While cardiac MRI (CMR) requires Electro Cardiogram (ECG) synchronization, elevated T waves inside a bore make it difficult to obtain good ECG waveforms, particularly in a high magnetic field generating device at or above 3 Tesla. Failure to accurately synchronize with R waves causes problems, such as low image quality and unworkable imaging. Nevertheless, the best way to place electrodes for vectorcardiography to control T waves elevation and gain high R waves at the same time has not yet been reported. In this study, arrangement of ECG electrodes, positional relation of electrodes and the heart, and ECG waveforms inside and outside a static magnetic field were evaluated in patients who underwent CMR. Then, important points in placing ECG electrodes and patient factors to pay attention to were selected. The study subjects were 25 patients (18 males and 7 females, 59.8±14.5 years old from 31 to 91 years of age) who underwent CMR between Apr. 1, 2015, and Oct. 21, 2015. ECG electrodes were placed on four positions surrounding the heart at the start of examination by a radiology technician with five or more years of experience in CMR. In reference to chest x-ray pictures, the electrodes were so positioned that the patient’s heart axis matches lead II whenever possible. ECG waveforms inside and outside a bore were recorded in all patients. Imaging was performed with a Philips 3.0T MRI system (Ingenia, Philips Healthcare, Best, Netherlands). Coronal-section image of the area including chest walls to the heart was taken with the Single Shot Turbo Spin Echo (SSTSE) Method. ECG waveforms were analyzed with Image J by measuring the amplitudes of R waves and T waves to obtain a ratio of them (R/T ratio). The amplitudes of S wave of V1 (SV1) and R wave of V5 (RV5) were recorded from a regular 12-leads ECG to examine the effects of cardiac diseases on ECG change. In analysis of MR image, thickness of body, direction of the heart, the angle of the heart axis, the angle between lead II and the heart axis, and the distance between the apex of the heart and the apex electrode were measured. The statistical significance of the relationship between these measurement results and the R/T ratio was assessed with Pearson’s correlation coefficient. The R/T ratio outside the MRI bore was 6.59 ± 5.3 (2.3 − 23). The R/T ratio inside the MRI bore was 2.24 ± 0.80 (1.36 − 4.67) and this confirmed the effect of T-wave elevation inside the bore. Among the patient factors, BMI (R=0.075); RV5 + SV1 (R=-0.0126); thickness of body (R=0.369; p = 0.091); the direction of the heart on transected image (R=-0.162); and the direction of the heart on sagittal-section image (R=-0.029) had no correlation to the R/T ratio. On the other hand, a strong, negative correlation between the angle of the heart axis and the R/T ratio was found. The closer the heart axis was to the body axis, such as bathycardia, the lower the R/T ratio became (R=-0.607; p < 0.001). Among the electrode placement factors, the angle of lead II had no effect on the R/T ratio (R=-0.3221; p = 0.117); however, both the angle difference between lead II and the heart axis (R=0.4297; p = 0.032) and the distance between the apex of the heart and the apex electrode (R=0.39644; p = 0.0498) had a correlation to the R/T ratio. The closer the lead II axis was to the heart axis or the apex of the heart to the apex electrode, the lower the R/T ratio became. The lower R/T ratio inside a bore are obtained in patients with bathycardia, a condition in which the heart axis is close to the body axis. In addition, the short distance between the apex of the heart and the apex electrode and closeness of lead II and the heart axis are factors of a worse R/T ratio. These two points should be improved to obtain waveforms with a high R/T ratio.