Acute Appendicitis on CT: Key Imaging Findings Every Radiology Resident Must Know

Acute Appendicitis – Condensed Summary

1. Etiology

Luminal obstruction is the most common cause (≥50%)

Fecalith (appendicolith)

Lymphoid hyperplasia (especially in children and young adults)

Rare: tumor, parasite, foreign body

Obstruction → continued mucus secretion → ↑ intraluminal pressure → impaired venous return → ischemia + bacterial overgrowth → risk of perforation

2. Pathophysiology

Obstruction + inflammation → wall edema → lumen dilatation (>6 mm)

Advanced stage: necrosis, perforation, periappendiceal abscess

Location alters clinical and imaging findings (retrocecal cases may have more subtle findings)

3. Clinical Findings

Classic: Periumbilical pain → shifts to the right lower quadrant within hours

Fever, nausea, anorexia

Localized tenderness (McBurney’s point)

Rebound tenderness + guarding (peritoneal irritation)

However, atypical locations may not present with classic clinical findings

---

4. Locations of the Appendix

(Radiologically important as they alter both symptoms and signs)

Retrocecal: Most common (up to 65%), difficult to visualize with US, CT is more useful

​​​​​​​​Pelvic: Adjacent to bladder; may present with dysuria or pelvic pain

Subcecal: Beneath the cecum, easy US window

Paracecal: Medial or anterior to the cecum

Preileal / Postileal: Rare; may mimic right upper quadrant pain

Promontory: Near the pelvic promontory, atypical location

Retrocecal

Subcecal

Paracecal

Pelvic

.

Promontory

5. US Findings (First-line, especially in young patients and pregnancy)

Blind-ended, non-compressible tubular structure >6 mm in diameter

Wall thickening (>2 mm)

Hyperechogenicity of periappendiceal fat (inflammation)

Appendicolith hyperechoic with posterior acoustic shadowing

Increased mural vascularity (Doppler)

Abscess appears as a heterogeneous mass

6. CT Findings (Gold standard, especially in atypical cases and obese patients)

Blind-ended, dilated appendix (>6 mm)

Wall thickening and contrast enhancement

Increased density (fat stranding) in periappendiceal fat planes

Appendicolith hyperdense (80–140 HU)

Complications: abscess, free air (perforation), phlegmon

Radiologic Finding of Acute Appendicitis

7. Special Pearls

Location changes both clinical and imaging findings → CT is essential if US is negative in atypical positions

Lymphoid hyperplasia is the most common cause in children and young adults

In pregnancy, the appendix location shifts upward with gestational age → diagnosis becomes more difficult

Presence of an appendicolith increases the risk of perforation

On CT, wall thickening and degree of fat stranding help in staging

8. Complications

1. Perforation

Definition: Disruption of the integrity of the appendiceal wall, with intraluminal contents and bacteria spreading into the peritoneal cavity.

US Findings:

Focal defect in the appendiceal wall

Irregularity of wall contour

Periappendiceal collection or free fluid

In advanced cases, the appendiceal lumen may not be visualized (collapse or fragmentation)

CT Findings:

Periappendiceal free air (most

specific finding)

Low-density fluid + marked inflammation in fat planes

Findings of localized or diffuse peritonitis

Focal loss of the appendiceal wall or loss of contrast enhancement

2. Periappendiceal Abscess

Definition: Localized purulent

collection formation following perforation.

US Findings:

Heterogeneous, complex, fluid-containing mass

Reverberation/dirty shadow if gas is present

Marked surrounding hyperemia

CT Findings:

Thick-walled fluid collection with peripheral contrast enhancement

Air bubbles inside (evidence of perforation)

Marked fat stranding in surrounding adipose tissue

3. Phlegmon (Inflammatory Mass)

Definition: Inflammatory conglomeration of the appendix, surrounding fat, bowel loops, and omentum.

US Findings:

Mass appearance with marked echogenicity increase

Appendiceal lumen may not be visualized

CT Findings:

Soft tissue density mass around the cecum

Marked inflammation of fat planes

May contain minimal fluid, no well-defined capsule

---

4. Diffuse Peritonitis

Definition: Widespread intra-abdominal infection caused by perforation.

US Findings:

Diffuse free fluid

Decreased peristalsis of bowel loops

CT Findings:

Diffuse free fluid + free air

Diffuse bowel wall thickening

Diffuse peritoneal contrast enhancement

5. Pylephlebitis (Septic Thrombosis of the Portal Vein) – Rare but lethal

Definition: Portal vein infection and thrombosis secondary to

appendiceal origin.

US Findings:

Hyperechoic/anechoic thrombus in the portal vein

Loss of flow on Doppler

CT Findings:

Filling defect within the portal vein (thrombus)

Septic embolic foci in the surrounding liver parenchyma

Presence of an accompanying abdominal infection source

💡 Pearl:

CT is superior in diagnosing complications of perforation, but in a hemodynamically unstable patient, US is critical for rapid screening.

If US is negative, CT must be performed, especially if the location is atypical or if complication is

suspected.