MRI of the normal appendix in children: data toward a new reference standard

David W. Swenson, Gary R. Schooler, Catherine Stamoulis, Edward Y. Lee

Magnetic resonance imaging (MRI) might prove useful in the diagnostic evaluation of pediatric appendicitis in the effort to avoid exposing children to the ionizing radiation of CT, yet there is a paucity of literature describing the normal range of appearances of the pediatric appendix on MRI. To investigate MRI characteristics of the normal appendix to aid in establishing a reference standard in the pediatric population. We conducted a retrospective study of children and young adults (≤18 years of age) who underwent lumbar spine or pelvis MRI between Jan. 1, 2013, and Dec. 31, 2013, for indications unrelated to appendicitis. Two board-certified radiologists independently reviewed all patients' MRI examinations for appendix visualization, diameter, intraluminal content signal, and presence of periappendiceal inflammation or free fluid. We used the Cohen kappa statistic and Spearman correlation coefficient to assess reader agreement on qualitative and quantitative data, respectively. Three hundred forty-six patients met inclusion criteria. Both readers visualized the appendix in 192/346 (55.5%) patients (kappa = 0.88, P < 0.0001). Estimated median appendix diameter was 5 mm for reader 1 and 6 mm for reader 2 ([25th, 75th] quartiles = [5, 6] mm; range, 2-11 mm; r = 0.81, P < 0.0001). Appendix intraluminal signal characteristics were variable. Periappendiceal inflammation was present in 0/192 (0%) and free fluid in 6/192 (3.1%) MRI examinations (kappa = 1.0). The normal appendix was seen on MRI in approximately half of pediatric patients, with a mean diameter of ~5-6 mm, variable intraluminal signal characteristics, no adjacent inflammatory changes, and rare surrounding free fluid.