Back pain is a common issue, and when it becomes persistent or severe, a magnetic resonance imaging (MRI) scan is often the first step in diagnosis. An MRI provides detailed pictures of soft tissues, making it the preferred method for viewing the spinal cord, nerves, and intervertebral discs. A herniated disc is the displacement of the disc’s inner material beyond its normal boundaries and is a frequent cause of nerve-related symptoms. Understanding how a herniated disc appears on an MRI helps translate the complex language of a radiological report.
Understanding the Healthy Spinal Disc
The intervertebral discs function as shock absorbers between the bony vertebrae of the spine. A healthy disc is composed of two distinct parts that appear differently on MRI. The tough, fibrous outer ring is called the annulus fibrosus, and it encases the inner core. The inner core, known as the nucleus pulposus, is a gel-like material rich in water content.
MRI uses strong magnetic fields and radio waves to create images, and its ability to highlight water is useful for disc assessment. On T2-weighted imaging, the water-rich nucleus pulposus of a healthy disc appears bright white, or hyperintense. This bright signal serves as the baseline for a normal, well-hydrated disc. The outer annulus fibrosus, which is less hydrated, appears darker, providing a clear contrast that defines the disc’s normal shape and boundaries.
Defining Types of Disc Displacement
A herniated disc is a general term for displacement, but radiologists classify the injury based on the physical appearance and extent of the material that has moved. The least severe type of displacement is a disc bulge, which is a broad, circumferential extension of disc material that typically involves more than 25% of the disc’s circumference. The disc material extends outward evenly, but the outer fibers often remain intact.
A disc protrusion is a more localized displacement where the base of the displaced material is wider than the displaced material itself. This focal displacement pushes outward into the surrounding space but often maintains an overall conical or dome shape. Protrusions can press directly on spinal nerves or the spinal cord.
The most advanced type is a disc extrusion, which occurs when the displaced material is wider than its base, or it has lost continuity with the parent disc. The extruded material may also migrate upward or downward away from the disc level, becoming a free fragment, which represents a severe breach of the annulus fibrosus. These specific distinctions correlate to the mechanism of injury and influence treatment decisions.
Interpreting the MRI Signal Changes
The visual evidence of a herniated disc on an MRI centers on two main observations: the signal intensity of the disc material and the physical shape of the displaced tissue. The most telling sign of disc damage, which often precedes herniation, is disc desiccation. Desiccation is the loss of water from the nucleus pulposus, causing the normally bright white signal on T2-weighted images to appear dark gray or black, known as hypointense.
This darkening indicates a degenerative change, signaling that the disc has lost its shock-absorbing qualities and is more susceptible to tearing. When the disc material is displaced, it appears as a dark or gray mass pushing out from the normal disc margin into the spinal canal or the neural foramen. The displaced material is most often dark because it is desiccated nucleus pulposus that has moved through a tear in the annulus.
The finding for symptomatic herniation is the effect of the displaced disc material on the surrounding structures. On the image, the dark herniated material can be seen flattening or displacing the bright white cerebrospinal fluid (CSF) surrounding the spinal cord and nerve roots. This displacement, known as compression or effacement, is the direct visual link between the structural problem and the patient’s symptoms. The extent of this compression is what the radiologist primarily assesses to determine the severity of the herniation.
Decoding Common Report Terminology
An MRI report uses specific language to describe the location and consequences of disc herniation. One frequently seen term is Annular Tear or Annular Fissure, which describes a break in the fibers of the outer annulus fibrosus. On an MRI, a tear sometimes appears as a high-intensity zone, or a small bright spot within the dark annulus, indicating localized inflammation or fluid accumulation.
The phrase Nerve Root Impingement or Compression is used when the displaced disc material physically presses against a spinal nerve root. This finding correlates directly with symptoms like sciatica, numbness, or weakness in the extremities.
Similarly, thecal sac effacement means the herniated material is pressing on and flattening the thecal sac, the membrane containing the cerebrospinal fluid and the spinal cord or cauda equina nerve roots.
Foraminal Stenosis describes the narrowing of the neural foramen, the small opening on the side of the spine where the nerve root exits. A herniated disc or bone spurs can reduce this space, leading to the stenosis that pinches the exiting nerve. Understanding these specific terms allows a person to connect the visual findings on the MRI with the ultimate medical diagnosis.