Department of Radiology, University of California San Diego, USA.
Contrast Media Mol Imaging. 2012 Jul-Aug;7(4):411-7. doi: 10.1002/cmmi.1467.
Iron oxide nanoparticles (IONPs) are widely used as MR contrast agents because of their strong magnetic properties and broad range of applications. The contrast induced by IONPs typically depends on concentration, water accessibility, particle size and heterogeneity of IONP distribution within the microenvironment. Although the latter could be a tool to assess local physiological effects at the molecular level, it renders IONP quantification from relaxation measurements challenging. This study aims to quantify IONP concentration using susceptibility measurements. In addition, further analysis of relaxation data is proposed to extract quantitative information about the IONP spatial distribution. Mesenchymal stem cells were labeled with IONPs and the IONP concentration measured by mass spectroscopy. MR relaxation parameters (T(1), T(2), T(2)) as well as magnetic susceptibility of cylindrical samples containing serial dilutions of mixtures of free and cell-internalized IONPs were measured and correlated with IONP concentration. Unlike relaxation data, magnetic susceptibility was independent of whether IONPs were free or internalized, making it an excellent candidate for IONP quantification. Using IONP concentration derived from mass spectroscopy and measured relaxation times, free and internalized IONP fractions were accurately calculated. Magnetic susceptibility was shown to be a robust technique to measure IONP concentration in this preliminary study. Novel imaging-based susceptibility mapping techniques could prove to be valuable tools to quantify IONP concentration directly by MRI, for samples of arbitrary shape. Combined with relaxation time mapping techniques, especially T(2) and T(2), this could be an efficient way to measure both IONP concentration and the internalized IONP fraction in vivo using MRI, to gain insight into tissue function and molecular imaging paradigms.
氧化铁纳米颗粒（IONPs）由于其强磁性和广泛的应用而被广泛用作磁共振对比剂。IONP 引起的对比通常取决于浓度、水可及性、粒径和 IONP 在微环境中的分布异质性。尽管后者可以作为评估分子水平局部生理效应的工具，但它使得从弛豫测量中定量 IONP 变得具有挑战性。本研究旨在使用磁化率测量来定量 IONP 浓度。此外，还提出了对弛豫数据的进一步分析，以提取关于 IONP 空间分布的定量信息。间充质干细胞用 IONP 标记，并通过质谱法测量 IONP 浓度。测量了含有游离和细胞内化 IONP 混合物的圆柱样品的磁共振弛豫参数（T(1)、T(2)、T(2)）和磁化率，并与 IONP 浓度相关联。与弛豫数据不同，磁化率与 IONP 是游离的还是内化的无关，使其成为 IONP 定量的理想候选物。使用从质谱法得出的 IONP 浓度和测量的弛豫时间，准确计算了游离和内化的 IONP 分数。磁化率是本初步研究中测量 IONP 浓度的稳健技术。基于成像的磁化率映射新技术可能成为通过 MRI 直接定量 IONP 浓度的有价值工具，适用于任意形状的样品。与弛豫时间映射技术相结合，特别是 T(2)和 T(2)，这可能是一种通过 MRI 测量体内 IONP 浓度和内化 IONP 分数的有效方法，从而深入了解组织功能和分子成像范例。
