Diagnostic Imaging: Ultrasound: Technique and Accuracy

Graded compression ultrasonography is one of the two imaging modalities commonly used in the assessment of clinically suspected appendicitis. A linear array transducer, usually 5 or 7 Mhz, is typically employed. The patient is initially examined in the supine position. Scanning should be initiated in the region of maximal pain indicated by the patient to expedite the sonographic evaluation (21). This self-localization technique has a sensitivity of 85%, specificity of 60%, and accuracy of 75% in diagnosing appendicitis. The overall accuracy of this technique in diagnosing appendicitis as well as other significant disease processes which may clinically mimic appendicitis is 86%. However, the self-localization technique is of limited value with retrocecal or perforated appendicitis due to the patient's inability to accurately localize the pain (21).

If no abnormality is found, then transverse and longitudinal images are obtained of the abdomen, including the right lower quadrant and the right lateral abdomen extending from the subhepatic location to the right pelvis. In all areas examined, the anterior abdominal wall is compressed slowly but firmly with the ultrasound transducer to displace normal bowel loops in an effort to locate an inflamed appendix. If an apparently normal appendix is identified, a careful survey of the entire length of the appendix should be performed to avoid a false negative examination when inflammation is confined to the tip of the appendix (22). A normal appendix was identified in 56% of children undergoing graded compression sonography who were subsequently shown not to have appendicitis (23 ).

Seen to best advantage in the transverse plane, alternating echogenic and hypoechoic concentric layers should be sought, corresponding to the various layers of bowel wall. Normally, five distinct layers of the bowel wall are visualized: an innermost hyperechoic layer which corresponds to the interface between the mucosa and intraluminal contents, a hypoechoic layer corresponding to muscularis mucosa, a middle hyperechoic layer corresponding to submucosa, an outer hypoechoic layer corresponding to the muscularis propria, and a peripheral hyperechoic layer corresponding to serosa (24). Fluid within the appendiceal lumen appears as an inner hypoechoic central sonolucency. A positive exam consists of reproducible identification of an abnormal, inflamed appendix.

In a select group of patients with atypical symptoms, color Doppler sonography can be a useful adjunct to gray scale sonography for improving observer confidence in the diagnosis of appendicitis. In children, gray scale ultrasound alone has a sensitivity of 87%, specificity of 92%, and accuracy of 90%. The addition of color Doppler ultrasound results in a sensitivity of 87%, specificity of 97%, and accuracy of 93% in the diagnosis of acute appendicitis in children. Therefore, color Doppler sonography should be used in conjunction with gray scale ultrasound in the evaluation of appendicitis (24).

The appearance of appendicitis at color Doppler sonography probably represents a continuum which changes with time and varies with the severity of disease. With initial appendiceal inflammation, there may be no detectable increase in color Doppler flow signal. Therefore, the absence of color Doppler flow signal is non-diagnostic as it can be seen in both normal and abnormal appendices (25). Visualization of increased color Doppler flow signal in the appendiceal wall and/or a right lower quadrant mass is supportive of a diagnosis of appendicitis (25). This likely reflects increasing hyperperfusion of the appendiceal wall accompanying worsening inflammation as well as peritoneal inflammation which may occur surrounding an inflammatory mass resulting from perforated appendicitis.

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Diagnostic Imaging: Ultrasound: Findings in Appendicitis

The sonographic diagnosis of acute appendicitis is based on identification of a tubular, noncompressible, aperistaltic bowel loop, which demonstrates a connection with the cecum and a distal blind end, with a diameter greater than 6 mm (12). A calcified appendicolith may be present, which may assist in differentiating dilated appendix from adjacent bowel loops. A calcified appendicolith appears sonographically as a curved, echogenic structure with posterior acoustic shadowing. Air or inspissated feces within the bowel lumen may produce posterior acoustic shadowing mimicking a calcified appendicolith. However, the posterior acoustic shadow found in association with air tends to be "dirtier" than that associated with calcification.

Hayden et al (6) and Quillin et al (25) evaluated sonographic findings in children with nonperforated and perforated appendicitis. Differentiating nonperforated appendicitis from perforated appendicitis is important for therapeutic planning (25).

*Significantly different incidence in nonperforated and perforated appendicitis.

Other findings identified in association with perforation include thickening of adjacent bowel wall, atonic bowel loops, interloop fluid pockets, no tenderness on pressure application, asymmetrical appendiceal wall thickening, prominent pericecal echogenicity, echo poor region around the appendix, and fluid collections containing debris (4,9,13). 39% of Hayden et al’s perforated appendicitis cases showed either no appendix or only questionable appendiceal remnants (6). Non-visualization of an abnormal appendix in perforated cases may be due to failure to recognize a decompressed, but abnormally thickened, appendix because of surrounding thickened, matted loops of small bowel and failure to recognize the decompressed and excessively necrosed remnants of the perforated appendix. Other factors contributing to non-visualization of an abnormal appendix are prevention of adequate compression owing to reflex abdominal rigidity caused by perforation and atonic dilation of bowel loops caused by peritonitis (6).

A collapsed, non fluid-filled non-enlarged (cross section diameter 2.5-4 mm) appendix was seen in 56% of the symptomatic children with no proven appendicitis (6).Mesenteric adenitis in the right lower quadrant is identifiable by ultrasound in 40% of appendicitis(13).

Sonographic false positive diagnoses is relatively uncommon, but may include such entities as: hydrosalpinx periappendicitis resulting from tubo-ovarian abscess or Crohn’s disease, psoas muscle fibers stimulating an enlarged appendix, inspissated stool mimicking an appendicolith, or resolving appendicitis (26).

Sonographic false negative diagnoses may occur in patients with a retrocecal appendicitis, ascites, ileus, small bowel obstruction or with a markedly enlarged or gas-filled appendix (26).

Factors which decrease the reliability of sonographic evaluation for appendicitis include the operator dependent nature of the modality, the presence of overlying bowel gas, and obesity of the patient.

Review case presentations for Ultrasound Findings in Appendicitis

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