In post-operative Tetralogy of Fallot (TOF) patients cMRI data are used for determining right ventricle (RV) dilation to justify the pulmonary valve placement. Echocardiogram analysis of RV volumes has inherent accuracy issues. There are no data exploring the relationship of transthoracic echocardiography tricuspid valve (TV) annulus size and cMRI RV dilation. Retrospective study in post-operative TOF patients. Patients less than 21 years old and with high-quality transthoracic echocardiogram and MRI within 6 months of each other were included. We collected demographic data, type of initial repair, echocardiographic, and cMRI data. A single reviewer measured echocardiogram TV size. Boston z-scores were used. 77 echocardiogram and cMRI pairs were analyzed. Average age was 14 years old. There is not a strong linear relationship between TV z-score and indexed right ventricular end diastolic volume (RVEDVi) (R2 = 0.31). Twenty patients had RVEDVi’s of greater than 150 ml/m2. From those 20 patients, 9 patients had a TV z-score of > 2. No patient had a TV z-score > 2 and a normal RVEDVi. For a TV z-score < 2, there was a positive predictive value of 100%, a negative predictive value of 83%, specificity of 100%, and sensitivity of 45%. In our study population, the TV z-score is not sufficient to predict RV dilation. However, TV z-score > 2 has a high positive predictive value for an RVEDVi of > 150 ml/m2. This would suggest that a cMRI should be done if the TV z-score is > 2 on echocardiography.