Sac Necrosis of Pancreas

Severe pancreatitis occurs in 20% to 30% of all patients with acute pancreatitis. It is characterize by protracted clinical course, multi organ failure and pancreatic necrosis [1].

In pancreatic necrosis there is severe inflammation causing necrosis of acinar cells, vessel walls and ducts leading to duct destruction and hemorrhage. Necrosis occurs early within 24 to 48 hours [1]. Development of pancreatic necrosis dramatically increases the mortality rate and the mortality risk depends on the extent of parenchymal injury. The necrosis can be peripheral or central which disrupt the duct system [2]. If the contamination does not occur in the necrotic tissue it can form a collection and remain stable. It can evolve into pseudocyst or resolve completely. Secondary infections occur in 5% to 10% of people with necrotizing pancreatitis and further increase the mortality rate [2].

A different entity of necrosis has been described as sac necrosis or the saccular necrosis of pancreas, where the necrotic pancreatic tissue form a well defined cystic lesion that is confined to the outline of the pancreas and communicate with the duct system [3].

Ultrasonography is of limited value in the assessment of pancreatic necrosis because in severe pancreatitis due to the presence of ileus which limits the sonographic visualization of pancreas. Differentiation cannot be made between the necrotic and the non necrotic pancreatic tissue by Ultrasonography. ultrasonography can be used to detect gall stones or any pancreatic collections [4].

Imaging method of choice for assessment of acute pancreatitis and its complications is contrast enhanced CT scan of abdomen. The role of CT is to demonstrate presence and the extent of the necrosis and other complications of acute pancreatitis. IV contrast enhanced CT is a better prognostic indicator than the numeric scoring system owing to its greater sensitivity and specificity [1, 2].

CT grading of pancreatitis is used to identify the group of patients who are at risk of morbidity and mortality by identifying necrotic pancreatic tissue. CT scans obtained three days after the clinical onset of pancreatitis is more accurate in depicting necrosis. It has a sensitivity of 87% and specificity of 100% in detecting pancreatic necrosis [1, 2].

Normal unenhanced pancreas has a ct attenuation value of 30-50 Hounsfield units (HU) and after contrast administration show homogenous enhancement which display attenuation value of 100-150 HU [1]. Pancreatic necrosis is defined as focal or diffuse area of nonenhancing pancreatic parenchyma.

The CT severity index scores necrosis as absent, minimal (when there is <30% of pancreatic necrosis) substantial (>30% of pancreatic necrosis) [1, 2]. The modified CT severity index found to be a stronger prognostic indicator and it includes presence or absence of acute fluid collections, pancreatic necrosis as a percentage and extrapancreatic manifestations such as ascites, pleural fluid, vascular complications or the involvement of gastrointestinal tract [1, 5].

MRI can be used as an alternative imaging modality where contrast enhanced CT is contraindicated or when the findings are equivocal and need further characterization. The normal pancreas has relatively high signal on T1W sequences compared to liver and muscle. On fat suppressed T1W images the relatively high signal intensity of the pancreas increases further. Normal pancreas on T2W images is slightly hyperintense to muscle and the T2W fat suppressed images show minimal contrast between pancreas and the surrounding suppressed fat. Contrast enhanced MR studies show intense enhancement in the arterial phase followed by rapid washout [1].

Morphologic changes of pancreatitis would be similar to that of CT. In acute pancreatitis fat suppressed T2W images are helpful in depicting focal or diffuse parenchymal changes accurately. T2W sequences will depict fluid collections, pseudocyst and areas of hemorrhage. Gadolinium enhanced T1W GRE images show pancreatic necrosis as areas of nonenhancing parenchyma [1].

MRCP can demonstrate possible choledocholithiasis, any duct disruption or leakage. And it shows the size, location and any possible communication of pseudocyst with the duct system [1].