1336886 - Myocardial Spin-Lock Dispersion mapping: Intra-subject repeatability and initial clinical evaluation

Background: Among non-contrast imaging biomarkers for myocardial infarction (MI), quantification of Spin-Lock (SL) dispersion has shown promise to increase specificity over conventional relaxometry¹. Recently, a single breath-hold (BH) pipeline was proposed to measure approximate myocardial SL dispersion at 3T by joint quantification of Relaxation Along a Fictitious Field in the 2^nd rotating frame (RAFF2) and T₂ times². To investigate clinical feasibility, we aim to evaluate this pipeline in healthy subjects and patients with different myocardial injuries.

Methods: All images were acquired at 3T (Magnetom Prisma, Siemens). Joint T_RAFF2 and T₂ mapping (Fig 1) was performed by acquiring 5 images: 1 with no preparation, 2 RAFF2-prepared (T_RAFF2p), 1 T2-prep (T_2p), each preceded by a 4s rest period, and a saturation-prepared image, to capture the effect of imaging pulses on the magnetization curve³. Group-wise registration was performed⁴ and T_RAFF2 and T₂ maps were generated using a 4-parameter fit. Approximate SL dispersion maps were calculated by subtracting T₂ and T_RAFF2, normalized by the RAFF frequency. In vivo SL dispersion maps were obtained for 8 healthy subjects (6 males; 35±4y), under end-expiration BH of 16 cardiac cycles using T_RAFF2p=12.9 and 25.7ms (peak RF pulse power=625Hz) and T_2p=50ms. Deep-learning-based myocardial segmentation was performed⁵ and mean T_RAFF2, T₂, and SL dispersion were extracted. To assess repeatability, SL dispersion mapping was repeated 3 times in separate BHs, and the coefficient of variation (CoV) was calculated. 3 patients (3 males; 63±8y) with suspected cardiac diseases were recruited. Pre-contrast MOLLI T₁ mapping and SL dispersion were acquired, and after contrast injection, LGE imaging was performed.

Results: In healthy subjects, simultaneous T_RAFF2 (80.8±7.4ms), T₂ (40.8±0.8ms), and SL dispersion (63.9±11.1ms²) were obtained with overall high visual map quality and a good depiction of the myocardium (Fig 2). Across healthy subjects, the CoV was 15±9, 11±6, and 31±21% for T_RAFF2, T₂, and SL dispersion, respectively. A representative case of a patient with MI is shown in Fig 3. Scar tissue is visible in the inferior segment of the LGE, T_RAFF2, and SL dispersion maps, while in native T₁ and T₂ no cardiac abnormality is seen.

Conclusion: Approximate myocardial SL dispersion mapping was obtained with good image quality and consistent repeatability across healthy subjects at 3T. Initial patient data indicates clinical feasibility and sensitivity to scar tissue, as identified by LGE. T₂ and T_RAFF2 are sensitive to different macromolecular interactions, showing differential contrast in disease. SL dispersion may be able to exploit this and allow for isolating contrast components, e.g., chemical exchange effects. These may allow for complementary and comprehensive clinical sensitivity to pathological remodeling, including potential differentiation between acute and chronic myocardial injury. Future clinical evaluation in a larger patient cohort is warranted.