Effects of reader experience will be evaluated by comparing the accuracy of local reading (single observer) to the accuracy of the expert central reading. We will evaluate patient acceptance of MRI in comparison to standard imaging practice as documented in the Dutch guidelines. For each examination participants are invited to rate their experience (including burden, discomfort and pain) using five-point Inhibitors,research,lifescience,medical Likert scales (none, mild,

moderate, severe, extreme). Differences between US, CT and MRI will be tested for statistical significance. Sample size calculation We anticipate an MRI sensitivity of 90% and specificity of 95%, based on accuracy results in published series of primarily pregnant women [11]. Inhibitors,research,lifescience,medical Approximately 60% of patients with suspected appendicitis are expected to have a final diagnosis of appendicitis, based on the findings in the OPTIMA trial, which had comparable

inclusion criteria and ran in similar hospitals [12]. To obtain sensitivity and specificity estimates with 95% confidence intervals not exceeding 10%, a study Inhibitors,research,lifescience,medical group of 230 patients is required. Of the 230 patients, 138 are anticipated to have acute appendicitis (60%), while MRI will see more correctly identify appendicitis in 124 (sensitivity 90%; 95% CI 84% to 94%) and correctly exclude appendicitis in 95 (specificity 95%; 95% CI 88% to 98%). Rationale for design It is widely recognized that imaging and other medical tests should be evaluated based on their ability to improve patient outcome or to reduce costs [13]. It is also acknowledged that evaluations of tests benefit from a phased approach, where an assessment of reproducibility and diagnostic validity precede evaluations of overall clinical utility and resource

use [14]. For these reasons we decided that an accuracy Inhibitors,research,lifescience,medical study, evaluating MRI next to the current best imaging strategy was in place. This will allow us to explore the likely utility of MRI in patients with suspected appendicitis, and to model various imaging scenarios with respect to their ability to identify patients with appendicitis while minimizing imaging costs and Inhibitors,research,lifescience,medical radiation exposure. When sufficient evidence has become available about the accuracy of MRI, a study Resminostat with initial US and randomization for CT or MRI in inconclusive cases can be considered as next research step. At present MRI is not a routine examination for acute abdomen in general and in suspected acute appendicitis in particular. A pilot study of 70 patients in Alkmaar Medical Center showed that performing MRI in patients with acute abdomen is very well feasible, also after office hours. In the present study two of six participating hospitals will perform MRI outside office hours. Different time windows of inclusion will most likely not be a source of bias. In the OPTIMA study the prevalence of appendicitis was independent of time of presentation (60% during vs. 59% after office hours) [12].