Appendicitis: Imaging Diagnosis

Appendicitis: Imaging Diagnosis

Introduction

Appendicitis is the most common abdominal surgical emergency worldwide, with a lifetime risk of approximately 7%. Imaging plays a crucial role in diagnosis, particularly in atypical presentations. This article details the imaging features of appendicitis across modalities, with emphasis on ultrasound (first-line in children and pregnant women) and CT (gold standard in adults).

  1. Ultrasound features - Graded compression technique findings
  2. CT features - Multi-detector CT diagnostic criteria
  3. Complicated appendicitis - Findings of perforation/abscess
  4. Differential diagnosis - Mimickers of appendicitis
  5. Special populations - Pediatric, pregnant, and elderly patients

1. Ultrasound Features

Graded compression ultrasound is the initial imaging modality of choice for children and pregnant patients with suspected appendicitis.

Technique

High-frequency linear transducer (5-12MHz) with gradual compression in RLQ to displace bowel gas and identify the appendix.

Diagnostic Criteria

  • Non-compressible tubular structure >6mm diameter
  • Target sign - concentric wall layers
  • Wall thickening (>3mm) with hyperemia on Doppler
  • Periappendiceal fat hyperechogenicity (inflammation)
  • Appendicolith - echogenic focus with posterior shadowing
Appendicitis ultrasound

Acute Appendicitis

  • Non-compressible dilated appendix (7.3mm)
  • Wall thickening and hyperemia
Appendicolith

Appendicolith

  • Echogenic focus with clean shadowing
  • Surrounding hypoechoic fluid


2. CT Features

Contrast-enhanced CT is the gold standard for diagnosing appendicitis in adults, with sensitivity of 94-98% and specificity of 95%.

Protocol

Multi-detector CT with IV contrast (oral/rectal contrast optional). 2-3mm reconstructions.

Diagnostic Criteria

  • Appendix diameter >6mm with wall thickening
  • Periappendiceal fat stranding (most sensitive sign)
  • Appendiceal wall enhancement with IV contrast
  • Appendicolith - calcified focus in lumen
  • Adjacent fascial thickening
CT appendicitis

Acute Appendicitis

  • Dilated fluid-filled appendix (arrows)
  • Surrounding fat stranding
CT appendicolith

Appendicolith

  • Calcified focus (arrow) within dilated appendix
  • Adjacent inflammatory changes

3. Complicated Appendicitis

Findings suggesting perforation, abscess formation, or peritonitis that may alter surgical approach.

Imaging Findings

  • Focal defect in appendiceal wall (direct sign of perforation)
  • Extraluminal air (pneumoperitoneum rare)
  • Phlegmon - ill-defined soft tissue mass
  • Abscess - fluid collection with enhancing rim
  • Extensive free fluid with debris

Management Implications

Perforation may require percutaneous drainage prior to interval appendectomy. Free perforation with peritonitis needs emergent surgery.

Perforated appendicitis

Perforated Appendicitis

  • Focal wall defect
  • Surrounding abscess formation
Appendiceal abscess

Appendiceal Abscess

  • Fluid collection with enhancing rim
  • Adjacent inflammatory changes

4. Differential Diagnosis

Several conditions can mimic appendicitis clinically and radiologically.

Common Mimickers

  • Mesenteric adenitis - enlarged lymph nodes, normal appendix
  • Omental infarction - fatty mass with inflammatory changes
  • Diverticulitis - left-sided in adults, right-sided cecal in elderly
  • Gynecological pathology - ovarian torsion, PID, ectopic pregnancy
  • Crohn's disease - terminal ileum involvement
Mesenteric adenitis

Mesenteric Adenitis

  • Cluster of enlarged mesenteric nodes
  • Normal appendix
Omental infarction

Omental Infarction

  • Fat density mass with inflammatory stranding
  • No identifiable appendix pathology


5. Special Populations

Diagnostic challenges in pediatric, pregnant, and elderly patients.

Pediatric Considerations

  • Higher perforation rates (15-30%)
  • Ultrasound first-line (avoids radiation)
  • Smaller diameter cutoff (>5.5mm)

Pregnancy Considerations

  • Appendix displaced superiorly by gravid uterus
  • Ultrasound first-line, MRI if indeterminate
  • CT reserved for complex cases after 1st trimester

Elderly Considerations

  • Atypical presentations common
  • Higher perforation rates at presentation
  • Increased malignancy risk as alternate diagnosis

Comparative Imaging Features

Feature Ultrasound CT
Appendix diameter >6mm non-compressible >6mm
Wall thickening >3mm with hyperemia Enhancing wall
Periappendiceal inflammation Hyperechoic fat Fat stranding
Appendicolith Echogenic with shadowing Calcified focus
Perforation Disrupted wall, abscess Focal defect, extraluminal air
.

References

  1. Pinto Leite N, Pereira JM, Cunha R, Pinto P, Sirlin C. CT evaluation of appendicitis and its complications: imaging techniques and key diagnostic findings. AJR Am J Roentgenol. 2005;185(2):406-417. doi:10.2214/ajr.185.2.01850406
  2. Doria AS, Moineddin R, Kellenberger CJ, et al. US or CT for Diagnosis of Appendicitis in Children and Adults? A Meta-Analysis. Radiology. 2006;241(1):83-94. doi:10.1148/radiol.2411050913
  3. Mostbeck G, Adam EJ, Nielsen MB, et al. How to diagnose acute appendicitis: ultrasound first. Insights Imaging. 2016;7(2):255-263. doi:10.1007/s13244-016-0469-6
  4. Kessler N, Cyteval C, Gallix B, et al. Appendicitis: evaluation of sensitivity, specificity, and predictive values of US, Doppler US, and laboratory findings. Radiology. 2004;230(2):472-478. doi:10.1148/radiol.2302021520
  5. Barger RL, Nandalur KR. Diagnostic performance of magnetic resonance imaging in the detection of appendicitis in adults: a meta-analysis. Acad Radiol. 2010;17(10):1211-1216. doi:10.1016/j.acra.2010.05.010
  6. Birnbaum BA, Wilson SR. Appendicitis at the millennium. Radiology. 2000;215(2):337-348. doi:10.1148/radiology.215.2.r00ma24337
  7. van Randen A, Bipat S, Zwinderman AH, Ubbink DT, Stoker J, Boermeester MA. Acute appendicitis: meta-analysis of diagnostic performance of CT and graded compression US related to prevalence of disease. Radiology. 2008;249(1):97-106. doi:10.1148/radiol.2483071652
  8. Drake FT, Mottey NE, Farrokhi ET, et al. Time to Appendectomy and Risk of Perforation in Acute Appendicitis. JAMA Surg. 2014;149(8):837. doi:10.1001/jamasurg.2014.77
  9. Rosen MP, Ding A, Blake MA, et al. ACR Appropriateness Criteria® Right Lower Quadrant Pain-Suspected Appendicitis. J Am Coll Radiol. 2011;8(11):749-755. doi:10.1016/j.jacr.2011.07.010
  10. American Institute of Ultrasound in Medicine (AIUM). AIUM practice guideline for the performance of ultrasound examinations of the abdomen and/or retroperitoneum. J Ultrasound Med. 2021;40(7):E1-E16. doi:10.1002/jum.15607