Diagnostic Ultrasound - Abdomen and Pelvis

Renal Abscess
TERMINOLOGY
Definitions
• Purulent &/or necrotic intraparenchymal or perinephric
collection arising from unresolved pyelonephritis
IMAGING
General Features
• Best diagnostic clue
○ Well-defined, centrally avascular hypoechoic area with
irregular wall and internal debris
• Location
○ Single > multiple; unilateral > bilateral
Ultrasonographic Findings
• Grayscale ultrasound
○ Complex cystic mass, may be sharply marginated or
more permeative
○ Anechoic/hypoechoic ± weak acoustic enhancement
○ May contain echogenic internal debris, septations, or
loculations
○ Internal echogenic foci with "comet tail" may represent
gas-forming organisms within abscess
○ Findings of pyelonephritis (renal enlargement, lack of
corticomedullary differentiation, and urothelial
thickening) may be present
○ Perinephric fat may appear hyperechoic and fascial
planes thickened due to inflammation
• Color Doppler
○ Rim may be hypervascular or vessels may course to edge
of lesion and stop
○ Little to no internal vascularity
CT Findings
• NECT
○ Round, well-defined, low-attenuation masses± gas
within collection
○ Perinephric inflammatory change
• CECT
○ Focal hypodense area ± wall enhancement
○ Perinephric reaction or extension
– Edema or obliteration of perinephric fat
– Thickened Gerota fascia and perinephric septa
Imaging Recommendations
• Best imaging tool
○ CT for perinephric extension; ultrasound for guided
aspiration
• Protocol advice
○ Initial examination is ultrasound; CT for further
investigation
DIFFERENTIAL DIAGNOSIS
Renal Cell Carcinoma (RCC)
• Hypervascular mass; usually asymptomatic
• Lack of inflammatory change favors this diagnosis over
abscess
Metastases and Lymphoma
• Hypovascular with variable echogenicity
• Often multiple, whereas most abscesses are solitary
Hemorrhagic Cyst or Proteinaceous Cyst
• Appearance of lesion itself indistinguishable from abscess
• Should not have associated inflammatory changes or
history of fever, flank pain, etc.
PATHOLOGY
General Features
• Etiology
○ Ascending urinary tract infections (80%)
– Corticomedullary abscess by Escherichia coliorProteus
species
○ Hematogenous spread (20%)
– Cortical abscess by Staphylococcus aureus
• Associated abnormalities
○ Urolithiasis (20-60%)
Microscopic Features
• Necrotic glomeruli & polymorphonuclear infiltration
CLINICAL ISSUES
Presentation
• Most common signs/symptoms
○ Fever, flank/abdominal pain, chills, and dysuria
Demographics
• Age
○ All; M = F
Natural History & Prognosis
• Complications: Abscess rupture into pelvis, perinephric
space, retroperitoneum, or peritoneal space
• Prognosis is good if therapy is prompt
○ Good (early therapy); poor (delayed or insufficient
therapy)
Treatment
• Antibiotic therapy, usually IV ± percutaneous drainage
• Surgical drainage or nephrectomy are rarely needed
DIAGNOSTIC CHECKLIST
Consider
• Clinical history & urinalysis for work-up of diagnosis
• Imaging-guided aspiration
Image Interpretation Pearls
• Many abscesses appear mass-like and may mimic
neoplasms but are usually associated with > 1-week history
of infection, minimal internal vascularity, and associated
inflammatory changes
SELECTED REFERENCES
1. Fontanilla T et al: Acute complicated pyelonephritis: contrast-enhanced
ultrasound. Abdom Imaging. 37(4):639-46, 2012
2. Heller MT et al: Acute conditions affecting the perinephric space: imaging
anatomy, pathways of disease spread, and differential diagnosis. Emerg
Radiol. 19(3):245-54, 2012
3. Demertzis J et al: State of the art: imaging of renal infections. Emerg Radiol.
14(1):13-22, 2007
Diagnoses: Urinary Tract
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