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 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 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.

National Institute of Health (NIH)
National Institute on Aging (NIA)
Investigator-Initiated Intramural Research Projects (ZIA)
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National Institute on Aging
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Spencer, Richard G; Pleshko, Nancy (2013) How do statistical differences in matrix-sensitive magnetic resonance outcomes translate into clinical assignment rules? J Am Acad Orthop Surg 21:438-9
Ramaswamy, Sharan; Gurkan, Ilksen; Sharma, Blanka et al. (2008) Assessment of tissue repair in full thickness chondral defects in the rabbit using magnetic resonance imaging transverse relaxation measurements. J Biomed Mater Res B Appl Biomater 86B:375-80
Velan, S Sendhil; Said, Nicholas; Durst, Christopher et al. (2008) Distinct patterns of fat metabolism in skeletal muscle of normal-weight, overweight, and obese humans. Am J Physiol Regul Integr Comp Physiol 295:R1060-5
Ramaswamy, Sharan; Uluer, Mehmet C; Leen, Stephanie et al. (2008) Noninvasive assessment of glycosaminoglycan production in injectable tissue-engineered cartilage constructs using magnetic resonance imaging. Tissue Eng Part C Methods 14:243-9