Characterizing biophysical properties of the Central Nervous System tissue with quantitative Magnetic Resonance Imaging

磁共振成像(MRI)是研究组织基本特性的最通用的成像技术之一。由于磁共振信号主要是由水分子中的质子产生的，所以磁共振成像可以用来研究不同环境下不同类型组织中的水行为，从而能够表征这些组织的生物物理性质。由于核磁共振固有的非侵入性，这种研究可以在活体中进行，使核磁共振成为非侵入性研究基础生物学和生理学的最强大的技术之一。定量T2磁共振成像已成功地应用于人脑中枢神经系统(CNS)组织的定性研究。我的实验室已经开发了类似的技术来研究啮齿动物的脊髓组织。在目前的应用中，我们建议进一步开发和验证定量核磁共振技术，以表征啮齿动物中枢神经组织的生物物理特性。*拟议研究计划的长期目标是开发、验证和应用定量核磁共振技术，以研究动物中枢组织的生物物理特性。*拟议研究的短期目标是：*1.在活体内开发大鼠脊髓的3D MWI。*我们将把我们的2D压缩传感髓鞘水成像(CS MWI)技术扩展到三维。CS算法将利用数据在切片维度上的稀疏性，以及2D CS算法中使用的两个面内维度和时间维度。*2.研究啮齿动物中枢神经系统组织中MWI的基本特征和局限性*我们将研究髓鞘间(即髓鞘双层之间的间隙)和轴突内/外间室之间潜在的水分子交换是否对我们的MWI技术从大鼠脊髓获得的髓鞘水分数有任何显著影响。我们将建立大鼠脊髓切除后的MWF值与脊髓温度的依赖关系。我们还将通过改变固定时间和磷酸盐缓冲液浓度来研究组织固定对MWF和T2值的影响。最后，我们将研究髓鞘致密化对MWI的影响。我们将在PLP缺失转基因小鼠模型中进行MWI测量，其特征是髓鞘之间的间隙变宽和不规则：类似于髓鞘碎片的模式。我们将比较PLP缺失的脊髓与对照脊髓和有髓鞘碎片的脊髓的MWF值和T2分布。*3.表征功能性髓鞘和髓鞘碎片之间的差异。*我们将研究用电子显微镜(EM)测量的髓鞘双层的超微结构与MWI之间的相关性。具体地说，我们将比较功能性髓鞘碎片和髓鞘碎片中的T2分布，并研究它们如何与EM测量的形态特征相对应。*4.比较相MRI和MWI在表征啮齿动物中枢神经系统组织生物物理特性方面的差异*我们将比较相MRI和MWI在区分髓鞘碎片和功能性髓鞘方面的各自能力。由于时相MRI对髓鞘和轴突的存在都很敏感，它区分功能性髓鞘和髓鞘碎片的能力可能优于MWI技术。我们将通过一系列涉及正常和异常大鼠脊髓的体内和体外实验来验证这一点。*拟议研究的意义：*拟议计划将进一步扩大中枢神经系统组织的非侵入性研究领域的研究能力，并将有助于拓宽我们对动物中枢神经系统生物学和生理学的理解。该方案还将为在跨学科环境中培训研究生提供极好的机会。

Magnetic Resonance Imaging (MRI) is one of the most versatile imaging techniques for studying fundamental properties of tissue. Since MRI signal is predominantly generated by protons in the water molecules, MRI can be used to study the water behaviour in different environments in different types of tissue, thus allowing characterization of biophysical properties of these tissues. Thanks to MRI's intrinsically non-invasive character, such studies can be carried out in living organisms, making MRI one of the most powerful techniques to non-invasively study basic biology and physiology. Quantitative T2 MRI has been successfully applied to characterize Central Nervous System (CNS) tissue in human brain. My laboratory has developed similar technology to study rodent spinal cord tissue. In the current application we propose to further develop and validate quantitative MRI techniques to characterize biophysical properties of the rodent CNS tissue.*Long term objective of the proposed research programme is the development, validation and application of quantitative MRI techniques to study biophysical properties of CNS tissue in animals. *Short term objectives of the proposed research are:*1. To develop 3D MWI of rat spinal cord in vivo.*We will extend our 2D Compressed Sensing Myelin Water Imaging (CS MWI) technique to three dimensions. The CS algorithm will take advantage of the data sparsity in the slice dimension, in addition to the two in-plane dimensions and the time dimension used in our 2D CS algorithm. *2. To study fundamental characteristics and limitations of MWI in rodent CNS tissue*We will study whether potential exchange of water molecules between the myelin compartment (i.e. the space in between the myelin bi-layers) and the intra-/extra-axonal compartment has any significant effect on Myelin Water Fraction values acquired with our MWI technique from rat spinal cords. We will establish the dependence of MWF values in excised rat spinal cords on the cord temperature. We will also study the effect of tissue fixation on MWF and T2 values by varying the fixation time and phosphate buffer concentration. Finally, we will study the effect of myelin compaction on MWI. We will carry out MWI measurements in PLP-null transgenic mouse model characterized by widening and irregularities of the spaces in between the myelin sheaths: similar pattern to myelin debris. We will compare the MWF values and T2 distributions from the PLP-null cords to the ones obtained from the control cords and cords with myelin debris present.*3. To characterize differences between functional myelin and myelin debris.*We will study correlation between ultra-structure of myelin bi-layers measured with electron microscopy (EM) and MWI. Specifically we will compare T2 distributions in functional myelin vs. myelin debris and study how they correspond to morphological features measured with EM. *4. To compare phase MRI and MWI in characterizing biophysical properties of rodent CNS tissue*We will compare phase MRI and MWI in their respective abilities to distinguish between myelin debris and functional myelin. Since phase MRI is sensitive to presence of both myelin and axons, its ability to distinguish between functional myelin and myelin debris may be superior to that of the MWI technique. We will verify this through a number of in vivo and ex vivo experiments involving normal and abnormal rat spinal cords. *Significance of the proposed research:*The proposed program will further expand research capabilities in the area of non-invasive studies of CNS tissue and will help to broaden our understanding of animal CNS biology and physiology. This programme will also provide excellent opportunities for training graduate students in an interdisciplinary environment.