Peptic Ulcer & Perforation
- A peptic ulcer is a crater eaten into the lining of the stomach or duodenum — usually by stomach acid plus H. pylori or NSAIDs.
- The disaster scenario is perforation: the crater drills all the way through the wall and the gut springs a leak.
- The classic imaging giveaway of perforation is free air outside the bowel — gas where gas should never be.
- An upright chest X-ray catches free air under the diaphragm; CT is far more sensitive and tells you where the hole is.
- Perforation is a surgical emergency. The picture confirms it; it does not get to slow anyone down.
Your stomach is basically a leather bag full of acid strong enough to dissolve a steak. The only reason it doesn't dissolve you is a thin, hard-working layer of mucus and bicarbonate coating the inside. A peptic ulcer is what happens when that protective coating loses the fight and the acid starts chewing a hole in the wall underneath. Most of the time that's a slow, miserable, treatable problem. Occasionally the acid digs all the way through — and that's when imaging gets very interesting, very fast.
What an ulcer actually is
"Peptic" just means "related to digestion and acid." An ulcer is a crater — a defect that goes deeper than the surface lining. They show up most often in the stomach (gastric) and the first part of the small bowel (duodenal). The two usual villains are Helicobacter pylori (a bug that colonizes the stomach and wrecks the defenses) and NSAIDs (ibuprofen and friends, which sabotage the mucus layer). Acid does the actual digging; these two just hold the door open.
Uncomplicated ulcers are largely a clinical and endoscopic story — a gastroenterologist looks at the crater directly with a camera, which beats any X-ray. Radiology's moment comes when an ulcer gets ambitious and complicates: it bleeds, it scars down and blocks the outlet, or it perforates.
The big one: perforation
Think of the bowel as a sealed garden hose carrying air and fluid. Perforation is a puncture in that hose. The contents — gas, acid, half-digested lunch — spill out into the peritoneal cavity, the sterile space wrapped around your organs. The patient typically goes from "bad stomach pain" to sudden, severe, all-over abdominal pain and a belly that's rigid as a board. Clinically, this is an emergency before any image is taken.
A suspected perforation is a "treat the patient, not the picture" situation. If someone has a rigid, board-like abdomen and is crashing, the surgeon is the priority. Imaging confirms and localizes — it should never delay the call.
Hunting for free air
Here's the elegant part. Inside your abdomen, gas belongs inside the bowel and nowhere else. So when the hose leaks, gas escapes into the peritoneal cavity, and that misplaced gas — pneumoperitoneum, or simply free air — becomes the fingerprint of perforation.
On an upright chest X-ray, gas floats to the highest point, which is right under the dome of the diaphragm. You get a thin, dark crescent of air sitting between the bright-white diaphragm and the liver below it. It's a beautiful sign precisely because it's so out of place.
One important honesty note: the upright chest film is convenient but not very sensitive — a small leak can hide. If perforation is suspected and the chest film is clean, that does not clear the patient.
CT: where the hole is
CT is the real workhorse. It finds far smaller volumes of free air than a plain film can, and it does something the X-ray can't: it points toward the culprit. Radiologists trace the free gas back to its source — bubbles clustered near a defect in the stomach or duodenal wall, a smudge of inflammatory fat-stranding around the leak, and sometimes free fluid pooling nearby. On the read, scrolling to the very top of the abdomen and looking for tiny black flecks trapped against the front of the body is a reliable trick — air rises, so it collects right under the anterior abdominal wall when the patient lies on their back.
Use a wide (lung) window when you're hunting for free air on CT. Tiny gas bubbles that vanish on soft-tissue windows light up clearly when you widen the window — black specks against gray.
The classic traps
Free air is high-stakes, so the mimics matter.
Not all free air means a perforated ulcer. Recent abdominal surgery leaves gas behind that can linger for days, and that's expected — not a leak. On a plain film, the Chilaiditi configuration (a loop of colon tucked between liver and diaphragm) can fake a crescent of free air, but look closely and you'll see bowel folds crossing it — real free air is featureless.
A perforated ulcer is not the only cause of pneumoperitoneum. Any breach of the gut wall can do it — including a perforation downstream from bowel obstruction, diverticulitis, or trauma. Free air tells you the gut is leaking somewhere; the rest of the scan tells you where and why.
Putting it together
| Question | Best first answer |
|---|---|
| Is there an uncomplicated ulcer? | Endoscopy, not radiology. |
| Is there free air at the bedside? | Upright chest X-ray — quick, but misses small leaks. |
| Where is the hole and how bad? | CT — sensitive, and localizes the source. |
If you remember one thing, remember the chain: acid digs a crater, the crater can drill through the wall, and the wall is the seal that keeps gas inside the gut. Break that seal and gas escapes — and that escaped gas, sitting somewhere it has no business being, is the whole diagnosis. Find the air, find the emergency.