Background: Transthoracic echocardiography (TTE) is a rapid and cost-effective method to quantify left ventricular ejection fraction (LVEF). Cardiac magnetic resonance imaging (cMRI) is the gold standard for LVEF quantification. While cMRI's expensive and low availability limit access, we seek to determine the correlation between cMRI and TTE in the clinical setting, and if cMRI changes device therapy decisions. Purpose: To compare LVEF quantification between TTE and cMRI. To demonstrate the utilisation of cMRI in determining if device therapy is indicated or not. Methods: Retrospective analysis of patients from one hospital, with TTE and cMRI within 10 days. Demographic and cardiac risk factor data also obtained. TTE LVEF calculated using Biplane Simpson's Method; cMRI LVEF calculated using cMRI software auto-trace mode with manual correction. Results: 44 patients included (57% male; age 46.2616 years; BMI 30.066.7 kg/m 2). 9 of 44 patients have variability .10% in absolute LVEF. In patients with LVEF .50%, there was significant variability (p,0.005) in LVEF quantification between TTE and cMRI. In patients with LVEF #50% (subgroups: ,30%, 30-35%, 36-50%), the mean LVEF was comparable between TTE and cMRI. However, 5 of 44 patients were reclassified using cMRI, on device therapy eligibility (LVEF #35%): 1 met criteria, 4 did not. Conclusion: For LVEF quantification, TTE often approximates cMRI, except for in patients with LVEF .50%. However, small discrepancies carry significant implications on device therapy indications in patients with moderately and borderline severely reduced LVEF.