Magnetic Resonance Imaging (MRI) is a diagnostic technique that provides detailed pictures of internal organs and structures without using ionizing radiation. The technology uses a large magnet and radiofrequencies to temporarily alter the alignment of hydrogen atoms in the body’s water molecules. As these atoms return to normal, they emit signals that a computer captures and converts into detailed images. MRI is particularly useful for assessing the spine because it excels at differentiating between soft tissues, such as the spinal cord, nerves, and intervertebral discs.
The Basics of Spinal MRI Imaging
MRI sequences are categorized into T1-weighted and T2-weighted, which determine the signal intensity and brightness of different tissues. T1-weighted images are generally used for assessing anatomical structure. On T1 scans, fat appears bright, while fluid-filled areas, such as cerebrospinal fluid (CSF), appear dark.
T2-weighted images are often preferred for identifying pathology because they highlight tissues with high water content. On a T2 scan, both fat and water appear bright, meaning CSF within the spinal canal shows up as a bright white signal. This bright signal surrounding the spinal cord makes lesions, inflammation, or injury easier to visualize.
The images are viewed in specific orientations, known as planes, which provide different perspectives of the spinal column. The sagittal view cuts the body from front to back, displaying a side profile useful for assessing vertebral alignment and disc height. The axial, or transverse, view slices the body horizontally, which is ideal for evaluating the spinal canal and exiting nerve roots. The coronal plane, which is less frequently used, divides the body into front and back sections.
Mapping the Spine: Segmental Anatomy
Spinal MRI labels rely on a standardized system that divides the vertebral column into distinct segments. The spine is composed of 33 individual bones called vertebrae, grouped into four main anatomical regions, each designated by a letter and a corresponding number.
The regions are:
- Cervical (C): Seven vertebrae (C1–C7) located in the neck.
- Thoracic (T): Twelve vertebrae (T1–T12) in the mid-back, connected to the rib cage.
- Lumbar (L): Five larger vertebrae (L1–L5) that bear the majority of the body’s weight.
- Sacral (S): Five fused vertebrae (S1–S5) that connect the spine to the hip bones.
Spinal labels most frequently refer to the junction points between two vertebrae, such as C5-C6 or L4-L5. These junctions are common sites for movement and potential degeneration. Numbering a specific level helps medical professionals precisely communicate findings across reports and images. This segmented approach allows for exact localization of abnormalities, such as disc issues or nerve root impingement.
Key Structures Revealed on a Labeled Scan
A labeled spinal MRI defines the components of the central supporting structure. The main bony components are the vertebral bodies, which appear as stacked, rectangular structures on a sagittal image. These segments provide support and protect the nervous tissue running through the spinal canal.
Between the vertebral bodies are the intervertebral discs, which function as shock absorbers and permit flexibility. Each disc has two distinct parts. The tough, fibrous outer ring is the annulus fibrosus, which helps contain the inner core. The inner structure is the nucleus pulposus, a gel-like center rich in water that provides cushioning capability. The healthy, water-rich nucleus pulposus typically displays a bright signal on T2-weighted images.
Running within the protective bony column is the spinal cord, a dense bundle of nervous tissue. Thin structures branching off the spinal cord are the nerve roots, which exit the spinal canal through small openings called intervertebral foramina. Other structures visible include the facet joints, which manage motion between vertebrae, and various ligaments that stabilize the column.
Deciphering Common Report Terminology
MRI reports use specific terminology to describe deviations from normal anatomy. Disc degeneration refers to the overall deterioration of the disc structure, often noted by a loss of the normal bright signal on T2-weighted images. This loss of brightness, called desiccation, indicates decreased water content in the nucleus pulposus, leading to reduced disc height and cushioning ability.
A common finding is a disc herniation, which describes the displacement of disc material beyond its normal boundaries. This general term encompasses three progressively severe categories:
Disc Bulge
A disc bulge is the mildest form, involving a broad-based, circumferential extension of the disc material. It typically affects more than 25% of the disc’s circumference.
Disc Protrusion
A protrusion is a type of herniation where the displaced material is focal, affecting a smaller area. Crucially, the base of the displaced material remains wider than the part extending outward.
Disc Extrusion
An extrusion represents a more significant displacement. Here, the base of the herniated material is narrower than the material extending into the spinal canal. The displaced material may even extend above or below the level of the disc space.
Another frequent term is spinal stenosis, which means a narrowing of the space available for the nerves within the spine. This narrowing can occur in the central spinal canal or in the smaller openings where the nerves exit, known as foraminal narrowing. Stenosis is often caused by a combination of disc changes, thickening ligaments, and bony overgrowth.