测量超极化 [1-13C]硫化的旋转-莱迪兹松弛磁场依赖性
Summary
提出了一种利用快速场循环松弛测定法，利用动态核极化方法超极化的13种C富集化合物自旋晶格松弛时间的磁场依赖性。具体来说，我们已经用[1-13C]丙酮进行了演示，但该协议可以扩展到其他超极化基板。
Abstract
超极化13C富集化合物在体内成像应用的基本限制是其有限的自旋晶格松弛时间。影响松弛速率的各种因素，如缓冲液成分、溶液pH、温度和磁场。在最后一方面，自旋晶格放松时间可以测量临床场强度，但在较低的领域，这些化合物从偏振器中分配并输送到MRI，松弛更快，难以测量。为了更好地了解运输过程中磁化损失的数量，我们采用快速场循环松弛测量法，在±0.75 T时对13C核进行磁共振检测，以测量超极化 [1-13C]丙酮的自旋晶格松弛时间。溶解动态核极化用于在浓度为80 mmol/L和生理pH（±7.8）下产生超极化的丙酮酸盐样品。这些溶液被迅速转移到快速场循环松弛计，以便使用校准的小翻转角度（3°-5°）测量样品磁化的松弛度作为时间的函数。为了绘制丙酮酸的C-1的T1色散图，我们记录了介于0.237 mT和0.705 T之间的不同松弛场的数据。利用这些信息，我们确定了一个经验方程，以估计上述磁场范围内超极化基板的自旋晶格松弛。这些结果可用于预测传输过程中磁化损失的数量，并改进实验设计，以尽量减少信号损耗。
Introduction
磁共振光谱成像（MRSI）可以生成光谱成像检测到的代谢物的空间图，但其实际应用往往受到其灵敏度相对较低的限制。这种体内磁共振成像和光谱方法的低灵敏度源于在体温下可实现的少量核磁化和合理的磁场强度。然而，通过使用动态核极化（DNP）来大大增强液体基质的体外磁化，从而利用MRSI1，2，这种基质被注射到体内代谢中去探索，从而克服这一限制。,3,4. DNP能够增强大多数核的磁化，具有非零核自旋，并用于提高13种C富集化合物（如丙酮5、6、碳酸氢碳酸盐）的体内MRSI敏感性7、8、富马9、乳酸10、谷氨酰胺11等，超过4级12级。其应用包括血管疾病成像 13，14，15，器官灌注13，16，17，18，癌症检测1，19，20，21，22，肿瘤分期23，24，并量化治疗反应2,6,23,24,25,26.
缓慢的旋转晶格放松对于MRSI体内检测至关重要。对于溶液中小分子中陀螺比低的原子核，旋转晶格松弛时间（T1s）在数十秒之间是可能的。几个物理因素影响核自旋过渡与其环境（晶格）之间的能量转移，导致松弛，包括磁场强度、温度和分子构象27。在分子中减少双极松弛，没有直接连接质子的碳位置，溶解介质的分解可以进一步减少分子间双极松弛。不幸的是，脱化溶剂在体内扩展放松的能力有限。由于化学移位各向异性，在高磁场强度下，碳基酸或碳化物酸（如丙酮酸）的松弛度会有所增强。极化后溶解过程中流体路径中存在的顺磁杂质会导致快速松弛，需要使用夹子避免或消除。
在低场中，13种含C化合物的松弛数据非常少，在低场中，自旋晶格的松弛速度会明显加快。然而，在低磁场测量T1，以了解在制备用于体内成像的制剂时的松弛，很重要，因为超极化造影剂通常是从靠近或位于地球的DNP仪器中分配的。领域。其他物理因素，如13C富集基板浓度、溶液pH、缓冲液和温度也会影响松弛，从而对制剂的配方产生影响。所有这些因素对于确定优化 DNP 溶解过程的关键参数以及计算样品从 DNP 设备到成像磁体过程中发生的信号损耗量至关重要。
核磁共振分散 （NMRD） 测量，即T1测量，作为磁场的函数，通常使用 NMR 光谱仪获得。为了获得这些测量，可以使用一种穿梭方法，当样品首先从光谱仪中穿梭出来，以在由其在磁体28、29、30边缘场中的位置决定的某个场中放松然后迅速移回 NMR 磁体以测量其剩余的磁化。通过在磁场中在同一点重复这个过程，但随着松弛时间的延长，可以得到一条松弛曲线，然后可以对其进行分析以估计T1。
我们使用一种称为快速场循环松弛测量31，32，33的替代技术来获取我们的NMRD数据。我们修改了商业场循环松弛计（见材料表），用于T1测量含有超极化13C核的溶液。与穿梭方法相比，场循环使该松弛计能够在较小的磁场范围（0.25 mT 至 1 T）上系统地获取 NMRD 数据。这是通过快速改变磁场本身，而不是磁场中的样品位置来实现的。因此，样品可以在高场强度下磁化，在较低的场强度下”放松”，然后在固定场（和Larmor频率）上采集自由感应衰减以最大化信号。这意味着样品温度可以在测量过程中控制，NMR 探头不需要在每个松弛场进行调谐，以便在整个磁场范围内自动采集。
本工作专注于在低磁场下分配和运输超极化溶液的影响，提出了使用快速测量超极化13C-硫化酸盐的自旋晶格松弛时间的详细方法磁场循环松弛测量，磁场范围为0.237 mT至0.705 T。使用这种方法的主要结果以前曾提出过[1-13 C]丙酸酯34和13C富集钠和碳酸氢钠35，其中其他因素，如基质浓度和溶解pH具有也进行了研究。
Protocol
Representative Results
Discussion
使用DNP增强信号采集是一种技术解决方案，解决13C核在有限浓度下产生的磁共振信号不足，就像动物注射中使用的一样，但也带来了其他实验挑战。图 7所示的每个松弛测量值都表示对独特制备样品的测量，因为它在溶解后不能重新极化以进行重新测量。由于样品和溶解介质在称重过程中样品制备的微小差异，或溶解过程本身的变化（如样品的不完全提取和彻底混合…
Disclosures
The authors have nothing to disclose.
Acknowledgements
作者要感谢安大略省癌症研究所、成像翻译项目和加拿大自然科学和工程研究理事会为这项研究提供资金。我们还要感谢与阿尔伯特·陈，GE医疗，加拿大多伦多，吉安尼费兰特，斯特拉尔，意大利，和威廉曼德，牛津仪器，英国有益的讨论。
Materials
| [1-13C]Pyruvic Acid | Sigma-Aldrich, St. Louis, MO, USA | 677175 | |
| 10mm NMR Tube | Norell, Inc., Morganton NC, USA | 1001-8 | |
| De-ionized water | |||
| Ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA) | Sigma-Aldrich, St. Louis, MO, USA | E5134 | |
| HyperSense Dynamic Nuclear Polarizer | Oxford Instruments, Abingdon, UK | Includes the following: "DNP-NMR Polarizer" software used to control and monitor the whole DNP polarizer; "RINMR" used to monitor the solid state polarization levels; "HyperTerminal" used to communicate the DNP software with the RINMR software that monitors the solid state polarization level. Also includes the MQC bench top spectrometer to monitor the liquid state polarization in conjunction with it own RINMR software | |
| MATLAB R2017b | MathWorks, Natick, MA | Include scripts for non-linear fitting of magnetization decay over time and T1 NMRD analysis of hyperpolarized pyruvic acid. | |
| OX063 Triarylmethyl radical | Oxford Instruments, Abingdon, UK | ||
| pH meter – SympHony | VWR International, Mississauga, ON., Canada | SB70P | |
| ProHance | Bracco Diagnostics Inc. | Gadoteridol, Gd-HP-DO3A | |
| Pure Ethanol (100% pure) | Commercial Alcohols, Toronto, ON, Canada | P016EAAN | |
| Shim Coil | Developed in-house | ||
| Sodium Chloride | Sigma-Aldrich, St. Louis, MO, USA | S7653 | |
| Sodium Hydroxide | Sigma-Aldrich, St. Louis, MO, USA | S8045 | |
| SpinMaster FFC2000 1T C/DC | Stelar s.r.l., Mede (PV) Italy | Includes the software "AcqNMR" that is used to set experimental parameters, monitor the tuning and matching of the RF coil, loading different pulse sequences, calibrate flip angle, data acquisition and curve fitting, among other functions. Also includes a depth gauge, some weights and a depth stopper. | |
| Trizma Pre-Set Crystals (pH 7.6) | Sigma-Aldrich, St. Louis, MO, USA | T7943 |
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