A herniated disc occurs when the soft, gel-like center (nucleus pulposus) pushes through a tear in the tougher outer ring (annulus fibrosus). This displacement presses on nearby spinal nerves, causing pain, numbness, or weakness in the extremities. When back pain persists or a neurological deficit is suspected, imaging is necessary to identify the exact cause and location of the compression. Imaging tests provide the visual confirmation needed to guide treatment decisions.
CT Scan Focus: Clarity on Bone and Calcification
Computed Tomography (CT) scans generate cross-sectional images of the spine using a rotating X-ray beam and computer processing. CT is highly effective at visualizing dense structures because X-rays are readily absorbed by materials like calcium. Consequently, CT is the preferred method for viewing the bony architecture of the spine, including the vertebral bodies and facet joints.
The scan provides exceptional detail regarding bone alignment, subtle fractures, and post-surgical changes, such as assessing hardware fusion. CT excels at identifying degenerative changes like bone spurs (osteophytes) that can narrow the spinal canal. It is also the most accurate tool for detecting calcification within the disc or surrounding ligaments.
Visualizing Soft Tissue: CT’s Challenge in Detecting Discs
While CT scans provide sharp images of bone, they present a challenge when attempting to visualize the soft tissues that make up a herniated disc. The disc material, the spinal cord, and the surrounding nerve roots are all composed of soft tissue with a similar density. This lack of substantial density difference results in poor contrast resolution, making it difficult to clearly distinguish the displaced disc from the nearby nerves and cerebrospinal fluid.
A large disc extrusion might be indirectly visible as a mass that displaces the epidural fat or the fluid-filled sac around the spinal cord. However, smaller bulges or the precise extent of a herniation are often obscured on a standard, non-contrast CT scan. To overcome this limitation, a technique called CT myelography can be used, which involves injecting a contrast dye directly into the space surrounding the spinal cord and nerve roots. The contrast agent highlights the nerve structures, allowing the herniated disc material to be seen as an indentation or block against the now-visible nerve roots.
The Definitive Imaging Tool: Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging (MRI) is the gold standard for diagnosing a herniated disc because of its superior ability to image soft tissue. Instead of X-rays, MRI uses powerful magnetic fields and radio waves to excite the hydrogen atoms, which are abundant in the water content of soft tissues. The scanner detects the energy released as these atoms return to their resting state, generating highly detailed images.
This mechanism creates excellent contrast between different soft tissue types, distinguishing the water-rich nucleus of the disc from the drier outer annulus and the surrounding nerves. The superior resolution allows physicians to see the exact size, shape, and location of the herniation, as well as its relationship to the spinal cord and exiting nerve roots. MRI can also reveal signs of disc degeneration, such as loss of water content, which appears as a darker signal on the scan.
Integrating CT into Disc Diagnosis and Treatment Planning
Despite the MRI’s clear advantage for soft tissue, the CT scan remains a valuable tool in the overall management of disc pathology. A CT scan may be performed when a patient has a contraindication for an MRI, such as a non-MRI-compatible pacemaker, certain metal implants, or severe claustrophobia. In these cases, CT myelography serves as the most effective alternative for visualizing nerve compression.
The CT scan’s strength in bone detail makes it indispensable for surgical planning, especially when fusion or instrumentation is involved. Surgeons rely on the precise, high-resolution bony images to assess the placement of screws or to evaluate the extent of bony stenosis. CT is also helpful in the differential diagnosis of acute trauma, where ruling out subtle fractures or bony instability is a higher priority than soft tissue detail.