Quantitative measurement of elasticity of the appendix using shear wave elastography in patients with suspected acute appendicitis

Abstract

Introduction: Shear wave elastography (SWE) has not been studied for diagnosing appendicitis. We postulated that an inflamed appendix would become stiffer than a normal appendix. We evaluated the elastic modulus values (EMV) by SWE in healthy volunteers, patients without appendicitis, and patients with appendicitis. We also evaluated diagnostic ability of SWE for differentiating an inflamed from a normal appendix in patients with suspected appendicitis.

Materials and methods: Forty-one patients with clinically suspected acute appendicitis and 11 healthy volunteers were prospectively enrolled. Gray-scale ultrasonography (US), SWE and multi-slice computed tomography (CT) were performed. The EMV was measured in the anterior, medial, and posterior appendiceal wall using SWE, and the highest value (kPa) was recorded.

Results: Patients were classified into appendicitis (n = 30) and no appendicitis groups (n = 11). One case of a negative appendectomy was detected. The median EMV was significantly higher in the appendicitis group (25.0 kPa) compared to that in the no appendicitis group (10.4 kPa) or in the healthy controls (8.3 kPa) (p<0.001). Among SWE and other US and CT features, CT was superior to any conventional gray-scale US feature or SWE. Either the CT diameter criterion or combined three CT features predicted true positive in 30 and true negative in 11 cases and yielded 100% sensitivity and 100% specificity. An EMV of 12.5 kPa for the stiffest region of the appendix predicted true positive in 28, true negative in 11, and false negative in two cases. The EMV (≥12.5 kPa) yielded 93% sensitivity and 100% specificity.

Conclusion: Our results suggest that EMV by SWE helps distinguish an inflamed from a normal appendix. Given that SWE has high specificity, quantitative measurement of the elasticity of the appendix may provide complementary information, in addition to morphologic features on gray-scale US, in the diagnosis of appendicitis.

Figure 1. Patient enrollment (n = 41).

Only the modified Alvarado scores and the CT results were considered during decision making for appendectomy. Positive CT results were defined when three radiologic features (≥6 mm in diameter, enhancement of periappendiceal fat, and wall thickening) were seen at the same time. CT, computed tomography; US, ultrasonography; SWE, shear wave elastography.

Figure 2. Gray-scale ultrasonography and shear wave elastography in a 47-year old female patient with appendicitis.

A. Gray-scale ultrasonography was 9.9 mm in diameter. The echogenicity of periappendiceal fat and the appendiceal wall thickening were also noted (not shown). B. Elastic modulus scales (mean Q-Box) by shear wave elastography were 26.0, 20.9, and 22.5 kilopascal (kPa) in the anterior, medial, and posterior wall of the appendix, respectively. The highest elastic modulus scale (26.0 kPa) was selected for analysis. The modified Alavarado score of this patient was 7, and computed tomography showed a larger diameter (≥6 mm), enhancement of periappendiceal fat and wall thickening. The histopathology result showed appendicitis.

Figure 3. Gray-scale ultrasonography and shear wave elastography in a 24-year-old male patient with no appendicitis.

A. Gray-scale ultrasonography was 5.8 mm in diameter. No echogenicity of periappendiceal fat or appendiceal wall thickening were noted. B. Elastic modulus scales (mean Q-Box) by shear wave elastography were 5.5, 11.2, and 4.2 kilopascal (kPa) in the anterior, medial, and posterior wall of the appendix, respectively. The highest elastic modulus scale (11.2 kPa) was selected for analysis. The modified Alavarado score in this patient was 5, and computed tomography showed a smaller diameter (<6 mm), no enhancement of periappendiceal fat, and the absence of appendiceal wall thickening. This patient was diagnosed with terminal ileitis and recovered without surgical intervention.