Noninvasive Quantification of Cell Density in Three-Dimensional Gels by MRI

Objective: For tissue engineering, there is a need for quantitative methods to map cell density inside three-dimensional (3-D) bioreactors to assess tissue growth over time. The current cell mapping methods in 2-D cultures are based on optical microscopy. However, optical methods fail in 3-D due to increased opacity of the tissue. We present an approach for measuring the density of cells embedded in a hydrogel to generate quantitative maps of cell density in a living, 3-D tissue culture sample. Methods: Quantification of cell density was obtained by calibrating the $\boldsymbol {^1}$ H $\boldsymbol {T_2}$ , magnetization transfer (MT) and diffusion-weighted nuclear magnetic resonance (NMR) signals to samples of known cell density. Maps of cell density were generated by weighting NMR images by these parameters post-calibration. Results: The highest sensitivity weighting arose from MT experiments, which yielded a limit of detection (LOD) of ${ {\boldsymbol{2.5}} \times {\text{10}}^8}$ cells/mL/ $\sqrt{\hbox{Hz}}$ in a 400 MHz (9.4 T) magnet. Conclusion: This mapping technique provides a noninvasive means of visualizing cell growth within optically opaque bioreactors. Significance: We anticipate that such readouts of tissue culture growth will provide valuable feedback for controlled cell growth in bioreactors.