A CT scan of the lumbar spine produces detailed cross-sectional images of the five lowest vertebrae in your back, revealing bone structure, the spinal canal, and surrounding tissues with high precision. It’s particularly good at showing fractures, bone spurs, spinal narrowing, and the alignment of your vertebrae, making it one of the most reliable tools for evaluating bony problems in the lower back.
Structures a Lumbar CT Reveals
The scan captures every bony element of your lower spine in fine detail. Each of the five lumbar vertebral bodies appears clearly, allowing radiologists to check that they’re square-shaped, consistent in height, and properly aligned with the vertebrae above and below. The posterior elements, including the facet joints (the small paired joints that connect one vertebra to the next), the pedicles (short bony bridges), the lamina (the bony plates that form the back wall of the spinal canal), and the spinous processes (the bumps you can feel along your back), are all individually visible.
The spinal canal itself shows up well on both axial (cross-section) and sagittal (side-view) images. This matters because the canal houses the spinal cord and nerve roots, and any encroachment into that space can cause pain, numbness, or weakness. Disc spaces between the vertebrae are visible too, though CT shows calcified or hardened disc material far better than it shows the softer, water-rich disc tissue that MRI excels at.
Conditions It Can Detect
CT is the imaging modality of choice when a fracture is suspected. It picks up complex fractures, nondisplaced fractures (where the bone cracks but doesn’t shift), and burst fractures where bone fragments may push backward into the spinal canal. For fractures that cause more than 50% loss of vertebral body height, CT has 100% sensitivity. However, for subtler compression fractures with less height loss, its sensitivity at the lumbar level drops to around 47%, which is why MRI is sometimes needed as a follow-up for minor or suspected occult fractures.
Beyond fractures, a lumbar CT commonly identifies:
- Spinal stenosis: narrowing of the central spinal canal or the side openings where nerves exit. This can result from bone spurs, thickened ligaments, disc changes, facet joint arthritis, or even fat buildup in the spinal canal (a condition called epidural lipomatosis). Some people are born with a naturally narrow canal from shortened pedicles, and CT shows this clearly.
- Spondylolisthesis: a condition where one vertebra slips forward over the one below it. CT can grade the severity of the slip precisely. The L5-S1 level (the lowest lumbar vertebra over the sacrum) is a common location.
- Degenerative changes: facet joint arthritis, bone spurs, and disc calcification that develop with age and can contribute to pain or nerve compression.
- Scoliosis: abnormal lateral curvature of the spine, with CT providing exact measurements of vertebral rotation and alignment.
When CT Is Preferred Over MRI
CT and MRI serve different roles for the lumbar spine, and your situation determines which one gives better answers. CT excels at bone detail. If your doctor suspects a fracture after trauma, needs to evaluate scoliosis geometry, or wants to assess hardened disc material pressing on a nerve, CT is typically the first choice. It’s also faster and more widely available in emergency settings.
MRI, on the other hand, is better for soft tissue: discs that haven’t calcified, ligament injuries, nerve root inflammation, infections, and tumors. For most general spinal problems, including a new episode of back pain with nerve symptoms, MRI is usually the preferred study. CT tends to be favored specifically when bone or joint arthritis is the primary concern.
Evaluating Spinal Surgery Hardware
If you’ve had spinal fusion, pedicle screws, or a disc replacement, CT is the most accurate way to check how things look after surgery. It shows whether screws are positioned correctly within the pedicles, whether the bone graft between fused vertebrae is solidifying, and whether the overall alignment of your spine has been maintained. It can also detect complications like screws that have loosened, fractured, or drifted out of position.
One particularly important post-surgical question CT answers is whether a fusion has truly healed or whether a “pseudoarthrosis” has developed, meaning the bones never fully joined. CT can precisely define cortical bone margins and remaining graft material to make that determination. Standard X-rays can screen for these problems, but CT is reported to be the more accurate modality for confirming them.
CT Myelography for Complex Cases
In some situations, a standard CT doesn’t provide enough information about the nerves themselves. A CT myelogram combines the scan with a contrast dye injected into the fluid-filled sac surrounding the spinal cord. This creates a detailed map of the thecal sac and nerve roots, showing exactly where compression is happening.
CT myelography is especially valuable when spinal hardware creates metal artifact that obscures MRI images. In one documented example, a disc extrusion compressing the L4 nerve root was completely hidden by streak artifact on MRI but clearly visible on the CT myelogram. It can also detect conditions like cerebrospinal fluid leaks, where contrast is seen leaking through a neural foramen into surrounding tissue, and nerve root avulsions where roots have been torn away. These are findings that standard CT or even MRI can miss entirely.
Radiation Exposure
A lumbar spine CT delivers an average effective dose of about 4.8 millisieverts (mSv). For context, you receive roughly 3 mSv per year from natural background radiation just from living on Earth. So a single lumbar CT is equivalent to about 1.5 years of natural exposure. This is a moderate dose compared to other CT exams, and it’s generally considered acceptable when the scan is clinically needed. It does, however, mean that repeated scans over time should be justified rather than routine.
What the Scan Is Like
The actual scanning portion takes only a few minutes on modern machines, though the entire visit, including positioning and any preparation, typically runs about 30 minutes. You’ll lie on a narrow table that slides into a large, ring-shaped scanner. The ring rotates around you but doesn’t touch you, and the process is painless.
Most lumbar CT scans don’t require contrast dye. If contrast is needed (as with a CT myelogram), you’ll be asked not to eat or drink for a few hours beforehand. If you have a known allergy to contrast material, your doctor will typically prescribe a steroid medication to take starting 12 hours before the exam to reduce the risk of a reaction. There’s no special preparation needed for a standard scan without contrast, and you can return to normal activities immediately afterward.