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

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To evaluate accelerated T1- and T2-mapping techniques for ultra-low-field MRI using low-rank reconstruction 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. 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. The use of low-rank reconstruction allows significant acceleration of T1 and T2 mapping in low-field MRI while maintaining image quality.