Acute pancreatitis ultrasound

This microchapter covers the role of ultrasound — including transabdominal ultrasound (TUS), contrast-enhanced ultrasound (CEUS), and endoscopic ultrasound (EUS) — in the evaluation of acute pancreatitis (AP). Recommendations are drawn primarily from the ACG 2024 Guidelines, ACR Appropriateness Criteria, AGA 2022 Clinical Practice Update, WSES 2019 Guidelines, and supporting systematic reviews.

• The ACG 2024 Guidelines recommend transabdominal ultrasound in all patients with AP to evaluate for biliary pancreatitis, with a repeat US if the initial examination is inconclusive (conditional recommendation, very low quality of evidence). For idiopathic AP, additional evaluation with repeat US, MRI/MRCP, and/or EUS is recommended.^[1]
• The ACR Appropriateness Criteria rate abdominal US as “usually appropriate” as the initial imaging modality for first-time presentation of suspected AP within 48–72 hours of symptom onset.^[2]
• The WSES 2019 Guidelines recommend US on admission or within the first 48 hours for all patients with AP.^[3]
• In pediatric AP, US is particularly favored as the initial imaging modality because it avoids ionizing radiation and sedation.^[4]

The primary role of transabdominal US in AP is etiologic (identification of gallstones, biliary sludge, and common bile duct dilation) rather than diagnosis of pancreatitis itself or severity staging.^[2]

• Gallstone detection: Sensitivity >95% in uncomplicated cases; falls to approximately 67%–87% in the setting of AP because of overlying bowel gas, ileus, and abdominal distension.^[5][6]
• Choledocholithiasis detection: Sensitivity ranges widely from 25%–90% depending on methodology, stone size, and population (ACR/Cucher: 25%–60%; Cochrane/Tse & Yuan: 27%–50%; Şurlin: 50%–80%; Fogel & Sherman: 25%–90%). A 2015 Cochrane diagnostic-accuracy review reported pooled sensitivity of 73% and specificity of 91% with substantial heterogeneity. Specificity is generally 90%–95%.^{[2][6][5][7][8]}
• Pancreatic parenchyma: The pancreas demonstrates features of AP on US in only ~20% of cases; it may be visualized yet appear sonographically normal.^[2]
• ALT + gallstones combination: ALT >3× upper limit of normal plus gallstones on US yields high sensitivity (95%–98%) for biliary pancreatitis (supported by older studies).^[6]

A complete abdominal US (not right-upper-quadrant only) should evaluate:^[4][9]

• Gallbladder – calculi, sludge, wall thickening, pericholecystic fluid
• Biliary tree – CBD diameter, filling defects, intrahepatic duct dilation
• Pancreas (when visible) – size, contour, echogenicity, pancreatic-duct diameter
• Peripancreatic region – edema, fluid collections
• Free fluid (flanks, pelvis)
• Vascular structures – focused Doppler can detect some cases of splenic/portal vein thrombosis or arterial pseudoaneurysm (CT/MRI remains definitive).^[9]

A right-upper-quadrant–only examination is inadequate and will miss peripancreatic findings.^[4] US may also identify alternative diagnoses (e.g., acute cholecystitis) when the clinical picture is atypical.^[2]

• Fasting ≥4 hours improves gallbladder distension and reduces bowel gas.^[9]
• Oral water immediately before scanning can displace gastric air and improve the pancreatic window.^[9]
• Left-lateral-decubitus or upright positioning and graded compression further optimize visualization.^[9]
• If the initial study is limited by ileus, repeat US after clinical improvement. Accuracy for biliary AP rose from 66% to 83% with a second examination in one prospective series, supporting the ACG recommendation for repeat imaging.^[1][10]

• Bowel gas and ileus obscure the pancreas in the majority of patients.^[2]
• Obesity degrades image quality.
• US underestimates extrapancreatic complications and cannot reliably distinguish interstitial edematous from necrotizing pancreatitis.
• US cannot stage necrosis; contrast-enhanced CT or MRI is required (ideally after 72 hours when clinically indicated).^[1][11]

CEUS with intravascular microbubble agents assesses pancreatic parenchymal perfusion and can differentiate edematous from necrotizing pancreatitis. A meta-analysis of 7 studies (421 patients) reported pooled sensitivity 92% and specificity 84% for severity assessment, with strong correlation to CT severity index (r = 0.93).^[12]

Limitations:

• Microbubble use for this indication is off-label (not FDA-approved).^[2]
• Same bowel-gas constraints as conventional US.
• Limited evaluation of extrapancreatic complications.
• ACR rates CEUS “usually not appropriate” for most AP scenarios given the availability of CT/MRI.^[2]

CEUS may be considered when contrast-enhanced CT is contraindicated (severe contrast allergy, pregnancy) or in resource-limited settings.

EUS is not part of the initial routine evaluation of AP but is central to the work-up of idiopathic and recurrent AP:

• Diagnostic yield for a potential etiology is 29%–88%, most often occult biliary lithiasis (microlithiasis, sludge, small CBD stones).^[13]
• A 2026 meta-analysis (8 studies) found EUS superior to MRCP for overall diagnostic yield (RR 2.01, 95% CI 1.42–2.85), driven by biliary etiologies (RR 3.67); MRCP performed better for pancreas divisum.^[14]
• Occult ampullary or pancreatobiliary malignancy may be identified in up to 5% after a single unexplained episode and up to 12% with recurrent AP, underscoring the value of EUS for cancer exclusion, especially in patients >40–60 years.^[13]
• AGA 2022 Best Practice Advice: EUS is the preferred diagnostic test for unexplained acute and recurrent pancreatitis; MRCP is a reasonable complementary or alternative test according to local expertise.^[13]
• Optimal timing is 2–6 weeks after resolution of the acute episode to allow inflammatory changes to subside.^[13]
• EUS can detect early chronic pancreatitis changes, although it may be oversensitive for this diagnosis.

1. Perform transabdominal US on admission (or within 48 hours) in every patient with AP to evaluate for gallstones and biliary dilation.^[1][3]
2. If the initial US is inconclusive, repeat it after clinical improvement or proceed to MRCP.^[1]
3. Do not use US to assess severity or necrosis; reserve contrast-enhanced CT or MRI for this purpose (preferably after 72 hours when indicated).^[1][11]
4. For idiopathic AP, obtain EUS (preferred) and/or MRCP 2–6 weeks after resolution to search for occult biliary disease, early chronic pancreatitis, anatomic variants, or neoplasia.^[13][1]
5. Consider CEUS only when CT/MRI is contraindicated and pancreatic perfusion assessment is required.

• The primary purpose of US in AP is etiologic (gallstones), not diagnosis or severity staging.
• A normal-appearing pancreas on US does not exclude AP; laboratory abnormalities precede imaging findings.
• Negative US for gallstones does not exclude a biliary etiology; sensitivity falls in the acute setting. Combine with liver enzymes (ALT >3× ULN has high positive predictive value).
• A right-upper-quadrant–only examination is insufficient; a complete abdominal US is required.
• In unexplained AP, pursue EUS to exclude occult malignancy, particularly in patients older than 40 years.

• Ordering a right-upper-quadrant US instead of a complete abdominal study.
• Attempting to “rule out” pancreatitis with US (features of AP are seen in only ~20% of cases).
• Excluding biliary etiology on the basis of a single negative US.
• Obtaining early CT solely because the US is limited; CT before 72 hours underestimates necrosis.
• Omitting EUS/MRCP after idiopathic AP and thereby missing occult biliary disease or neoplasia.