4D flow MRI at the cardiac level on mouse models remains very difficult due to the very small size of vessels and the extremely high cardiac rhythm. Furthermore, the saturation of the spin into the heart leads to a lack of signal which limits the study of abnormal blood flow in the heart or in the descending aorta. To overcome this problem we proposed to use a new 3D time-resolved Phase Contrast UTE sequence in combination with an injection of Ultra Small Particles of Iron Oxide (USPIO)¹ to obtain a positive signal in blood. Various undersampling factors were also tested to limit the total acquisition time.Flow velocity measurements with the 4D FLOW UTE sequence were performed on a rectilinear tube at 7T filled with solutions of of MnCl2 at a concentration of 0/1/2/4/6/8 mM to assess the robustness of measurements against T2* effect and the measurement linearity. Undersampling effects on streamline were also analyzed on a curved tube (radius = 0.75 mm) with multiple undersampling factors (2, 3 and 4). Flow rate was imposed to 0.3 m/s and pixel resolution on reconstructed images was equal to 0.234 mm. Flow quantification of the mouse cardiovascular system was performed with a 100 µL injection of USPIO at a dose of 200 µmol Fe/kg with a 4D FLOW sequence triggered on ECG in order to reconstruct 10 cine images and a number of projections equal to 12096.In the phantom, using the UTE sequence, the maximum difference of the average velocity was inferior to 0.8 cm/s whatever was the concentration of MnCl2 or velocity encoding values. Furthermore, flow velocity measurements showed an excellent linearity (R2 = 0.9998). Even if the spatial resolution was decreased with the undersampling factor, we were still able to extract flow trajectories from the data, even with the lowest number of projections (6048) as shown in figure 1. On the mouse heart, the UTE sequence gave images without flow dephasing artifacts which enabled a good measurement of anatomical parameters such as the diameter of the aortic cross or the volume of the ventricles. The high quality of magnitude and phase images enabled us to trace the streamlines of blood in the aortic cross and in the pulmonary arteries as shown in figure 2.We demonstrated that combining the injection of iron nanoparticles with 3D Time-Resolved Phase Contrast UTE sequences generated a strong positive contrast between blood and surrounding tissues and could be used to quantify flow velocity. These properties were exploited to produce images and quantify blood flow velocity of the cardiovascular system in small animals at high magnetic fields with a high spatial (<200 µm)3 and temporal (16 ms) resolutions. Total acquisition time could be reduced (25 min) by limiting the number of acquired projections per cine image. This approach might be useful to measure functional cardiac parameters or to assess anatomical modifications to the blood vessels or blood flow velocities in cardio-vascular disease models.