Magnetic resonance neurography

Source: Wikipedia, the free encyclopedia.
Bilateral Split Sciatic Nerve

Magnetic resonance neurography (MRN) is the direct imaging of

diffusion tensor imaging
.

History and physical basis

Magnetic resonance imaging (MRI) is based on differences in the physical properties of

water molecules in different tissues in the body. The protons and the water molecules of which they are part have subtly different movement characteristics that relate to their biophysical
surroundings. Because of this, MRI is capable of differentiating one tissue from another; this provides "tissue contrast." From the time of the first clinical use of MRI in the mid-1970s until 1992, however, despite the active work of many thousands of researchers, there was no reliable method for visualizing nerve. In some parts of the body, nerves could be observed as areas of absent signal delineated by bright fat, or as bland grey structures that could not be reliably distinguished from other similar-appearing structures in cross sectional images.

In 1992, Aaron Filler and Franklyn Howe, working at

pulse sequence techniques that would make nerves imageable as well. Further, because they demonstrate water signal arising in the neural tissue itself, they can also reveal abnormalities that affect only the nerve and that do not affect surrounding tissues. More than three million patients seek medical attention every year for nerve-related disorders such as sciatica, carpal tunnel syndrome or various other nerve injuries, yet before 1992, no radiologists were trained to image nerves.[5]

There are two main physical bases for the imaging discovery. Firstly, it was known at the time that water diffused preferentially along the long axis of neural tissue in the brain – a property called "

T2
water" in the nerve and that this mostly affected isotropic water.

The endoneurial fluid compartment in nerve can be unmasked by similar techniques resulting in a "T2" based neurography[6] as well as the original diffusion based neurography technique. Endoneurial fluid increases when nerve is compressed, irritated or injured, leading to nerve image hyperintensity in an magnetic resonance neurography image. Subsequent research has further demonstrated the biophysical basis for the ability of MR Neurography to show nerve injury and irritation.[7]

Measurements of the T2 relaxation rate of nerve by Filler and Howe revealed that previous reports of a short relaxation time were wrong and that—once signal from

fat suppression
sequences used for neurography nerve imaging.

Within a few months of the initial findings on diffusion-based nerve imaging, the diffusion technique for nerve imaging was adapted to permit for visualization of neural tracts in the spinal cord and brain via Diffusion Tensor Imaging.

Clinical uses

The most significant impact of magnetic resonance neurography is on the evaluation of the large proximal nerve elements such as the

cervical spine and the underarm that innervate shoulder, arm and hand),[9] the lumbosacral plexus (nerves between the lumbosacral spine and legs), the sciatic nerve in the pelvis,[10] as well as other nerves such as the pudendal nerve[11]
that follow deep or complex courses.

Neurography has also been helpful for improving image diagnosis in spine disorders. It can help identify which spinal nerve is actually irritated as a supplement to routine spinal MRI. Standard spinal MRI only demonstrates the anatomy and numerous

bone spurs or stenoses that may or may not actually cause nerve impingement symptoms.[12][13]

Many nerves, such as the median and ulnar nerve in the arm or the tibial nerve in the tarsal tunnel, are just below the skin surface and can be tested for pathology with electromyography, but this technique has always been difficult to apply for deep proximal nerves. Magnetic resonance neurography has greatly expanded the efficacy of nerve diagnosis by allowing uniform evaluation of virtually any nerve in the body.[14][15][16][17]

There are numerous reports dealing with specialized uses of magnetic resonance neurography for nerve pathology such as traumatic brachial plexus root avulsions,[18] cervical radiculopathy, guidance for nerve blocks,[19] demonstration of cysts in nerves,[20] carpal tunnel syndrome, and obstetrical brachial plexus palsy.[21] In addition several formal large scale outcome trials carried out with high quality "Class A" methodology[22][23][24] have been published that have verified the clinical efficacy and validity of MR Neurography.

Use of magnetic resonance neurography is increasing in neurology and neurosurgery as the implications of its value in diagnosing various causes of sciatica becomes more widespread.

New England Journal of Medicine in July 2009 published a report on whole body neurography using a diffusion based neurography technique.[28] In 2010, RadioGraphics - a publication of the Radiological Society of North America that serves to provide continuing medical education to radiologists - published an article series taking the position that Neurography has an important role in the evaluation of entrapment neuropathies.[29]

Magnetic resonance neurography does not pose any diagnostic disadvantage relative to standard magnetic resonance imaging because neurography studies typically include high resolution standard MRI image series for anatomical reference along with the neurographic sequences. However, the patient will generally have a slightly longer time in the scanner compared to a routine MRI scan. Magnetic resonance neurography can only be performed in

claustrophobic
patients. Although it has been in use for fifteen years and is the subject of more than 150 research publications, most insurance companies still classify this test as experimental and may decline reimbursement, resulting in the need to file appeals. Patients in some plans obtain standard insurance coverage for this widely used procedure.

References

External links