We describe the clinical case of a thymoma, surgically removed after diagnosis, staging, and preoperative assessment performed by means of cardiac electrocardiogram-gated multidetector computed tomography. This technique allowed a very accurate assessment of the mass, proving superior to conventional computed tomography thanks to the possibility of identifying the relationships of the mass with the mediastinal structures, including the large vessels and coronary arteries. It also established the origin of the arterial vascularization from a tributary branch of the left internal mammary artery, visualizing the pathway and the relationship of the vessel with the mass and the point where it is penetrated. Finally, we analyzed the anatomy and patency of the coronary arteries, essential data in this patient with a high risk of coronary artery disease.