A “slipped disc,” or herniated/bulging disc, occurs when a soft cushion of tissue between spinal bones extends beyond its normal boundaries. This can cause discomfort, especially if it presses on nearby nerves. Magnetic Resonance Imaging (MRI) is a primary diagnostic tool for visualizing these spinal issues, offering detailed insights into the disc’s condition and its relationship to surrounding structures.
The Anatomy of a Healthy Disc
The human spine consists of vertebrae separated by intervertebral discs. These discs act as natural shock absorbers, protecting the vertebrae from daily impacts.
Each intervertebral disc consists of two distinct parts. The outer portion is a tough, fibrous ring known as the annulus fibrosus, providing structural integrity. Encased within this ring is the nucleus pulposus, a soft, gel-like substance rich in water. The nucleus pulposus helps distribute pressure evenly and contributes to spinal flexibility.
How MRI Creates Disc Images
Magnetic Resonance Imaging (MRI) uses powerful magnets and radio waves to generate detailed images of internal body structures, particularly soft tissues. The process involves aligning hydrogen atoms, abundant in the body’s water and fat molecules, within a strong magnetic field. Radio waves are briefly pulsed, causing these atoms to temporarily shift alignment. When turned off, atoms return to original alignment, releasing signals detected by the scanner and converted into detailed images.
MRI is effective for visualizing discs because it excels at showing differences in water content and tissue density. In a healthy disc, the water-rich nucleus pulposus appears bright on T2-weighted images, while the annulus fibrosus, with less water, appears darker. On T1-weighted images, water appears dark, making these sequences useful for anatomical detail where fat appears bright. This distinction in signal intensity allows radiologists to assess the disc’s hydration and structural integrity.
Identifying a Slipped Disc on MRI
MRI provides clear visual cues for a “slipped disc.” One common finding is disc bulging, where the outer annulus fibrosus extends broadly beyond its normal vertebral boundaries. On MRI, a bulging disc appears as a generalized, symmetrical protrusion of the disc material. This outward expansion does not necessarily involve a tear in the outer fibrous ring.
In disc herniation, a more distinct displacement of the nucleus pulposus occurs, typically involving a tear in the annulus fibrosus, allowing the inner gel-like material to push outward. On MRI, the herniated disc material appears as an abnormal extension or mass beyond the normal disc confines. This displaced material can press on adjacent spinal nerves or the spinal cord, visible as spinal canal narrowing or neural structure displacement.
Other signs of disc issues include loss of disc height, indicating disc degeneration and compression. Degenerated discs may appear darker on T2-weighted images due to dehydration. Inflammation or edema around the affected area can also be seen as bright signals on T2-weighted images, indicating an active injury response. These visual characteristics help pinpoint the location and severity of the disc problem.
Variations in Disc Appearance on MRI
MRI differentiates various types of disc pathology by their distinct appearances. A disc bulge is a broad, symmetrical outward extension of the disc material, where the outer annulus fibrosus remains intact. It appears as a generalized widening of the disc contour beyond the vertebral endplates.
Disc Protrusion
Disc protrusion is a localized displacement where the base of the displaced disc material is wider than the actual extent of the protrusion. On MRI, this appears as a focal bulge, but the nucleus pulposus has not fully ruptured through the annulus. The outer fibers of the annulus fibrosus may still contain the displaced material.
Disc Extrusion
A disc extrusion is an advanced form of herniation where the nucleus pulposus has broken through the annulus fibrosus. The neck of the herniated material, at its exit point, is narrower than the body of the extruded material. This appears as a distinct, mushroom-shaped mass extending outside the disc.
Disc Sequestration
The most severe form is disc sequestration, where a fragment of extruded disc material has separated entirely from the main disc and migrated away. On MRI, this detached fragment can appear as a free-floating piece within the spinal canal, sometimes distant from the original disc space. Identifying these variations helps clinicians determine disc damage and plan management.