Seeing the Unseen:
Revelations From Peripheral Nerve MRI

MRI showing peripheral nerve lesions

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For 200 years, the anatomical basis of intraneural ganglia was not fully understood. Now it is. Intraneural ganglia, mucinous cysts in the epineurium of nerves, have been difficult to treat because of a high rate of recurrence after surgical intervention. New MRI techniques at Mayo Clinic have revealed the reasons why and have added substance to a new theory about the pathogenesis of intraneural ganglia. These same techniques, unique to Mayo Clinic, are also changing the management of peripheral nerve lesions and improving outcomes.

Kimberly K. Amrami, M.D., a Mayo Clinic musculoskeletal radiologist, and her colleagues have dramatically improved peripheral nerve lesion identification and localization by using improved high-resolution and interval MRI technology combined with MR neurography.

The MR neurographic technique, developed at the University of Washington, Seattle, in the mid-1990s, is a direct imaging technique that produces images of individual nerves and reveals actual nerve pathologic characteristics. Previous techniques relied on inferences derived from muscle denervation. Advances at Mayo Clinic have turned inferences into precision targeting and have shed light on pathogenesis.

Depending on individual case requirements, advanced MR nerve imaging techniques at Mayo Clinic include the following:

• Custom-made dedicated-radiofrequency coils, fabricated at Mayo Clinic
• Multiplanar MR images with the highest possible spatial resolution through 2-D and 3-D techniques
• Advanced postprocessing for optimal image reformatting and reconstruction

Customized imaging protocols

Imaging protocols are individualized, taking into account all of the patient's available clinical and diagnostic information. This individualization is particularly important, given both the distance that some peripheral nerves travel from the spinal cord to target muscles and the fact that there may be bilateral and multifocal involvement.

Dr. Amrami points out that the localization of a foot drop due to a lower motor neuron lesion may be anywhere from the L4 nerve to the deep branch of the peroneal nerve. Thus, determining how much anatomy to include for MR imaging depends on the patient's history, findings on physical examination (including presence of a Tinel sign), and results from EMG and other tests.

Dr. Amrami notes that balancing the precision gained from using the smallest possible radio frequency coils against the need to image large anatomical areas can be challenging. Multiple imaging sequences and, sometimes, several imaging sessions may be required for optimal results.

Examples of impact on outcomes

Schwannomas
Schwannomas, the most common type of benign tumor in the peripheral nervous system, have long been thought to be based on single-fiber involvement. Technical improvements in the past 20 years have made it possible to resect schwannomas at the fascicular level. Yet, Mayo neurosurgeon Robert J. Spinner, M.D., notes that resection has not always had the predicted outcome, regardless of surgical expertise. "The question is 'why?' And the answer turns out to be that not all schwannomas are the same," he says.

A retrospective review at Mayo Clinic, published in the February 2010 issue of Journal of Neurosurgery, found that in a subgroup of schwannomas, several fibers — not just 1 — were involved. Tumor resection in such cases can result not only in nerve loss, but also in loss of function. Preoperative and intraoperative assessment may not reveal the distinguishing features of this subgroup, but enhanced imaging techniques such as those used at Mayo Clinic can help guide the surgeon and influence preoperative patient counseling.

Localization for targeted biopsies
MR technologic advances have also greatly improved lesion localization for targeted fascicular biopsy. In the past, sampling was nonspecific, with results that did not necessarily enhance diagnosis or that caused further neurologic deficit. Dr. Amrami explains that, rather than identifying a general location such as the brachial plexus, her team typically can specify the size of the lesion and its precise location. In an increasing number of cases, their experience and expertise with a wide range of peripheral nerve disorders allow Dr. Amrami and her colleagues to diagnose the entity, as confirmed by biopsy performed in a safe manner.

Intraneural perineuriomas
Intraneural perineuriomas are a benign form of focal hypertrophic neuropathy affecting young people. Most often affecting major nerves, such as the sciatic, radial, and ulnar nerves and the brachial plexus, this condition is frequently underrecognized. Diagnosis rests on identifying the focal nerve enlargement through imaging and a targeted fascicular biopsy. MRI is particularly important for localization.

In the August 2009 issue of Brain, a Mayo Clinic study reported that standard MRI failed to reveal abnormalities in many of the 32 subjects, yet these abnormalities were revealed with Mayo's advanced high-field (3-Tesla) MRI with custom coils designed to image specific nerves.

Intraneural ganglion cysts
The 3-D imaging techniques used at Mayo Clinic have helped to substantiate a theory of intraneural ganglion cyst pathogenesis, the unifying articular (synovial) theory, formulated by Dr. Spinner and his colleagues in 2003. The theory, further elaborated on in the October 2009 issue of Neurosurgery, holds that synovial joints are the origin of the cysts that dissect along an articular branch of the involved nerve into the parent nerve. Without advanced imaging, the extent of such cysts and their connection to the joint via the twiglike articular branch could not be seen.

A prospective study of every case of peroneal intraneural ganglia treated at Mayo Clinic in the past 10 years (>30 cases), as well as a retrospective study (>30 cases), showed that the joint itself was always the source of the cyst. This finding helps explain previous recurrence after resection: typically, only part of the cyst was removed. Now, because the joint connection of the intraneural cyst has been established and can be seen on imaging, the articular branch connection can be purposefully disconnected at surgery, significantly improving surgical outcomes.

Intraneural ganglion cysts are rare; only 400 cases have been reported in the medical literature. However, major advances in imaging and radiology at Mayo Clinic are improving surgical outcomes and revealing pathogenesis for far more common peripheral nerve lesions, both benign and malignant.