Diagnostic Ultrasound - Abdomen and Pelvis

Pyogenic Hepatic Abscess
TERMINOLOGY
Definitions
• Localized collection of pus in liver due to bacterial
infectious process with destruction of hepatic parenchyma
and stroma
IMAGING
General Features
• Best diagnostic clue
○ "Cluster" sign: Cluster of small pyogenic abscesses that
coalesce into a single large cavity
• Location
○ Portal origin: Right lobe (65%); left lobe (12%); both
lobes (23%)
○ Biliary origin: Both lobes (90%), near biliary ducts
○ If infection following an interventional procedure: In
vicinity of procedure site
• Size
○ Varies from few mm to 10 cm; single or multiple
– Portal origin: Usually solitary larger abscess
– Biliary origin: Multiple small abscesses
– Direct extension and trauma: Solitary large abscess
• Morphology
○ Spherical, multiseptated mass
Ultrasonographic Findings
• Grayscale ultrasound
○ Variable in shape and echogenicity
○ Usually spherical or ovoid in shape
○ Borders may be well-defined to irregular
○ Wall may be thin/thick, hypoechoic/mildly echogenic
○ Echogenicity of abscesses
– Anechoic (50%), hyperechoic (25%), hypoechoic (25%)
○ Fluid level or debris, internal septa, and posterior
acoustic enhancement
○ May see gas in an abscess: Bright echogenic foci with
posterior reverberation artifact
○ Early lesions tend to be echogenic and poorly
demarcated
○ May evolve into well-demarcated, nearly anechoic lesions
○ Hepatic parenchyma adjacent to abscess:
Heterogeneous and hypoechoic due to edema
○ Associated right pleural effusion
• Color Doppler
○ Vascularity may be demonstrable in abscess wall
○ Edematous parenchyma adjacent to abscess may be
hypervascular
• Contrast-enhanced ultrasound
○ Usually no internal enhancement
– May be useful in differentiating abscess from
hypovascular hepatic tumor, which shows diffuse or
peripheral intratumoral enhancement
CT Findings
• NECT
○ Simple abscess: Well-defined, round, hypodense mass (0-
45 HU)
○ "Cluster" sign
– Small abscesses aggregate to coalesce into a single
big cavity, usually septated
○ Complex pyogenic abscess: "Target" lesion
– Hypodense center (pus), hyperdense capsule
○ Abscess with central gas (< 20% of cases)
– Seen as air bubbles or an air-fluid level
○ Large air-fluid or fluid-debris level
– Often associated with bowel communication or
necrotic tissue
• CECT
○ "Double target" sign
– Hypodense central abscess cavity surrounded by inner
hyperdense ring/outer hypodense zone,
enhancement of surrounding hepatic parenchyma
○ Well-defined, round or lobulated, hypodense mass
○ Rim/capsule enhancement and septal enhancement
○ Transient segmental hepatic parenchymal enhancement
surrounding abscess
○ Right lower lobe atelectasis, pleural effusion
MR Findings
• T1WI
○ Hypointense
• T2WI
○ Variably hyperintense mass
○ Hyperintense perilesional edema
• T1WI C+
○ Hypointense pus in center
○ Rim or capsule enhancement
○ Small abscesses