Magnetic Resonance Neurography (MRN) is an advanced imaging technique designed to provide highly detailed pictures of the body’s peripheral nerves. Unlike standard Magnetic Resonance Imaging (MRI), which focuses on surrounding soft tissues like muscle and bone, MRN is tailored to visualize the nerves themselves in high resolution. By optimizing the signal from the water within the nerve, MRN is an important tool for localizing and characterizing nerve abnormalities, such as inflammation or injury, that standard MRI often misses.
How Magnetic Resonance Neurography Visualizes Nerves
MRN achieves specialized imaging by employing specific sequences that enhance the signal coming directly from the nerve tissue. A key difference from conventional MRI involves the use of highly T2-weighted sequences. Nerves that are irritated, compressed, or inflamed accumulate excess fluid, resulting in a prolonged T2 relaxation time, which causes them to appear bright or hyperintense on the image.
MRN uses fat suppression techniques, such as Short Tau Inversion Recovery (STIR), to increase contrast. Fat surrounds the nerves and produces a bright signal that can obscure the nerve. The STIR sequence applies a specific radiofrequency pulse to nullify the signal from fat molecules, making the background dark. This allows the bright signal from the inflamed or damaged nerve tissue to stand out clearly.
Advanced Protocols
Some advanced MRN protocols incorporate Diffusion Tensor Imaging (DTI) and tractography. DTI measures the direction and magnitude of water molecule movement within the nerve fibers, which injury can alter. Tractography uses this directional information to create three-dimensional, color-coded maps that visualize the entire pathway of a nerve bundle, assessing its continuity and structural integrity non-invasively. Specialized phased-array surface coils are also used, positioned close to the body part being scanned. This provides the superior resolution needed to distinguish small nerve structures from surrounding soft tissues.
Conditions Diagnosed Using Neurography
MRN is utilized to diagnose structural pathologies of the peripheral nervous system, especially when symptoms point toward a specific anatomical issue. It is effective at pinpointing the location and nature of nerve entrapment syndromes, such as carpal tunnel syndrome, cubital tunnel syndrome, and thoracic outlet syndrome.
The technique evaluates complex nerve structures like the brachial plexus and lumbosacral plexus. It assesses traumatic injuries, identifying if a nerve has been stretched, partially torn, or completely severed (discontinuity). MRN also characterizes nerve sheath tumors, such as schwannomas or neurofibromas. It distinguishes them from other types of masses and provides information about their size and relationship to the surrounding nerve tissue.
Neurography provides anatomical information necessary for surgical planning by revealing inflammation, swelling, and physical compression. For instance, MRN can diagnose piriformis syndrome, where the sciatic nerve is compressed by a deep buttock muscle, especially in cases of sciatica unresponsive to typical treatment. This structural insight helps clinicians determine if a patient requires conservative management or surgical intervention.
Preparing For and Undergoing the Scan
Preparation for an MRN scan requires the removal of all metallic objects, such as jewelry, watches, and items with metal fasteners. Patients must inform staff about any implanted medical devices, such as pacemakers or aneurysm clips, as the strong magnetic field may be contraindicated or require procedural modifications.
The scan takes place within the magnetic resonance imaging machine, which resembles a large tunnel. Because high-resolution sequences are needed to capture nerve detail, the procedure is lengthy, typically lasting 45 minutes to over an hour. Patients must remain completely still during the scan to prevent image blurring.
The machine generates loud knocking noises, so earplugs or headphones are provided for hearing protection. In some cases, a gadolinium-based contrast agent may be injected intravenously. This helps visualize nerve abnormalities or differentiate nerves from adjacent structures, especially when tumors or significant inflammation are present.
Neurography Compared to Standard Nerve Diagnostics
Magnetic Resonance Neurography offers structural information, differing from the functional data provided by standard nerve diagnostics like Electromyography (EMG) and Nerve Conduction Studies (NCS). EMG and NCS measure the electrical activity of nerves and muscles, assessing signal conduction and muscle response to determine physiological function.
MRN provides a physical map of the nerve, showing its size, shape, surrounding tissues, and internal damage. While EMG/NCS confirms a functional problem, MRN visually localizes the exact anatomical cause. The tests are complementary and often used together for a complete diagnosis.
MRN is useful when functional tests are inconclusive or symptoms persist despite normal EMG/NCS results. MRN can detect structural changes, such as nerve swelling, much earlier than EMG/NCS. Functional tests typically require a waiting period of two to three weeks after an injury for changes to become detectable. This early, precise anatomical localization makes MRN an invaluable tool, especially when surgical intervention is considered.