Introduction: Utilization of CT imaging has risen dramatically with increases in availability, but without corresponding improvements in patient outcomes. Previous attempts to improve imaging appropriateness via guideline implementation have met with limited success, with commonly cited barriers including a lack of confidence in patient outcomes, medicolegal risk, and patient expectations. The objective of this project is to improve CT utilization and appropriateness by addressing common barriers through clinical decision support (CDS) embedded in clinical practice. Methods: This matched-pair cluster-randomized trial saw 12 Alberta EDs with CT scanners randomized to receive CDS for diagnostic imaging. After extensive site engagement to recruit emergency medicine and diagnostic imaging leadership and stakeholders and understand local contexts, half of the sites received CDS for mild traumatic brain injury (MTBI) based on the Canadian CT Head Rule, while the remainder received CDS for suspected pulmonary embolism (PE), including the Pulmonary Embolism Rule-out Criteria (PERC), Wells Score, age-adjusted D-dimer and CT pulmonary angiography (CTPA) use. Hardcopy CT order forms including quantitative decision support, source literature and patient handouts were developed and adapted and integrated into workflow as per local site preference. Regular physician and site report cards on CT utilization and CDS use were also provided. The primary outcome was diagnostic imaging utilization for patients with MTBI and suspected PE. Results: During the study period, 144 emergency physicians at 6 EDs saw 3,278 patients with MTBI and 146 emergency physicians at six matched comparison EDs saw 18,606 patients with suspected PE. Use of CDS was highly variable by site, ranging from 0% to 29% of CT orders for MTBI and from 13% to 75% of CTPA orders for suspected PE. Impact on CT utilization, appropriateness, diagnostic yield is currently under investigation, but is expected to be limited at many sites given the variable adoption of decision support. Conclusion: A comprehensive CDS intervention to improve evidence-based imaging has met with variable uptake. Meaningful and widespread sustained improvements in practice will likely require incentives, accountability measures and leadership authority to enforce change.