Free-flowing ascites conforms to shape of surrounding structures and flows to dependent recesses
Simple fluid signal on T1WI (hypointense) and T2WI (hyperintense) MR
No appreciable complexity within ascites fluid
• Exudative ascites: Typically demonstrates increasing density of fluid with increasing protein content
Often mildly hyperdense (15-30 HU) relative to transudative ascites
May demonstrate complexity, including septations, peritoneal thickening/enhancement, and loculation
Loculated ascites fluid exerts mass effect and displaces adjacent structures (such as bowel loops)
Usually simple fluid signal on T1WI and T2WI MR, but internal complexity and protein can result in intermediate T1 and T2 signal
Chylous ascites can measure < 0 HU or demonstrate fat-fluid levels
PATHOLOGY
• Ascites is typically divided into 2 types
Transudative ascites is caused by high portal venous pressures and is characterized by low protein, low LDH, normal glucose, and low specific gravity
– Transudative ascites is simple ascites fluid most often caused by cirrhosis, hepatitis, heart failure, renal failure, hypoproteinemia, etc.
Exudative ascites is characterized by high protein and high specific gravity
– Exudative ascites related to multiple causes, such as infection, ischemia, peritoneal carcinomatosis, peritonitis, or pancreatitis
(Left) Axial CECT shows ascites due to hepatic cirrhosis, with large varices and splenomegaly. Notice the relatively simple, uncomplicated appearance of this transudative ascites.
(Right) Axial CECT shows massive ascites due to right heart failure. Like other forms of transudative ascites, note that the fluid appears simple without evidence of complexity, nodularity, or adjacent peritoneal thickening/enhancement.
(Left) Ultrasound shows large anechoic ascites in the lower abdomen displacing bowel loops in a patient with cirrhosis. As in this case, simple transudative ascites is classically anechoic, freely mobile, and shows acoustic enhancement.
(Right) Ultrasound shows complicated ascites in a cirrhotic patient. The fluid was non-mobile and loculated on real-time scanning, and there are multiple internal septations , suggesting this is not simple transudative ascites. The ascites fluid was found to be infected.
TERMINOLOGY
Definitions
• Pathologic accumulation of fluid within peritoneal cavity
IMAGING
General Features
• Best diagnostic clue
Diagnostic paracentesis (in cases where either infection or tumor is suspected)
• Location
Free-flowing ascites fluid flows to most dependent recesses of abdomen and pelvis
– Morison pouch (hepatorenal fossa): Most dependent recess in upper abdomen
– Rectouterine or rectovesical space: Most dependent spaces in pelvis
– Paracolic gutters and subphrenic spaces
– Lesser sac usually does not fill with ascites, with the exception of cases with tense ascites or ascites due to a local source (gastric ulcer or pancreatitis)
Otherwise, lesser sac fluid suspicious for ascites related to carcinomatosis or infection
• Morphology
Free-flowing ascites: Conforms to shape of surrounding structures and does not deform normal shape of adjacent organs or exert mass effect
– Fluid insinuates itself between organs
Loculated fluid: Fluid demonstrates rounded contour and appears encapsulated
– Loculated ascites fluid exerts mass effect and displaces adjacent structures (such as bowel loops)
• Key concepts and descriptors
Ascites typically divided into 2 types
– Transudative ascites is caused by high portal venous pressures and is characterized by low protein, low LDH, normal glucose, and low specific gravity
Transudative ascites is simple ascites fluid that is most often caused by cirrhosis, hepatitis, heart failure, renal failure, or hypoproteinemia
– Exudative ascites characterized by high protein and high specific gravity
Exudative ascites related to multiple causes, such as infection, ischemia, peritoneal carcinomatosis, peritonitis, and pancreatitis
• Abdominal radiograph: Insensitive for fluid, as diagnosis of ascites on radiographs requires a substantial amount of fluid to be present (usually > 500 cc)
Diffuse haziness and increased density of abdomen
Poor visualization of normally visualized soft tissue structures, such as psoas and renal outline
– Obliteration of hepatic and splenic angles
Bulging of flanks due to fluid distending abdomen
Separation of small bowel loops with centralization of floating gas-containing small bowel
Hellmer sign: Lateral edge of liver medially displaced from adjacent thoracoabdominal wall
Symmetric densities on sides of bladder (dog ears sign)
Medial displacement of ascending and descending colon
Lateral displacement of properitoneal fat line
CT Findings
• Transudative ascites
Typical transudative ascites has density of 0-15 Hounsfield units (HU) and appears free flowing
Small amounts of ascites typically seen in right perihepatic space, Morison pouch, and pouch of Douglas
Larger amounts of fluid accumulate in paracolic gutters
– Ascites can cause centralization of bowel loops
– Ascites accumulating in central abdomen may have triangular configuration within leaves of mesentery
Massive ascites distends peritoneal spaces
Look for associated evidence of liver, heart, or kidney failure (most common causes of transudative ascites)
• Exudative ascites
Exudative ascites typically demonstrates increasing density of fluid with increasing protein content
– Exudative ascites often mildly hyperdense relative to transudative ascites (15-30 HU)
– May demonstrate complexity, including septations, peritoneal thickening/enhancement, and loculation
Ascites related to peritoneal carcinomatosis: Complex ascites with nodularity and thickening of adjacent peritoneum and frequent loculation of ascites
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due to hepatic cirrhosis, with large varices and splenomegaly. Notice the relatively simple, uncomplicated appearance of this transudative ascites.
in the lower abdomen displacing bowel loops 
in a patient with cirrhosis. As in this case, simple transudative ascites is classically anechoic, freely mobile, and shows acoustic enhancement.
in a cirrhotic patient. The fluid was non-mobile and loculated on real-time scanning, and there are multiple internal septations 
, suggesting this is not simple transudative ascites. The ascites fluid was found to be infected.
Small amounts of ascites typically seen in right perihepatic space, Morison pouch, and pouch of Douglas
Exudative ascites typically demonstrates increasing density of fluid with increasing protein content
Ascites related to peritoneal carcinomatosis: Complex ascites with nodularity and thickening of adjacent peritoneum and frequent loculation of ascites
