Page 56 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis

Chapter 3
type has also been reported (16). The reliability of relaxometry measurements is further complicated by deviations from a mono exponential decay in signal intensity (11,16,19–23).
We further assume that the effects of biologically relevant variations of spatial distribution and compartmentalization of iron oxide particles may additionally affect the relationship between relaxation rates and Fe concentration. Specifically in dividing cells, both cells and intracellular SPIO content are subject to various evolution pathways. Cells may divide, sharing their SPIO content to daughter cells (24), or simply may translocate to nonlabeled regions, resulting in a mixture of labeled and nonlabeled cells. Some cells may die and disintegrate, transferring their content to the extracellular space, while may be taken up by macrophages or other surrounding cells. As these pathways may be differently represented, their effects on the SPIO compartmentalization and spatial distribution may result in different R2 and R2* values at the same SPIO concentration (25). Under these circumstances, relaxometry may lead to a misinterpretation of iron concentrations and hence to false conclusions on how tissue develops.
In our study, we address the effects of these evolution pathways by investigating the resulting change in transversal relaxation times. For this purpose, suspensions of labeled and unlabeled cells were used as a model to mimic cell translocation effects. We studied the effect of cell division by monitoring the evolution of labeled cells. Aggregation effects were studied by suspensions of SPIO, MPIO and SPIO-complexes. To create different spatial distribution of labeling SPIOs, we varied cell labeling and extracellular SPIO content and used different cell densities.
Materials and Methods
Cell culture and cell labeling
As a model system for dividing cells, Brown Norway 175 (BN175) sarcoma cells were used. Cells were grown in advanced RPMI medium (Invitrogen, Breda, The Netherlands) with 1% (v/v) penicillin– streptomycin (10 000 U penicillin ml-1 + 10 000 ug streptomycin ml-1; Cambrex, Verviers, Belgium) and 10% (v/v) fetal
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