A prerequisite for cardiac magnetic resonance imaging (CMR) is adequate synchronization of image acquisition with the cardiac cycle, usually achieved using the spatio-temporal information of an electrocardiogram (ECG). However, ECG is an inherently electrical measurement and distortions increase with higher magnetic field strengths, mainly caused by the magneto-hydro-dynamic (MHD) effect that occurs when a conductive fluid such as blood travels through a magnetic field1. Hence, ECG triggered CMR is limited by R-wave misregistration. Doppler ultrasound (DUS), on the other hand, is a promising alternative as it measures the physiological motion of the heart rather than electrical activation2. Therefore, it is not objected to MHD effects and does not interact with the image acquisition. However, the aluminum coating of the transducer and connecting cable, placed directly underneath a transmit/receive (Tx/Rx) RF coil, may introduce safety concerns at 7T. The purpose of this work was to evaluate the feasibility of DUS for CMR image synchronization at 7T.A custom build cardiotocograph was used to derive DUS signals (Fig. 1). Since common-mode currents affect image homogeneity and may cause serious patient burns3, six cable traps tuned to 297 MHz were placed within the first 60 cm (each cable trap placed every 10 cm) of the transmission line connecting the ultrasound transducer. To evaluate potential distortions of the transmit RF field, large field-of-view coronal gradient echo images4 and DREAM flip angle (FA) maps5 were acquired in a large elliptic body phantom containing tissue-simulating liquid. In addition, potential effects on the H- and especially SAR-relevant E-field distribution by the transducer or transmission line were measured with field probes (H3DV7 and ES3DV2, SPEAG, Zurich, Switzerland) outside the MR system using a computer-controlled positioning device and the same 8-channel RF Tx/Rx body coil as used for MR imaging (Fig. 2). Line plots (3 mm step size, 71 data points) were acquired close to the phantom surface (1 cm distance), with and without transducer and transmission line present. Cardiac MRI was performed at a 7T whole-body research system (Magnetom 7T, Siemens Healthcare GmbH, Germany) in 3 healthy subjects after signing informed consent. The ultrasound transducer made out of one single ceramic (radius = 5 mm) was placed in an apical location under the RF coil to record transmitral flow. The E-wave in early diastole was selected as a trigger time point. For validation of the trigger signal, ECG, pulse, and DUS signals were recorded simultaneously outside of the MR room and compared in terms of RR interval length and time delay. Breath hold 2D cine FLASH sequences (matrix: 240x240, voxel size: 1.5×1.5×3.0 mm3, single slice, 25 phases, TR/TE 40.9/4.76 ms) were acquired in short axis and four chamber view. To assess the image quality, endocardial blurring (EB) was measured in the left ventricle as a mean over all cardiac phases.No signs of common-mode currents were visible along the transducer and transmission line in the gradient echo images (Fig. 2 A,B). Moreover, the B1 maps remained unaffected by the transducer and transmission line (Fig. 2 C, D). The measured E-and H-field distribution at the test bench yielded high agreement with maximal differences in the E-field of 0.75 V/m and 6 mA/m for the H-field, corresponding to a maximal change of 5 % with and without transducer and transmission line (Fig.2 F). 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 (Fig.3). The DUS signal showed a mean time delay compared to the R-wave of 516±20 ms and a similar variation of 51 ms. The DUS signal was not disturbed by RF pulses or gradients during image acquisition. Exemplary in vivo images are shown in Fig. 4 with a corresponding plot over all cardiac phases with marked EB (yellow). Analysis of endocardial blurring between ventricular blood and myocardium resulted in 3.4±0.8 pixel.Doppler Ultrasound was applied as a new trigger method in cardiac MRI at 7T. The DUS transmission line and transducer were approved for RF safety and successfully tested for CMR image synchronization at 7T. As the DUS signal remained unaffected by the MHD effect and electromagnetic interferences, it represents a promising alternative for ultra-high field CMR. In future, this method needs to be evaluated in more detail in a larger patient population.