Computed tomography evaluation of the normal appendix: Comparison of low-dose and standard-dose unenhanced helical computed tomography

Abstract

OBJECTIVE: To determine the visualization rate of the normal appendix on low- and standard-dose unenhanced computed tomography (CT) and to evaluate the variables that may influence the identification of the appendix. METHODS: The study population was derived from the previous study, approved by the internal review board, investigating the effectiveness of low-dose CT (LDCT) in diagnosis of urolithiasis. Sixty-eight patients presenting with acute flank pain underwent 2 unenhanced dual-slice CT examinations. Standard-dose CT (SDCT) scans were obtained using 2 × 5-mm collimation, 120 kVp, and 170 effective mAs and followed by LDCT using 30 or 50 effective mAs. Two independent board-certified radiologists retrospectively recorded the visualization, outer diameter, and the wall thickness of normal appendices. The diameters, circumference, and cross-sectional area of the abdomen were measured. RESULTS: The prevalence of appendectomy was 8.8% (6 of 68 patients). The means of the 2 reviewers' sensitivity, specificity, positive and negative predictive values, and accuracy for visualization of normal appendix at SDCT versus LDCT respectively were 78% versus 73%, 100% versus 92%, 100% versus 99%, 31% versus 24%, and 80% versus 74% (P = 0.39-0.75). The interobserver agreement was good at both SDCT (? = 0.61) and LDCT (? = 0.74). Overall 40% to 58% of appendices at LDCT and 33% to 47% at SDCT was larger than 6 mm. There was no significant correlation in the appendix visualization neither with abdominal dimensions nor with visceral or pericecal fat at both dose sets. The calculated mean effective radiation dose at LDCT was 70% to 82% less than SDCT. CONCLUSIONS: Low- and standard-dose nonenhanced helical CT can visualize a normal appendix with high accuracy and good interobserver agreement. The diameter of normal appendix overlaps with that of appendicitis at CT. A diameter of 10.0 mm should be considered as the upper limit of normal in the absence of any other CT signs of appendicitis. © 2007 Lippincott Williams & Wilkins, Inc.