Magnetic Resonance Analysis of Connective Tissue and Muscle

在一个具体的项目中，我们试图评估使用磁共振成像(MRI)测量临床适用的组织工程软骨系统中进化的糖胺多聚糖(GAG)含量的可靠性。用聚环氧乙烷-二丙烯酸酯(PEoda)水凝胶样品包裹牛软骨细胞(240万个/样品)。用MRI-Gd排除法测定发育软骨的固定电荷密度(FCD)。在孵育9、16、29、36、43和50天后对标本进行MRI实验。这些时间点的样本随后通过生化程序进行分析，以便将MRI得出的FCD测量结果与组织中真实的GAG含量相关联。还对样本的组织切片进行了处理，以揭示GAG浓度的时间差异。我们发现，FCD和GAG含量之间存在很强的相关性(R2=0.85)，直到36天。然而，当延长到50d时，相关性显著下降(R2=0.54)。由此，我们得出结论，在培养的前36天，软骨细胞包裹的水凝胶结构的FCD与真实的GAG含量有很好的相关性。这项研究表明，MRI衍生的FCD测量可以可靠地解释可注射软骨组织工程系统的早期评估(5周)。 在进一步的工作中，我们试图确定细胞在组织工程构建中的分布。这很难通过非破坏性手段进行研究，例如植入后所需的手段。然而，用含铁颗粒标记细胞可能被证明是解决这个问题的有用方法，因为已经证明使用磁共振成像可以很容易地检测到这种标记细胞的区域。在这项研究中，我们使用FDA批准的超顺磁性氧化铁(SPIO)试剂Feridex与转染剂相结合，标记并可视化了两种不同组织工程结构中的MRI软骨细胞。建立了MRI和组织学确定的标记细胞位置之间的对应关系。主要软骨基质成分的表型、活性和产量不受SPIO标记过程的影响。我们相信这种可视化和跟踪软骨细胞的方法可能会在软骨组织工程疗法的进一步发展中发挥作用。 最后，MR也是对人体进行详细代谢研究的一种很好的方法。我们感兴趣的是体重、脂肪代谢和健康风险之间的联系。人们对此知之甚少，也很难研究。磁共振波谱(MRS)可以对脂类代谢进行非侵入性研究。我们扩展了现有的二维MRS技术，以允许量化受试者的肌内(IMCL)和肌外(EMCL)脂室及其不饱和程度，并将这些结果与BMI相关联。以肌酸(Cr)为标准，每组8名受试者的IMCL/Cr值与BMI(n=8)显著相关，分别为5.91.7(BMI&lt；25)、10.9 1.82(25&lt；BMI&lt；30)和13.1-0.87(BMI&gt；30)。同样，IMCL不饱和度随着BMI的增加而显著降低(p&lt；0.01)，分别为1.51±0.08、1.30±0.11和0.90±0.14。我们的结论是，二维MRS可以评估脂代谢的重要方面，并提出非侵入性测量的不饱和度可以作为肥胖相关脂代谢缺陷的生物标志物。

In one specific project, we sought to assess the reliability in measurement of evolving sulfated glycosaminoglycan (GAG) content in a clinically applicable tissue engineered cartilage system using magnetic resonance imaging (MRI). Samples of the hydrogel, poly(ethylene oxide) diacrylate (PEODA) were used to encapsulate bovine chondrocytes ( 2.4 million cells/ sample). The fixed charge density (FCD) of the developing cartilage was determined using the MRI gadolinium exclusion method. MRI experiments were performed on samples following 9, 16, 29, 36, 43 and 50 days of incubation. Samples from these timepoints were subsequently analyzed via biochemical procedures in order to correlate the MRI-derived FCD measurements with the true GAG content in the tissue. Histological sections of the samples were also processed to reveal temporal differences in the GAG concentration. We found a strong correlation (R2 = 0.85) between FCD and GAG content was determined up to 36 days. However, when extended to 50 days, the correlation decreased significantly (R2 = 0.54). From this, we conclude that FCD of chondrocyte-encapsulated hydrogel constructs correlate well with true GAG content during the first 36 days of incubation. This study demonstrates that MRI-derived FCD measurements can be reliably interpreted in the early stage evaluation ( 5 weeks) of injectable cartilage tissue engineering systems. In further work, we sought to define the distribution of cells within tissue engineered constructs. This is difficult to study through non-destructive means, such as would be required after implantation. However, cell labeling with iron-containing particles may prove to be a useful approach to this problem, since regions of such labeled cells have been shown to be readily detectable using magnetic resonance imaging. In this study, we used the FDA-approved superparamagnetic iron oxide (SPIO) agent, Feridex, in combination with transfection agents to label and visualize with MRI chondrocytes in two different tissue engineered constructs. Correspondence between labeled cell location as determined by MRI and by histology was established. The phenotype, viability and production of major cartilage matrix constituents were found to be unaffected by the SPIO-labeling process. We believe that this method of visualizing and tracking chondrocytes may be useful in the further development of cartilage tissue engineering therapeutics. Lastly, MR is also an excellent modality for detailed metabolic investigations in human subject. We were interested in the link between body weight, lipid metabolism, and health risks. This is poorly understood and difficult to study. Magnetic resonance spectroscopy (MRS) permits non-invasive investigation of lipid metabolism. We extended existing two-dimensional MRS techniques to permit quantification of intramyocellular (IMCL) and extramyocellular (EMCL) lipid compartments and their degree of unsaturation in human subjects, and correlated these results with BMI. Using muscle creatine (Cr) for normalization, a statistically significant (p < 0.01) increase in IMCL/Cr with BMI (n=8 subjects per group) was observed, with values of 5.9 1.7 (BMI < 25), 10.9 1.82 (25 < BMI< 30) and 13.1 0.87 (BMI > 30). Similarly, the degree of IMCL unsaturation decreased significantly (p < 0.01) with BMI, with respective values of 1.51 0.08, 1.30 0.11, and 0.90 0.14. We conclude that important aspects of lipid metabolism can be evaluated with 2-dimensional MRS and propose that degree of unsaturation measured noninvasively may serve as a biomarker for lipid metabolic defects associated with obesity.