What are the CT imaging findings characteristic of acute appendicitis?

CT Imaging Findings of Acute Appendicitis

CT abdomen and pelvis with IV contrast demonstrates acute appendicitis with high diagnostic accuracy (91-99% sensitivity, 91-98% specificity), identifying key findings including enlarged appendix >6-9mm diameter, appendiceal wall thickening with enhancement, and periappendiceal fat stranding 1.

Primary Diagnostic CT Features

The most diagnostically powerful CT findings for acute appendicitis, in order of significance:

Core Findings (Highest Diagnostic Value)

• Enlarged appendix: Diameter >6mm (some studies suggest >9mm as abnormal cutoff)

  • Sensitivity 93%, Specificity 92% 2
  • Most powerful single parameter 3
  • Average diameter in acute appendicitis: 12.6mm (range varies) 4
  • Normal appendix: 4-7mm diameter 4

• Appendiceal wall thickening:

  • Sensitivity 66%, Specificity 96% 2
  • Correlates directly with appendiceal diameter 4

• Periappendiceal fat stranding:

  • Sensitivity 87%, Specificity 74% 2
  • Lower specificity (40%) when used alone 5

• Appendiceal wall enhancement (with IV contrast):

  • Sensitivity 75%, Specificity 85% 2
  • Present in 100% of phlegmonous appendicitis 6
  • Reduced to 66.7% in gangrenous appendicitis 6

Secondary Supportive Findings

Additional CT signs that support the diagnosis:

• Appendicolith: Specificity 96-100% 5, 2
• Focal cecal apical thickening 2
• Appendiceal intraluminal air 2
• Periappendiceal fluid collections 1
• Lymphadenopathy 2

CT Findings Indicating Complicated Appendicitis

Appendiceal wall enhancement defect is the strongest predictor of perforation and complicated disease 5, 7:

High-Specificity Signs of Perforation (96-100% specificity):

• Appendiceal wall enhancement defect: OR 3.39 for complicated appendicitis 7
• Extraluminal appendicolith: Specificity 96-100% 5
• Extraluminal air: Specificity 96-100% 5
• Periappendiceal abscess: Specificity 96-100% 5

Moderate-Specificity Signs:

• Periappendiceal phlegmon or fluid collection: Specificity 94% 5
• Intramural air 2

Combined Criteria Approach:

When baseline criteria (periappendiceal abscess, extraluminal air, extraluminal appendicolith) are combined with either phlegmon OR wall enhancement defect, sensitivity for perforation increases to 94-96% 5.

Technical Considerations

CT with IV contrast is the preferred protocol 1, 8, 9:

• IV contrast is "usually appropriate" for suspected appendicitis 8, 9
• Unenhanced CT has 92.7% sensitivity and 96.1% specificity but is limited in characterizing complications (perforation, abscess) 5
• No studies directly compare enhanced vs unenhanced CT for appendicitis diagnosis 1
• Oral contrast is optional; IV contrast alone is sufficient 1

Field of view matters:

• Full abdomen and pelvis CT provides broad assessment of complications including upper abdominal collections 1
• Focused RLQ CT may miss complications extending beyond lower abdomen/pelvis 1

Clinical Correlation Points

Important caveats:

• Wider appendiceal diameter and wall enhancement defects correlate with increased length of stay 4
• Appendiceal wall enhancement is present in all phlegmonous appendicitis but only 67% of gangrenous appendicitis—absence doesn't exclude severe disease 6
• CT cannot reliably exclude occult appendiceal neoplasms (present in up to 3.2% of cases) 10
• Microperforations may be underestimated on CT 10

Population-specific performance:

• In elderly patients (>65 years): CT sensitivity 100%, specificity 99.1%, PPV 95.7%, NPV 100% 5
• Mortality in elderly with perforated appendicitis: 11.9-15% vs 1.5-3% for non-perforated 5