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

Abstract

Purpose 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.Methods 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 degrees flip angle, and two saturation recovery T1-weighted contrasts with 10 degrees 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.Results Excellent agreement is observed between iNAV-based SAVA T1 mapping at 0.55 T and spin echo reference in phantom, with a R2=0.998$$ {R}2=0.998 $$ 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%).Conclusion 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.