Introduction: Combination of computed tomography angiography (CTA) and adenosine stress CT myocardial perfusion (CTP) allows for coronary artery lesion assessment as well as myocardial ischemia. Nowadays, ischemia on CTP is assessed semi-quantitatively by visual analysis. The aim of this study was to fully quantify myocardial ischemia and the subtended myocardial mass on CTP. Methods: We included 33 patients referred for a combined CTA and adenosine stress CTP with good or excellent imaging quality on CTP. Firstly, the coronary artery tree was automatically extracted from CTA and the relevant coronary artery lesions (≥ 50%) were manually defined (fig 1A). Secondly, epi- and endocardial contours along with CTP deficits were manually defined in short-axis images (fig 1D, 1E). Thirdly, a Voronoi-based algorithm was used to quantify the subtended myocardial mass (fig 1B). Fourthly, the perfusion defect and subtended myocardial mass were spatially registered to the CTA and measured in grams (fig 1F, 1C). Finally, this can be used to quantitatively correlate the perfusion defect to the subtended myocardial mass. To assess reproducibility, left ventricular epicardial and endocardial contours along with perfusion defects were re-drawn. Results: Voronoi-based segmentation was successful in all cases. We assessed a total of 64 relevant coronary artery lesions. Average values for left ventricular mass, total subtended mass and perfusion defect mass were 118, 69 and 7 grams respectively. In 19/33 patients (58%) the total perfusion defect mass could be distributed over the relevant coronary artery lesion(s). Results were highly reproducible (r≥0.822, p<0.01). Conclusions: Quantification of myocardial ischemia and subtended myocardial mass using a Voronoi-based segmentation algorithm seem feasible at adenosine stress CTP and allows for quantitative correlation of coronary artery lesions to corresponding areas of myocardial hypoperfusion.