On the limitations of echo planar 4D flow MRI

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Fluid Dynamics
• Medical Imaging Technology

Background:

• Four-dimensional flow MRI (4D Flow MRI) is crucial for assessing cardiovascular hemodynamics.
• Gradient Echo (GRE) sequences are standard for 4D Flow MRI, but their acquisition speed can be limiting.
• Echo Planar Imaging (EPI) offers faster acquisition but its performance in high-velocity flow is not well-characterized.

Purpose of the Study:

• To compare the performance of EPI and GRE readouts in high-flow velocity regimes.
• To evaluate the impact of EPI and GRE on measurement accuracy in 4D Flow MRI.
• To assess displacement artifacts and effective spatial resolution for both sequences.

Main Methods:

• Phase-contrast MRI sequences for EPI and GRE were simulated using computational fluid dynamics (CFD) velocity data.
• Simulations assessed displacement artifacts and effective spatial resolution.
• In silico findings were validated experimentally using a steady flow phantom.

Main Results:

• EPI factor 5 with simulated stenotic flow (peak velocity 2.2 m/s) showed 7.3 mm misregistration and reduced resolution by factors of 5-8 compared to GRE.
• In vitro experiments revealed a maximum velocity difference of 0.97 m/s between EPI factor 5 and GRE.
• EPI readout demonstrated considerable velocity misregistration and spatially varying resolution degradation.

Conclusions:

• EPI readout in 4D Flow MRI leads to significant velocity misregistration.
• EPI readout causes spatially varying degradation of spatial resolution.
• Standard GRE sequences are superior to EPI for 4D Flow MRI in high-flow velocity regimes.