Electrodes for vectorcardiography are used in cardiac MRI (CMR) to synchronize with ECG. In a high magnetic field at or above 3 Tesla, T-wave amplitude increases inside a bore compared with outside a bore, and R-wave amplitude decreases relatively. This results in poor ECG synchronization, such as double triggering and erroneous synchronization with T-waves. Since poor synchronization leads to low image quality or discontinuation of examinations, it is necessary to place electrodes so that an R/T-wave amplitude ratio (R/T ratio) becomes higher in 3 Tesla CMR. In a survey for ten radiology technicians from multiple facilities who were experienced in CMR imaging, 14 out of 14 answered that it was good to line up the basal and apex electrodes (namely, lead II) with the heart axis. Also, 11 out of 14 answered that synchronization was poorer in obese patients whose heart axis is away from the head-tail body axis due to eventration of the diaphragm. However, the clinical study in our facility¹⁾ found that the closer the heart axis was to the body axis, the poorer synchronization was and that the wider the angle between the heart axis and lead II axis was, the better T waves were controlled to gain high R waves. Taking the results above into consideration, we examined ECG electrode placement positions to increase the R/T ratio in healthy volunteers.The study subjects were eight healthy volunteers (three males and five females, 28.1±3.2 years old). ECG waveforms were recorded inside and outside a bore with a 3.0 Tesla MRI system (Ingenia, Philips Healthcare, Best, The Netherlands). As shown in Figure 1, ECG electrodes were placed in the following five positions: first, the basic placement position of lead II in line with the heart axis, the second and third positions were variations of the angle of lead II axis against the heart axis (angle of intersection), and the fourth and fifth positions were where the lead II was fixed at the angle in line with the heart axis, electrodes were moved in parallel to the left to create variations of the distance between the apex electrode and the apex of the heart. Then, R/T ratios in all five positions were compared, placed in the descending order of R/T ratio, and statistically analyzed with the Kruskal-Wallis test.R/T ratios in the basic placement position were 3.04±1.25 outside a bore and 1.12±0.28 inside a bore. T-wave elevation inside a bore was verified in healthy volunteers without cardiac disease as well. The angle of intersection in the basic placement position was 0.63 ± 3.7 degrees. When the angle of intersection was 12 ± 3.2 degrees, the R/T ratio was 2.15 ± 0.66, and when the angle of intersection was 29.7 ± 7.6 degrees, the R/T ratio was 2.11 ± 0.48. The relationship between the angle of intersection and the R/T ratio was that the R/T ratio increased as the angle of lead II axis became further away from the head-tail body axis. When the distance between the apex electrode and the apex of the heart was 26.0 ± 7.1 mm, the R/T ratio was 1.79 ± 0.32 and when the distance between the apex electrode and the apex of the heart was 28.3 ± 5.7 mm, the R/T ratio was 1.88 ± 0.59. The relationship of the distance between the apex electrode and the apex of the heart to the R/T ratio was that the R/T ratio increased as the apex electrode and the apex of the heart became a little further apart than the basic placement position of 18.8 ± 7.2 mm. In all subjects, the basic placement position was the lowest in the order of R/T ratios among the five kinds of electrode positions (average order 5.0, p < 0.01). The results above found that there was a discrepancy between the ECG electrode placement position considered ideal by radiology technicians engaged in CMR and the placement position that actually increases R/T ratio.To obtain the highest R/T ratio, the angle of lead II axis against the heart axis must be closer to the horizontal plane and the apex electrode and the apex of the heart must be apart. Further studies are necessary to decide which one of the two points is more effective.