The complexity of biologic tissues, with multiple compartments each with its own diffusion and relaxation properties, requires complex formalisms to model water signal in most magnetic resonance imaging or magnetic resonance spectroscopy experiments. In this article, we describe a magnetic susceptibilityinduced shift in the resonance frequency of extracellular water by the introduction of a gadolinium contrast agent to medium perfusing a hollow fiber bioreactor.

Diffusion-weighted MRI is commonly used in the diagnosis and evaluation of ischemic stroke because of the rapid decrease observed in the apparent diffusion coefficient (ADC) of tissue water following ischemia. Although this observation has been clinically useful for many years, the biophysical mechanisms underlying the reduction of tissue ADC are still unknown. To help elucidate these mechanisms, we have employed a novel three-dimensional (3D) hollow-fiber bioreactor (HFBR) perfused cell culture system that enables cells to be grown to high density and studied via MRI and MRS. Since the discovery of ischemia-induced decreases in diffusion coefficients measured in brain tissue, diffusionweighted (DW) MRI has been successfully used to diagnose and evaluate acute clinical stroke (1,2). The apparent diffusion coefficient (ADC) of tissue water drops significantly following cerebral ischemia, enabling easy identification of ischemic tissue in DW images. Although this observation has been clinically useful over the last several years, there is not a comprehensive understanding of the biophysical mechanisms underlying the reduction of tissue ADC. Many conditions could effectively slow down the movement of water in ischemic tissue and result in a measured drop in the ADC. These include: ischemia-induced cellular swelling, which results in an increase in the intracellular volume fraction (IVF) and increases in the tortuosity of the extracellular space; decreases in temperature; decreases in membrane permeability; and decreases in the intrinsic energy-dependent motion of intracellular water (i.e., cytosolic streaming) (Ref. 3 and references therein). A number of theoretical models have been developed that attempt to take many relevant physiological parameters into account while modeling the results of DWMRI experiments, including: cell shape and size; unique intrinsic diffusion coefficients and T 2 relaxation times of the water in the intracellular and extracellular spaces; membrane permeability; and shape-dependent tortuosity factors of the extracellular space (4 -9).To effectively evaluate these models and deepen our understanding of how each of these parameters affect experimentally measured DWMRI results, it would be very useful to have experimental determination of as many of the modeling parameters as possible. In particular, the diffusion coefficient of water in the intracellular and extracellular space has been the focus of a significant amount of research. Because intracellular and extracellular water are typically not resolved spectroscopically, multiexponential analysis of signal decay curves from the total water signal has been used to assign diffusion properties of the intra-and extracellular spaces in cell cultures (10) and in living tissue (11,12). Contrast agents that differentially affect relaxation times in the intra-and extracellular spaces have also been used in conjunction with diffusion experiments to differentiate diffusion in the intra-and extracellular spaces (13-15). Finally, many surrogate marker...

This work reports results of experiments in hollow-fiber bioreactor C6 glioma cell cultures where the apparent diffusion coefficient (ADC) of intracellular water (iADC) was measured at diffusion times between 0.83 and 40 ms. The experiments were carried out before and after the onset of permanent ischemia. The changes in iADC following ischemia were dependent on the diffusion time employed in the experiment. An ischemia-induced decrease in the iADC was measured at short diffusion times, while at long diffusion times the iADC increased. Decreases in the iADC measured at short diffusion times are interpreted to be a result of a decrease in the intrinsic diffusivity of intracellular water due to energy failure. Increases in iADC measured at long diffusion times, are interpreted to result from cell swelling.