Acute pancreatitis versus chronic pancreatitis on imaging

How can acute pancreatitis be distinguished from chronic pancreatitis on imaging?

Acute

US: US may be limited in the initial evaluation of acute pancreatitis because of overlying bowel gas, resulting in incomplete visualization of the pancreas and underestimation of the extent of peripancreatic fluid collections compared with CT. If pancreatic visualization is not impeded by bowel gas, early or mild pancreatitis often appears normal. In more severe cases of pancreatis, the pancreas may appear enlarged and hypoechoic.

MDCT: CT is not performed to diagnose early or mild pancreatis as it may be normal. Occasionally, the pancreas may appear enlarged and slightly heterogeneous, with increased attenuation in the peripancreatic fat (“dirty fat”) caused by inflammation. CT with MPR is the preferred study for patients with clinically severe pancreatitis, especially to evaluate for necrosis or other complications. NCCT is initially performed to detect pancreatic ductal or parenchymal calcifications and hemorrhage, and to provide a baseline HU for any masses. Imaging in the late HAP and the PVP is then performed. In more severe disease, intraglandular intravasation of pancreatic fluid causes intrapancreatic fluid collections. Extravasation of fluid results in peripancreatic inflammation, thickened fascial planes, and peripancreatic fluid collections, most commonly in the anterior pararenal space (left greater than right) and lesser sac. Fluid extending into the pararenal space can result in the Grey Turner sign (flank ecchymosis) and fluid extending into the gastrohepatic and falciform ligaments can result in Cullen sign (periumbilical ecchymosis). Posterior leakage of fluid can present with a pleural effusion, classically on the left.

Chronic

US: Calcifications, ductal dilatation, heterogeneous hyperechoic echotexture, focal mass lesions, and pseudocysts may be present. The gland usually atrophies with focally enlarged areas.

MDCT: Intraductal calcifications are the most reliable CT indicator of chronic pancreatitis. The gland size is variable, but focal enlargement caused by a chronic inflammatory mass may necessitate biopsy to exclude carcinoma. The pancreatic duct can be dilated (> 3 mm) to the level of the papilla and may appear beaded, irregular, or smooth. Pseudocysts may be seen within or adjacent to the gland.

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