T-Hex: Tilted hexagonal grids for rapid 3D imaging

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Signal Processing

Background:

• Fast 3D k-space sampling in MRI typically uses 2D readouts, limiting speed.
• Existing methods face challenges with flexible segmentation and uniform k-space density.
• Minimizing magnetic field (B0) effects is crucial for image quality.

Purpose of the Study:

• To develop and demonstrate a 3D MRI encoding strategy balancing speed, segmentation flexibility, and uniform k-space density.
• To address benign B0 effects in fast 3D MRI acquisition.
• To improve signal-to-noise ratio (SNR) efficiency in 3D MRI.

Main Methods:

• Introduced Tilted Hexagonal (T-Hex) sampling for acquiring more k-space volume per excitation.
• Controlled k-space volume coverage per shot via a tilting angle.
• Employed a 3D iterative SENSE reconstruction incorporating field dynamics and off-resonance effects.
• Compared T-Hex imaging speed and noise performance against established 3D schemes.

Main Results:

• T-Hex acquisition demonstrated superior imaging speed compared to existing alternatives, especially with spiral trajectories.
• Successfully addressed the interplay of 3D trajectories, array detection, and off-resonance effects.
• Achieved greatest utility for high-speed imaging at intermediate resolutions (1-4 mm).
• T-Hex EPI combined extended coverage per shot with enhanced robustness against off-resonance.

Conclusions:

• Tilted hexagonal grid sampling is a feasible method for achieving 3D imaging speed with near-optimal SNR efficiency.
• The technique offers benign depiction properties suitable for various MRI applications.
• T-Hex sampling is particularly promising for time-resolved imaging, such as functional MRI (fMRI).