Magnetic Resonance Imaging (MRI) is a non-invasive medical test that uses a powerful magnetic field and radio waves to create detailed images of organs and tissues within the body. When it comes to the nervous system, a specialized version of this test, often called Magnetic Resonance Neurography (MRN), can show nerve damage. The effectiveness of MRN depends on the specific nature and location of the injury, but the imaging technique provides a detailed look at the physical structure of the nerve and the surrounding environment.
How MRI Visualizes Nerve Tissue
MRI generates images by manipulating the body’s water molecules, specifically the hydrogen protons, within a strong magnetic field. The machine emits radiofrequency pulses, causing these protons to briefly align and then release energy as they return to their natural state. The varying speed at which different tissues release this energy allows the system to generate contrast in the final image.
Two primary types of images, T1-weighted and T2-weighted, are used to distinguish tissues. T1-weighted images visualize overall anatomy, while T2-weighted images are designed to highlight pathology. A damaged or irritated nerve often contains increased water content due to inflammation, which shows up as a bright signal on T2-weighted sequences, directly indicating an abnormality within the nerve itself.
The normal nerve is typically seen as a small, ovoid structure of intermediate signal intensity, easily distinguished from the surrounding muscle and fat. When a nerve is injured, this normal appearance is altered, providing visual evidence of damage. High-resolution MRN sequences are effective at differentiating the nerve from adjacent soft tissues, allowing for the detection of subtle changes in size or signal intensity.
Structural Nerve Injuries Visible on MRI
MRI is effective for visualizing nerve damage that involves a physical, structural alteration to the nerve or its environment. This is particularly true for nerve compression, which is one of the most common causes of nerve pain. Herniated discs in the spine, for example, can be clearly seen pressing against spinal nerve roots, a condition known as radiculopathy.
In cases of nerve entrapment syndromes, such as Carpal Tunnel Syndrome or ulnar nerve entrapment at the elbow, MRI can show the narrowing of the anatomical tunnel. The imaging can reveal secondary signs of compression, including localized swelling or flattening of the nerve at the point of impingement. Visualizing the exact cause of external compression is necessary for precise surgical planning.
Direct trauma can cause severe structural damage, and MRI excels at identifying these injuries. A complete nerve transection, where the nerve is severed, or a severe crush injury resulting in internal scarring (neuroma formation) are visible on the scan. Furthermore, space-occupying lesions, like tumors, cysts, or hematomas, that are directly affecting the nerve can be accurately localized and characterized by MRI.
Functional Damage and Complementary Diagnostic Tools
While MRI is exceptional for visualizing the structure of a nerve, its primary limitation is its inability to measure the nerve’s function, or how well it transmits electrical signals. Functional damage, such as mild demyelination—where the insulating layer around the nerve axon is slightly damaged but the overall nerve structure remains largely intact—may not cause enough physical change to be detectable on an MRI scan. Similarly, subtle forms of peripheral neuropathy, where the electrical conduction is impaired without major structural deformity, can result in normal MRI findings despite the patient experiencing significant symptoms.
This is where complementary diagnostic tests become necessary to provide a complete picture of nerve health. Electromyography (EMG) and Nerve Conduction Studies (NCS) are functional tests that measure the electrical signaling of the nerve and muscle.
NCS involves stimulating a nerve with a small electrical impulse and measuring the speed and strength of the signal as it travels along the nerve. This directly assesses the nerve’s ability to conduct a signal, which is a measure of its function.
EMG involves inserting a fine needle electrode into a muscle to evaluate its electrical activity both at rest and during contraction. Since muscles lose their normal electrical input when the nerve supplying them is damaged, EMG can pinpoint the location and severity of nerve damage by showing abnormal muscle activity. These electrophysiological tests can detect nerve dysfunction even in cases where the MRI shows no structural abnormality. The combination of structural information from MRI and functional data from EMG/NCS allows physicians to accurately diagnose the type, location, and severity of nerve damage, leading to the most appropriate treatment plan.