Free-running 3D whole heart myocardial T1 mapping with isotropic spatial resolution

Abstract

Purpose: To develop a free-running (free-breathing, retrospective cardiac gating) 3D myocardial T₁ mapping with isotropic spatial resolution.

Methods: The free-running sequence is inversion recovery (IR)-prepared followed by continuous 3D golden angle radial data acquisition. 1D respiratory motion signal is extracted from the k-space center of all spokes and used to bin the k-space data into different respiratory states, enabling estimation and correction of 3D translational respiratory motion, whereas cardiac motion is recorded using electrocardiography and synchronized with data acquisition. 3D translational respiratory motion compensated T₁ maps at diastole and systole were generated with 1.5 mm isotropic spatial resolution with low-rank inversion and high-dimensionality patch-based undersampled reconstruction. The technique was validated against conventional methods in phantom and 9 healthy subjects.

Results: Phantom results demonstrated good agreement (R² = 0.99) of T₁ estimation with reference method. Homogeneous systolic and diastolic 3D T₁ maps were reconstructed from the proposed technique. Diastolic septal T₁ estimated with the proposed method (1140 ± 36 ms) was comparable to the saturation recovery single-shot acquisition (SASHA) sequence (1153 ± 49 ms), but was higher than the modified Look-Locker inversion recovery (MOLLI) sequence (1037 ± 33 ms). Precision of the proposed method (42 ± 8 ms) was comparable to MOLLI (41 ± 7 ms) and improved with respect to SASHA (87 ± 19 ms).

Conclusions: The proposed free-running whole heart T₁ mapping method allows for reconstruction of isotropic resolution 3D T₁ maps at different cardiac phases, serving as a promising tool for whole heart myocardial tissue characterization.

Keywords: 3D radial; free-running; inversion recovery; myocardial T1 mapping.

Figure 1

(A) Schematic diagram of the proposed free‐running myocardial 3D T₁ mapping sequence. After inversion recovery (IR) preparation, spoiled gradient echo (SPGR) readout with low flip angle (θ) was performed using 3D golden angle radial trajectory. T_gap is the time between the IR pulse and the first excitation and T_ex is the time interval between the last excitation in the SPGR readout and the next IR pulse. IRTR is the IR repetition time. (B) Data sorting process for reconstruction of multiple T₁ contrasts for a given cardiac phase, which includes 3 steps: 1D respiratory motion estimation from k‐space center of all radial spokes and cardiac motion extraction from ECG log; respiratory motion correction of k‐space using motion parameters estimated by 3D translational image registration of respiratory bin images at diastole; binning the respiratory motion corrected k‐space into different T₁ contrasts for a given cardiac phase

Figure 2

(A) The 5 independent components (IC) extracted from the k‐space center of all spokes using independent component analysis. (B) The spectral power of the signal components in (A). (C) The IC (IC 4) with highest spectral power in the respiratory frequency range (0.1–0.5 Hz) is selected and band‐pass filtered and compared with respiration bellow. (D) Example of low‐resolution self‐navigated images of 5 respiratory bins from intermediate reconstruction for 3D translational respiratory motion estimation. Setting bin 5 as the reference, its difference with other respiratory bins are shown in the second row to demonstrate the extent of respiration‐induced motion of the heart in this subject. The difference images are demonstrated with the same gray level range

Figure 3

(A) The singular values obtained by singular value decomposition of the dictionary simulated according to the imaging and reconstruction parameters of the proposed 3D T₁ mapping technique using Bloch simulation. (B) The singular value images corresponding to the 3 largest singular values in (A) reconstructed by direct back projection and HD‐PROST (high‐dimensionality 3D patch‐based undersampled reconstruction) algorithm

Figure 4

(A and B) Phantom T₁ maps of 2D inversion recovery spin echo (2D IR‐SE) and the proposed 3D T₁ mapping approach. (C) Linear correlation of phantom T₁ estimation with the proposed 3D method in comparison with reference 2D IR‐SE sequence. (D) Bland‐Altman plot showing the difference between the 2 methods and their average. The grey solid line indicates the mean difference, and the grey dotted lines indicate the 95% confidence intervals of limits of agreement

Figure 5

Proposed 3D T₁ mapping technique at diastolic cardiac phase for a representative healthy subject. Eight short‐axis slices, from apex to base of the left ventricle and the reformatted long‐axis view are shown. Uniform T₁ distribution across the myocardium can be observed

Figure 6

Short‐axis 2D MOLLI, 2D SASHA, and the proposed 3D T₁ mapping results at diastolic cardiac phase for 2 healthy subjects. Long‐axis view is also included for the proposed 3D T₁ mapping technique

Figure 7

(A) Mean septum T₁ values of all 9 healthy subjects measured with 2D MOLLI, 2D SASHA, and the proposed free‐running 3D T₁ mapping technique. (B) SDs of the septal T₁ measurements from all the subjects for the 3 methods. The mean ± SD across all the subjects are shown on top of each method (**P < 0.01)

Figure 8

Representative diastolic and systolic T₁ maps of basal, mid and apical short‐axis views using the proposed 3D T₁ mapping sequence from 2 healthy subjects (A, B). Uniform myocardial T₁ distribution can be observed on the T₁ maps, both in diastole and systole.

Figure 9

(A) AHA bull's‐eye plots showing the myocardium T₁ distribution across the left ventricle at diastole and systole with the proposed free‐running 3D T₁ mapping technique. The mean values obtained by averaging across all the subjects are shown in each segment (*P < 0.05, **P < 0.01). (B) Box plots showing the median, 25th, and 75th percentiles and range of the diastolic and systolic T₁ in each AHA segment. A, anterior; AS, anteroseptal; IS, inferoseptal; I, inferior; IL, inferolateral; AL, anterolateral; S, septal; L, lateral