UCSF

Introduction: Information from MR blood flow quantification can be used to evaluate patient health, inform surgical decisions and provide boundary conditions for numerical simulations. Thus, it is important to determine the accuracy and precision of the flow measurement and its contributing factors. In this study, we investigated the reproducibility of the measurement by comparing flow results from 2D MRV, 4D MRV and CFD. Also, we investigated the sensitivity of the measurements at different positions in the scanner, since these could be affected by gradient field imperfections.

Methods: A phantom which was an exact replica of a patient intracranial aneurysm was set up with a flow system where flow was driven by a gravity-fed pressure head to provide constant flow. A contrast-enhanced MRA image was obtained at 0.5 mm isotropic resolution to determine the geometry. 4D MRV within the phantom was measured under 3 conditions: cine-MR at the isocenter (0 cm offset) for 5 time points, continuous measurement at the isocenter, and continuous measurement 10cm offset from the isocenter. CFD simulation was performed on commercial software COMSOL. Image post-processing was done using in-house Python tools. Qualitative comparisons were made using Paraview while quantitative comparisons were assessed by correlation and Bland-Altman plots.

Results: Imaging at 10 cm from isocenter was found to adequately visualize secondary flows such as jets in the aneurysm. Total flow through the inlets obtained from the 2D- and 4D- MRV acquisitions were 4.03 ± 0.07 mL/s and 3.65 ± 0.12 mL/s, which were significantly larger than the directly measured flow of 3.1 mL/s. The overall velocity field from the CFD results underestimate the measurements from 4D MRV, suggesting they provided inlet boundary conditions are too low. The greatest differences between the CFD and 4D MRV results appear at the vessel walls.

Conclusion: We developed a pipeline for evaluating flow measured under different conditions and modalities. A combination of noise and partial volume effects compromise velocity measurements, particularly in voxels at the vessel edge. Additionally, CFD can provide precise measurements at the wall, but the accuracy of the results depends highly on the image-derived geometric and flow boundary conditions.