Free-breathing three-dimensional whole-heart adiabatic T1ρ mapping for non-contrast tissue characterization at 0.55T

Fig. 1. Framework of the proposed 3D adiabatic T1ρ mapping research sequence. (A) Pulse sequence diagram. T1ρ preparation is performed with adiabatic SL that consists of multiple pairs of HS pulses. Four interleaved 3D volumes are acquired in repeated cycles of four heartbeats with electrocardiogram triggered SPGR acquisition. The first three volumes (IMG1-IMG3) are prepared with saturation pulse with the same recovery time (TSAT), while the last volume (IMG4) is prepared with saturation pulse without delay time. IMG2 and IMG3 are also prepared with adiabatic-SL consisting of 2 and 4 HS pulses with SL duration (τSL) of 2τHS and 4τHS, respectively. 2D iNAVs are performed before data acquisition in each heartbeat, and a variable-density Cartesian trajectory with spiral-like profile order (VD-CASPR) is adopted with 4-fold under-sampling. (B) T1ρ map calculation. The four volumes with different contrast weighting (IMG1-IMG4) are used to calculate a 3D T1ρ map voxel-by-voxel using a 3-parameter fitting method. SL spin-lock, 2D two-dimensional, 3D three-dimensional, HS hyperbolic-secant, SPGR spoiled gradient echo

Fig. 2. Phantom results. (A) Phantom T1ρ maps acquired with 2D fully sampled reference sequence and the proposed 3D T1ρ mapping sequence with simulated heart rate of 60 bpm and saturation recovery time (TSAT) of 600 ms. (B) Linear regression plot of phantom T1ρ measured by 3D T1ρ mapping in comparison with 2D reference values. (C, D) Phantom T1ρ mean value and CV measured by 3D T1ρ mapping with TSAT ranging from 300 to 1100 ms and corresponding simulated heart rates of 120 to 40 bpm. Phantom T1ρ mean values indicate negligible sensitivity to heart rate especially for phantom vials that mimic myocardium (vial #1–6). T1ρ CVs were generally elevated with shorter TSAT delays at higher heart rates, primarily due to reduced recovery of longitudinal magnetization. This effect is more pronounced for phantom vials with longer T1 and T1ρ values that mimic the blood pool (vial #8, 9). However, CVs were less sensitive to heart rate variations in phantom vials mimicking myocardium, remaining below 4% across all TSAT values. CV coefficient of variation, 2D two-dimensional, 3D three-dimensional

Fig. 3. Representative T1ρ maps of a healthy volunteer measured by the proposed 3D T1ρ mapping sequence. (A) 3D T1ρ maps in different orientations including short-axis views from apex to base, coronal view, 2-chamber, and 4-chamber view. (B) Bull's-eye plots of whole-heart T1ρ. The mean T1ρ values in the 16 American Heart Association segments are presented. (C) Histogram of the myocardial T1ρ over the whole left ventricle. The T1ρ mean, SD and CV across the whole left ventricle are shown. SD standard deviation, CV coefficient of variation, 3D three-dimensional

Fig. 4. Results of in-vivo experiments. (A) Representative short-axis T1ρ maps of three healthy subjects obtained with the proposed 3D T1ρ mapping sequence and a breath-hold 2D T1ρ mapping sequence. 3D T1ρ maps were comparable with 2D T1ρ maps for the depiction of cardiac structure such as left ventricle myocardium and papillary muscles, but 3D images presented visually less noise than 2D images. (B) Bull’s-eye plots of the myocardial T1ρ mean and CV measured by 3D and 2D sequences. The results in the 16 American Heart Association segments are averaged across all healthy volunteers. The CVs in the inferior-lateral segments were slightly higher for both sequences, potentially due to the attenuation of coil sensitivity in those regions. CV coefficient of variation, 2D two-dimensional, 3D three-dimensional