Magnetic Resonance Analysis of Connective Tissue and Muscle

在一个特定的项目中，我们试图利用磁共振成像（MRI）评估在临床应用的组织工程软骨系统中测量不断变化的硫酸氨基多糖（GAG）含量的可靠性。水凝胶样品，聚（环氧乙烷）二丙烯酸酯（PEODA）被用来包封牛软骨细胞（240万个细胞/样品）。采用MRI钆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天后对样品进行MRI实验。随后通过生化程序分析这些时间点的样本，以便将mri衍生的FCD测量结果与组织中真实的GAG含量相关联。对样品的组织学切片也进行了处理，以揭示GAG浓度的时间差异。我们发现FCD和GAG含量在36天内具有很强的相关性（R2 = 0.85）。然而，当延长到50天，相关性显著下降（R2 = 0.54）。由此，我们得出结论，在孵育的前36天，软骨细胞包封水凝胶结构的FCD与真正的GAG含量密切相关。该研究表明，mri衍生的FCD测量可以在可注射软骨组织工程系统的早期评估（5周）中可靠地解释。

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.