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Radiology of Acute Pancreatitis and Its Complications
151
2. Pancreatic Pseudocyst
An encapsulated collection of fluid with a well-defined inflammatory wall
usually outside the pancreas with minimal or no necrosis. This entity usually
occurs more than 4 weeks after the onset of interstitial oedematous
pancreatitis. CECT criteria: well circumscribed usually round or oval.
Homogeneous fluid density. No non-liquid component. Well-defined wall,
which is completely encapsulated. Maturation usually requires more than 4
weeks after onset of acute pancreatitis, occuring after interstitial oedematous
pancreatitis.
3. ANC (Acute Necrotic Collection)
A collection containing variable amounts of both fluid and necrosis
associated with necrotizing pancreatitis, the necrosis can involve the
pancreatic parenchyma and/or the peripancreatic tissues. CECT criteria: occurs
only in the setting of acute necrotizing pancreatitis. Heterogeneous and non-
liquid density of varying degrees in different locations (some appear
homogeneous early in their course). No definable wall encapsulating the
collection. Location could be intrapancreatic and/or extrapancreatic.
4. WON (Walled-Off Necrosis)
A mature encapsulated collection of pancreatic and/or peripancreatic
necrosis that has developed a well-defined inflammatory wall. WON usually
occurs more than 4 weeks after onset of necrotizing pancreatitis. CECT
criteria: heterogeneous with liquid and non-liquid density with varying degrees
of loculations (some may appear homogeneous). Well-defined wall, which is
completely encapsulated. Location could be intrapancreatic and/or
extrapancreatic. Maturation usually requires 4 weeks after onset of acute
necrotizing pancreatitis [10].
OVERVIEW OF IMAGING TECHNIQUES
Imaging techniques commonly used for the diagnosis and severity
assessment of AP include abdominal ultrasound, contrast-enhanced CT, and
magnetic resonance imaging [17]. Plain film of the abdomen is a non-specific
examination.
Clinical suspicion of acute pancreatitis should be confirmed by checking
for elevated pancreatic enzyme levels in the blood serum, in the urine, or
Kamil Zeleňák and Martin Števík
152
peritoneal fluid, although the increase is not specific. However, abdominal
symptomatology accompanied by three times elevated amylase is very likely
in the diagnosis of acute pancreatitis. In the diagnosis of severe acute
pancreatitis imaging methods are essential [13].
Plain film. When acute pancreatitis is suspected, chest and abdominal X-
ray examinations should be performed to check for the presence of any
abnormal findings caused by acute pancreatitis. Abdominal X-ray
examinations in acute pancreatitis visualize ileus, localized sentinel loop signs
in the left upper abdomen, enlarged duodenal loops and gas collections, colon
cutoff signs in the right colon, retroperitoneal gas collection, calcified
gallstones, and pancreatolithiasis [32].
Abdominal ultrasonography. The pancreas may appear completely normal
in mild cases of acute pancreatitis. Because of the increased amount of gas in
the intestine (paralytic ileus) in the early stage of the disease, ultrasound is not
useful for the early detection of acute pancreatitis. Ultrasound’s role is to
detect gallstones after the first episode of AP [5]. In the first line, ultrasound is
used to exclude biliary aetiology, and then in distinguishing fluid and semi-
solid local complications (Figure 4). The advantage of ultrasound is its
simplicity and the fact that it is a non-invasive examination. More definitive
findings include a diffusely enlarged gland and reduced parenchymal
echogenicity. Focal enlargement of the pancreatic head and body may also be
seen.
In severe form the area of peripancreatic fluid collections or
intrapancreatic localization of liquid compositions may be evidence of
necrosis. The spleen can be used as an acoustic window to image the
pancreatic tail. The Doppler technique should be used to assess vascular
complications of acute pancreatitis, such as venous thrombosis and
pseudoaneurysm formation. A pseudoaneurysm has turbulent arterial flow
within the mass. Complications of acute pancreatitis may be identified.
Peripancreatic free fluid collections look like ill-defined anechoic collections.
The fluid collections may demonstrate internal echoes/debris or septations if
haemorrhage or a superimposed infection has occurred. Pseudocysts look like
well-defined round or oval anechoic fluid collections with through
transmission. Infected and non-infected pseudocysts are indistinguishable from
each other sonographically. Ultrasonography is often used to monitor the
resolution of pancreatic pseudocysts. A pancreatic abscess may appear as a
complex cystic structure with internal debris, septations and, possibly,
echogenic gas bubbles [16].
Radiology of Acute Pancreatitis and Its Complications
153
CEUS (contrast enhanced ultrasound) has been proven to be an accurate
method for evaluating the presence and extent of necrosis in acute pancreatitis
[4].
EUS (endosonography) allows to accurately predict the severity and risk
of mortality in nearly 90% of cases with AP [3].
Computed tomography. Computed tomography scan with contrast medium
is considered the gold standard in the diagnosis of local involvement,
especially of pancreatic necrosis. An early CT scan is only recommended
when the diagnosis is uncertain, or in case of suspected early complications,
such as bowel perforation or ischaemia. Typical CT findings in acute
pancreatitis include focal or diffuse enlargement of the pancreas,
heterogeneous enhancement of the gland due to oedema or necrosis, irregular
or shaggy contour of the pancreatic margins, blurring of peripancreatic fat
planes with streaky soft tissue stranding densities, thickening of fascial planes,
and the presence of intraperitoneal or retroperitoneal fluid collections
(Figure 5).
Figure 4. Axial ultrasound image of interstitial oedematous pancreatitis in a 10 year-
old boy. The image shows homogeneous pancreatic parenchyma without necrosis
(asterisk) and hypoechoic peripancreatic fat stranding (arrow). VL – lienal vein; Ao –
aorta.
Kamil Zeleňák and Martin Števík
154
Figure 5. Axial CT image of interstitial oedematous pancreatitis in a 49-year-old man.
The image shows homogeneous enhancement of the pancreatic parenchyma without
necrosis (asterisk) and mild peripancreatic fat stranding (arrow).
Fluid collections are most commonly found in the peripancreatic and
anterior pararenal spaces. They can extend from the mediastinum down to the
pelvis. Clavien et al. reported a 92% sensitivity and 100% specificity in
diagnosing acute pancreatitis via CECT [19]. Balthazar et al. reported an
overall accuracy of 80–90% in the detection of pancreatic necrosis [8].
The
Atlanta classification defined pancreatic necrosis as a deposit of the pancreatic
tissue, the density of which does not exceed 50 Hounsfield units (HU) and
covers at least 30% of the pancreatic tissue and an area of more than 3
centimetres in diameter [29]. An early CT may be misleading regarding the
morphologic severity of the pancreatitis, because it may underestimate the
presence and amount of necrosis. Necrosis of peripancreatic tissue can be very
difficult to diagnose, but is suspected when the collection is inhomogeneous,
i.e., there are various densities on CT. Necrosis is stabilized within 96 hours
after the onset of symptoms, and can therefore contrast CT findings within the
first 24 hours can give a false negative and be underestimated. A pancreatic
abscess can manifest as a thick-walled low-attenuation fluid collection with
gas bubbles or a poorly defined fluid collection with mixed
densities/attenuation. Gas bubbles are not specific for infection. A suspected
pancreatic abscess requires needle aspiration because it cannot be ruled out on
the basis of CT morphology. Necrotic pancreatic tissue is not enhanced after
the administration of intravenous contrast. Normal unenhanced pancreas has
CT attenuation measuring 30–50 HU and after intravenous contrast, the
pancreas should display attenuation measuring 100–150 HU [8].
Radiology of Acute Pancreatitis and Its Complications
155
Early detection of infection in acute pancreatitis affects the choice of
treatment and clinical outcome. Positron emission tomography-computed
tomography (PET/CT) is useful in detecting infection in pancreatic or
peripancreatic fluid collections in patients with AP [14, 40].
Local complications of acute pancreatitis can be recognized using CECT
[29], such as necrosis, infected necrosis (difficult to differentiate from aseptic
necrosis, often only by the presence of bubbles of gas), abscesses (collection
density of over 10 HU), pseudocysts, haemorrhage (hyperdense fluid
collection retroperitoneal or peripancreatic tissue) (Figure 6). A pseudocyst
appears as an oval or round water density collection with a thin or thick wall,
which may be enhanced. Venous thrombosis can be identified through a
failure of the peripancreatic vein (splenic vein, portal vein) to enhance or as an
intraluminal filling defect. Associated gastric varices may be identified. A
pseudoaneurysm usually appears as a well-defined round structure with a
contrast-enhancement pattern similar to that of the aorta and other arteries.
Haemorrhage appears as a high-attenuation fluid collections. Active bleeding
is seen as contrast extravasation [16].
Figure 6. (Continued).
Kamil Zeleňák and Martin Števík
156
Figure 6. (Continued).
Radiology of Acute Pancreatitis and Its Complications
157
Figure 6. Serial axial CT scans (A−E). Acute necrotizing pancreatitis (pancreas gland
and peripancreatic necrosis) in a 28-year-old man. Typical sequence of events in a case
of central gland necrosis, pseudocyst formation and walled-off necrosis. Collections
(white arrows) in lesser sac and left anterior and posterior pararenal compartment.
Grade E, less than 30% parenchymal necrosis; modified CTSI 8 − severe acute
pancreatitis.
(A) Initial CT shows perfusion defects of the body of the pancreas (asterix),
indicating parenchymal necrosis with a fluid collection extending into the left
pararenal compartment (arrows) and the root of the transverse mesocolon.
The fluid collection lacks a well-defined wall. Acute peripancreatic fluid
collection (<4 weeks). Acute necrotic collection (ANC) was suspected.
(B) Follow-up axial CT 10 days later reveals definitive parenchymal body
necrosis (asterix). An increase of the partially encapsulated progressive
peripancreatic heterogeneous acute necrotic collection (arrows) is observed.
Significant fat necrosis reaching the transverse mesocolon and left posterior
peritoneal cavity space was observed (black asterix).
(C) Follow-up axial CT examination 60 days after onset of symptoms. Walled-off
necrosis in peripancreatic space (>4 weeks) (arrows). (A) CT depicts well-
defined heterogeneous collection (arrows) with primarily fluid density and
scattered areas of fat density and fully encapsulated collection, indicative of
necrosis of peripancreatic tissues. This was diagnosed as walled-off necrosis
(WON). The patient suffered from fever despite antibiotic treatment.
This heterogeneous and fully encapsulated collection (arrows) was classified as
pancreatic necrosis, pseudocyst, or even pancreatic abscess (all may have a similar
appearance on CT).
(D) Follow-up axial CT scan. Three months after the acute attack a persistent
heterogeneous collection is observed (arrows). Drainage operation – pseudo-
cystostomi (arrow) was performed between WON and stomach, which is
partially diminished in size.
Kamil Zeleňák and Martin Števík
158
(E) Follow-up CT 4 months later shows a recurrent collection. The patient
developed fever and elevated white blood cell count, suggestive of infected
collection. CT shows extensive necrosis (30−50%) and an encapsulated
heterogeneous collection (arrows) with liquid and non-liquid (fat) densities
(small arrows), as well as impacted gas bubbles (arrowheads), indicative of
infected necrosis. At surgery, infected necrosis was debrided and pus was
drained.
Indications for Use of CT in Patients with Acute Pancreatitis
An initial CT scan is indicated in patients in cases where there is doubt
about the clinical diagnosis (best at least 48 hours after onset of symptoms): in
patients with clinical signs of severe pancreatitis; in patients who do not show
adequate improvement within 72 hours of initiation of the conservative
medical treatment; in patients who develop a sudden change in clinical status
(fever, pain, decreasing hematocrit, hypotension, etc.) after initial
improvement, suggesting complications.
Follow-up CT scan should be obtained after 7 to 10 days and/or before
hospital discharge in all patients with a grade D or E pancreatitis (CTSI score
of 3−10) to exclude clinically silent complications; in patients with no or
inadequate clinical improvement or deterioration under treatment; in patients
with grade A−C pancreatitis (CTSI 0−2) only when complications are
suspected [41].
Magnetic resonance image is preferred for patients where the CT is
contraindicated. The morphologic changes of acute pancreatitis are similar on
CT and MRI. The pancreas may be enlarged focally (usually the pancreatic
head) or diffusely. An MRI is superior to CT in differentiating between fluid
and solid necrotic debris (Figures 7, 8).
Magnetic resonance imaging has a better resolution of solid material
within fluid collections, and provides a better identification of local
complications. Magnetic resonance cholangiopancreatography (MRCP) is also
helpful in the diagnosis of choledocholithiasis and pancreas divisum. Magnetic
resonance imaging may also be better than CT in detecting areas of sterile
pancreatic necrosis in what appear to be simple pseudocysts on CT. Acute
inflammatory changes appear as strands of low signal intensity in the
surrounding peripancreatic fat. Complications of acute pancreatitis can also be
identified. Haemorrhage is characterized by T1 shortening or high signal
intensity on T1-weighted images (WI) with fat suppression. Peripancreatic
fluid collection, pseudocyst, and abscess are recognized by their high signal
Radiology of Acute Pancreatitis and Its Complications
159
intensity on T2-weighted images. Devascularized or necrotic portions of the
pancreas fail to enhance on dynamic gadolinium-enhanced images [42]. The
T1−WI is present variable decrease in the intensity signal in case of acute
pancreatitis and hyperintensity of haemorrhage. If the collections show
homogeneous high signal intensity on a fat-suppressed T2−weighted MRI
image, they contain clear fluid (i.e., pseudocysts). If a T2−weighted MRI
sequence shows that the collection has a low signal intensity, most likely this
is necrotic fat tissue (i.e., sterile necrosis or walled-off necrosis). The necrosis
and pseudocyst are without enhancement in T1−WI [16]. Diffusion-weighted
magnetic resonance imaging (DW-MRI) can be used as a non-invasive
technique for the detection of infection in acute pancreatitis-associated
collections [28].
Figure 7. Axial MR T1−weighted fat suppressed image of acute interstitial oedematous
pancreatitis in a 56-year-old woman. The image shows homogeneous signal of the
pancreatic parenchyma (asterisk) without necrosis and mild peripancreatic fat
stranding (white arrow). Fluid collection can be observed in the left anterior pararenal
space (black arrow). Grade D (Balthazar), modified CTSI 3 - mild acute pancreatitis.
Kamil Zeleňák and Martin Števík
160
Figure 8. Axial MR T1−weighted fat suppressed image of acute necrotizing
pancreatitis (peripancreatic necrosis alone) in a 28-year-old woman. MR shows normal
pancreas (asterisk), tail is surrounded by heterogeneous acute necrotic collections
(arrows) with hyperintense signal due to a high level of protein, suggestive of
peripancreatic necrosis alone. This collection can be observed in the left anterior
pararenal space and the root of the transverse mesocolon. Grade D (Balthazar),
modified CTSI 4 − moderate acute pancreatitis.
ANGIOGRAPHY AND INTERVENTIONAL RADIOLOGY
Vascular complications of acute pancreatitis result from the proteolytic
effects of the pancreatic enzymes that cause the erosion of blood vessels,
which often results in pseudoaneurysm (PSA) formation. The splenic artery,
followed by the pancreaticoduodenal and gastroduodenal arteries, are most
commonly affected. The bleeding point is identified by noting free contrast
extravasation [27]. Haemorrhagic complications are usually manifestations of
the progress of severe pancreatitis [50]. It is recommended that all
pseudoaneurysms be repaired [24].
Endovascular therapy remains the first-line option for known arterial
bleeding (preferably in a patient with a stable haemodynamic status) and is
considered a safe and effective modality to treat visceral pseudoaneurysms.
Metallic coil embolization is preferable distal and proximal to the site of
arterial extravasation (the so-called sandwich technique), thereby preventing
backflow from the collateral circulation [25, 26].
A PSA with narrow neck can be embolized selectively (Figures 9−13).