Theoretically, electrocardiogram-gated computed tomography (ECG-CT) can more precisely characterize the motion of a beating heart. However, cardiac motion involves both pulsatile cardiac activity and respiratory-induced movement. ECG-CT has high temporal resolution and fast scanning speed, requiring image acquisition to be completed within a single cardiac cycle. As a result, it is challenging to assess the full range of respiratory motion. In contrast, respiratory-gated four-dimensional computed tomography (4D CT) enables synchronized integration of both cardiac pulsation and respiratory motion. This study aimed to compare cardiac substructure motion using ECG-CT versus 4D CT, evaluate a gating plan for breast cancer patients, and explore an image registration method in cardiac radiation.A total of 20 left-sided breast cancer patients underwent both ECG-CT and 4D CT scanning. Parameters such as the absolute motion, volume increment ratios, the Dice similarity coefficient (DSC), and the Hausdorff distance (HD) of cardiac substructures were evaluated. Additionally, unlike ECG-CT, 4D CT offered inherent gating capability. Therefore, three optimal phases from 4D CT were utilized for the gating plan to assess the dose of cardiac substructures.Except for the left anterior descending (LAD) artery, the motion of other cardiac substructures analyzed by ECG-CT in the superior-inferior (SI) direction was significantly smaller than that assessed by 4D CT. The motion evaluated by ECG-CT for ascending aorta (AAo), left atrium (LA), left ventricle + ventricular wall (LV_S), and superior vena cava (SVC) in right-left (RL) and anterior-posterior (AP) directions and pulmonary artery (PA) in RL direction was significantly larger than that of 4D CT. Based on 4D CT, the gating plan showed significant dose advantages compared to the average intensity projection (AIP) plan (1,685.85±355.99 vs. 1,844.05±394.43 cGy, P<0.01).4D CT is a superior method for evaluating the motion of cardiac substructures compared to ECG-CT in the SI direction. A 4D CT-based gating plan provides a viable alternative to reducing the LAD dose when deep-inspiration breath-hold (DIBH) is unavailable. Furthermore, the heart, descending aorta (DAo), and RA + RV can serve as reference structures for cardiac radiation ablation.Copyright © 2025 AME Publishing Company. All rights reserved.