MRI contrast for molecular and cellular imaging of the brain

人们对开发分子成像方法越来越感兴趣，这些方法使传统的放射成像技术能够获得有关正常和患病组织中发生的分子和细胞过程的广泛信息。 一系列信息被认为很重要，例如监测细胞迁移的能力、能够对基因表达进行成像的报告基因的开发、对受体成像的稳健策略的开发以及可用于检测特定酶的存在或监测离子状态变化的环境敏感剂的开发。 这项工作的长期目标是开发策略，使 MRI 对比对广泛的分子和细胞过程敏感。 这项工作建立在 30 多年的工作基础上，我们展示了第一个用于检测基因表达的 MRI 策略、第一个用于监测钙流入替代物的 MRI 方法、第一个用于执行神经元轨迹追踪的 MRI 方法，以及第一个用于监测体内单细胞迁移的 MRI 方法。 这些都代表了任何能够测量这些过程的放射成像技术的初步报告。 这些技术广泛应用于多种疾病的临床前模型成像。 过去一年，我们在所有具体目标上都取得了进展。 目标 1：开发基于氧化铁的造影剂，用于标记内源性神经干细胞的迁移并对其进行成像。 在过去的几年中，我们已经展示了微米级氧化铁颗粒用于特定细胞 MRI 的独特优势。 可以检测单个细胞，并且实际上可以检测单个细胞内的单个颗粒。 细胞迁移 MRI 的主要范例是离体标记细胞并监测移植到动物体内后的迁移。检测单个粒子的能力使得标记策略效率低下。特别是，在过去的几年里，我们已经证明，将粒子注射到大鼠脑室中可以使粒子被脑室下区的神经前体细胞吸收，并且 MRI 可以监测细胞向嗅球的迁移。 发表的一篇论文测量了单侧鼻阻塞和恢复过程中新神经元迁移的变化。在鼻阻塞和恢复过程中，球解剖结构和细胞迁移之间存在微妙的耦合。鼻孔闭塞后球的重新生长需要新的神经元。 我们现在正在研究鼻孔阻塞后引入特定气味是否会改变细胞迁移到球部的模式。在一个受研究这些内源性新神经元启发的新项目中，我们发现皮质和中脑前体细胞可以在成人脑脊液中生长，形成完全整合且外观正常的脑组织。在过去的一年里，我们已经证明，如果将这种新组织放置在靠近纹状体的脑脊液中，这种新组织可以整合到运动通路中，并且当放置在靠近头端迁移流到球茎的脑脊液中时，可以整合到嗅觉通路中。 第二个主要项目对整个大脑进行成像，以研究免疫大脑相互作用。我们成功地在小鼠模型中检测到病毒感染期间 T 细胞在单细胞水平上迁移到大脑中。在许多情况下，T 细胞积聚与小出血有关，这开启了 T 细胞浸润脑实质是否与血脑屏障破坏相关的问题，即使涉及的细胞很少。 目标 2：应用微加工技术来制造可能对 MRI 对比有价值的独特金属结构。 常用于 MRI 的氧化铁颗粒是非常有效的造影剂，能够检测单微米大小的颗粒。 然而，由于颗粒的本体相制造，它们不是很均匀，并且它们不包含很高含量的金属。 该问题的解决方案是使用现代微加工技术来制造基于金属的微米级造影剂。 在过去的几年里，我们已经证明，双环形、圆柱体和椭圆体结构为区分颗粒提供了独特的优势，并且这些结构可以转变为 pH 传感器。我们已经证明，用于细胞示踪的简单微加工镀金铁盘既可用于新神经元成像，也可用于免疫细胞浸润。最后，我们在 NIST 的合作者 Gary Zabow（前研究员）开发了制造此类 MRI 造影剂的新方法，并正在 NIH 进行测试。在过去的一年里，我们与 M. Barbic 合作，证明了磁热材料具有独特的特性，有可能对细胞跟踪的 MRI 研究非常有用。 目标 3：开发新的传递机制以扩展锰增强 MRI 的适用性。 在过去的十年中，我们已经证明了锰离子在 MRI 对比方面的显着效用。 锰离子通过配体或电压门控钙通道进入细胞，因此可用作 MRI 试剂来监测钙流入。 一旦进入神经元内部，锰将沿顺行方向移动并跨功能突触，使神经元网络能够通过 MRI 进行成像。 最后，系统地给予锰可以提供有关啮齿动物大脑的细胞结构信息。 这些成功使我们对拓宽将锰离子输送到细胞的方式感兴趣。锰增强 MRI 的一个主要限制是所需的浓度。这限制了翻译在人类成像中的使用。 在过去的几年里，我们通过两种方式解决了这个问题，我们发表了一项初步研究，表明锰正电子发射同位素将使 PET 能够获得与锰增强 MRI 类似的信息，包括神经追踪和组织的功能激活。已经开始讨论决定是否将其转化为人类研究。 我们与 Daniel Reich 合作完成了第一项研究，以测试 FDA 批准的释放锰的药物是否可用于疾病检测。第一篇关于普通志愿者的手稿出版。针对多发性硬化症患者和癫痫患者的研究正在进行中。请注意，所有人类研究都是根据我们合作者的协议进行的，这就是本工作报告的相关人类使用数据的地方。 最后，我们与 Dorian McGavern 合作，继续探索利用颅骨复杂的血管/骨髓系统在不破坏颅骨的情况下向大脑添加 Mn2 的新方法。 目标 4：制定策略，使细胞过程能够改变 MRI 造影剂的弛豫度。 我们继续探索对 Aim 2 下产生的微加工粒子进行调制的方法。 在过去的几年里，我们完成了一项研究，证明微加工颗粒可以制成传感器。在过去的一年里，我们一直在寻找方法，使这种非常有趣的传感器的制造更加稳健。此外，我们还开发了一种新型的 Gd 螯合物聚合物，它具有高弛豫率但分子量相对较低。它已附加到一类经典神经示踪剂（CTB 和葡聚糖）上，以实现新的 MRI 神经示踪剂。 一种新颖的微型无线 MRI 探测器已经开发出来，它在可能植入 MRI 探测器的情况下显示出很大的应用前景。 已经展示了两种工程解决方案。在过去的一年里，我们与 NCI 的一个小组合作，证明了这种方法在体内 EPR 实验中的有用性。与 Chittiboina 博士一起研究介入性垂体成像线圈的工作正在进行中，如果成功，我们将使该线圈无线化以帮助外科手术。

There continues to be increasing interest in developing molecular imaging approaches that enable traditional radiological imaging techniques to obtain a wide range of information about molecular and cellular processes that occur in normal and diseased tissue. A range of information is considered important such as the ability to monitor cell migration, the development of reporters that enable imaging of gene expression, the development of robust strategies to image receptors, and the development of environmentally sensitive agents that can be used to detect the presence of specific enzymes or monitor changes in ion status. The long term goals of this work are to develop strategies that enable MRI contrast that is sensitive to a wide range of molecular and cellular processes. This work builds on over 30 years of work where we have demonstrated the first MRI strategy for detecting gene expression, the first MRI approach for monitoring a surrogate of calcium influx, the first MRI approach for performing neuronal track tracing, and the first MRI approach for monitoring the migration of single cells in vivo. These all represented initial reports by any radiological imaging technique which enabled these processes to be measured. These techniques are finding widespread application to imaging pre-clinical models of a broad range of diseases. Over the past year we have made progress in all of the specific aims. Aim 1: Develop iron oxide based contrast for labeling and imaging the migration of endogenous neural stem cells. Over the past few years we have demonstrated the unique advantages of micron sized iron oxide particles for MRI of specific cells. Single cells can be detected and indeed, single particles within single cells can be detected. The main paradigm for MRI of cell migration is to label cells ex vivo and monitor migration after transplantation into an animal. The ability to detect a single particle enables inefficient labeling strategies. In particular, over the past few years we have demonstrated that injection of particles into the ventricles of the rat brain enables particles to be taken up by neural precursors in the subventricular zone and MRI can monitor the migration of cells to the olfactory bulb. A paper got published that measured the changes in migration of new neurons during unilateral nasal occlusion and recovery There was exquisite coupling between bulb anatomy and cell migration both during nasal block and recovery. New neurons were required for bulb re-growth after naris occlusion. We are now studying whether introduction of specific odors after naris oclusion alters the pattern of migration of the cells into the bulb. In a new project inspired by studying these endogenous new neurons we have shown that cortical and mid-brain precursor cells can be grown in the adult CSF to form fully integrated and normally appearing brain tissue. Over the past year we hav shown that this new tissue can integrate into the motor pathway if plasced in CSF near tehstriatum and can integrate into the olfactory pathway when placed in CSF neat the rostral migratory stream to the bulb. A second major project images the entire brain to study immune brain interactions. We have succeeded in detecting T cell migration into the brain at single cell level during virus infection in mouse models. Intreting in many cases T cell accumulation is associated with small bleeding opening the issue of whether T cell infiltration into brain parenchyma is associated with BBB breakdown even when very few cells are involved. Aim 2: Apply microfabrication techniques to manufacture unique metal structures that may be valuable for MRI contrast. Iron oxide particles commonly used for MRI are very potent contrast agents enabling detection of single micron sized particles. However, due to bulk phase manufacture of particles they are not very uniform and they do not contain very high content of metal. A solution to this problem is to use modern microfabrication techniques to manufacture metal based, micron sized contrast agents. Over the past few years we have shown that double doughnut, cylinders, and ellipsoid structures offer unique advantages for distinguishing particles and that these structures can be turned into sensors for pH. We have demonstrated that simple microfabricated gold coated iron discs for cell tracing can be used both for imaging new neurons as well as immune cell infiltration. Finally, our collaborator at NIST, Gary Zabow (former fellow)has developed novel ways to manufacture this class of MRI contrast that are being tested here at NIH. Over the past year, in collabortation with M. Barbic, we have demonstrated that magnetocaloric materials have unique properties that have potential to be very useful for MRI studies of cell tracking. Aim 3: Develop novel delivery mechanisms to extend the applicability of manganese enhanced MRI. Over the past ten years we have demonstrated the remarkable utility of the manganese ion for MRI contrast. Manganese ion enters cells on ligand or voltage gated calcium channels and so can be used as an MRI agent to monitor calcium influx. Once inside of neurons, manganese will move in an anterograde direction and cross functional synapses enabling neuronal networks to be imaged with MRI. Finally, manganese given systemically gives cytoarchitectural information about the rodent brain. These successes have us interested in broadening the ways in which manganese ion can be delivered to cells. A major limitation of manganese enhanced MRI are the concentrations required. This limits translation for use in human imaging. Over the past couple of years we have tackled this problem in two ways, we have published an initial study that demonstrates that manganese positron emitting isotopes will enable PET to obtain similar information that can be obtained with manganese enhanced MRI, including neural tracing and functional activation of tissue. Discussion have begun to decide if to translate this to human studies. We completed our first study in collaboration with Daniel Reich to test if an FDA approved agent that releases Mn might be useful for disease detection. The first manuscript on normal volunteers was published. Studies on MS patients and patients with epilepsy are underway. Please note all human studies are done under the protocols of our collaborators and that is where the relevant human use data is reported for this work. Finally, in collaboration with Dorian McGavern we continue pursuing novel approaches to add Mn2+ to the brain without disruption of the skull using the intricate vessel/marrow system of the skull. Aim 4: Develop strategies that enable cellular processes to alter the relaxivity of MRI contrast agents. We continue to explore ways in which the microfabricated particles produced under Aim 2 can be modulated. Over the past few years we have completed a study that demonstrates that the microfabricated particles can be made into a sensors. Over the past year we have sought ways to make the manufacture of this very interesting class of sensors more robust. In addition, we have developed a novel polymer of Gd chelates that has high relaxivity but relatively low molecular weight. This has been attached to a class of classical neural tracers (CTB and dextrans) to enable new MRI neural tracers. A novel, miniature wireless MRI detector that shows much promise for use where it may be possible to implant an MRI detector has been developed. Two engineering solutions have been demonstrated. Over the past year we have collaborated with a group at NCI to demonstrate the usefulness of this approach for in vivo EPR experiment. Work on an interventional Pituritary imaging coil with Dr. Chittiboina is proceeding and if succesful, we will make this coil wireless to help in surgical procedures.