Rapid quantitative MRI at 46 mT: Accelerated T1 and T2 mapping with low-rank reconstructions.

Purpose: To evaluate accelerated T₁- and T₂-mapping techniques for ultra-low-field MRI using low-rank reconstruction methods.

Methods: Two low-rank-based algorithms, image-based locally low-rank (LLR) and k-space-based structured low-rank (SLR), were implemented to accelerate T₁ and T₂ mapping on a 46 mT Halbach MRI scanner. Data were acquired with 3D turbo spin-echo sequences using variable-density poisson-disk random sampling patterns. For validation, phantom and in vivo experiments were performed on six healthy volunteers to compare the obtained values with literature and to study reconstruction performance at different undersampling factors and spatial resolutions. In addition, the reconstruction performance of the LLR and SLR algorithms for T₁ mapping was compared using retrospective undersampling datasets. Total scan times were reduced from 45/38 min (R = 1) to 23/19 min (R = 2) and 11/9 min (R = 4) for a 2.5 × 2.5 × 5 mm³ resolution, and to 18/16 min (R = 4) for a higher in-plane resolution 1.5 × 1.5 × 5 mm³ for T₁/T₂ mapping, respectively.

Results: Both LLR and SLR algorithms successfully reconstructed T₁ and T₂ maps from undersampled data, significantly reducing scan times and eliminating undersampling artifacts. Phantom validation showed that consistent T₁ and T₂ values were obtained at different undersampling factors up to R = 4. For in vivo experiments, comparable image quality and estimated T₁ and T₂ values were obtained for fully sampled and undersampled (R = 4) reconstructions, both of which were in line with the literature values.

Conclusions: The use of low-rank reconstruction allows significant acceleration of T₁ and T₂ mapping in low-field MRI while maintaining image quality.