Residency · Residency · Diagnostic Radiology
Acute Appendicitis: Imaging Diagnosis and Pitfalls
Role of Imaging
Clinical Context
Acute appendicitis is the most common surgical emergency of the abdomen. The classic presentation involves periumbilical pain that migrates to the right lower quadrant, accompanied by fever and leukocytosis. Imaging is recommended when the clinical diagnosis is uncertain, and its use has reduced the negative appendectomy rate from 15-25% to less than 5%. CT is the primary modality in adults, while ultrasound is first-line in children and pregnant patients.
Imaging Modality Selection
| Modality | Patient Population | Sensitivity | Specificity |
|---|---|---|---|
| CT (abdomen/pelvis with IV contrast) | Most adults | 94-98% | 95-98% |
| Ultrasound (graded compression) | Children, thin adults, pregnant patients | 75-90% | 95% |
| MRI (without gadolinium) | Pregnant patients (second-line) | 90-97% | — |
CT of the abdomen and pelvis with intravenous contrast is the modality of choice in most adults, with a sensitivity of 94-98% and specificity of 95-98%. Ultrasound using the graded compression technique is first-line in children, thin adults, and pregnant patients, offering a sensitivity of 75-90% and specificity of 95%. MRI without gadolinium is the preferred second-line modality in pregnant patients when ultrasound is inconclusive, with a sensitivity of 90-97%.
CT Findings of Acute Appendicitis
Primary Findings
The most reliable CT criterion is a dilated appendix exceeding 6 mm in diameter, measured from outer wall to outer wall. Appendiceal wall thickening and enhancement are characterized by wall thickness greater than 3 mm with circumferential mural enhancement. An appendicolith, a calcified structure within the appendiceal lumen, is present in 25-30% of cases and is associated with a higher perforation risk. Intraluminal fluid or an air-fluid level within a dilated appendix provides additional supporting evidence.
Secondary Findings
Periappendiceal fat stranding refers to inflammatory changes in the surrounding mesenteric fat and is one of the most commonly seen secondary findings. A small amount of periappendiceal free fluid may be visible adjacent to the appendix. Cecal wall thickening represents inflammatory changes extending to the cecal apex and produces the arrowhead sign, which is focal cecal apical thickening pointing toward the appendiceal orifice. A phlegmon or abscess indicates complicated or perforated appendicitis. Extraluminal air near the appendix suggests perforation. Reactive lymphadenopathy manifests as enlarged right lower quadrant mesenteric lymph nodes.
Ultrasound Findings
Technique: Graded Compression
The graded compression technique involves gradually compressing the right lower quadrant with the linear transducer to displace bowel gas and identify the appendix. A normal appendix is compressible, measures less than 6 mm in diameter, and appears as a blind-ending tubular structure arising from the cecal base.
Positive Findings
The key ultrasound finding is a non-compressible, blind-ending tubular structure exceeding 6 mm in diameter. An appendicolith appears as a hyperechoic focus with posterior shadowing. Periappendiceal hyperechoic fat indicates inflammation. The target sign describes concentric rings on cross-section representing the inflamed appendiceal wall layers. Free fluid in the right lower quadrant and increased flow on color Doppler (hyperemia of the appendiceal wall) are additional supportive findings.
Perforated Appendicitis
CT Features
Perforation may be identified by a defect in the enhancing appendiceal wall, a periappendiceal abscess (rim-enhancing fluid collection), an extraluminal appendicolith, free air or extraluminal air bubbles, phlegmonous changes (an ill-defined soft tissue inflammatory mass replacing the normal appendiceal anatomy), or loss of the normal appendiceal wall contour.
Management Implications
Uncomplicated appendicitis is treated with laparoscopic appendectomy. Perforated appendicitis with abscess is managed with percutaneous drainage plus intravenous antibiotics, followed by interval appendectomy typically 6-8 weeks later. A small contained perforation without abscess may be managed with antibiotics alone in select patients.
Pitfalls and Mimics
Tip Appendicitis
Tip appendicitis occurs when inflammation is limited to the tip of the appendix while the base appears normal. It is easily missed if the entire appendix is not visualized, and the appendix must be traced from its cecal origin to its tip on CT to avoid this pitfall.
Stump Appendicitis
Stump appendicitis is inflammation of the residual appendiceal stump after prior appendectomy and can occur months to years after surgery. CT shows an inflamed tubular structure at the cecal base in a patient with prior appendectomy history.
Epiploic Appendagitis
Epiploic appendagitis results from inflammation of an epiploic appendage, a fatty appendage of the colon. On CT it appears as an oval fat-density lesion with a hyperattenuating ring (inflamed peritoneum) adjacent to the colon, often with a central dot of high attenuation representing a thrombosed central vessel. It is self-limited and managed conservatively without surgery. Although most commonly found on the left adjacent to the sigmoid colon, it can occur on the right and mimic appendicitis.
Omental Infarction
Omental infarction is a segmental infarction of the greater omentum. On CT it appears as a large, cake-like area of fat stranding in the omentum, usually right-sided. It is self-limited and managed conservatively.
Mesenteric Adenitis
Mesenteric adenitis presents as enlarged right lower quadrant mesenteric lymph nodes (greater than 5 mm short axis) with a normal appendix. It is often caused by a viral etiology such as Yersinia or adenovirus, is self-limited, and is common in children and young adults.
Right-Sided Diverticulitis
Cecal or ascending colon diverticulitis can mimic appendicitis. CT shows pericolic fat stranding centered on a diverticulum rather than the appendix. This entity is more common in Asian populations.
Appendicitis Mimics: Key Differentiating Features
| Condition | Key CT Feature | Management |
|---|---|---|
| Epiploic appendagitis | Oval fat-density lesion with hyperattenuating ring, central dot | Conservative |
| Omental infarction | Large cake-like fat stranding in omentum | Conservative |
| Cecal diverticulitis | Pericolic stranding centered on diverticulum, normal appendix | Usually conservative |
| Mesenteric adenitis | Enlarged RLQ lymph nodes, normal appendix | Conservative |
| Right-sided diverticulitis | Inflamed diverticulum, normal appendix | Conservative/antibiotics |
Other Mimics
Additional mimics of acute appendicitis include Meckel diverticulitis, Crohn disease (terminal ileitis), tubo-ovarian abscess, right ovarian torsion, ruptured ovarian cyst, ectopic pregnancy, and endometriosis.
<image>An axial CT image diagram of acute appendicitis. A dilated appendix (greater than 6 mm in diameter) is shown in the right lower quadrant with a thickened, enhancing wall and a round hyperdense appendicolith within its lumen. Surrounding periappendiceal fat stranding is depicted as hazy increased attenuation of the mesenteric fat. A small amount of free fluid is visible adjacent to the appendix. The cecal apex shows mild wall thickening (arrowhead sign). Each finding is labeled with arrows and annotations.</image>
<image>A comparison panel showing three right lower quadrant pathologies that mimic appendicitis on CT. Panel 1 (Epiploic appendagitis): an oval fat-density lesion with a hyperattenuating ring sign and central dot adjacent to the cecum, with a normal appendix visible separately. Panel 2 (Omental infarction): a large area of fat stranding in the right anterior omentum with a streaky, cake-like appearance. Panel 3 (Cecal diverticulitis): pericolic fat stranding centered on an inflamed cecal diverticulum with a normal appendix identified. Each panel has labels identifying the key distinguishing features and a note stating whether surgical or conservative management is indicated.</image>
Clinical Pearls
An appendiceal diameter greater than 6 mm with periappendiceal fat stranding on CT is the most reliable combination of findings for diagnosing acute appendicitis. The appendix must always be traced from base to tip on CT, because tip appendicitis is easily missed if only the proximal appendix is evaluated. An appendicolith is not diagnostic of appendicitis by itself but is associated with higher perforation risk when appendicitis is present. Epiploic appendagitis and omental infarction are self-limited conditions that mimic appendicitis on imaging, and recognizing their characteristic CT features avoids unnecessary surgery. In pregnant patients, the appendix may be displaced superiorly by the gravid uterus, and MRI without gadolinium is the preferred modality when ultrasound is inconclusive. A non-visualized appendix on ultrasound is an indeterminate result, not a negative study, and CT or MRI should be considered if clinical suspicion remains high.
References
- Pinto Leite N, et al. "CT Evaluation of Appendicitis and Its Complications." RadioGraphics, 2005
- ACR Appropriateness Criteria: Right Lower Quadrant Pain, 2022
- Singh AK, et al. "Acute Epiploic Appendagitis and Its Mimics." RadioGraphics, 2005
- Burns BJ, et al. "Appendicitis in Pregnancy: Imaging Evaluation." Emergency Radiology, 2007
