UCSF

Background: 4D flow (4DF) magnetic resonance imaging (MRI) offers a promising way to evaluate blood flow patterns in intracranial aneurysms, although long scan times present a major limitation to broad clinical implementation. Compressed sensing (CS), an accelerated imaging technique using strategically undersampled data for data reconstruction, offers a possible solution to reduce scan times. The aim of this study was to understand the effects and limitations of varying compressed sensing acceleration factors, R, at different resolutions in in vitro 4D flow acquisitions.

Methods: This study employed a phantom depicting a saccular aneurysm. Experiment 1 evaluated the reliability of 4D flow with varying levels of compressed sensing acceleration factors (R=7.6, 12.8, and 16.6). Experiment 2 assessed the effects of varying resolutions (0.5, 1.0, 1.5, and 2.0 mm) with a compressed sensing R=12.8. Qualitative analysis included a visual assessment of velocity vectors and streamlines. Quantitative analysis compared the velocity components, peak velocity, flow rate, and wall shear stress in each experiment. All studies were post-processed using a clinically-geared software as well as with an In-House engineering pipeline, with the purpose of understanding the advantages and disadvantages of each approach and validating any results.

Results: The addition of compressed sensing reduced scan times to approximately 4-7 minutes. The In-House processing pipeline is superior to the clinical software in visualizing of velocity; visual analysis showed velocity overestimations in the R=16.8 streamlines, indicating the limits of compressed sensing to be 7.6 < R < 12.8. As expected, comparison of velocity components reflects a decrease in linear regression slopes and correlations as acceleration factors increased (m > 0.90 for all acceleration factors except R=16.6, and all r > 0.9). Experiment 2 highlights partial voluming effect at R=12.8: as resolutions decrease, velocities along the wall are less reliable than velocities furthest from the wall, which retain high slope and correlation values (both above 0.9 at 1.0 mm and 1.5 mm resolutions). High variability peak velocity, flow rate, and wall shear stress in both pipelines point to the need for a reliable way to post-process 4D flow.

Conclusion: This study showed reliable velocity data can be obtained from 4D flow studies acquired with compressed sensing lower to moderate acceleration factors at higher resolutions. With clinically-acceptable scan times, the focus now shifts towards establishing a robust and validated workflow for 4D flow studies before clinical implementation can truly be feasible.