Marrow Edema & AVN
- Bone marrow is mostly fat in adults, so on MRI the trick is simple: anything that pushes that fat out and replaces it with water lights up bright on the water-sensitive (fluid) sequences and goes dark on T1.
- "Marrow edema" isn't a diagnosis — it's a signpost. The job is figuring out why the marrow is waterlogged: trauma, infection, a stress reaction, or dying bone.
- Avascular necrosis (AVN), also called osteonecrosis, is bone that died because its blood supply got cut off — most famously the femoral head.
- The classic MRI tell for AVN is the double line sign: a wavy, two-toned border walling off the dead segment, best seen on T2.
- The thing you dread is the dead bone collapsing once it's lost its scaffolding — that's the difference between "watch it" and "the patient needs a new hip."
Here's a fact that reframes a lot of bone MRI: adult marrow is basically a fat depot with some blood-making bits sprinkled in. So when I read a bone MRI, I'm really watching the fat. Anything that evicts the fat and moves water in — swelling, pus, blood, dying tissue — changes the brightness, and that change is the whole game.
Why marrow lights up (and the one trick that makes it obvious)
Normal yellow marrow is full of fat. Fat is bright on T1. So a healthy adult femoral head looks reassuringly bright and even on T1 — like a slab of well-marbled steak.
Now flood that marrow with water. On T1, the fat signal drops, so the area goes dark. On a fluid-sensitive sequence — T2 with fat saturation, or STIR, which is built to make fat disappear and water shout — that same waterlogged spot goes bright. Dark on T1, bright on fluid-sensitive. That two-sequence pairing is the bread and butter of marrow imaging.
"Marrow edema" on MRI just means water has moved into the marrow. It is a pattern, not a diagnosis — your next move is always to ask what dragged the water in.
The honest part nobody loves: edema is gloriously nonspecific. A bone bruise, an early stress reaction, infection, inflammatory arthritis, a tumor, and a healing fracture can all produce a bright marrow blush. Context — the history, the location, what the rest of the picture is doing — is what turns the signpost into an answer.
AVN: bone that lost its blood supply
Avascular necrosis — the radiologists also say osteonecrosis — is exactly what the name promises: an area of bone died because its blood supply was choked off. Picture a section of lawn over a buried pipe that bursts; cut off the water and that patch browns out while the lawn around it stays green. Bone does the same, except bone can't just regrow — it has to be slowly remodeled, and in the meantime it's structurally fragile.
The femoral head is the poster child because its blood supply is precarious and end-arterial, so it's vulnerable when things go wrong upstream — a displaced hip fracture, steroids, alcohol, sickle cell disease, and a long list of others. (Plenty of cases are simply idiopathic, which is the radiologist's dignified word for "we don't know.")
X-rays are late to this party. Early AVN can have a completely normal radiograph, because the dead bone hasn't yet changed density or shape. MRI sees it far sooner. So if the hip hurts, the X-ray is clean, and there's a reason to worry, the next step is often MRI rather than reassurance.
The signs worth memorizing
On MRI, the line you're hunting for is the double line sign: on T2, a dead segment of marrow gets outlined by a serpiginous (wavy, map-like) border that shows two parallel lines — a dark outer line of sclerotic, reactive bone and a bright inner line of granulation tissue. That double line is fairly specific for osteonecrosis and is the finding that lets you commit.
On radiographs and CT, the late tells are a crescent-shaped lucency just under the joint surface (the crescent sign, from the dead bone starting to separate) and, eventually, flattening and collapse of the femoral head.
Don't confuse two look-alikes. A subchondral insufficiency fracture (common in older or osteoporotic patients) also makes subchondral edema and can collapse — but it tends to show a low-signal fracture line paralleling the joint surface rather than a true serpiginous double line. And a bone infarct in the shaft is the same process as AVN, just away from a joint, with that same wavy rind. Same disease, different real estate.
Why the staging obsession — it's all about collapse
Everyone frets over staging AVN, and the reason is brutally practical: has the bone collapsed yet? Before collapse, the joint surface is still round and there's a real shot at saving the native hip. Once that subchondral bone caves in, the smooth ball goes lumpy, the cartilage above it follows, and you're sliding toward end-stage arthritis and a hip replacement. So the report's most load-bearing sentence isn't "there is osteonecrosis" — it's whether the surface is still intact.
When you spot AVN in one hip, deliberately look at the other one. The insults that cause it — steroids, alcohol, sickle cell — are usually systemic, so bilateral disease is common, and the second hip is often the silent one that gets missed.
Putting it together
When you see bright marrow on a fluid-sensitive sequence, resist calling it anything until you've asked why. A young runner with shin pain points toward a stress reaction or stress fracture; fever, a track of pus, and adjacent soft-tissue inflammation point toward osteomyelitis. And a serpiginous double line walling off a wedge of dead bone — especially in a femoral head, especially in someone on steroids — is osteonecrosis until proven otherwise. Marrow edema tells you where to look. The surrounding story tells you what it is.