Cardiac catheterization is systematically recommended and remains the gold standard to confirm the diagnosis of PAH, establish its pathophysiology and prognosis but also for the follow-up under medical treatment of these patients in tertiary centres every 6 months. Some centres repeat invasive hemodynamic assessment in the event of clinical aggravation before considering a change in drug management or overall management. This is the reference examination for the calculation of PVRs, an extremely rigorous methodology is required in patients with intracardiac shunt: the Fick principle must then be applied to calculate Qp. Usually, it is calculated by thermodilution but is no longer applicable in the case of intracardiac shunt, as is significant valvulopathy or massive tricuspid leakage. Qp is equal to the oxygen consumption (MVO2) divided by the difference in oxygen content (CaO2) between the pulmonary artery and the pulmonary veins (VP). Qp= MVO2/ (CaO2VP- CaO2 AP). They must be carried out with a rigorous methodology adapted on a case-by-case basis according to the anatomy and hemodynamics of the heart disease and a precise knowledge of the limits of the technique, in expert centres in PAH and congenital heart disease and must not be seen as without consequences for the patient . Although this tool is the gold standard, rigorously carried out in our reference centre, with rare accidents or potential risks, it remains an invasive examination often poorly experienced by patients .

Magnetic resonance imaging (MRI) has emerged in recent years as the reference technique for non-invasive morphological and functional evaluation of the right ventricle. It also seems relevant for the study of pulmonary arterial hemodynamics while providing prognostic values for PAH. Its regular implementation is indicated in the 2015 ESC guidelines for the diagnosis and follow-up of patients, with more particularly an injection of gadolinium for the search for myocardial fibrosis, a marker of right ventricular remodelling. Also considered are: decrease in right ventricular ejection fraction, paradoxical septal kinetics, decrease in left ventricular dimensions, compliance of the pulmonary artery (mL/mmHg) and mean flow velocity (cm/s). Carrying out this examination using conventional techniques (2D MRI) has some limitations. Extrinsic, linked to operators and observers. The quality of the examinations here depends on the quality of the acquisitions made by the radiology manipulators (sequences / section plans) as well as an adapted analysis of these cardiopathies. All of this involves a high degree of technical expertise on the part of paramedical staff, as well as expertise in congenital heart disease and cardiac imaging for medical staff. Finally, this examination has intrinsic limitations, since it does not allow the characteristics of intracardiac or vascular flows (turbulence, direction, vortex) to be explored. All this implies an examination time of about 30 minutes 4D Flow MRI is a promising imaging that allows the acquisition of anatomical, volume, right ventricular remodeling and intracardiac flow information in a single step with 2D (only 8 minutes extra), in free breathing and totally autonomous mode. It is a sequence allowing a quasi-isotopic, retrospective analysis, providing information on blood flow in the 3 planes of the space, during the cardiac cycle. It is not influenced by the quality of the medical teams and, above all, it allows an unlimited post-processing capacity. This innovative imaging method is therefore particularly suitable for the study of cardiac output such as Qp. The right ventricular volume is difficult to analyze due to the complexity of its geometry, automatic segmentation is still under development, this element seems to be a promising hemodynamic parameter since it is quantitatively calculated and seems to be correlated with the systolic function of the left ventricle and inaccessible to conventional techniques due to the particular geometry of the right ventricle. Previous studies have demonstrated the possibility of qualitative analysis of intraventricular blood flows by studying retrograde flows, vortices and their kinetic energy, which are involved in diastolic dysfunction of the right ventricle .Studies have shown that these structures can be altered in PAH dog models but also in repaired Fallot tetralogy series and Fontan type circulations .

Thus, at the same time as the realization of a 2D MRI, essential for the diagnosis and follow-up of PAH, with an additional 8 minutes for 4D flow, the investigators could have additional fundamental information on pulmonary cardiac output but also prognostic markers of right ventricular dysfunction turning dramatic in pulmonary vascular disease.

Our objective is to evaluate the contribution of 4D MRI flow imaging to the measurement of pulmonary cardiac output compared to the gold standard which is cardiac catheterization using the Fick method. But also, to compare 2D and 4D MRI flow for the evaluation of volumes, function and right ventricular remodeling, as well as the quantification of pulmonary, tricuspid and systemic flows.