Lindale Jacob R, Smith Loren L, Mammen Mathew W, Eriksson Shannon L, Everhart Lucas M, Warren Warren S
Department of Chemistry, Duke University, Durham, NC 27708.
Department of Physics, Duke University, Durham, NC 27708.
Proc Natl Acad Sci U S A. 2024 Apr 2;121(14):e2400066121. doi: 10.1073/pnas.2400066121. Epub 2024 Mar 27.
The inherently low signal-to-noise ratio of NMR and MRI is now being addressed by hyperpolarization methods. For example, iridium-based catalysts that reversibly bind both parahydrogen and ligands in solution can hyperpolarize protons (SABRE) or heteronuclei (X-SABRE) on a wide variety of ligands, using a complex interplay of spin dynamics and chemical exchange processes, with common signal enhancements between 10 and 10. This does not approach obvious theoretical limits, and further enhancement would be valuable in many applications (such as imaging mM concentration species in vivo). Most SABRE/X-SABRE implementations require far lower fields (μT-mT) than standard magnetic resonance (>1T), and this gives an additional degree of freedom: the ability to fully modulate fields in three dimensions. However, this has been underexplored because the standard simplifying theoretical assumptions in magnetic resonance need to be revisited. Here, we take a different approach, an evolutionary strategy algorithm for numerical optimization, multi-axis computer-aided heteronuclear transfer enhancement for SABRE (MACHETE-SABRE). We find nonintuitive but highly efficient multiaxial pulse sequences which experimentally can produce a sevenfold improvement in polarization over continuous excitation. This approach optimizes polarization differently than traditional methods, thus gaining extra efficiency.
核磁共振（NMR）和磁共振成像（MRI）固有的低信噪比问题目前正通过超极化方法来解决。例如，基于铱的催化剂能够在溶液中可逆地结合仲氢和配体，利用自旋动力学和化学交换过程的复杂相互作用，使多种配体上的质子（SABRE）或异核（X-SABRE）实现超极化，信号通常增强10到10倍。这尚未达到明显的理论极限，进一步增强在许多应用中（如体内mM浓度物质的成像）将很有价值。大多数SABRE/X-SABRE实现所需的场强（μT - mT）远低于标准磁共振（>1T），这提供了一个额外的自由度：能够在三维空间中完全调制场强。然而，这一点尚未得到充分探索，因为磁共振中标准的简化理论假设需要重新审视。在这里，我们采用了一种不同的方法，即用于数值优化的进化策略算法、用于SABRE的多轴计算机辅助异核转移增强（MACHETE-SABRE）。我们发现了非直观但高效的多轴脉冲序列，通过实验，其极化效果比连续激发提高了七倍。这种方法与传统方法优化极化的方式不同，从而获得了额外的效率。
