Acute Pancreatitis and Complications

Interstitial edematous pancreatitis (IEP) (70-80% of cases): Normal enhancement of pancreas without necrosis

• Pancreas typically enlarged and edematous

• Peripancreatic fat stranding, edema, and free fluid

• Mild edematous pancreatitis can appear normal on CT

Necrotizing pancreatitis (NP) (20-30% of cases): Areas of nonenhancing parenchymal necrosis

• May necrose pancreatic duct as well

• Necrosis usually develops within 3-4 days after symptom onset

Complications

• Infected pancreatic necrosis : Ectopic gas, in absence of intervention, is highly suggestive of infected necrosis

• Central necrosis : Necrosis of central portion of gland/duct with intact pancreas/duct in head and tail

• Pseudoaneurysm : Most common locations are splenic (50%) and gastroduodenal (20%) arteries

• Venous thrombosis : Splenic vein most common, but portal vein or superior mesenteric vein can be involved

• Fluid collections : Nomenclature depends on age of collection and edematous vs. NP

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    Acute peripancreatic fluid collection : Fluid collection within first 4 weeks after acute IEP

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    Pseudocyst : Fluid collection persisting > 4 weeks

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    Acute necrotic collection : Nonloculated but containing internal necrotic debris and blood

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    Walled-off necrosis (WON) : Loculated, complex fluid collection persisting > 4 weeks after NP

Infiltrating pancreatic adenocarcinoma

Perforated duodenal ulcer

"Shock" pancreas

Lymphoma

Alcohol and gallstones account for vast majority of cases

Many other causes, including metabolic disorders, infection, trauma, drugs, anatomic variants, neoplasm, and ERCP

Multiphasic/multiplanar CT is best for initial evaluation

MR with MRCP is better for evaluation of pancreatic duct and complex fluid collections

Acute inflammation of pancreas with variable involvement of other regional tissues or remote organs

Best diagnostic clue

• Enlarged, edematous pancreas with peripancreatic fluid, fat stranding, and fluid collections

Location

• Pancreas and surrounding peripancreatic soft tissues

Size

• Pancreas usually increased in size (either focal or diffuse)

Morphology

• 2 subtypes: Interstitial edematous, and necrotizing pancreatitis

Revised Atlanta classification in 2012 standardized nomenclature used to describe acute pancreatitis

2 primary subtypes of acute pancreatitis

• Interstitial edematous pancreatitis (IEP) (70-80%)

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    Pancreas typically enlarged and edematous

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    Peripancreatic fat stranding, edema, and free fluid

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    Usually involves entire gland but can be focal

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    Normal enhancement of pancreas without necrosis

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    Normal CT does not exclude pancreatitis

• Necrotizing pancreatitis (NP) (20-30% of cases): Areas of parenchymal necrosis that are either nonenhancing or severely hypoenhancing (usually < 30 HU)

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    Usually more peripancreatic inflammation than IEP

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    Differentiate cases with ≤ 30% parenchymal necrosis from > 30% necrosis for patient prognosis

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    Necrosis may not be present initially but can develop within 3-4 days after symptom onset

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      Early CT can underestimate or miss necrosis

Necrosis both parenchymal and peripancreatic in 75%, peripancreatic alone 20%, parenchymal alone 5%

• Parenchymal necrosis alone in 5%

• Peripancreatic necrosis alone in 20%

Complications

• Infected pancreatic necrosis

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    Implies superinfection of necrotic parenchyma

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    Ectopic gas in pancreatic bed, in absence of intervention or fistula, virtually diagnostic

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    No other specific findings, although inflammation usually greater in cases with infected necrosis

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      May require aspiration for culture

• Central necrosis (disconnected duct syndrome)

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    Necrosis of parenchyma and duct in body with intact pancreas/duct in head and tail

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    Results in encapsulated fluid/debris collection with continual leakage of pancreatic fluid into collection

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    Diagnosis should be suggested based on distribution of necrosis

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    Collection may require either internal drainage or surgery (usually distal pancreatectomy)

• Extrapancreatic fat necrosis

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    Pancreatic enzymes cause fat necrosis

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    Usually low density with heterogeneous fluid and solid components but can appear nodular and mass-like, mimicking carcinomatosis

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    Most often peripancreatic, mesenteric, retroperitoneal

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    Carries better prognosis than parenchymal necrosis but worse than IEP

• Pseudoaneurysm

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    Small contrast-filled outpouching arising next to artery ± adjacent hematoma (due to leak or rupture)

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    Mostly splenic (50%) and gastroduodenal (20%) arteries

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    Unexplained hemorrhage in pancreatic bed should prompt careful search for pseudoaneurysm

• Venous thrombosis

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    May occur due to direct intimal injury or mass effect from adjacent collections

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    Splenic vein > portal vein or superior mesenteric vein (SMV)

• Fluid collections

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    Acute peripancreatic fluid collection : Fluid collection first 4 weeks after acute IEP

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      Simple, nonloculated with no internal debris

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    Pseudocyst : Fluid collection persisting > 4 weeks

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      Loculated with well-defined, enhancing wall usually in lesser sac or pararenal spaces

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      Can rarely be found in unusual locations distant from pancreas, such as thorax

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      Simple fluid attenuation with no internal debris

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    Acute necrotic collection : Fluid collection within first 4 weeks after acute NP

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      Nonloculated but containing internal necrotic debris and blood

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      Acute complex fluid collection with internal debris and solid material in setting of a normally enhancing gland suggests ANC due to extrapancreatic necrosis

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    Walled-off necrosis (WON) : Loculated fluid collection persisting > 4 weeks after NP

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      Heterogeneous collection with well-defined wall and internal necrotic debris/blood products

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    "Pancreatic abscess" : Term no longer utilized in revised Atlanta classification

• "Hemorrhagic" pancreatitis :Term not included in Atlanta classification

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    Small amounts of blood frequently present in peripancreatic fluid collections and has no direct impact on disease severity

Pancreas appears enlarged with increased signal on T2WI and abnormally low signal on T1WI due to edema

• Fat suppression very important in highlighting edema and fluid on T2WI

T1WI C+ images similar to CECT in detection of necrosis and nonenhancement

T2WI offers advantage (over CT) of allowing differentiation of simple fluid collections from those with internal solid debris (i.e., WON)

MRCP can evaluate integrity of pancreatic duct, unlike CT

• May delineate communication between collection and pancreatic duct

• Can delineate anatomic variants which might predispose to pancreatitis, including pancreatic divisum

• Very sensitive for gallstones and other biliary pathology

Acute pancreatitis may be associated with restricted diffusion (lower ADC values than normal pancreas)