Imaging in pancreatitis: when and what
- Finding
Pancreatitis is diagnosed by pain and blood tests; ultrasound looks for gallstones, and CT is kept for doubt or a patient who is not getting better.
What it is
Acute pancreatitis is diagnosed when two of three criteria are met: typical upper abdominal pain, lipase or amylase at least three times the upper limit of normal, and typical imaging. If the first two are present, no imaging is needed to make the diagnosis. Every patient should have an abdominal ultrasound early to look for gallstones, the commonest cause, because that changes treatment. Contrast-enhanced CT is ordered when the diagnosis is uncertain or when the patient does not improve or worsens after the first two or three days; done too early it underestimates necrosis, which takes about three days to show. CT shows an enlarged, oedematous pancreas with fat stranding (interstitial oedematous pancreatitis) or areas that do not enhance (necrotising pancreatitis), and the fluid collections and vascular complications that follow. In chronic pancreatitis, imaging shows calcifications, an irregular dilated main duct and a shrunken gland, and it must exclude a cancer.
What it looks like
- Interstitial oedematous pancreatitis: a swollen gland that enhances evenly, with peripancreatic fat stranding and fluid.
- Necrotising pancreatitis: parts of the gland or the surrounding fat do not enhance after contrast.
- Collections in the first four weeks: acute peripancreatic fluid collection (oedematous type) or acute necrotic collection (necrotising type).
- After four weeks: a pseudocyst (fluid only, with a wall) or walled-off necrosis (fluid and solid debris, with a wall).
- Gas bubbles inside a necrotic collection suggest infected necrosis.
- Chronic pancreatitis: calcifications in the gland (also visible on plain film), a beaded dilated main duct, side-branch ectasia and atrophy.
Why it looks that way
- Enzymes leaking from injured acinar cells inflame the gland and the fat around it, causing oedema and fluid.
- Where the microcirculation fails, tissue dies and no longer takes up intravenous contrast; this takes days to declare.
- Repeated injury replaces the gland with fibrosis; protein plugs calcify inside the ducts, and strictures make the duct beaded.
| Collection | Time | Contents | Wall |
|---|---|---|---|
| Acute peripancreatic fluid collection | Under 4 weeks | Fluid (oedematous pancreatitis) | No |
| Pseudocyst | Over 4 weeks | Fluid only | Yes |
| Acute necrotic collection | Under 4 weeks | Fluid and necrotic tissue | No |
| Walled-off necrosis | Over 4 weeks | Fluid and necrotic tissue | Yes |
Differential and pitfalls
- Early CT (in the first day or two) adds little when the diagnosis is clear and may miss necrosis that appears later.
- Ultrasound often cannot see the pancreas because of bowel gas, but it can still see gallstones and a dilated common bile duct.
- Vascular complications: splenic vein thrombosis and arterial pseudoaneurysm, which can bleed suddenly; CT angiography finds them.
- A focal mass, a cut-off duct, or both the bile duct and pancreatic duct dilated (double duct sign) raise concern for pancreatic cancer.
- Dense lumps on a plain film in the upper abdomen across the spine are typical of chronic calcific pancreatitis.
Next step
- Typical pain and lipase at least three times normal: diagnosis made; order an abdominal ultrasound for gallstones.
- Diagnosis uncertain at presentation: contrast-enhanced CT of the abdomen.
- Not improving or worsening after the first two or three days, or new fever or organ failure: contrast-enhanced CT to look for necrosis and collections.
- Suspected stone in the common bile duct without cholangitis: MRCP; cholangitis or persistent obstruction: call gastroenterology for ERCP.
- Gas in a collection, bleeding or a suspected pseudoaneurysm: call the surgeon and interventional radiology.
Test yourself
Think of the answer, then tap the question to see it.
When is imaging needed to diagnose acute pancreatitis?
Only when one of the other two criteria is missing: typical pain or lipase or amylase at least three times the upper limit of normal.
Why is a CT on the day of admission often unhelpful in acute pancreatitis?
Necrosis takes about three days to show, so early CT may underestimate it, and it rarely changes management when the diagnosis is clear.
How does a pseudocyst differ from walled-off necrosis?
Both appear after four weeks with a wall, but a pseudocyst holds only fluid, while walled-off necrosis holds fluid and solid necrotic debris.
Which imaging findings are typical of chronic pancreatitis?
Calcifications in the gland, a dilated beaded main pancreatic duct, ectatic side branches and an atrophic gland.
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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Acute Pancreatitis (StatPearls (NCBI Bookshelf), 2025); Chronic Pancreatitis (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.