A pipeline from MRI 4D Flow sequence to numerical simulation
Résumé
To facilitate the evolution diagnosis of intracranial aneurysms, it is relevant to consider patient-specific hemodynamic factors. MRI is used to assess blood velocity with the 4D Flow sequence, which is a dynamic extension of a Phase Contrast sequence with a flow encoding in the three directions of
space triggered by the cardiac cycle. This sequence was first developed for large vessel therefore it must be optimised for intracranial arteries. To facilitate the execution of a multitude of tests without the need for patients to be constrained, a vascular phantom is employed. A vascular
phantom is an MRI-compatible structure that simulates the circulation of a fluid within vessels, with the aid of a pump.
The optimisation of the parameters of the 4D flow sequence is necessary to enable this sequence to be used in clinical practice. This is achieved by acquiring data on a vascular phantom with each parameter and its range of values and then comparing it with a control sequence. Subsequently, a comparative analysis is conducted on the reconstructed volumes. This involves calculating the inter-sequence correlation in the velocity distribution, as influenced by the application of acceleration factors. These tests led to the identification of an optimised sequence with a minimum correlation index of 0.88 with the control sequence. The sequence was accelerated by 24.6 times while minimising the impact of the various parameters on the flow images obtained.
Subsequently, Computational Fluid Dynamics (CFD) was used, as a reference method for biomarkers of aneurysm progression, to generate a simulation of the blood flow, which was compared with the MRI images. The reconstructed amplitude sequence of 4D Flow allows us to focus on the vascular geometry of the phantom. A binary mask volume is extracted from this sequence, which is then processed to obtain a numerical mesh. The simulation of blood flow is conducted using the Finite Element Method (FEM) for solving the Navier-Stokes equations with incompressible, Newtonian, and laminar flow, rigid walls, and non-slip boundary conditions which are compatible with the structure of the phantom used in MRI.
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