Diagnostic and Interventional Imaging (2014) 95, 17—26 REVIEW / Neuroradiology Current and future imaging of the peripheral nervous system a,∗ b c d M. Ohana , T. Moser , A. Moussaouï , S. Kremer , e f d R.Y. Carlier , P. Liverneaux , J.-L. Dietemann a Department of Radiology, Nouveau Civil Hospital — Strasbourg University Hospitals, 1, place de l’Hôpital, 67000 Strasbourg, France b Department of Radiology, Montreal University Hospitals —Notre-Dame Hospital, 1560 Sherbrooke East, Montreal (Quebec), Canada c Department of Radiology, Sainte-Odile Clinic, 6, rue Simonis, 67100 Strasbourg, France d Department of Radiology II, Hautepierre Hospital — Strasbourg University Hospitals, avenue Molière, 67098 Strasbourg, France e Department of Radiology, Raymond-Poincaré Hospital, 104, boulevard Raymond-Poincaré, 92380 Garches, France f SOS main, CCOM — Strasbourg University Hospitals, 10, avenue Baumann, 67403 Illkirch, France KEYWORDS Abstract Peripheral nervous system (PNS) imaging is usually carried out by ultrasound and MRI. Peripheral nervous Thanks to its wide availability and excellent spatial resolution, ultrasound is a mature investiga- system; tion with clearly established indications, particularly in entrapment syndromes and tumors. MRI Ultrasound; is generally a second-line examination, which provides decisive additional information thanks to MRI; its excellent contrast resolution and its multiplanar abilities. This review describes the current Neurography methods for imaging the PNS, concentrating on acquisition techniques, normal results and basic pathological semiology. Ongoing and future developments are described in order to underline the forthcoming changes in this very dynamic field of musculoskeletal radiology. © 2013 Éditions françaises de radiologie. Published by Elsevier Masson SAS. All rights reserved. The nervous system is divided into the central nervous system (CNS) which consists of the brain, spinal cord and retina, and the peripheral nervous system (PNS) which is made up of all of the other nervous structures [1]. The PNS is therefore made up of nerves, ganglions, various receptors and synaptic and motor nerve endings, which carry CNS commands to the effector organs and return internal and external sensory informations. Each structure is essential in transmitting the nerve impulse, but because of its length, specific histology function and exposure to diseases, the peripheral nerve remains the main focus of PNS imaging. ∗ Corresponding author. E-mail address: [email protected] (M. Ohana). 2211-5684/$ — see front matter © 2013 Éditions françaises de radiologie. Published by Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.diii.2013.05.008 18 M. Ohana et al. Macrophages then clean up the cell debris and release A few pathophysiologic concepts growth factors which stimulate axonal development. Sev- Histology eral buds therefore develop from the axon stump. One of these buds becomes predominant and keeps advancing in a A peripheral nerve is a rope-shaped organ which has a spe- proximal to distal direction, guided and stimulated by the cific concentric organization designed to guide, protect and remaining endoneural environment. Nerve regrowth occurs nourish neuronal fibers within it. The main structure is the at a rate of 1 to 2 mm per day and its effectiveness increases axon [1] which is a long extension of the neuronal cell body in positive correlation with the closeness of the section mar- specialized in transporting the nerve impulse. It is constantly gins. Should the nerve margins be too far apart, the axonal sheathed in a support cell, the Schwann cell, whose special- buds will no longer have a guide and may grow anarchically, isation differentiates two types of fibers: forming a mass, the amputation neuroma. • unmyelinated fibers, surrounded by a single small diam- This clearly defined degeneration followed by axonal eter layer of Schwann cytoplasm, which conducts slowly regrowth sequence only occurs completely and effectively and mostly has sensory (pain) and vegetative functions; in clear-cut nerve section or focal nerve compression. This • myelinated fibers, around which the Schwann cell wraps can be seen in the experimental setting (by tying off the in multiple layers to form the myelin sheath. These nerve in the nerve crush model [4]) or clinically in a wound myelinated fibers vary in diameter, are generally involved from a sharp object such as a blade or a broken piece of in rapid, responsive conduction and constitute all the glass. somatic motor fibers and a large proportion of the sensory fibers. Classification of post-traumatic peripheral nerve injuries These myelinated and unmyelinated peripheral nerve fibers lie in a loose connective tissue called the Traumatic nerve injuries, however, involve a far greater endoneurium, made up of fibroblasts, a collagen matrix spectrum of damage, ranging from simple compression to and blood capillaries. They group in bundles, each bun- total destruction with loss of tissue. The more disorganized dle being demarcated by a solid concentric cell layer, the the ultrastructure of the nerve has become, the less likely perineurium. Several fascicles (from a few units to a hun- nerve regrowth is to occur. dred) are grouped together to form the actual nerve trunk, Several nerve damage classifications have been described which is separated from the surrounding environment by to reflect this relationship. The most widely used is the Sun- a dense fibrous connective tissue, the epineurium, within derland classification (1978) inspired by the work of Seddon which many blood vessels, the vasa nervorum, circulate. (1943) [5]. It consists of five stages [6]: This concentric histology [2] is shown schematically in Fig. 1. • Sunderland I (neuropraxia according to Seddon): a local conduction block secondary to focal demyelination, therefore involving destruction of the myelin sheath but Wallerian degeneration with no damage to the axon or rupture of the endoneural After section or crush, axons forming the peripheral nerves support tissue. Recovery occurs fully within 12 weeks; • are able to regenerate slowly [1]. Sunderland II (axonotmesis according to Seddon): this In the first hours after an injury, the axon and the involves a loss of axonal continuity, with complete dis- Schwann cells forming its myelin sheath begin to disinte- tal Wallerian degeneration. The supporting connective grate. This starts immediately distal to the site of injury and tissue is preserved and the endoneural tubes guide the follows a proximal to distal path. This phenomenon, due to proximal to distal axonal regrowth (‘‘Schwann stakes’’). the post-traumatic interruption of axonal flow, and there- Complete recovery usually occurs but requires several fore the shortage of the essential substances for the axon months, depending on the distance which has to be ren- to survive, is known as Wallerian degeneration [3] and lasts ervated; • between one and two weeks, depending on the length of the Sunderland III: the damage locally destroys the axon, the nerve involved. myelin sheath and the endoneural tubes. The perineurium Figure 1. Schematic anatomy of the peripheral nerve. Current and future imaging of the peripheral nervous system 19 and epineurium remain intact. Regrowth is variable but generally incomplete because of incorrect orientation of the fibers and trapping in the endoneural scar; • Sunderland IV: only the epineurium remains intact with endoneural scarring and loss of endoneural and perineural continuity. The scarring prevents regrowth of the fibers and results in the formation of a neuroma; • Sunderland V (neurotmesis according to Seddon): the nerve is totally dislocated, affecting the endoneurium, epineurium and perineurium. As in stage IV, only surgery can offer a hope of nerve regrowth. Muscle denervation The section of a motor nerve has direct consequences on the innervated skeletal muscle, the first one being paralysis. Wallerian degeneration ultimately reaches the neu- romuscular junction, which is thereby destroyed. The denervated muscle fiber will at first develop vasogenic edema [7]. These abnormalities are seen electromyographi- cally from 48 hours [8], and a typical histological appearance is seen after 3 weeks [9]. These changes usually reverse Figure 2. Sixty-one-year-old man. Curvilinear reconstructions of when the endplate regenerates. the right sciatic nerve from a lower limb CT angiogram. If the denervation persists, metabolic changes within the muscle will progress to muscle fiber atrophy and an Table 1 Advantages and disadvantages of CT scanning increase in their fat content. Chronic denervation leads to to examine the peripheral nervous system (PNS). diffuse fatty infiltration of the affected muscles after sev- eral months [10]. Advantages Disadvantages Good spatial resolution Irradiation Excellent bone contrast Poor contrast Current PNS imaging methods Large field of view resolution Volume acquisition with Occasionally requires PNS imaging relies mostly on ultrasound and MRI: com- multiplanar iodinated contrast puted tomography has a more limited input. Conventional Reconstructions (MPR) injection radiography has only indirect applications, researching bony Readily available Generally provides only causes in entrapment syndromes (fracture, supernumerary Quickly performed indirect information on bone, bone callus, etc.) [11,12]. Interventional guiding for PNS disease diagnostic and therapeutic Computed tomography procedures CT is useful in diagnosing intradural, intraforaminal