–G Coronal reconstructed 3D STIR SPACE (1D) showing sciatic nerves (arrows). Axial T1w (1E) and T2 SPAIR (1F) images showing bilateral femoral nerves (arrows) and lateral femoral cutaneous nerves (arrowheads). MIP coronal 3D STIR SPACE (1G) image showing bilateral LS plexus nerve roots, femoral nerves (arrowheads) and sciatic nerves (arrows). peripheral nerves is feasible, aiding in ventral rami of lumbar (L1–4 +/– T12), and lateral femoral cutaneous nerves pre-surgical evaluation and appropriate sacral (S1–4 and LS trunk, L4–S1) and (L2, 3)]. All branches exit lateral to the patient management [3, 4]. This article the lower sacrococcygeal (S2–4 and C1) psoas muscle and course under the provides a pertinent discussion of the nerves, respectively [5]. The plexus is inguinal ligament, except the obturator LS plexus anatomy, current role of MRN formed lateral to the intervertebral nerve and LS trunk, which exit medial to in evaluation of plexopathy and foramina and various branches course the psoas muscle. The sacral plexus gives describes the respective imaging find- through the psoas major muscle. The rise to anterior branches, namely the ings of various pathologies using 3T MR ventral rami are further split into ante- tibial part of the sciatic nerve (L4–S3), Neurography. rior and posterior divisions. The anterior pudendal nerve (S1–4) and medial part divisions give rise to the iliohypogastric of the posterior femoral cutaneous Anatomic considerations (L1), ilioinguinal (L1), genitofemoral nerve (S1–3) and, posterior branches, The LS plexus is comprised of lumbar (L1-L2) and obturator nerves (L2–4). namely the common peroneal part of the plexus, sacral plexus and the pudendal The posterior divisions combine to form sciatic nerve (L4–S2), superior (L4–S1) plexus, with contributions from the the posterior branches [femoral (L2–4) and inferior gluteal nerves (L5–S2), MAGNETOM Flash · 3/2011 · www.siemens.com/magnetom-world 27 Clinical Orthopedic Imaging Table 1: S The 3T MR imaging protocol employed in our institution of the evaluation of the lumbosacral plexus. Sequence Slice Field-of-view Voxel size (mm3) TR/ TE (ms) Turbo factor (cm) Axial T1 TSE BL 33 0.64 800/12 6 Axial T2 SPAIR BL 33 1.00 4500/80 17 Coronal PD SPAIR BL 36-38 0.6 4980/38 7 Coronal T1 TSE BL 36-38 0.5 550/10 3 3D Coronal STIR SPACE BL 36-38 1.45 1500/103 61 Lumbar 3D Sagittal T2 SPACE 28 1.45 1000/99 69 spine Coronal 3D VIBE * BL 36-38 0.58 4.39/2.01 – Abbreviations are: TSE = Turbo Spin Echo, SPAIR = spectral adiabatic inversion recovery, STIR = short tau inversion recovery, 3D SPACE = three-dimen- sional Sampling Perfection with Application optimized Contrasts using constantly varying flip angle Evolutions, VIBE = Volume Interpolated Breath- hold Exam (* optional), BL = bilateral lateral part of the posterior femoral clinical and laboratory findings. MRN guidance during perineural and intra- cutaneous nerve (S1–3) and the nerve may be used in the latter case to confirm muscular medication injection. to the piriformis muscle (L5, S2). The lumbar plexitis / plexopathy in clinically above two plexuses connect via the LS confusing presentation and underlying Clinical fi ndings trunk to form the LS plexus [2, 5]. known systemic condition. In addition, a LS plexopathy most often presents with primary or idiopathic form of LS plexop- asymmetric weakness, pain and/or par- Pathologic conditions and athy may occur, possibly due to altered esthesias in the lower extremities involv- indications of MRN immunological response and is consid- ing multiple contiguous LS nerve root The LS plexus is relatively protected by ered analogous to idiopathic brachial distributions. Generally, unilateral local- the axial skeleton and entrapment neu- plexopathy [6–8]. The entity has a favor- ization of symptoms indicates a local ropathy is much less common than bra- able outcome and spontaneous recov- pathology, whereas bilateral symptoms chial plexopathy. It is considered as the ery, whereas its diagnosis is based upon suggest a systemic process. The clinical counterpart of the brachial plexus in the exclusion of other etiologies and some- picture often varies depending upon the lower body and is affected by similar times may require confirmation with location and degree of plexus involve- types of diseases. The LS plexus may be MRN in case of indeterminate electrodi- ment. In cases of involvement of the involved by local processes in the viscin- agnostic results. An important indication upper nerve roots, patients predomi- ity of the plexus, such as extrinsic com- of MRN is in patients under consider- nantly present with femoral and obtura- pression by space occupying lesions, ation for surgery for peripheral nerve tor nerve symptoms. LS trunk and upper injury or infiltration by tumor / infec- lesions (piriformis syndrome/meralgia sacral plexus lesions result in foot drop tious process – which is an indication for paresthetica), post abdominal surgery depending on the extent of involvement MRN; or in systemic conditions, such entrapment of ilioinguinal/genitofemo- and weakness of knee flexion or hip as metabolic, autoimmune, vasculitis, ral nerves, or after injury to a large abduction. Sensory symptoms may vary ischemic or inflammatory disorders – branch (sciatic, femoral, obturator). based on the individual nerves involved, which are usually diagnosed based on Finally, MRN is increasingly used for which may include numbness or 28 MAGNETOM Flash · 3/2011 · www.siemens.com/magnetom-world Orthopedic Imaging Clinical 2A 2B 2B 2 Normal vs abnormal LS Plexus. Coronal MIP 3D STIR SPACE images in a nor- mal subject (2A) and another sub- ject with schwan- nomatosis (arrows in 2B). 3A 3B 3 Piriformis syndrome – large LS plexus branch nerve abnormality. 33-year-old man with right buttock and pelvic pain, suspected piriformis syndrome. Coronal T1w (3A) and coronal MIP 3D STIR SPACE (3B) images through the pelvis show split right sciatic nerve by an accessory slip of the right piriformis muscle (arrow). Notice abnormal T2 hyperintensity of the sciatic nerve in keeping with entrapment neuropathy. dysesthesia in the anterolateral thigh MRN technique and normal tures under interrogation, it is essential from lateral femoral cutaneous nerve appearances to use high resolution imaging with a involvement; in the mons and labia Compared to 1.5T systems 3 Tesla combination of 2D (dimensional) and 3D majora from genitofemoral nerve (MAGNETOM Verio and MAGNETOM Trio, isotropic spin echo type imaging for involvement and in the lower abdomen Siemens, Erlangen, Germany) imaging optimal assessment. In the presence of and inguinal area, upper medial thigh or is preferred for most MRN examinations known metal* in the area of imaging, pelvis from damage to the ilioinguinal, by the authors due to high quality scans 1.5T imaging is preferred. Table 1 and iliohypogastric, or pudendal nerves, obtained by these systems due to better Fig. 1 show the 3T MRN imaging protocol respectively. Rarely, there may be associ- signal-to-noise ratio and contrast reso- employed at Johns Hopkins for the eval- ated bowel and bladder incontinence as lution available in short imaging times. uation of the LS plexus. For fascicular well as sexual dysfunction [6–8]. Due to the relatively small nerve struc- architecture and subtle signal intensity MAGNETOM Flash · 3/2011 · www.siemens.com/magnetom-world 29 Clinical Orthopedic Imaging changes, high- resolution 2D axial T1w planning and understanding of the adjacent vascular signal intensity, or and T2 SPAIR (Spectral Adiabatic Inver- referring physicians. Side-side imaging asymmetric SI to the other side, given sion Recovery) images are ideal. Fascicu- differences of the nerves can also be variations due to non-uniform fat satura- lar appearance of nerves is consistently highlighted on maximum intensity pro- tion); as well as indirect features such as seen with T2 SPAIR images in larger jection (MIP) images. Gadolinium admin- effacement of perineural fat planes due branches, such as femoral nerves and istration is reserved for cases of sus- to focal fibrosis/mass lesions (Figs.