Diagnostic Ultrasound - Abdomen and Pelvis

Ascites
TERMINOLOGY
Definitions
• Abnormal accumulation of fluid within peritoneal cavity
IMAGING
General Features
• Best diagnostic clue
○ Free fluid in peritoneal cavity
• Location
○ Fluid collects in most dependent locations unless there
are loculations
○ Dependent spaces: Pouch of Douglas in pelvis; Morison
pouch (hepatorenal fossa) in upper abdomen, paracolic
gutters
○ Subphrenic spaces:Not dependent, but fill due to
suction effect of diaphragmatic motion
○ Lesser sac usually does not fill with ascites unless tense
ascites, local source (gastric ulcer or pancreatitis)
– Otherwise due to carcinomatosis or infected ascites
• Morphology
○ Free-flowing: Fluid insinuates itself between organs and
is shaped by surrounding structures
– No mass effect
○ Loculated: Rounded, bulging contour, encapsulated
– Does not conform to organ margins
– Mass effect on adjacent organs
○ Fluid can also be chylous, hemorrhagic, bilious,
pancreatic, urine, or cerebrospinal fluid
○ Malignant ascites and pseudomyxoma peritonei more
complex with solid components
Ultrasonographic Findings
• Grayscale ultrasound
○ US accurate at detecting, localizing, and characterizing
ascites; quantification more subjective
– Fluid in dependent recesses, shifts with patient
movement, compresses with increased transducer
pressure
○ Characterization of ascites: Simple or complicated
– Simple: Anechoic; homogeneous, freely mobile, deep
acoustic enhancement
□ Usually transudate
– Complicated: Echogenic fluid with coarse or fine
internal echoes, layering debris or particulate material,
septa
□ Usually exudate
□ Appearance of hemorrhagic ascites varies with time
of onset and transducer frequency
□ Can be anechoic or hyperechoic initially, ±clots,
anechoic later
– Loculated ascites: Encapsulated, internal thick or thin
septa
□ Secondary to adhesions, chronic ascites,
malignancy, infection
□ Rounded margins with mass effect, frequently
displacing adjacent structures, less compressible
□ Malignant ascites: Tethered matted bowel,
peritoneal masses; concordant fluid in greater and
lesser sac
• Small free fluid in cul-de-sac; physiologic in women
• Massive ascites: Small bowel loops arrayed on either side of
vertically floating mesentery
• Transverse & sigmoid colon usually float on top of fluid
• Cerebrospinal fluid ascites: Small amounts of free fluid
normal with ventriculoperitoneal shunt
○ Localized/loculated collection around tip of shunt tube is
pathologic, implies adhesions
• Pancreatic ascites: Peripancreatic, lesser sac, anterior
pararenal space
○ Disruption of pancreatic duct or severe pancreatitis
Radiographic Findings
• Plain abdominal films insensitive for small amounts of
ascites
○ Direct signs
– Obliteration of hepatic angle
– Hellmersign: Displacement of lateral edge of liver
medially, away from thoracoabdominal wall
○ Indirect signs: Diffuse abdominal haziness; bulging of
flanks; poor visualization of psoas & renal outline
– Centralization of floating gas-containing small bowel
or separation of small bowel loops
CT Findings
• Simple ascites: Low-density free fluid 0-30 Hounsfield units
○ Centralization of bowel loops; triangular configuration
within leaves of mesentery
○ Massive ascites; distends peritoneal spaces
• Complex ascites
○ Exudates: Density of ascitic fluid increases with
increasing protein content
○ Hemorrhagic ascites: High density with layering ± active
bleeding
○ Chylous ascites: Less than 0 HU
○ Bilious ascites: Less than 20 HU; typically in right or left
supramesocolic spaces
○ Urine ascites: Nonspecific appearance but delayed CECT
can confirm diagnosis
Nonvascular Interventions
• US-guided therapeutic & diagnostic paracentesis
Imaging Recommendations
• Best imaging tool
○ US for detection & characterization of peritoneal fluid
collections
○ US guidance for paracentesis
• Protocol advice
○ Look for associated hepatic disease, peritoneal masses,
or adherent bowel
DIFFERENTIAL DIAGNOSIS
Hemoperitoneum
• Trauma, ruptured aneurysm, ruptured ectopic pregnancy,
ruptured liver mass, postsurgical bleeding, anticoagulant
therapy
• Fluid debris level may develop if patient in supine position
for a long time
• Massive hemorrhage: Large echogenic mass (clots), later
heterogeneous (lysis)
Diagnoses: Abdominal Wall/Peritoneal Cavity
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