Motion corrected 3D whole-heart SAVA T₁ mapping at 0.55 T

To propose a novel highly efficient isotropic-resolution 3D whole-heart saturation-recovery and variable-flip-angle (SAVA) T₁ mapping sequence at 0.55 T, incorporating image navigator (iNAV)-based non-rigid motion correction and dictionary matching.

The proposed iNAV-based isotropic-resolution 3D whole-heart SAVA T₁ mapping sequence at 0.55 T acquires three gradient echo T₁-weighted volumes sequentially: an equilibrium contrast with 4° flip angle, and two saturation recovery T₁-weighted contrasts with 10° flip angles and different saturation delays. Sequence parameters were optimized for the lower field strength by simulations and phantom experiments. Two-dimensional iNAVs are acquired at each heartbeat to enable respiratory motion estimation and correction and 100% respiratory scan efficiency. The T₁ mapping is computed by dictionary matching, using subject-specific dictionaries based on Bloch equations simulations. Non-rigid motion correction is implemented based on respiratory bins reconstructed by iterative-SENSE and subsequent patch-based low-rank denoising, for each contrast separately. The proposed approach was evaluated in a standardized T₁ phantom and 10 healthy subjects, in comparison to spin-echo reference and 2D MOLLI, respectively.

Excellent agreement is observed between iNAV-based SAVA T₁ mapping at 0.55 T and spin echo reference in phantom, with a $R^2=0.998$ for all phantom vials. Good image quality was obtained in vivo for the contrast images and corresponding T₁ maps in a scan time of 6:30 min ±40 s. Average and SD of myocardial T₁ values across subjects and segments was 706 ± 41 ms, which is comparable to acquired 2D MOLLI values of 681 ± 26 ms, and previously reported 2D MOLLI values of 701 ± 24 ms. Coefficient of variation values (12%) are higher than those previously reported for diaphragmatic navigator-based non-isotropic SAVA T₁ mapping at 3 T (7.4%).

The proposed iNAV-based SAVA approach achieves free-breathing motion-corrected 3D whole-heart T₁ mapping at 0.55 T in approximately 7 min scan time for an isotropic resolution of 2 mm. In vivo experiments showed that the proposed sequence achieves good map quality, with comparable T₁ values and spatial variability compared to 2D MOLLI T₁ mapping. Further evaluation is warranted in patients with cardiovascular disease.