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Objective:
Ultrasound (US) is the current primary modality for assessment of acute appendicitis in children; however, magnetic resonance imaging (MRI) has high diagnostic performance (96.5% sensitivity, 96.1% specificity) that rivals and may surpass US depending on site and provider experience (Moore, 2016). Amidst national shortages of sonographers and growing availability of rapid abdominal MRI protocols, developing a pathway for MRI as a first-line approach in certain patients is worth consideration.

Materials and Methods:
In this retrospective study, we examined MRI and US appendicitis exams in the emergency department of an urban children's hospital with a well established MRI appendicitis service between 2016-2023. We evaluated relative volumes of US versus MRI and compared patient characteristics and exam times across imaging pathways. We established prospective age and weight thresholds to assess simulated volumes of an MRI-first pathway.

Results:
We reviewed 5,032 US and 1,556 MRI appendicitis exams in 5,856 patient visits (median age 10 years, range 0-29 years, 54% female). Patients were categorized by: US only (71.1%), MRI only (6.7%), or US+MRI in the same visit (22.2%). Patients who had both US+MRI were, on average, 9 kg heavier and 1.29 years older (both p<0.001) than US-only. Mean total time from order to completed exam in the US+MRI group was 1.93 hours longer than US-only, and 1.61 hours longer than MRI-only (p<0.001). As compared with males, mean US time from order to completed exam was 45 minutes longer in females (p< 0.001) whereas mean MRI time was 7 minutes shorter (p=0.043). Age (p<0.001) and weight (p<0.001) both showed moderate positive correlation with US time but not with MRI time (age, p=0.809, weight, p=0.569). To explore potential MRI volume increases with an MRI-first pathway, we analyzed patients over 13 years old and above 45 kg. Of the 1276 patients (67.0% female) in this group, 29.4% had MRI+US, 6.8% MRI only, and 63.8% US only. Applying these parameters to an MRI-first pathway would predict an increase in MRI appendicitis exams by 52.5% annually (114 additional patients/year, 10/month, 3/week). In a more conservative approach, we evaluated a subgroup over 15 years old, above 65 kg, consisting of 368 patients (63.3% female). Using this threshold would increase MRI appendicitis exams by 14.3% annually (31 additional patients/year, 3/month, 1/week).

Conclusion:
Implementing the proposed or similar criteria for an MRI-first appendicitis pathway could optimize time for both patients and staff. This would especially benefit female patients who face significantly longer US times and those who are more likely to require secondary imaging following inconclusive US. The volume estimates for our thresholds appear to present a reasonable increase in load for MRI capacity. Selection criteria could be further refined by incorporating clinical data and the expertise of the care team and medical imaging staff. Our results justify the inclusion of age, weight, and sex as criteria for an MRI-first appendicitis pathway and provide an estimate for predicted volumes of such an approach.