Accelerated free-breathing whole-heart 3D T2 mapping with high isotropic resolution

Abstract

Purpose: To enable free-breathing whole-heart 3D T₂ mapping with high isotropic resolution in a clinically feasible and predictable scan time. This 3D motion-corrected undersampled signal matched (MUST) T₂ map is achieved by combining an undersampled motion-compensated T₂ -prepared Cartesian acquisition with a high-order patch-based reconstruction.

Methods: The 3D MUST-T₂ mapping acquisition consists of an electrocardiogram-triggered, T₂ -prepared, balanced SSFP sequence with nonselective saturation pulses. Three undersampled T₂ -weighted volumes are acquired using a 3D Cartesian variable-density sampling with increasing T₂ preparation times. A 2D image-based navigator is used to correct for respiratory motion of the heart and allow 100% scan efficiency. Multicontrast high-dimensionality undersampled patch-based reconstruction is used in concert with dictionary matching to generate 3D T₂ maps. The proposed framework was evaluated in simulations, phantom experiments, and in vivo (10 healthy subjects, 2 patients) with 1.5-mm³ isotropic resolution. Three-dimensional MUST-T₂ was compared against standard multi-echo spin-echo sequence (phantom) and conventional breath-held single-shot 2D SSFP T₂ mapping (in vivo).

Results: Three-dimensional MUST-T₂ showed high accuracy in phantom experiments (R² > 0.99). The precision of T₂ values was similar for 3D MUST-T₂ and 2D balanced SSFP T₂ mapping in vivo (5 ± 1 ms versus 4 ± 2 ms, P = .52). Slightly longer T₂ values were observed with 3D MUST-T₂ in comparison to 2D balanced SSFP T₂ mapping (50.7 ± 2 ms versus 48.2 ± 1 ms, P < .05). Preliminary results in patients demonstrated T₂ values in agreement with literature values.

Conclusion: The proposed approach enables free-breathing whole-heart 3D T₂ mapping with high isotropic resolution in about 8 minutes, achieving accurate and precise T₂ quantification of myocardial tissue in a clinically feasible scan time.

Keywords: T2 quantification; fast imaging; isotropic resolution; motion correction; myocardial T2 mapping.

Figure 1

Schematic overview of the proposed free‐breathing 3D motion‐corrected undersampled signal matched (MUST) T₂ technique for whole‐heart myocardial T₂ mapping. Three T₂‐prepared volumes are acquired sequentially with increasing TE_T2prep ([0,28,55] ms). A nonselective saturation pulse is applied immediately after the electrocardiogram (ECG) R‐wave to avoid recovery heartbeats. A 2D image navigator is acquired to enable translational respiratory motion correction of the heart and shorter and predictable scan times. A golden‐angle shifted variable‐density Cartesian undersampling is used to achieve clinically feasible scan times. All T₂ prepared volumes are reconstructed simultaneously with high‐dimensionality undersampled patch‐based reconstruction (HD‐PROST).18 A dictionary is then simulated and matched to the measured signal to generate the whole‐heart T₂ maps. Abbreviations: AW, acquisition window; bSSFP, balanced SSFP; iNAV, image‐based navigator; TD, trigger delay; T_SAT, saturation time

Figure 2

Results from extended phase graph (EPG) simulations show the effect of the saturation pulse on the MR signal evolution. A, Simulated magnetization obtained with the EPG formalism for different recovery times (ranging from 0 to 9 seconds) when the saturation pulse is not used. The signals were generated for tissues with a T₂ of 50 ms, varying T₁s (ranging from 700 ms to 1200 ms), TE_T2prep = 50 ms, and a simulated heart rate of 60 bpm. For long T₁s, a minimum of about 6 idle heartbeats are needed to allow for full recovery of the longitudinal magnetization. B, When the saturation pulse is applied at every heartbeat, idle heartbeats are not required for signal recovery, at the cost of lower signal intensity. C, Evolution of the matched T₂ values obtained with the proposed 3D MUST‐T₂ mapping sequence over different simulated heart rates (ranging from 50 bpm to 100 bpm) for each phantom vial. The proposed approach is mostly insensitive to heart‐rate variations, even for long T₁s. D, Effect of different heart rates across all healthy subjects (N = 10) on mean T₂ values. Abbreviations: BPM, beats per minute

Figure 3

Phantom accuracy for the proposed 3D MUST‐T₂ sequence. Plots compare the mean T₂ values derived from the 9 vials for 5 different acceleration factors with the ground‐truth T₂ values (measured by spin echo [SE] with 8 TEs from 10‐640 ms29), conventional 2D T₂p‐SSFP mapping (green), and the proposed 3D MUST‐T₂ sequence. The T₂ accuracy is preserved with the proposed approach with excellent agreement with the reference T₂ values, even for high acceleration (×5). The T₂ values for the last tube (T₂ = 250ms) were out of range (> 300 ms) for the 2D T₂p‐SSFP sequence and therefore are not shown

Figure 4

The T₂ maps obtained using the proposed free‐breathing 3D MUST‐T₂ sequence and the conventional breath‐held 2D T₂p‐SSFP sequence are shown for 3 healthy subjects. The 3D MUST‐T₂ slices were reformatted to short axis to match the 2D T₂ map acquisitions. Good visualization of the myocardium and surrounding structures can be observed on the 3D MUST‐T₂ maps. Acquisition times are expressed as minutes:seconds. Abbreviations: AT, acquisition time

Figure 5

Accuracy and precision of the proposed 3D MUST‐T₂ mapping sequence. A, The T₂ accuracy of the proposed 3D MUST‐T₂ sequence versus conventional 2D T₂p‐SSFP, as measured by the mean T₂ value, are shown in the left ventricular segmentation. The T₂ values are in good agreement with the literature (T₂ = 50 ± 4 ms33). The averaged T₂ relaxation times over the whole myocardium are shown in the bull's eye plots' center. Accuracy (B) and precision (C) of T₂ relaxation times (ms) obtained in the myocardial septum with the proposed 3D MUST‐T₂ and the conventional 2D T₂p‐SSFP are shown for the 10 healthy subjects

Figure 6

Three‐dimensional visualization of the acquired T₂‐weighted images and the corresponding T₂ volume. Representative T₂‐weighted images for subject 2 (acquisition time: 10 minutes, heart rate = 38 bpm), and the corresponding T₂ maps obtained by the proposed 3D MUST‐ T₂. Eight reformatted short‐axis slices that cover the heart from apex to base are shown. Uniform distribution of T₂ values through the slices over the whole left ventricle can be observed. The color scale indicates T₂ values between 0 ms and 120 ms

Figure 7

Representative T₂‐prepared images for subject 2 and the corresponding T₂ maps obtained with the proposed 3D MUST‐T₂ sequence. Reformats in short‐axis, vertical long‐axis (VL), 3‐chamber, and 4‐chamber views are shown

Figure 8

Short‐axis T₂ maps at apical, midventricular, and basal level for 2 patients acquired with the proposed free‐breathing 3D MUST‐T₂ framework and the conventional breath‐hold 2D T₂p‐SSFP sequence. The septal T₂ relaxation times for each slice are reported as mean ± SD