Cardiac catheterization is the gold standard technique to measure systolic pressure gradient (SPG) in patients with aortic coarctation (AoCo), but it is an invasive technique, non-exempt of risk, patients are exposed to x-rays and is difficult to reproduce. 3D PC-MRI has the capacity to measure non-invasively the 3D-spatial and temporal evolution of complex flow patterns and analyze quantitative hemodynamics parameters, including relative pressures¹. The aim of this work was to evaluate the accuracy of relative pressures obtained from 3D PC-MRI in a realistic aortic phantom with different grades of AoCo at rest and stress conditions. Further, we evaluate the accuracy of the relative pressure when subjected to different aortic segmentations and spatial resolutions.3D PC-MRI were acquired in a 1.5 T MRI system (Philips) using a 4-channel body coil and retrospective cardiac gating. Experiments were performed in a pulsatile aortic phantom setup^2,3 in 8 settings (see Figure 1): The normal phantom (without AoCo) and 13, 11 and 9 mm AoCo phantom at rest (75 bpm) and stress (136 bpm) conditions. Acquisition parameters were: acquired and reconstructed spatial resolution of 1.79x1.83x1.80 mm3 and 0.89x0.89x0.90 mm3, acquired and reconstructed temporal resolution of 52.2 ms and 35 ms, field of view of 200x200x114 mm, TR/TE of 6.5/3.8 ms, flip angle of 6.5°, Venc of 150-400 cm/s, 25 time frames, and 127 slices. Additionally, a second 3D PC-MRI acquisition of the 13 mm AoCo phantom, at rest and stress conditions, was obtained in order to study the effects of different aortic segmentation and different spatial resolutions: reconstructed isotropic resolution of 0.9 mm, 1.4 mm and 2.0 mm and acquired resolution of 1.8, 2.0 and 2.5 mm. The different aortic segmentations involved removing areas of turbulent flow in the ascending aorta as shown in Figure 2. Analyses were performed with the commercial software GTFlow 2.2.15 (Gyrotools, LLC). Relative pressures were calculated generating a region of the vessel of interest and calculating automatically a relative pressure map solving the Navier-Stokes equation. In the pressure map, a region of interest (ROI) in the ascending aorta and post aortic coarctation was generated, obtaining the relative pressure mean value of the voxels inside the ROI. Streamlines were generated in order to visualize the flow velocity field along the phantom at peak systole. The phantom was equipped with a catheterization unit (two Arrow catheters, 4 Fr) to measure invasively and simultaneously the pressures in the ascending aorta and post aortic coarctation. These values were considered as gold standard. Systolic and diastolic pressures and SPG obtained with cardiac catheterization and from 3D PC-MRI are summarized in table 1. Excellent accuracy was obtained with the 13 and 11 mm AoCo phantoms at rest and stress conditions. However, we observed large errors in the 9 mm AoCo phantom at rest and stress conditions (Figure 1). Figures 2 and 3 show the systolic pressure gradients and streamlines of the 13 mm AoCo phantom (second acquisition) for different aortic segmentation and spatial resolutions. The systolic pressure gradients measured with catheterization were 10 and 14 mmHg under rest and stress conditions respectively. Relative pressures were in excellent accuracy with catheterization values when the aortic segmentation only included laminar flow and with higher spatial resolution at rest and stress conditions.We observed that the accuracy of relative pressure form the 3D PC-MRI was excellent compared with catheterization values in cases of mild to moderate AoCo at rest and stress conditions. In the 9 mm AoCo, the SPG were under-estimated in 48.3 % and 47.7 % at rest and stress conditions respectively, probably by the higher flow turbulence after the coarctation and the lower number of voxels in the effective orifice of the 9 mm. Also, the accuracy increase when the aortic segmentation exclude turbulent flow and with higher spatial resolutions.Relative pressures measured from 3D PC-MRI were in excellent agreement with gold standard values for cases of mild to moderate AoCo, however its accuracy greatly decreased for severe cases.