The glycosaminoglycan (GAG) content of engineered cartilage is a determinant of biochemical and mechanical quality. The ability to measure the degree to which GAG content is maintained or increases in an implant is therefore of importance in cartilage repair procedures. The gadolinium exclusion magnetic resonance imaging (MRI) method for estimating matrix fixed charge density (FCD) is ideally suited to this. One promising approach to cartilage repair is use of seeded injectable hydrogels. Accordingly, we assess the reliability of measuring GAG content in such a system ex vivo using MRI. Samples of the photopolymerizable hydrogel, poly(ethylene oxide) diacrylate, were seeded with bovine chondrocytes (*2.4 million cells/sample). The FCD of the constructs was determined using MRI after 9, 16, 29, 36, 43, and 50 days of incubation. Values were correlated with the results of biochemical determination of GAG from the same samples. FCD and GAG were found to be statistically significantly correlated (R 2 = 0.91, p < 0.01). We conclude that MRI-derived FCD measurements of FCD in injectable hydrogels reflect tissue GAG content and that this methodology therefore has potential for in vivo monitoring of such constructs.

Labeling of cells with superparamagnetic iron oxide nanoparticles permits cell tracking by 1H MRI while 31P MRS allows non-invasive evaluation of cellular bioenergetics. We evaluated the compatibility of these two techniques by obtaining 31P NMR spectra of iron-labeled and unlabeled immobilized C2C12 myoblast cells in vitro. Broadened but usable 31P spectra were obtained, and peak area ratios of resonances corresponding to intracellular metabolites showed no significant differences between labeled and unlabeled cell populations. We conclude that 31P NMR spectra can be obtained from cells labeled with sufficient iron to permit visualization by 1H imaging protocols and that these spectra have sufficient quality to be used in assessing metabolic status. This result introduces the possibility of using localized 31P MRS to evaluate the viability of iron-labeled therapeutic cells as well as surrounding host tissue in vivo.