Imaging for Peripheral Nerve Disorders

Published on 13/03/2015 by admin

Last modified 22/04/2025

Print this page

This article have been viewed 2390 times

Chapter 196 Imaging for Peripheral Nerve Disorders

Nerves can be seen in computed tomography (CT) scans and in routine magnetic resonance imaging (MRI) scans. However, because they are similar in size, shape, and location to other structures such as blood vessels and lymphatics, they cannot be identified reliably at all locations along their length unless specialized tissue-specific imaging strategies are applied. The optimal strategies provide both definitive identification of a nerve at any point along the nerve and also demonstrate pathology based on an intrinsic image signal from the nerve itself.

The basis for tissue-specific MRI of nerve derives from the existence of a variety of unique types of tissue water in nerve. The three major types of neurographic techniques that have been described are diffusion neurography, diffusion tensor tractography–neurography, and T2-based neurography. It is conventional to use the term tractography for neural tracts in the brain and spinal cord and to use the term neurography for peripheral nerve; however, peripheral nerve tractography is an appropriate description when the nerve images are based on assembling and linking calculated vectors rather than on direct image intensity measurements.

Diffusion neurography and the closely related diffusion tensor tractographic technique were the first methods reported.¹^–³ These methods have extremely high selectivity for nerves and should be very sensitive to a variety of types of more subtle pathology. However, the technical demands necessary to perform diffusion neurography have delayed its clinical application.⁴^–⁶ In addition, diffusion images tend to degrade in detail at locations of more severe pathology.

T2-based neurography, however, can be applied on a reliable basis on many existing top-quality clinical scanners with minor modifications. Even when the technical equipment requirements can be met, there may be limitations on the provision of MR neurography because of the lack of expertise in prescribing, postprocessing, interpreting, and acting upon the MR data. Once technical limitations are resolved, however, T2 neurography has the advantage of improving in quality of detail as the severity of the nerve abnormality increases, which is the opposite of diffusion- based neurography–tractography techniques.

Diffusion and Diffusion Tensor Neurography Techniques

Neural tissue is among several tissues in the body that demonstrate the property of linearly correlated water diffusion. Instead of diffusing freely in any direction (isotropic diffusion), the water molecules in neural tissue are restricted in their diffusion so that they tend to move in a directed fashion along the main longitudinal axis of the neural tissue (anisotropic diffusion). The detailed biophysical basis of this restricted diffusion is still not fully understood because water molecules should move freely through cell membranes.

In diffusion-based imaging, tissue contrast arises because it is possible to use the relative degree of diffusional isotropy and anisotropy in a given tissue to affect the rate of signal decay in that tissue. MR images can be sensitized to anisotropy by applying magnetic field gradients during the pulse sequence of the image acquisition process. When diffusion carries a proton to a new position in the gradient, it experiences a slightly different local magnetic field strength, and so its spin rate changes, resulting in desynchronization of the spins. When water diffuses equally in all directions in a tissue, the rate of this cause of signal decay is not affected by how the gradient is oriented.

When there is anisotropic diffusion, however, there is a large impact on the decay rate if the gradient is directed strictly perpendicular or strictly parallel to the direction of the nerve. When the uniform components of the gradient are perpendicular to the nerve and the slope of the gradient is parallel to the nerve (progressing from strong to weak along the length of the nerve), the nerve protons move rapidly to new positions in the gradient and their signal decays more rapidly than in an isotropic tissue. When the reverse is true, then the protons stay in the same field strength region of the gradient as they diffuse along the longitudinal axis of the nerve and they show little or no signal decay relative to surrounding isotropic tissues.

When a strong diffusion gradient is applied in the appropriate direction along with fat suppression and heavy T2 weighting, it is possible to produce a relatively pure nerve image. This is a method that has been applied to produce whole-body neurography images.^7,⁸

For both peripheral and central nervous system (CNS) neural tissue, Filler and colleagues ⁹^–¹² pointed out that it was possible to measure each voxel (three-dimensional pixel) in an image volume using three or more gradients in order to calculate both the magnitude and the direction in three-dimensional space of the anisotropy. These two data components could then be used to depict actual curving neural tracts rather than to simply display two-dimensional contrast differences in cross section. When six or more gradient directions are used, the vector processing can be accomplished using a tensor model and the result becomes very robust across any curve or angle of progression. Vector models are used with as many as 256 directions of data acquisition. The end result is to determine the orientation of the primary characteristic vectors (eigenvector in tensor models) in a voxel. This can be done to determine more than one axonal direction in a voxel (crossing fibers) when large numbers of acquisition directions are used in vector methods.

Until this technologic development took place, the relative amount of apparent diffusion had been used as a signal intensity to assign contrast to a given image pixel in a cross section or to assign colors to regions of neural tissue that responded to a given gradient direction. In tractography, however, each voxel can be represented by an arrow that is a vector in three-dimensional space that provides a summary estimate of the main direction of the complex tensor that describes the entire pattern of diffusion in the nerve. The arrow also has a magnitude that encodes the degree to which water in that voxel is diffusing in an anisotropic fashion relative to isotropic diffusion.

When the arrows are followed from voxel to voxel, the result is a linear tract image that moves through the three-dimensional volume of the tissue, following the course of the neural tissue. A variety of three-dimensional computer graphic techniques are now used to assemble tractographic images from these linear anisotropy traces. This is the basis of tractographic diffusion tensor imaging (DTI).

When diffusion images are obtained in multiple different directions for a DTI study, the raw diffusion images can be helpful for identifying the nerve. This step can be done using the original images because they suppress signal from vessels and other structures that are bright on T2-weighted fat-suppressed images, leading to relatively pure nerve images depending on the orientation of the nerve and the image at a given location. Unlike fully processed DTI images, the raw diffusion component images remain embedded in the original DICOM (Digital Imaging and Communications in Medicine) format spatial grid and can be precisely correlated with T2 neurographic images obtained in the same imaging session (Fig. 196-1).

FIGURE 196-1 Use of diffusion tensor image (DTI) data for nerve identification. This series of images demonstrates the use of diffusion-weighted components of a DTI acquisition (a-ii) for nerve identification in the same patient as in Figure 196-4. The asterisks track the L5 spinal nerve as it progresses through to the sciatic nerve (f to ii). The arrows track the L3 spinal nerve (left arrow) and L4 spinal nerve (right arrow) as they progress into the femoral nerve (a to f) and then mark the femoral nerve (f to ii). The arrowheads (g to ii) show the progress of the obturator nerve as it forms from anterior L4 and L5 branches then descends through the obturator foramen. The lines track the S2 contribution to the pudendal nerve (l to dd). Appropriately selected DTI component images contain relatively pure nerve images with very significant suppression of vessels and other structures that are bright on T2-weighted images. Note that the matched B0 (non-diffusion) component of the DTI acquisition obtained a few seconds earlier in the same patient shows numerous bright structures at the inguinal ligament that make identification of the femoral nerve and obturator nerve impossible without reference to the diffusion image.

T2-Based Neurography

Once the diffusion method was understood, it was possible to show that structures with long decay times (imaged at a relatively long echo time) in fat-suppressed spin-echo images were, in fact, nerves. Previously, nerves had been misinterpreted as exhibiting short decay times on T2 imaging.¹³ This was because nerves are a mixture of different tissues including protein-laden axoplasmic water, myelin, fatty interfascicular epineurium, and connective tissues. Older methods allowed the image signal from these various component tissues to mix. In a variety of different imaging techniques, the result of the image signal mixing was a featureless gray image of the nerve that left nerves difficult to distinguish clearly in an image and caused confusion about the fundamental imaging characteristics of nerve.

In a number of settings, MR neurography has proved to be more efficacious than electrodiagnostic studies for identifying nerve compressions that will improve with surgical treatment. This is true both in diagnoses that are typically evaluated by electrodiagnostics, such as carpal tunnel syndrome,^14,¹⁵ and in diagnoses in which electrodiagnostics have proved difficult to rely on, such as thoracic outlet syndrome, piriformis syndrome, and related sciatic nerve entrapments and pudendal nerve entrapment syndromes.^11,¹⁶^–¹⁸

Utility of MR neurography has now been established in evaluation of entrapment syndromes,^16,¹⁹^–²² nerve injury and evaluation of repair,²³ and assessment of nerve tumors,²⁴^–²⁶ as well as in the setting of neuritis and a variety of neuropathies.²⁷ It has also proved effective for evaluating nerve disorders affecting young pediatric patients such as obstetric brachial plexus palsy.²⁸

Classes of Image Findings

Image findings in MR neurography studies include regions of nerve hyperintensity or nerve swelling, which result from edema at the fascicular level (Fig. 196-2). Distortions of normal nerve course, abnormal contours, and alterations of nerve caliber are also readily seen, any of which can be classed by the degree or severity of the abnormality. These findings can indicate entrapment or adhesions as well as post-traumatic effects.

FIGURE 196-2 Varying degrees of severity of brachial plexus entrapment in thoracic outlet syndromes. A, Linear plexus with short segment of mild irritative changes near the later border of the scalene triangle. B, Evidence of more-restrictive fibrosis associated with narrowing and brightening of plexus elements near the scalene border. Note the linear plexus despite the elevated shoulder. C, Short segment of marked hyperintensity with slight swelling. D, Severe multiple element abnormality with narrowed and swollen segments and marked hyperintensity. E, Linear normal plexus with isolated focal impingement of C5 spinal nerve, just proximal to the scalene triangle. F, Fibrous band causing sharp downward distortion of the mid and lower trunk proximal to the scalene triangle, with second sharp upward distortion of the lower trunk near the scalene insertion at the first rib. G, Moderate restrictive impingement of the plexus at the scalene triangle causing generalized distortion of the course of the plexus with short segment of focal hyperintensity. H, Patient presenting with severe pain, numbness, and weakness from progressive thoracic outlet syndrome. Multiple points of sharp nerve course distortion with edema and hyperintensity affecting multiple brachial plexus elements.

(From Filler AG: Imaging for peripheral nerve disorders. In Winn HR (ed): Youmans Neurological Surgery. Philadelphia: Elsevier, 2011, pp 2413-2426.)

In trauma, assessments of nerve continuity and/or location of severed nerve endings are feasible. With the elapse of time after injury or repair, the development of traumatic neuromas may be readily appreciated (Fig. 196-3).

FIGURE 196-3 Confirmation of total nerve disruption in trauma. A, The right brachial plexus of a 15-year-old with flail arm, 2 months after a motorcycle accident. The image demonstrates gross discontinuities in the upper plexus elements (ue), meningocoeles proximally (me), and bright swollen nerve trunks (st). B, Disconnected and retracted lower trunk (lt) in traumatic injury of brachial plexus.

(From Filler AG, Maravilla KR, Tsuruda JS: MR neurography and muscle MR imaging for image diagnosis of disorders affecting the peripheral nerves and musculature. Neurol Clin 22(3):643-82, vi-vii, 2004.)

Imaging in The Setting of Nerve Entrapment and Pain

Thoracic Outlet Syndromes

Thoracic outlet syndromes have been problematic diagnostically because of the difficulty of obtaining surgically useful localization of brachial plexus compression sites. Indeed, the limitations in the quality of diagnostic techniques have led to ongoing disagreements about the very existence of this class of nerve entrapment syndromes.

In part these problems arise because of the difficulty of placing diagnostic electrodes for electrophysiologic studies in this region. Also, because many cases of thoracic outlet syndrome are due to nerve compression by the scalene muscle, it is often the case that patients present with pain elicited by motions such as arm raising but do not have sufficient motor symptoms to produce an abnormal electromyogram (EMG). Finally, the diagnosis of thoracic outlet syndrome is greatly complicated because the term describes a variety of diverse underlying pathologies that have in common only the fact that they affect some part of the brachial plexus.

MR neurography has shown promising capabilities for discriminating among several different types of thoracic outlet syndrome lesions and in many cases can depict well-delineated and readily treatable specific pathologies.^29,³⁰ These include distortions of the course of the proximal elements at the scalene triangle (see Fig. 196-2A-D), fibrous band entrapments affecting C8 and T1 spinal nerves and the lower trunk of the brachial plexus (see Fig. 196-2F), gross distortions of the mid-plexus (see Fig. 196-2G and H), irritation at the level of the first rib, and distal plexus irritation.

MR neurography is also helpful when it demonstrates a completely normal course and caliber for all nerve elements that are under suspicion. In the presence of normal findings, it is possible to take a firm stance against surgical treatment of the hand, arm, or shoulder problems that might have been tentatively attributed to a thoracic outlet syndrome.

Brachial Plexus Neuritis

In addition to surgically treatable entrapments, MR neurography is also helpful in identifying patients with complaints attributable to nerve inflammations.^27,³¹ These patients often present with painless hand atrophy, with no associated sensory abnormality. This has sometimes been termed a Gilliatt-Sumner hand syndrome. Brachial plexus elements appear bright and swollen but demonstrate no distinct evidence of impingement.

Lumbar Foraminal Pathology

The normal course of the lumbar spinal nerves is in a smooth straight line. In patients with a nondiagnostic MRI and myelogram, MR neurography can demonstrate distortions of the course of the exiting nerve in the distal foramen.

Even when myelography is capable of demonstrating a nerve root cut off, an MR neurogram can provide considerable additional information. The full length of the foraminal impingement requiring treatment can often be better appreciated in a neurographic image. In addition, the ability of MR neurography to demonstrate hyperintensity in the lumbar root adjacent to the impingement helps confirm the clinical significance of the impingement.²⁷

Piriformis, Pudendal and Ilioinguinal Pelvic Entrapments

MR neurography has had a significant impact on diagnosis of nerve entrapments in the pelvis.^16,^17,³²^–³⁴ Although sciatic pathologies have been an important part of the advance, the ability of MR neurography to track other nerve elements in the pelvis such as the ilioinguinal nerve, the pudendal nerve, and the lateral femoral cutaneous nerve has gone a long way toward resolving what had been a troublesome black box. Imaging of the complete course of the L4 spinal nerve as it progresses into the femoral nerve has made it possible to search for abnormalities along the intra-abdominal and intrapelvic course that were previously almost impossible to diagnose (see Figs. 196-1 and 196-4).

A single imaging study can demonstrate the full course of the L5 and S1 spinal nerves and their descendants in the lumbosacral plexus and in the proximal sciatic nerve. It is very reliable for demonstrating the presence of a split nerve–split muscle configuration in the setting of a possible piriformis syndrome. Focal sciatic impingements at the ischial margin, at the tendon of the obturator internus, at the distal ischial tunnel on the lateral aspect of the ischial tuberosity, and at various locations in the thigh are readily distinguished.^17,^17a

Reliable identification of anatomic variants of the sciatic nerve now plays a critical role in improving the safety of surgeries for the release of pelvic sciatic nerve entrapment. Isolated section of a single piriformis segment in patients with a split nerve passing through a split muscle can cause severe nerve compromise from some types of piriformis surgery if this condition is not detected in advance.

Identification of the presence or absence of pudendal nerve irritation at its S2 or S3 spinal nerve origin in the variant condition of passage through the piriformis muscle, in the Alcock canal along the medial aspect of the obturator internus muscle or at the rectal branch of the pudendal nerve proximal to its entrance to the Alcock canal has also been quite useful clinically.^18,³⁴

Distal Entrapments

Distal entrapments benefit from MR neurography imaging when physical examination or electrodiagnostic studies show that locations other than the most routine sites may be involved. Such entrapments include less-common problems such as posterior interosseous nerve entrapment of the distal radial nerve in the forearm as well as common issues such as peroneal nerve entrapment at or above the fibular head, tarsal tunnel syndrome, cubital tunnel syndrome. and carpal tunnel syndrome. For instance, treatment of median nerve entrapment in the distal forearm can fail if only the flexor retinaculum is addressed. Ulnar entrapment in Guyon’s canal and proximal peroneal nerve entrapments along the tendon of the biceps femoris just distal to the sciatic bifurcation are other specialized issues that can best be investigated by imaging.

Identification of distal entrapments of cranial nerves can also be accomplished with neurographic imaging. This includes the inferior alveolar nerve in the jaw and the glossopharyngeal nerve as it descends in the upper neck, as well as imaging of proximal and distal branches of intracranial and extracranial facial nerve segments.

Nerve Trauma

MR neurography can be fairly efficient for establishing the existence of significant nerve injury ³⁵ and for demonstrating true discontinuities in nerve (see Fig. 196-3B). At present, there still exists a limitation in that hemorrhage immediately after a trauma can obscure some details of the nerve image in the first few days after an injury.

There is still no completely reliable means of confirming a true nerve root avulsion by imaging.³⁶ An MR neurography study includes an effective MR Myelogram (see Fig. 196-3A) and can therefore demonstrate the meningoceles that may be associated with nerve root avulsions. The procedure is less invasive than a CT myelogram and provides information of similar quality.

Obstetric Brachial Plexus Injury

A particularly complex situation arises in the management of obstetric brachial plexus injuries. Imaging of infants is difficult and the need for sedation entails significant additional risks. However, it has proved possible to identify growing terminal neuromas in some locations and to demonstrate continuity of nerves at other sites. As computer techniques for postprocessing of images have progressed, it has become possible to generate fairly detailed image representations of complex brachial plexus injuries in children younger than four months.²⁸

Follow-up of Nerve-Grafting Procedures

After nerve grafting or after primary early repair of a nerve laceration, several months must elapse before it becomes clear whether the graft has been successful. MR neurography has proved helpful for evaluating a nerve suture repair site when recovery did not ensue.²³

Nerve Tumors

MR neurography permits a directed examination for nerve tumors that has a greater sensitivity than other MR methods because of its reliability for showing that a given mass is actually inside a nerve or in direct continuity with it (Fig. 196-4), as well as for locating safe entry approaches to the mass by demonstrating the position of nerve fascicles on the exterior surface of the mass. In some cases, the level of detail also allows distinguishing among types of tumors such as the reported capability of distinguishing perineuriomas³⁷ from schwannomas and for identifying very small tumors that just slightly expand the caliber of a nerve. Imaging can play a role in the initial diagnosis of neurofibromatosis and is also helpful in tracking and analyzing advanced stages of the disease.

FIGURE 196-4 Complex tumor anatomy. This patient has a neurofibroma in the L5 spinal nerve, as well as a neurofibroma in the proximal femoral nerve, just distal to the intersection of the L3 and L4 spinal nerves in the upper portion of the lumbosacral plexus. FeN, femoral nerve.

Imaging of Denervated Muscle

In a variety of situations, MRI can be a useful adjunct to electrodiagnostic studies for evaluating muscle denervation. Denervated muscle becomes hyperintense when imaged with a standard T2-based MR neurography protocol. When this phenomenon is present, it greatly simplifies the task of identifying individual muscles involved in a selective focal or distal injury.

The development of signal alterations in muscle after denervation was first reported by Shabas ³⁸ and by Jolesz and colleagues.³⁹ It is now known that abnormal muscle appearance may be detected as early as 4 days after an axonotmetic or neurotmetic injury. The signal abnormalities become progressively more intense as months pass, reaching a maximum about 4 months from the date of injury. In the 1- to 2-year range, the quality of the abnormality changes but will persist unless the nerve regrows or is restored through grafting.

Additional information about clinical utility of muscle imaging appears in a report on imaging of 32 cases by West and colleagues.⁴⁰ They document a range of alterations in image appearance in a case study format.

Open MR-Guided Injections for Diagnosis and Treatment

Open MR-guided injections can be carried out in a low field (0.25 Tesla) imager that has a small-diameter magnet and in-room monitor ⁴¹ (Fig. 196-5). If the diameter of the magnet is about 3 feet, then it is possible for the surgeon to easily reach into the center of the system as imaging progresses. The in-room monitor allows the surgeon to watch the image in real time. These typically involve a projector that provides a large crisp image showing multiple adjacent image frames.

FIGURE 196-5 Optiguide Panorama 0.23T interventional magnetic resonance imaging (MRI) system. A, The system includes an open horizontal disk configuration magnet with an optical monitoring stand, a keyboard and operator stand and in-the-room projector-based monitor. B, The drum of the magnet is fitted with optical tracking spheres for reference, and similar spheres can be attached to a variety of instruments or pointers. C, The system has a table that provides side access and can be detached to roll out of the MR room. D, The monitor provides sufficient view for ready observation at a distance. E, Specialized interventional MRI software assists in the use of the system and integration of the optically acquired information.

Injection is done using a 22 gauge, 15-cm, or 10-cm titanium alloy needle. An injection should be about 10 ml of 0.5% bupivacaine and 1 ml of betamethasone into the piriformis muscle, 5 ml of bupivacaine for an anterior or middle scalene muscle, or 7 ml for an obturator internus muscle. Botulinum toxin can also be injected into selected deep muscle targets in this fashion. Injection along the course of the sciatic nerve in the ischial tunnel should be just 3 ml of bupivacaine , but it can include 300 units of hyaluronidase in 2 ml of sterile saline. Because of the large volume of bupivacaine, procedures should be carried out in a surgicenter setting.

The imaging during procedures is monitored with fast (10 to 15 second) three-slice image sets: one working image and the two adjacent image slices. These are obtained using a type of MRI called FLASH (fast low-angle shot) or FFE (fast field echo). The needle advance must be maintained in the center slice of the three-slice set so that an accurate depiction of needle depth is seen. Because the piriformis muscle is often no more than 1 to 2 cm thick (Fig. 196-6) and because the bowel is often adjacent just deep to the muscle, it is necessary to re-image once or twice with each movement of the needle. Lower-accuracy alternatives to open MRI include x-ray fluoroscopy, CT guidance, or EMG with ultrasound.⁴²

FIGURE 196-6 Examination under magnetic resonance imaging (MRI) and image guided injections. A, Examination under MRI carried out to distinguish piriformis muscle from sacrotuberous ligament tear as source of pain. Axial T1-weighted acquisition in open MR. ^∗, examiner’s finger; FH, femoral head; Pir T, piriformis tendon; Sa, sacrum; STL, sacrotuberous ligament. B, Open MR-guided injection of a very thin piriformis muscle adjacent to underlying bowel. Axial T1-weighted acquisition in open MR. C, colon; HR, hip replacement artifact in ischium; L, Lufkin titanium needle; Pir, piriformis muscle; Sci, sciatic nerve.

(Filler AG, Hill HG: Piriformis syndrome, obturator internus syndrome, pudendal nerve entrapment and other pelvic entrapments. In Winn HR (ed): Youmans Neurological Surgery, ed 6th. Philadelphia: Elsevier, 2011, pp 2447-2455.)