Real-time cardiac synchronization with fixed volume frame rate for reducing physiological instabilities in 3D FMRI

Area of Science:

• Magnetic Resonance Imaging
• Neuroimaging
• Biomedical Engineering

Background:

• 2D echo-planar imaging (EPI) is standard for functional MRI (fMRI), but 3D readouts offer higher resolution and SNR.
• 3D spoiled gradient-echo (SPGR) and balanced steady-state free-precession (bSSFP) sequences are less prone to distortion, making them suitable for deep brain structures.
• However, these multi-shot 3D sequences suffer from physiological instabilities, particularly cardiac pulsation, affecting signal quality in inferior brain regions.

Purpose of the Study:

• To characterize the source of signal instabilities in 3D SPGR and bSSFP fMRI.
• To develop and evaluate a novel method for reducing these instabilities.
• To improve the temporal signal-to-noise ratio (tSNR) in challenging brain regions.

Main Methods:

• Acquisition of rapidly sampled single-slice data to capture respiratory and cardiac waveforms.
• Simulations to assess the impact of cardiac cycle synchronization on signal stability.
• Development of a real-time cardiac synchronization technique using parallel imaging without reducing volume acquisition rate.

Main Results:

• Cardiac pulsation was identified as the primary source of signal instabilities in 3D fMRI.
• Real-time cardiac synchronization significantly reduced instabilities.
• In bSSFP, brainstem tSNR increased by 45%; in SPGR, subcortical and cortical temporal stability improved by ~20%.

Conclusions:

• Cardiac synchronization is a viable strategy to mitigate physiological noise in 3D fMRI.
• The developed real-time method enhances temporal stability in deep brain structures and cortical areas.
• This technique offers a powerful alternative for fMRI in susceptibility-prone regions.