Cs + ADC in rat brain decreases markedly at death

Spectroscopic resolution of intracellular and extracellular compartments can be used to probe the kinetic environment of those spaces and the compartment-specific changes that occur following injury. This is important for understanding the biophysical mechanisms that underlie the remarkable diffusion-weighted MRI contrast of injured central nervous system (CNS) tissue. Cesium-133 is a physiologic analog of potassium that is actively taken up by cells and resides primarily in the intracellular space. The ¹³³Cs⁺ signal can, thus, be exploited to probe the kinetic environment of the intracellular space. Two principal ¹³³Cs ⁺ resonances were observed at 11.74 T. These resonances arise separately from ¹³³Cs⁺ in brain and temporalis muscle. The apparent diffusion coefficient (ADC) of Cs⁺ in brain decreased from 1.0 ± 0.2 μm²/ms in healthy tissue to 0.24 ± 0.04 μm²/ms following global ischemia (average ADC ± average uncertainty), while there was no significant change in the ADC of Cs⁺ in temporalis muscle after injury. This finding under-scores the tissue-specific nature of the decrease in ADC that accompanies brain injury. Further, as the Cs⁺ ADC should reflect water ADC in the intracellular space, these results strongly support the hypothesis that the decrease in water ADC associated with CNS injury arises largely from kinetic changes taking place in the intracellular space.