Magnetic Resonance Imaging (MRI) is a diagnostic technique that provides highly detailed pictures of the body’s internal structures. Unlike X-rays or Computed Tomography (CT) scans, MRI does not use ionizing radiation; instead, it employs a powerful magnetic field and radio waves to generate images. This makes it particularly effective for visualizing soft tissues in the back, such as the spinal cord, nerve roots, and intervertebral discs. The resulting cross-sectional images allow physicians to evaluate the spine in multiple planes, offering a comprehensive view of complex anatomy.
Visualizing the Spinal Foundation: Discs and Vertebrae
A primary function of a back MRI is to assess the condition of the intervertebral discs, which act as cushions between the bony vertebrae. A healthy disc is highly hydrated, causing it to appear bright white on T2-weighted MRI sequences. The earliest sign of degenerative change is disc desiccation, a loss of water content that causes the disc to appear noticeably darker on the scan, often referred to as a “black disc.”
The scan clearly distinguishes between a disc bulge and a disc herniation, which represent different degrees of structural failure. A disc bulge is a generalized outward extension of the disc material, involving more than 25% of the disc’s circumference, while the tough outer fibrous layer remains intact. In contrast, a disc herniation, or ruptured disc, is a focal event where the soft inner material pushes through a tear in the outer layer. Herniations are further classified as a protrusion (base wider than displaced material) or an extrusion (displaced material wider than its neck).
Beyond the soft tissues, MRI also provides valuable information about the vertebral bones. While CT scans are superior for acute bone trauma, MRI excels at identifying fractures not visible on standard X-rays, particularly those related to osteoporosis. An acute vertebral compression fracture is visualized by the presence of bone marrow edema, which appears as a bright signal on specialized fluid-sensitive sequences. This edema indicates a fresh injury, helping to differentiate a recent, painful fracture from an older, healed one. The MRI can also detect associated soft tissue injuries, such as paravertebral soft tissue edema (swelling in the muscles and ligaments surrounding the spine).
Detecting Nerve and Spinal Cord Compression
The MRI is the imaging modality of choice for evaluating pressure on the neurological elements of the back. The spinal cord and nerve roots are surrounded by cerebrospinal fluid (CSF), which appears as a bright white outline on T2-weighted images. This contrast allows physicians to clearly see when and where space is compromised.
A common finding is spinal stenosis, which is a narrowing of the bony canal housing the spinal cord and nerve roots. Central canal stenosis occurs when the main passageway narrows, often due to thickened ligaments, disc bulging, or bone spur formation. As the condition progresses, the white CSF space is gradually obliterated, and in severe cases, the nerve roots can appear clustered together.
Narrowing can also occur where individual spinal nerves exit the column, a condition known as neural foraminal stenosis. This lateral narrowing is typically caused by overgrowth of the facet joints (osteoarthritis) or disc material protruding sideways. The MRI provides a clear view of this constricted space, allowing the physician to pinpoint the exact location and cause of the nerve compression.
When a nerve root is compressed and irritated, leading to symptoms like pain or weakness (radiculopathy), the MRI may show signs of inflammation. While a standard scan shows mechanical pressure, a contrast-enhanced MRI can reveal a breakdown in the nerve’s protective barrier. This allows the contrast agent to leak into the nerve, showing up as nerve root enhancement, which correlates with active irritation. Severe compression of the spinal cord can also cause an abnormal bright signal within the cord itself, representing edema or swelling of the neural tissue.
Identifying Less Common Spinal Pathology
MRI is highly effective at identifying less frequent but serious pathologies that can affect the spine. Spinal infections, such as discitis (infection of the disc) and osteomyelitis (infection of the bone), are clearly visualized due to the resulting inflammatory response. These conditions typically show a bright signal in the intervertebral disc and adjacent vertebral bone marrow on T2-weighted images, indicating edema. Contrast administration is often used to confirm the diagnosis by showing enhancement in the infected disc space.
Tumors, both primary and metastatic, are another set of conditions that MRI is uniquely suited to diagnose and characterize. Metastatic disease, which has spread from another part of the body, is the most common spinal tumor and is visualized as an abnormal replacement of the normal fatty bone marrow. These lesions are easily detected because they appear differently than the bright background of healthy fatty marrow.
The scan helps to define the tumor’s extent, including whether it is contained within the bone or has spread into the spinal canal to compress neural structures. Primary spinal tumors, though rare, also have specific appearances on MRI that help differentiate them. MRI can also reveal inflammatory conditions like axial spondyloarthritis, characterized by bone marrow edema at the vertebral corners, known as Romanus lesions.