Applied Radiology

Dr. Holz and Dr. Bowen are in the Department of Radiology at the University of Miami School of Medicine in Miami, FL.

A ccurate assessment of the brachial plexus (BP) and sacral plexus (SP) requires thorough understanding of the complex anatomy. What follows is a discussion of the anatomy, current imaging techniques, and pathologic conditions of these two regions.

Brachial plexus

Anatomy --The brachial plexus is formed from the ventral rami of C5 to T1 spinal nerves, with variable contributions from C4 and T2. The ventral rami are the five roots of the BP. At approximately the lateral border of the anterior scalene muscle, the C5 and C6 roots form the upper or superior trunk, the C7 root forms the middle trunk, and the C8 and T1 roots form the lower trunk. The trunks are located superior and posterior to the subclavian artery as the plexus emerges from the interscalene triangle between the anterior and middle scalene muscles and the first rib (figure 1). Unlike the subclavian artery, which runs with the BP, the subclavian vein courses anterior to the anterior scalene muscle (figure 2A). Beyond the first rib, the subclavian vessels become the axillary vessels, with the subclavian vein remaining anteroinferior to the artery. ^1-5

The three trunks divide into three anterior and three posterior divisions (figure 2B). These six divisions subsequently join to form three cords in the medial aspect of the axilla. The plexus enters the axilla by passing between the clavicle and first rib.

The cords are named for their relationship to the axillary artery. The anterior divisions of the upper and middle trunks form the lateral cord, the anterior division of the lower trunk forms the medial cord, and the posterior divisions of all three trunks form the posterior cord. Near the lateral margin of the pectoralis minor muscle, each cord ends in two major terminal branches: the musculocutaneous nerve (from the posterior cord), axillary nerve (from the lateral cord), radial nerve (from the posterior cord), median nerve (from the lateral and medial cords), and ulnar nerve (from the medial cord). ^1-5

Imaging technique --The BP can be difficult to image due to its complex anatomy, oblique orientation, and location in the neck and axilla where there are often magnetic susceptibility effects. The goal of imaging the brachial plexus is to produce high spatial resolution with adequate signal-to-noise ratio (SNR) while keeping scan time acceptable. To that end, it is important to strike a balance in the number of sequences, matrix size, number of excitations (NEX), and field of view (FOV) (Table).

Following an axial localizing scan, coronal and sagittal images are obtained. Coronal images are approximately parallel to the long axis of the plexus. Sagittal or oblique sagittal images, on the other hand, are oriented approximately perpendicular to the long axis of the plexus; these are useful to evaluate the cross-sectional anatomy of the BP and adjacent landmarks. A small FOV (14 to 26 cm) is used for both planes. Slice thickness ranges from 4 to 5 mm with a 0.5 to 2 mm interslice gap in the sagittal plane, and from 3 to 4 mm with a 0.5 mm gap in the coronal plane. Matrix size is high: generally 256 (frequency) * 256 (phase) for a 14- to 16-cm FOV and 512 * 256 to 512 for an 18- to 26-cm FOV. Standard, commercially available respiratory motion compensation is used along with a spatial presaturation band over the heart and aortic root to suppress flow artifacts. A phased-array radiofrequency coil is preferred to a body or conventional surface coil in order to obtain high SNR and adequate coverage. ^1,2

T1-weighted spin-echo (SE) images are useful to identify anatomic landmarks and evaluate regional anatomy. These are acquired in both the coronal and oblique sagittal planes. T2-weighted fast spin-echo (FSE) images with fat saturation are used to detect abnormally increased signal in injured nerves. Fat saturation is performed using either a frequency-selective (chemical shift) or short-tau inversion recovery (STIR) technique. While chemical shift fat saturation has a higher SNR and fewer blood-flow related artifacts than the STIR technique, it suffers from inhomogeneous fat suppression--a result of bulk susceptibility effects in the neck and shoulder region. In our experience, the STIR method provides more homogeneous fat suppression while main-taining excellent T2 contrast. ¹ Additional STIR images in the axial plane are excellent for the evaluation of neural foramina and exiting nerve roots in cases of suspected nerve root avulsion/pseudomeningocele. Postcontrast, T1-weighted frequency-selective fat saturation images are obtained for cases involving intraspinal/transforaminal spread of tumor or infection, in postoperative evaluation, and in cases of stretch injury or nerve entrapment. ^1,2,4,5,7 The standard contrast dose of 0.1 mmol/kg is injected into the antecubital vein on the contralateral side.

Pathologic conditions --In normal individuals, the major peripheral nerves are bilaterally symmetric, linear (in-plane images) (figure 1), oval or round (cross-sectional images) (figure 2) structures often delineated by surrounding fat. They generally are isointense on T1-weighted images and mildly hyperintense to adjacent muscle on heavily T2-weighted images. ^1,4-6 Using high resolution technique, the rod-like fascicles within the nerve usually are visible on cross-sectional images. Enlargement of the nerve, alteration in a fascicular pattern, or markedly increased signal intensity on STIR and postcontrast images are abnormal findings. ¹ Abnormal hyperintense signal has been shown to correlate with clinical dysfunction. ⁷

Pathologic changes result from traumatic or atraumatic processes. Blunt or penetrating trauma with fracture, dislocation, or hematoma formation may produce compression, stretching, or avulsion of the BP with diffusely increased signal on T2-weighted images and postcontrast enhancement (figure 3). Long, uniform involvement of the nerve without eccentric masses favors the diagnosis of stretch injury over neoplasia. Occasionally, a posttraumatic neuroma may develop. These are usually well circumscribed, mass-like areas of T2 hyperintensity within the nerve. Nerve root avulsions are associated with a dural tear resulting in the formation of a traumatic pseudo-meningocele, seen as a focal CSF collection extending through the neural foramen. Unlike stretch injuries, in which the degree of stretching usually is insufficient to cause permanent paralysis, nerves completely avulsed from the spinal cord produce fixed deficits. ⁸

Atraumatic processes include neoplasms--either primary tumors of neural origin or other tumors--which secondarily involve the plexus. For example, schwannoma and neurofibroma are benign primary neoplasms of neural origin. Schwannoma is typically solitary and causes focal enlargement of a nerve. The nerve is isointense or slightly hyperintense to muscle on T1-weighted images and markedly hyperintense on T2-weighted images (figure 4). Plexiform neurofibromas usually are multiple and may diffusely enlarge many components of the BP. Neurofibrosarcoma is a malignant nerve sheath neoplasm seen in 3% to 13% of patients with neurofibromatosis. Unfortunately, because it has similar signal characteristics to benign nerve tumors, it cannot reliably be distinguished. ⁵

Tumors that may invade the brachial plexus, ribs, and vessels include superior sulcus (Pancoast) carcinoma, breast carcinoma, lymphoma, and spinal metastases. As with other pathologic processes, these are typically isointense to muscle on T1-weighted images and hyperintense on T2-weighted images. ^1,2,4-6,8 Radiation plexopathy is unlikely to occur with less than 6000 Rads of exposure, although other factors, including dose regimen and previous regional surgery, may facilitate radiation injury. Unlike metastases which are often painful and involve the lower trunk (C8-T1), radiation plexopathy is usually painless and involves the upper trunk (C5-6) or the entire plexus. On imaging, diffuse symmetric thickening or irregularity of the BP without a focal mass is noted. These changes usually are isointense to muscle on T1-weighted images and can show postcontrast enhancement. On T2-weighted images, nerves and surrounding connective tissue may be hyperintense or isointense to muscle. ²

Sacral plexus

Anatomy --The sacral plexus is formed from the ventral rami (roots) of L4 to S4. It innervates muscles of the buttocks and posterior thigh, as well as muscles below the knee. The plexus roots are situated anterior to the sacrum and the piriformis muscle. They split into anterior and posterior divisions which subsequently give rise to anterior and posterior branches of the plexus. Anterior branches include the tibial part of the sciatic nerve and pudendal nerve, and the medial part of the posterior femoral cutaneous nerve. Posterior branches include the common peroneal part of the sciatic nerve, the superior and inferior gluteal nerves, the lateral part of the posterior femoral cutaneous nerve, and the nerve to the piriformis muscle.

Individual nerves may be difficult to distinguish on axial T1-weighted images, as they may interdigitate with serrations in the muscle. At the level of the greater sciatic foramen, the sacral plexus and surrounding vessels generally have a dot-dash configuration in the premuscular fat (figure 5). ^1,9-11

The sciatic nerve is the continuation of the sacral plexus. It is formed at the inferior margin of the piriformis muscle, from the L4 to S3 roots. It exits the pelvis through the greater sciatic foramen along with the piriformis muscle and the superior and inferior gluteal vessels and nerves (figure 6). The sciatic nerve courses laterally in the gluteal region, reaching the posterior portion of the upper thigh. It then courses inferiorly, dividing into the common peroneal and tibial nerves at the lower third of the femur. ^1,9-11

Imaging technique --Using a phased-array RF coil, the sacral plexus is routinely scanned in two planes (Table). First, a pad is placed beneath the patient's knees to decrease lumbar lordosis. Spatial RF pulses are placed superior and inferior to the imaging volume to decrease signal intensity from flowing blood. Following a sagittal localizing image, axial and coronal precontrast and fat suppressed postcontrast T1-weighted images are obtained. Axial STIR or fat suppressed, FSE T2-weighted images are usually obtained because susceptibility effects are less pronounced in the pelvis than at the cervicothoracic junction. Oblique "coronal" images, oriented parallel to the plane of the sacrum, may be obtained to aid in evaluation of the sacral foramina and proximal S1 to S4 roots. Slice thickness ranges from 4 to 5 mm, with no interslice gap for both cross-sectional (axial) images and in-plane (coronal) images. The FOV is usually 22 to 26 cm with a 512 * 256 to 512 matrix. ^1,9-12

Pathologic conditions --Sacral plexopathy may be due to a variety of etiologies including vascular, degenerative, infectious, and neoplastic disease. Trauma is less likely to produce a sacral rather than a brachial plexopathy. ⁹ Nervous compression may result from retroperitoneal or psoas hematoma, aneurysm or pseudoaneurysm of the iliac or gluteal arteries, lumbar disc herniation, or vertebral osteomyelitis with associated psoas or iliacus abscess.

Neoplastic disease includes primary nerve sheath tumors (schwannoma [figure 7] or neurofibroma), intraperitoneal tumors (colon carcinoma), retroperitoneal tumors (renal cell carcinoma, lymphoma), extraperitoneal tumors (uterine leiomyoma/leiomyosarcoma, prostate carcinoma, cervical carcinoma, rectal carcinoma), and distant metastases (lung and breast carcinoma) (figure 8). Clinically, sarcomas tend to produce a lumbar plexopathy (L1-L4), colorectal neoplasms a sacral plexopathy (L5-S3), and genitourinary tumors a lumbosacral panplexopathy. ⁹ On imaging, T1-weighted sequences demonstrate obliteration or replacement of fat planes around the nerves. T1-weighted, postcontrast, fat suppressed images help to distinguish enhancing tumor from nonenhancing muscle and to detect perineural spread. ^1,9,12 AR

References

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2. Bowen B: Brachial plexus. In: Bradley WG, Stark DD (eds): Magnetic Resonance Imaging, ed 3, pp 1821-1832. Philadelphia, Mosby-Year Book, 1999.

3. Blair D, Rapoport S, Sostman HD, et al: Normal brachial plexus: MR imaging. Radiology 165:763-767, 1987.

4. Sherrier R, Sostman HD: Magnetic resonance imaging of the brachial plexus. J Thorac Imaging 8(1):27-33, 1993.

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6. Gupta RK, Mehta VS, Banerji AK, Jain RK: MR evaluation of brachial plexus injuries. Neuroradiology 31:377-381, 1989.

7. Hiehle J Jr., Tsuruda J, Kilot M, et al: MR evaluation of the brachial plexus using high resolution phased array coils. Presented at the 32nd Annual Meeting of the American Society of Neuroradiology, Nashville, TN, May 1-7, 1994.

8. Rapoport S, Blair D, McCarthy S, et al: Brachial plexus: Correlation of MR imaging with CT and pathologic findings. Radiology 167:161-165, 1988.

9. Bowen B: Lumbosacral plexus. In: Bradley WG, Stark DD (eds): Magnetic Resonance Imaging, ed 3, pp 1907-1916. Philadelphia, Mosby-Year Book, 1999.

10. Gierada D, Erickson S, Haughton V, et al: MR imaging of the sacral plexus: Normal findings. Am J Roentgenol 160:1059-1065, 1993.

11. Blake L, Robertson W, Hayes C: Sacral plexus: Optimal imaging planes for MR assessment. Radiology 199:767-772, 1996.

12. Gierada D, Erickson S: MR imaging of the sacral plexus: Abnormal findings. Am J Roentgenol 160:1067-1071, 1993.