The Medical Ethical Committee of the Academic Medical Center in Amsterdam approved our study protocol. All participating hospitals gave their consent after assessment of local feasibility. Only

patients who give written informed consent will be included in the study. Standard care The findings of clinical assessment, the clinical diagnosis and possible alternative diagnoses, and the level of confidence (certainty) of the clinical diagnosis of acute appendicitis will be prospectively documented Inhibitors,research,lifescience,medical by the treating physician in an on line case record form (CRF). Subsequently a staff radiologist or radiological resident will perform an ultrasonography (US). This US concerns a complete examination of the abdomen, including the use of the graded compression technique. In case of a non diagnostic US, an find more abdominal computed tomography (CT) of the complete abdomen will be performed. All CT scans will be performed using a multi-detector row 4, 16 or 64 slice CT scanner (4-slice SOMATOM Volume Zoom, 16-slice SOMATOM sensation, Siemens Medical Systems, Forchheim, Germany; 16-slice Inhibitors,research,lifescience,medical MX 8000, 64-slice Brilliance,

Philips Medical Systems, Best, The Netherlands; 64-slice Aquilion, Toshiba Medical Systems, Tokyo, Japan) and intravenous contrast medium. No oral or Inhibitors,research,lifescience,medical rectal contrast medium is routinely administrated. The radiologist will record imaging features of the appendix, presence or absence of appendicitis, level of confidence of the diagnosis, and possible alternative diagnoses separately in our Inhibitors,research,lifescience,medical online CRF for US or CT. MRI examination Consenting patients will undergo MRI at 1.5 T (MAGNETOM Avanto 1,5 T MRI, Siemens Medical Systems, Forchheim, Germany; Intera 1.5 T MRI, Philips Medical Systems, Best, The Netherlands) within two hours of admission to the emergency department. The MRI examination will comprise breath hold axial and coronal T2 weighted sequences (HASTE: slice thickness 6 mm, FOV 400 mm, TR1500 ms, TE 90 ms, 256 × 256 matrix, flip angle 170; HASTE SPAIR: slice thickness 6 mm, FOV 400 MM, TR 1400 ms,

TE 93 ms, 256 × 256 matrix, flip angle 160) and Inhibitors,research,lifescience,medical free breathing axial and coronal diffusion weighted sequences (DWI: slice thickness 6 mm, FOV 400 mm, TR 3900 aminophylline ms, TE 75 ms, B-values 50 – 400 – 800, 192 × 192 matrix). A pilot study in one of the participating institutions has indicated the potential of DWI for acute appendicitis (unpublished data). No intravenous contrast medium is administrated. In-room time will be approximately 15 minutes. In two hospitals (AMC, MCA) MRI examinations will be performed between 8 AM and 11 PM, in the other hospitals during office hours. MRI interpretation All MR scans will be prospectively read by two independent radiologists, blinded for each other’s findings, US and CT results. These selected radiologists will be trained to adequately appraise the MR scan for presence or absence of appendicitis.