Vertebral Marrow Lesions
- Vertebral bone marrow is mostly fat in an adult, so on a normal T1-weighted MRI the vertebrae glow brighter than the disc — that's your baseline.
- A marrow lesion is anything that shoves the fat out and replaces it with cells or fluid, which turns the bone dark on T1.
- The single most useful rule of thumb: a focal lesion that stays brighter than (or equal to) normal disc/muscle on T1 is reassuringly fatty; one that goes darker than disc earns a hard second look.
- Common benign culprits — hemangioma, focal fatty marrow, bone island, Schmorl node — have signatures that let you call them and move on.
- The scary trio is metastasis, myeloma, and infection; when in doubt, fat-suppressed and post-contrast sequences sort the suspicious from the boring.
Open up any vertebra and you'll find the bone marrow doing two jobs at once: making blood cells (red marrow) and storing fat (yellow marrow). In a healthy adult spine, the fat has mostly won. So the very first thing your eye should do on a spine MRI is treat the vertebral bodies as a row of soft, fatty marshmallows — and any marshmallow that suddenly turns to charcoal is the one worth investigating.
Why fat is the whole game
Here's the trick that makes marrow imaging click: fat is bright on T1. Normal adult vertebrae are full of fat, so they're naturally bright on a T1-weighted sequence — reliably brighter than the intervertebral disc and the nearby muscle. That gives you a free, built-in reference standard sitting right there in every image.
A marrow lesion, almost by definition, is something moving in and evicting the fat. Tumor cells, plasma cells, edema, pus — none of them are fat, so wherever they pile up, the bright T1 signal drops. The mental model I use: the marrow is a cozy apartment full of fluffy yellow furniture (fat), and disease is the unwelcome roommate who hauls all the furniture out to the curb. The emptier of fat the room gets, the darker it looks on T1.
On T1, normal fatty marrow should be brighter than the adjacent disc. A focal lesion that is darker than disc is the one that should make you sit up straight.
The benign regulars you can wave through
Most marrow lesions are nothing — incidental findings you'll see all day. The reassuring ones tend to either be fat or contain fat:
| Lesion | T1 | T2 | The tell |
|---|---|---|---|
| Vertebral hemangioma | Bright | Bright | Coarse vertical "corduroy" trabeculae on CT; "polka-dot" on axial. |
| Focal fatty marrow | Bright | Variable | Follows fat on every sequence; suppresses with fat saturation. |
| Bone island (enostosis) | Dark | Dark | Dense sclerotic dot on CT with spiky brush borders; cold on bone scan. |
| Schmorl node | Variable | Variable | Disc material herniated through an endplate; continuous with the disc. |
A vertebral hemangioma is the classic "don't panic" lesion — it's bright on both T1 and T2 because it's a tangle of vessels with a generous helping of fat. On CT it shows that unmistakable coarse vertical striping radiologists lovingly call corduroy (or polka-dots when you look at it end-on). It looks dramatic and means almost nothing.
The trio that ruins your afternoon
When the fat gets pushed out and the lesion goes dark on T1 and bright on the fluid-sensitive (STIR/T2 fat-sat) sequences, you're in the territory that matters: metastasis, multiple myeloma, and infection. These are the reason marrow imaging exists.
- Metastasis loves the spine — it's one of the most common places tumors spread, because the vertebrae keep red marrow with its rich blood supply. Lesions are often multiple, randomly scattered, and respect (initially) the disc space.
- Myeloma is plasma cells run amok; it can look like scattered focal lesions, a diffuse "salt-and-pepper" or just-too-dark marrow, or be nearly invisible — which is exactly why it's sneaky.
- Infection (discitis-osteomyelitis) is the one that crosses the disc, lighting up two adjacent endplates and the disc between them. That pattern is its signature.
Tumor and infection both darken marrow on T1 and brighten it on STIR, so signal alone won't separate them. The classic discriminator is the disc: metastases tend to spare the disc early, while infection devours the disc and the two endplates around it. Use that pattern, not just the brightness.
This is also where the "is it cancer or just a healing crack?" question lives. A benign acute compression fracture floods the marrow with edema that mimics tumor on STIR. The reassuring clues for benign: a horizontal band of preserved normal fatty marrow, no soft-tissue mass bulging out behind the bone, and signal that normalizes over weeks as it heals. A pathologic fracture tends to replace the whole body, bulge out the back wall, and may bring company at other levels.
Diffuse marrow that's darker than the disc on T1 isn't always tumor. Reconverted red marrow — from anemia, smoking, athletic training, or marrow-stimulating drugs — can repaint the spine darker, and so can the normal childhood spine. Context and symmetry matter; a single odd-vertebra-out is far more worrying than a uniformly busy spine.
How to actually look
You don't need ten sequences; you need the right two or three working together. T1 finds fat loss. STIR or fat-saturated T2 finds water/edema, screaming where T1 only whispers. Post-contrast fat-saturated T1 shows which dark lesions actually enhance (live tumor or infection) versus which don't. And whenever a marrow lesion gets aggressive, the very next question is mechanical: is it pushing on the spinal cord? That's why a suspicious vertebral lesion and the search for cord compression almost always travel together.
If you remember one sentence, make it this: normal adult marrow is bright fat on T1, and the lesion you cannot ignore is the one that turns that fat dark — then you reach for STIR and contrast to decide whether it's a boring roommate or a dangerous one. The systematic version of this hunt lives in the approach to a bone lesion, which works for the spine just as well as the rest of the skeleton.