Magnetic resonance neurography

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Bilateral Split Sciatic Nerve Bilateral Sciatic Neurography.jpg
Bilateral Split Sciatic Nerve

Magnetic resonance neurography (MRN) is the direct imaging of nerves in the body by optimizing selectivity for unique MRI water properties of nerves. It is a modification of magnetic resonance imaging. This technique yields a detailed image of a nerve from the resonance signal that arises from in the nerve itself rather than from surrounding tissues or from fat in the nerve lining. Because of the intraneural source of the image signal, the image provides a medically useful set of information about the internal state of the nerve such as the presence of irritation, nerve swelling (edema), compression, pinch or injury. Standard magnetic resonance images can show the outline of some nerves in portions of their courses but do not show the intrinsic signal from nerve water. Magnetic resonance neurography is used to evaluate major nerve compressions such as those affecting the sciatic nerve (e.g. piriformis syndrome), the brachial plexus nerves (e.g. thoracic outlet syndrome), the pudendal nerve, or virtually any named nerve in the body. A related technique for imaging neural tracts in the brain and spinal cord is called magnetic resonance tractography or diffusion tensor imaging.

Contents

History and physical basis

Magnetic resonance imaging (MRI) is based on differences in the physical properties of protons in 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 St. George's Hospital Medical School in London, succeeded in identifying the unique water properties of nerve water that would make it possible to generate tissue-specific nerve images. [1] [2] [3] [4] The result was an initial "pure" nerve image in which every other tissue was made to disappear leaving behind only the image of the nerves. The initial pure nerve image served as the basis of image processing techniques leading to discovery of a series of other MRI 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 "anisotropic diffusion". Diffusion MRI had been developed to take advantage of this phenomenon to show contrast between white matter and grey matter in the brain. However, diffusion MRI proved ineffective for imaging of nerves for reasons that were not initially clear. Filler and Howe discovered that the problem was that the most of the image signal in nerve came from protons that were not involved in anisotropic diffusion. They developed a collection of methods to suppress the "isotropic signal" and this resulted in allowing the anisotropic signal to be unmasked. This was based on the discovery that Chemical Shift Selection could be used to suppress "short 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 lipid protons was suppressed—the primary image signal from nerve had long T2 relaxation rates best imaged with pulse sequence echo times in the range of 50 to 100  milliseconds. In addition, they later showed that T2-neurography differs from most other MR imaging in that the conspicuity or relative prominence of nerve is affected by the angle of voxel orientation during the acquisition of the image. When acquisitions are done with echo times below 40 milliseconds, there can be "magic angle effects" [8] that provide some spurious information, so MR Neurography is always done with echo times greater than 40 milliseconds. The need for long echo times also characterizes the type of inversion recovery 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 brachial plexus (the nerves between 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 disk bulges, 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. [25] [26] There are 1.5 million lumbar MRI scans performed in the US each year for sciatica, leading to surgery for a herniated disk in about 300,000 patients per year. Of these, about 100,000 surgeries fail. Therefore, there is successful treatment for sciatica in just 200,000 and failure of diagnosis or treatment in up to 1.3 million annually in the US alone. The success rate of the paradigm of lumbar MRI and disk resection for treatment of sciatica is therefore about 15%(Filler 2005). Neurography has been applied increasingly to evaluate the distal nerve roots, lumbo-sacral plexus and proximal sciatic nerve in the pelvis and thigh to find other causes of sciatica. It is increasingly important for brachial plexus imaging and for the diagnosis of thoracic outlet syndrome. [27] Research and development in the clinical use of diagnostic neurography has taken place at Johns Hopkins, the Mayo Clinic, UCLA, UCSF, Harvard, the University of Washington in Seattle, University of London, and Oxford University (see references below) as well as through the Neurography Institute. Recent patent litigation concerning MR Neurography has led some unlicensed centers to discontinue offering the technique. Courses have been offered for radiologists at the annual meetings of the Radiological Society of North America (RSNA), and at the International Society for Magnetic Resonance in Medicine and for surgeons at the annual meetings of the American Association of Neurological Surgeons and the Congress of Neurological Surgeons. The use of imaging for diagnosis of nerve disorders represents a change from the way most physicians were trained to practice over the past several decades, as older routine tests fail to identify the diagnosis for nerve related disorders. The 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 1.5 tesla and 3 tesla cylindrical type scanners and can't really be done effectively in lower power "open" MR scanners - this can pose significant challenges for 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.

Related Research Articles

<span class="mw-page-title-main">Brachial plexus</span> Network of nerves

The brachial plexus is a network of nerves formed by the anterior rami of the lower four cervical nerves and first thoracic nerve. This plexus extends from the spinal cord, through the cervicoaxillary canal in the neck, over the first rib, and into the armpit, it supplies afferent and efferent nerve fibers to the chest, shoulder, arm, forearm, and hand.

<span class="mw-page-title-main">Sciatica</span> Lower back pain that extends down leg

Sciatica is pain going down the leg from the lower back. This pain may go down the back, outside, or front of the leg. Onset is often sudden following activities like heavy lifting, though gradual onset may also occur. The pain is often described as shooting. Typically, symptoms are only on one side of the body. Certain causes, however, may result in pain on both sides. Lower back pain is sometimes present. Weakness or numbness may occur in various parts of the affected leg and foot.

<span class="mw-page-title-main">Thoracic outlet syndrome</span> Medical condition

Thoracic outlet syndrome (TOS) is a condition in which there is compression of the nerves, arteries, or veins in the superior thoracic aperture, the passageway from the lower neck to the armpit, also known as the thoracic outlet. There are three main types: neurogenic, venous, and arterial. The neurogenic type is the most common and presents with pain, weakness, paraesthesia, and occasionally loss of muscle at the base of the thumb. The venous type results in swelling, pain, and possibly a bluish coloration of the arm. The arterial type results in pain, coldness, and pallor of the arm.

<span class="mw-page-title-main">Pancoast tumor</span> Medical condition

A Pancoast tumor is a tumor of the apex of the lung. It is a type of lung cancer defined primarily by its location situated at the top end of either the right or left lung. It typically spreads to nearby tissues such as the ribs and vertebrae. Most Pancoast tumors are non-small-cell lung cancers.

<span class="mw-page-title-main">Piriformis syndrome</span> Medical condition

Piriformis syndrome is a condition which is believed to result from compression of the sciatic nerve by the piriformis muscle. The largest and most bulky nerve in the human body is the sciatic nerve. Starting at its origin it is 2 cm wide and 0.5 cm thick. The sciatic nerve forms the roots of L4-S3 segments of the lumbosacral plexus. The nerve will pass inferiorly to the piriformis muscle, in the direction of the lower limb where it divides into common tibial and fibular nerves. Symptoms may include pain and numbness in the buttocks and down the leg. Often symptoms are worsened with sitting or running.

In medicine, a stinger, also called a burner or nerve pinch injury, is a neurological injury suffered by athletes, mostly in high-contact sports such as ice hockey, rugby, American football, and wrestling. The spine injury is characterized by a shooting or stinging pain that travels down one arm, followed by numbness and weakness in the parts of the arms, including the biceps, deltoid, and spinati muscles. Many athletes in contact sports have suffered stingers, but they are often unreported to medical professionals.

<span class="mw-page-title-main">Diffusion MRI</span> Method of utilizing water in magnetic resonance imaging

Diffusion-weighted magnetic resonance imaging is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast in MR images. It allows the mapping of the diffusion process of molecules, mainly water, in biological tissues, in vivo and non-invasively. Molecular diffusion in tissues is not random, but reflects interactions with many obstacles, such as macromolecules, fibers, and membranes. Water molecule diffusion patterns can therefore reveal microscopic details about tissue architecture, either normal or in a diseased state. A special kind of DWI, diffusion tensor imaging (DTI), has been used extensively to map white matter tractography in the brain.

<span class="mw-page-title-main">Nerve block</span> Deliberate inhibition of nerve impulses

Nerve block or regional nerve blockade is any deliberate interruption of signals traveling along a nerve, often for the purpose of pain relief. Local anesthetic nerve block is a short-term block, usually lasting hours or days, involving the injection of an anesthetic, a corticosteroid, and other agents onto or near a nerve. Neurolytic block, the deliberate temporary degeneration of nerve fibers through the application of chemicals, heat, or freezing, produces a block that may persist for weeks, months, or indefinitely. Neurectomy, the cutting through or removal of a nerve or a section of a nerve, usually produces a permanent block. Because neurectomy of a sensory nerve is often followed, months later, by the emergence of new, more intense pain, sensory nerve neurectomy is rarely performed.

<span class="mw-page-title-main">Brachial plexus injury</span> Medical condition

A brachial plexus injury (BPI), also known as brachial plexus lesion, is an injury to the brachial plexus, the network of nerves that conducts signals from the spinal cord to the shoulder, arm and hand. These nerves originate in the fifth, sixth, seventh and eighth cervical (C5–C8), and first thoracic (T1) spinal nerves, and innervate the muscles and skin of the chest, shoulder, arm and hand.

<span class="mw-page-title-main">Spinal disc herniation</span> Injury to the connective tissue between spinal vertebrae

A spinal disc herniation is an injury to the cushioning and connective tissue between vertebrae, usually caused by excessive strain or trauma to the spine. It may result in back pain, pain or sensation in different parts of the body, and physical disability. The most conclusive diagnostic tool for disc herniation is MRI, and treatment may range from painkillers to surgery. Protection from disc herniation is best provided by core strength and an awareness of body mechanics including posture.

Plexopathy is a disorder of the network of nerves in the brachial or lumbosacral plexus. Symptoms include pain, muscle weakness, and sensory deficits (numbness).

Neural fibrolipoma is an overgrowth of fibro-fatty tissue along a nerve trunk that often leads to nerve compression. These only occur in the extremities, and often affect the median nerve. They are rare, very slow-growing, and their origin is unknown. It is believed that they may begin growth in response to trauma. They are not encapsulated by any sort of covering or sheath around the growth itself, as opposed to other cysts beneath the skin that often are. This means there are loosely defined margins of this lipoma. Despite this, they are known to be benign. Neural fibrolipomas are often more firm and tough to the touch than other lipomas. They are slightly mobile under the skin, and compress with pressure.

<span class="mw-page-title-main">Nerve compression syndrome</span> Human disease

Nerve compression syndrome, or compression neuropathy, or nerve entrapment syndrome, is a medical condition caused by chronic, direct pressure on a peripheral nerve. It is known colloquially as a trapped nerve, though this may also refer to nerve root compression. Its symptoms include pain, tingling, numbness and muscle weakness. The symptoms affect just one particular part of the body, depending on which nerve is affected. The diagnosis is largely clinical and can be confirmed with diagnostic nerve blocks. Occasionally imaging and electrophysiology studies aid in the diagnosis. Timely diagnosis is important as untreated chronic nerve compression may cause permanent damage. A surgical nerve decompression can relieve pressure on the nerve but cannot always reverse the physiological changes that occurred before treatment. Nerve injury by a single episode of physical trauma is in one sense an acute compression neuropathy but is not usually included under this heading, as chronic compression takes a unique pathophysiological course.

<span class="mw-page-title-main">Tarlov cyst</span> Medical condition

Tarlov cysts, are type II innervated meningeal cysts, cerebrospinal-fluid-filled (CSF) sacs most frequently located in the spinal canal of the sacral region of the spinal cord (S1–S5) and much less often in the cervical, thoracic or lumbar spine. They can be distinguished from other meningeal cysts by their nerve-fiber-filled walls. Tarlov cysts are defined as cysts formed within the nerve-root sheath at the dorsal root ganglion. The etiology of these cysts is not well understood; some current theories explaining this phenomenon have not yet been tested or challenged but include increased pressure in CSF, filling of congenital cysts with one-way valves, inflammation in response to trauma and disease. They are named for American neurosurgeon Isadore Tarlov, who described them in 1938.

<span class="mw-page-title-main">Magnetic resonance imaging of the brain</span>

Magnetic resonance imaging of the brain uses magnetic resonance imaging (MRI) to produce high quality two-dimensional or three-dimensional images of the brain and brainstem as well as the cerebellum without the use of ionizing radiation (X-rays) or radioactive tracers.

<span class="mw-page-title-main">Intravoxel incoherent motion</span> Concept and a method initially introduced and developed by Le Bihan et al

Intravoxel incoherent motion (IVIM) imaging is a concept and a method initially introduced and developed by Le Bihan et al. to quantitatively assess all the microscopic translational motions that could contribute to the signal acquired with diffusion MRI. In this model, biological tissue contains two distinct environments: molecular diffusion of water in the tissue, and microcirculation of blood in the capillary network (perfusion). The concept introduced by D. Le Bihan is that water flowing in capillaries mimics a random walk (Fig.1), as long as the assumption that all directions are represented in the capillaries is satisfied.

The history of magnetic resonance imaging (MRI) includes the work of many researchers who contributed to the discovery of nuclear magnetic resonance (NMR) and described the underlying physics of magnetic resonance imaging, starting early in the twentieth century. MR imaging was invented by Paul C. Lauterbur who developed a mechanism to encode spatial information into an NMR signal using magnetic field gradients in September 1971; he published the theory behind it in March 1973.

<span class="mw-page-title-main">MRI sequence</span>

An MRI sequence in magnetic resonance imaging (MRI) is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance.

A nerve decompression is a neurosurgical procedure to relieve chronic, direct pressure on a nerve to treat nerve entrapment, a pain syndrome characterized by severe chronic pain and muscle weakness. In this way a nerve decompression targets the underlying pathophysiology of the syndrome and is considered a first-line surgical treatment option for peripheral nerve pain. Despite treating the underlying cause of the disease, the symptoms may not be fully reversible as delays in diagnosis can allow permanent damage to occur to the nerve and surrounding microvasculature. Traditionally only nerves accessible with open surgery have been good candidates, however innovations in laparoscopy and nerve-sparing techniques made nearly all nerves in the body good candidates, as surgical access is no longer a barrier.

<span class="mw-page-title-main">Deep gluteal syndrome</span> Medical condition

Deep gluteal syndrome describes the non-discogenic extrapelvic entrapment of the sciatic nerve in the deep gluteal space. It is an extension of non-discogenic sciatic nerve entrapment beyond the traditional model of piriformis syndrome. Symptoms are pain or dysthesias the buttocks, hip, and posterior thigh with or without radiating leg pain. Patients often report pain when sitting. The two most common causes are piriformis syndrome and fibrovascular bands, but many other causes exist. Diagnosis is usually done through physical examination, magnetic resonance imaging, magnetic resonance neurography, and diagnostic nerve blocks. Surgical treatment is an endoscopic sciatic nerve decompression.

References

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  2. ↑ Filler AG, Howe FA, Hayes CE, Kliot M, Winn HR, Bell BA, Griffiths JR, Tsuruda JS (March 1993). "Magnetic resonance neurography". Lancet. 341 (8846): 659–61. doi:10.1016/0140-6736(93)90422-D. PMID   8095572. S2CID   24795253.
  3. ↑ Filler AG, Tsuruda JS, Richards TL, Howe FA: Images, apparatus, algorithms and methods.GB 9216383 Archived 2009-06-26 at the Wayback Machine , UK Patent Office, 1992.
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  19. ↑ Raphael DT, McIntee D, Tsuruda JS, Colletti P, Tatevossian R (December 2005). "Frontal slab composite magnetic resonance neurography of the brachial plexus: implications for infraclavicular block approaches". Anesthesiology. 103 (6): 1218–24. doi: 10.1097/00000542-200512000-00017 . PMID   16306735. S2CID   30662280.
  20. ↑ Spinner RJ, Atkinson JL, Scheithauer BW, et al. (August 2003). "Peroneal intraneural ganglia: the importance of the articular branch. Clinical series". J. Neurosurg. 99 (2): 319–29. doi:10.3171/jns.2003.99.2.0319. PMID   12924707.[ dead link ]
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  22. ↑ Filler AG, Haynes J, Jordan SE, et al. (February 2005). "Sciatica of nondisc origin and piriformis syndrome: Diagnosis by magnetic resonance neurography and interventional magnetic resonance imaging with outcome study of resulting treatment". J Neurosurg Spine. 2 (2): 99–115. doi:10.3171/spi.2005.2.2.0099. PMID   15739520.
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  29. ↑ Petchprapa CN, Rosenberg ZS, Sconfienza LM, Cavalcanti CF, LaRoccaVieira R, Zember JS (2010). "MR imaging of entrapment neuropathies of the lower extremity: Part1. The pelvis and hip". RadioGraphics. 30 (4): 983–1000. doi:10.1148/rg.304095135. PMID   20631364. S2CID   207734661.
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