###### Abstract  
Background: This work aims at defining the Effective Office Area (EOA) derived from the acoustic source term (AST) method from 4D Phase-Contrast MRI data to provide a reference for the assessment of MRI valvular prostheses as part of a comprehensive cardiac exam. Methods: Three different Bileaflet Mechanical Heart Valves (BMHV) and a dysfunctional BMHV were tested in-vitro using 4D Phase-Contrast MRI and a numerical design of the experimental study was performed, including the influence of internal diameter, stroke volume, and heart rate. The EOA AST was computed based on the MRI 4D Phase-Contrast acquisition. Results: EOAAST values vary by 15 mm from the BMHV center and should be computed between 8 to 10 mm after the metallic \"blurring\" artefact (ranging from 18.9 to 23.4 mm from the BMHV). EOAAST values were found to be lower compared with numerical results in the appropriate plane. Detection of the BMHV dysfunction by continuity equation computed from 4D flow acquisition is limited based on EOAAST computations, while EOAc and velocities after the valves could more directly highlight a blocked leaflet. Conclusion: This multi-disciplinary study demonstrates the suitability of the EOA AST method to assess BMHV function using MRI. Translation to the clinic is feasible using an optimized 2D Phase-Contrast flow stack or a 4D Phase-Contrast minimal volume based on the recommendations provided. bi-leaflet mechanical valve; effective orifice area; magnetic resonance imaging; acoustic source term +
Footnote †: journal: Medical Imaging Department, University Hospital of Dijon, 21079 Dijon, France
Echocardiographic criteria for the assessment of valvular prosthesis are mostly based on effective orifice area (EOA) and transvalvular pressure gradient [4]. EOA computation is performed from stroke volume measurement and from Doppler continuous wave velocity measurements through the valve location. Using the continuity equation, the EOA should be computed at the vena contracta of the valve, which is the contracted area formed by the flow when going through an orifice [5]. MRI, in addition to providing the capacity to assess the cardiac function, also enables measurement of velocity fields during the cardiac cycle through a 4D Phase-Contrast flow sequence [6], while metallic blurring still limits the study of metallic structured prostheses. Limitations of the 4D Phase-Contrast flow measurements also arise from the temporal resolution and acquisition time. A model of the prosthesis will generate a different blur of the valve depending upon the composition of the alloy of the valve. As far as we know, no study addressed the distance from the prosthesis orifice at which the acquisition of the velocity time integral (VTI) is performed or the length of the blur due to the valve structure. Then, cardiac magnetic resonance is reported to have limited interest in the diagnostic evaluation of left-sided prosthetic heart dysfunction in comparison with other techniques, such as multidetector-row CT, fluoroscopy, or echocardiography [7]. A specific single-point ramped imaging with T1 enhancement (SPRITE) MRI sequence was used to investigate flow through a BMHV [8]. This technique used the axial velocity component through the BMHV to diagnose dysfunction of the valve and is reported to be immune to magnetic susceptibility. In the case of a BMHV, the orifice is divided into three parts, and the blood divides into three contracted jets after the orifice. BMHV manufacturers provide the diameter of the internal orifice and valve label. However, those labels cannot be used for comparison between models [4]. Geometric orifice area (GOA) has been computed from MRI before [9; 10]. The main functional assessment for BMHV consists of computing the EOA and the transvalvular pressure gradient, as well as checking the motion of the leaflets. Echocardiographic guidelines [11] have been reported to show discrepancies between EOA computation given by echo guidelines and the real EOA.