Myocardial Approximate Spin-lock Dispersion Mapping using a Simultaneous T2 and TRAFF2 Mapping at 3T MRI

Abstract

Ischemic heart disease (IHD) is one of the leading causes of death worldwide. Myocardial infarction (MI) represents a third of all IHD cases, and cardiac magnetic resonance imaging (MRI) is often used to assess its damage to myocardial viability. Late gadolinium enhancement (LGE) is the current gold standard, but the use of gadolinium-based agents limits the clinical applicability in some patients. Spin-lock (SL) dispersion has recently been proposed as a promising non-contrast biomarker for the assessment of MI. However, at 3T, the required range of SL preparations acquired at different amplitudes suffers from specific absorption rate (SAR) limitations and off-resonance artifacts. Relaxation Along a Fictitious Field (RAFF) is an alternative to SL preparations with lower SAR requirements, while still sampling relaxation in the rotating frame. In this study, a single breath-hold simultaneous T_RAFF2 and T₂ mapping sequence is proposed for SL dispersion mapping at 3T. Excellent reproducibility (coefficient of variations lower than 10%) was achieved in phantom experiments, indicating good intrascan repeatability. The average myocardial T_RAFF2, T₂, and SL dispersion obtained with the proposed sequence (68.0±10.7 ms, 44.0±4.0 ms, and 0.4±0.2 ×10^-4 s², respectively) were comparable to the reference methods (62.7±11.7 ms, 41.2±2.4 ms, and 0.3±0.2x 10^-4s², respectively). High visual map quality, free of B₀ and B₁⁺ related artifacts, for T₂, T_RAFF2, and SL dispersion maps were obtained in phantoms and in vivo, suggesting promise in clinical use at 3T. Clinical relevance - and imaging promises non-contrast assessment of scar and focal fibrosis in a single breath-hold using approximate spin-lock dispersion mapping.

Fig. 1.

A) Pulse sequence diagram for the proposed simultaneous myocardial T_RAFF2 and T₂ mapping sequence. B) Schematic representation of the amplitude of the RF pulses. C) The difference between the T_RAFF2 and T₂ map divided by the RAFF2 pulse amplitude results in an approximation for a spin-lock (SL) dispersion map.

Fig. 2.

A) T₂, T_RAFF2, and spin-lock (SL) dispersion maps for the T1MES phantom using the proposed simultaneous sequence (top row) and the single-parameter reference maps (bottom row). B) Correlation and Bland-Altman plots for each parameter, showing an excellent correlation between the relaxation properties in phantom, in the expected in vivo range.

Fig. 3.

Coefficient of variation (CV) across three measurements using the proposed simultaneous T_RAFF2 and T₂ mapping sequence. All vials show a CV lower than 10% demonstrating excellent reproducibility of the proposed sequence.

Fig. 4.

A) Baseline images for the proposed simultaneous T_RAFF2 and T₂ mapping sequence with the corresponding preparation times. B) In vivo myocardial T_RAFF2, T₂, and spin-lock (SL) dispersion maps were obtained with the proposed sequence and with the single-parameter sequences (Reference). Following image registration, the dispersion map is obtained by subtracting T₂ from T_RAFF2 and dividing by the RAFF2 pulse power. All maps present high visual image quality, homogeneous myocardial signal and no $B_1^{+}$ or B₀ artifacts are visible.

Fig. 5.

Standard deviation (σ) maps computed from the fit residuals to obtain a spatially resolved estimation of the T_RAFF2, T₂, and spin-lock (SL) dispersion precision.