Frequency of visualization and thickness of normal appendix at nonenhanced helical CT

To evaluate the frequency of visualization, thickness, and features of the normal appendix at nonenhanced helical computed tomography (CT).

Three radiologists blinded to patient surgical history retrospectively reviewed CT scans obtained for renal colic assessment in 187 consecutive patients. No contrast material was administered. The frequency of visualization and the two-wall thickness of normal appendices were recorded. Interobserver agreement and effect of adequacy of intraperitoneal fat on identification of the appendix were assessed.

The prevalence of appendectomy was 10.7% (20 of 187 patients). The means for the three reviewers' sensitivity, specificity, positive and negative predictive values, and accuracy of visualization of normal appendix were 79% (CI: 73%, 84%), 90% (CI: 78%, 96%), 98% (CI: 97%, 99%), 34% (CI: 22%, 47%), and 80% (CI: 74%, 86%), respectively. There was no significant difference among the three reviewers (P >.05) according to conditional logistic regression and exact McNemar test results. For all reviewers, the frequency of appendix visualization was significantly lower in patients with less intraperitoneal fat (P =.01-.001, chi(2) test). The mean thickness of normal appendix if no intraluminal content was visualized was 6.6 mm +/- 1.0 (SD), and the mean thickness, excluding visualized intraluminal content, was 3.6 mm +/- 0.8. The nonweighted kappa value for interobserver agreement for normal appendix visualization was 0.69-0.75 among the three reviewers, which indicated good to excellent agreement.

Most normal appendices are seen at nonenhanced helical CT. The thickness of normal appendix, when the content is not recognizable, overlaps the values currently used to diagnose appendicitis at CT.