Bronchiectasis
- Bronchiectasis is permanent, abnormal widening of the bronchi — airways stretched out of shape and unable to spring back.
- The defining CT sign is the signet ring: a dilated airway sitting bigger than the artery running beside it.
- Normal airways taper as they head outward; bronchiectatic ones don't, and they keep their caliber way too far toward the chest wall.
- The hallmark plain-film finding is tram-track lines — thickened, parallel airway walls that don't fade away.
- Once it's there, it doesn't reverse. The job of imaging is to spot it, describe its shape and location, and hunt for the cause.
Think of a healthy bronchus as a fresh garden hose: round, springy, and tapering gently from the spigot out to the nozzle. Bronchiectasis is what you get when that hose has been left out in the sun for ten summers — stretched, baggy, permanently floppy, and never going back to its original snap. The walls are thickened, the lumen is too wide, and the airway has lost the discipline to narrow as it travels outward. That's the whole disease in one image. Everything else is figuring out why the hose got wrecked.
What "dilated" actually means
The word bronchiectasis just bolts together "bronchus" and "-ectasis" (Greek for stretching out). So: stretched-out airways. Simple enough. The trouble is that airways are supposed to be a little bit invisible — thin-walled tubes of air threading through lung — so you need a reliable trick to decide when one has gotten too big.
That trick is the artery riding next to it. In the lung, each small bronchus travels with a pulmonary artery branch of roughly the same diameter, like two pipes taped together. Cut across that pair on CT and you see a circle (the airway) next to a dot (the vessel). Normally they're about the same size. In bronchiectasis the airway balloons bigger than its partner artery, and the pair starts to look like a ring with a gemstone next to it.
The radiologists call this the signet ring sign — the dilated, ring-shaped bronchus is the band, and the smaller adjacent artery is the "stone." It's the single most useful CT clue that an airway is abnormally wide, and it's worth burning into memory.
How to spot it
There are three things I look for, and they reinforce each other.
| Sign | What you're seeing | Where |
|---|---|---|
| Signet ring | Airway wider than its buddy artery | Cross-section on CT |
| Lack of tapering | Airway keeps the same caliber outward instead of shrinking | Along the airway's length |
| Airways near the edge | Visible airways within ~1 cm of the pleura | Lung periphery on CT |
That last one is sneaky-good. Healthy small airways get too tiny to see before they reach the chest wall, so if you can clearly make out a tube right up against the pleura, something has dilated it. On a plain chest radiograph, the same dilated, wall-thickened airways show up as tram-track lines — two parallel white rails that should have faded but didn't.
The flavors (cylindrical, varicose, cystic)
Bronchiectasis comes in three shapes, and they're really just a spectrum of how badly the hose has been mangled.
- Cylindrical — mildest. The airway is uniformly widened but still roughly tubular, like a slightly over-inflated straw.
- Varicose — the wall develops beaded bulges, so the airway looks like a string of irregular pearls or a knobbly varicose vein.
- Cystic (saccular) — worst. The airway ends balloon out into grape-like clusters of cysts, sometimes with little air-fluid levels pooled inside.
You don't need to agonize over which is which for the diagnosis — the presence of dilatation is what matters. But naming the shape and noting how much lung is involved is part of a useful report.
Why airways go baggy
Two engines drive most bronchiectasis, and they feed each other in a nasty loop. There's infection/inflammation chewing up the airway wall from the inside, and there's traction — scar tissue in the surrounding lung yanking airways open from the outside. Picture either gnawing the hose from within or guy-wires pulling its walls apart; either way it ends up wide and floppy.
Traction bronchiectasis is the version you'll meet inside fibrotic lung disease — when stiff scar contracts, it drags nearby airways open. That's why you'll see it riding shotgun with UIP/IPF and other fibrosis. The infective/inflammatory version shows up with chronic conditions: cystic fibrosis (classically upper-lobe predominant), prior severe pneumonia or tuberculosis, allergic bronchopulmonary aspergillosis (central, upper-lobe), and immune problems. Distribution is a genuine clue here, so it's always worth noting where the damage lives.
Bronchiectasis on imaging is a finding, not a final answer. When you see it, the next move is to read the pattern and location — upper-lobe vs lower-lobe, central vs peripheral — because that's what steers you toward the underlying cause the clinicians actually need.
The traps
Reversible isn't real bronchiectasis. During an acute infection, airways can transiently dilate and then recover once the patient is better — so-called reversible bronchial dilatation. True bronchiectasis is permanent, so be cautious about calling it on a single scan taken mid-pneumonia; a follow-up once things settle keeps you honest.
One more impostor worth naming: severe traction bronchiectasis viewed end-on, clustered in scarred lung, can masquerade as honeycombing. The tell is that you can usually trace a bronchiectatic airway along its length, whereas honeycomb cysts are stacked, share walls, and sit subpleurally.
If you remember one thing, make it the signet ring: an airway that has grown bigger than the artery it travels with has lost its shape for good, and your job is to name it, map it, and help find out why.