The detection of congenital heart diseases by ultrasound varies from 45% to 74% and an alternative imaging modality would be desirable. Fetal cardiac MRI has the potential to visualize anatomy and to assess functional parameters of the fetal heart. External fetal cardiac gating using a newly developed Doppler ultrasound sensor (DUS) has been introduced in previous studies. The purpose of this study was to perform fetal cardiac MRI as well as MR flow measurement within great vessels using for external fetal cardiac gating in human fetus and to optimize the device.Six cable traps tuned to 297 MHz were placed on the previously described DUS device within the first 60 cm of the transmission line connecting the ultrasound transducer to avoid heating of the cable and to reduce the artefacts due to eddy current. To evaluate potential distortions of the transmit RF field, gradient echo images and DREAM flip angle (FA) and SNR maps were acquired. 7 pregnant volunteer (gestation week 30 to 34) were examined at 1.5 T MRI to evaluate the DUS gating method for fetal cine MRI and MR flow measurement. To obtain a gating signal from the fetal heart, an MRI compatible ultrasound transducer of a cardiotocogram of a standard CTG was employed for cardiac triggering. Trigger signals were processed based on a newly developed algorithm and transmitted to the physiologic unit of the MRI for cardiac gating. Initially scout images were obtained in axial, coronal and sagittal orientations with HASTE sequences (TR 1080 ms, TE 55 ms, Flip angle 150°, FoV 400 mm). Afterwards, retrospective cine cardiac MRI was performed by using a steady-state free precession (SSFP) sequences in two-, three-, and four-chamber long-axis orientations and in the short-axis orientation not only to assess the morphology but also to assess the left ventricular function. The typical scan parameters were: TR 34.91ms; TE 1.34ms; FoV 400 ms; Flip-angle 55°; slice thickness 3mm. Left ventricular function parameters were assessed by cardiac cine MRI and compared to parameters obtained from consecutively performed standard ECG. For the velocity measurement, a retrospectively triggered gradient echo sequence was used: TR 5.5ms, TE 3.2ms, FOV 200 mm, acquisition matrix 100x100, reconstructed to a 256x256 image matrix, slice thickness 6 mm, pulse angle of 15°. 30 heartphases were recorded, 6 measurements were averaged. The maximum encoding velocity used was 100 cm/s. The flow velocity was measured in the ascending and descending aorta, umbilical vein, superior caval vein, and in pulmonary arteries. No signs of common-mode currents were visible along the transducer and transmission line in the gradient echo images. Moreover, the B1 maps remained unaffected by the transducer and transmission line (Fig. 1). As a consequence, no interferences were observed between DUS and MRI during CMR imaging. The validation of the DUS trigger signal resulted in a high correlation to the ECG signal of r=0.99 with a p-value of 0.9 and a mean difference in RR-interval length between ECG and DUS of 0.1±1 ms. Cardiac gating signals from the fetus could be reliably detected, in concordance with the US signal (Fig 2). No artefacts and interferences were observed, resulting in very good image quality (Fig. 3). The synchronous contraction of the ventricles was clearly visualized from the apex to the base with an average R-R interval of 464 ± 94 ms. The average end-systolic and end-diastolic volumes calculated from cine cardiac MRI and ECG were 0.58 ml and 3.17 ml, yielding stroke volumes of 2.60 ml with an ejection fraction of 80 % and cardiac output of 334 ml/min, respectively. There was no significant difference between the DUS triggered cardiac MRI and the echo-cardiography. Using the ultrasound the mean flow velocity could be also measured (Fig. 4). Within the umbilical vein and thoracic aorta was 24.78 cm/s and 47.33 cm/s, respectively. The aorta, the pulmonary arteries, as well as the supraaortal vessels, i.e. carotid arteries, could be identified with MR Angiography. MR flow analyses revealed a mean flow velocity in the fetal aorta with 102 cm/min. The mean flow velocity in the pulmonary arteries was 50 cm/min.Cine cardiac MRI as well as MR flow measurement could be performed in human fetuses using the optimized DUS device and dedicated software for fetal cardiac triggering. Fetal cardiac functional parameters revealed high agreement in comparison with standard fetal echocardiography. No signs of common-mode currents were visible along the transducer and transmission line in the gradient echo images. Fetal cardiac MRI has the high potential to detect cardiovascular malformations and to evaluate fetal cardiac function.