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

Magn Reson Med. 2021 May;85(5):2507-2523. doi: 10.1002/mrm.28600. Epub 2020 Dec 3.


Purpose: The purpose of this work is to devise and demonstrate an encoding strategy for 3D MRI that reconciles high speed with flexible segmentation, uniform k-space density, and benign T2 effects.

Methods: Fast sampling of a 3D k-space is typically accomplished by 2D readouts per shot using EPI trains or spiral readouts. Tilted hexagonal (T-Hex) sampling is a way of acquiring more k-space volume per excitation while maintaining uniform sampling density and a smooth T2 filter. The k-space volume covered per shot is controlled by the tilting angle. Image reconstruction is performed with a 3D extension of the iterative SENSE approach, incorporating actual field dynamics and static off-resonance. T-Hex imaging is compared with established 3D schemes in terms of speed and noise performance.

Results: Tilted hexagonal acquisition is found to achieve greater imaging speed than known alternatives, particularly in combination with spiral trajectories. The interplay of the proposed 3D trajectories, array detection, and off-resonance is successfully addressed by iterative inversion of the full signal model. Enhanced coverage per shot is of greatest utility for high speed in an intermediate resolution regime of 1 to 4 mm. T-Hex EPI combines the benefits of extended coverage per shot with increased robustness against off-resonance effects.

Conclusion: Sampling of tilted hexagonal grids is a feasible means of gaining 3D imaging speed with near-optimal SNR efficiency and benign depiction properties. It is a particularly promising technique for time-resolved applications such as fMRI.

Keywords: 3D encoding; algebraic image reconstruction; magnetic field monitoring; spiral imaging.

MeSH terms

  • Algorithms
  • Brain / diagnostic imaging
  • Computer Systems
  • Image Processing, Computer-Assisted*
  • Imaging, Three-Dimensional*
  • Magnetic Resonance Imaging