Brain stroke belongs to the most common causes of human disability and death, and though we know a lot of its mechanisms, it still needs to be more effectively diagnosed, treated and prevented. The standard method of brain infarct diagnosis is magnetic resonance imaging (MRI), which detects magnetization answer of particles (typically hydrogen nuclei) in tissues. The standard MRI method applied in brain stroke diagnosis is diffusion weighted imaging (DWI), which shows changes as early as 11 minutes after ischemia. However, analytical steps from ‘what is being observed’ to understanding what truly lies beneath, should be undertaken to establish a proper and prompt diagnosis and implement a personalized treatment strategy. The cellular swelling mechanism leading to reduction of extracellular space and hence the restriction of diffusion is adopted as a sort of paradigm explaining water diffusion restriction, and the enhancement of DWI signal in the early phase of brain ischemia. According to widely accepted theory, stating that the early increase in DWI signal is caused by cellular (‘cytotoxic’) edema, the pathological reality is much more complicated – different, small changes of magnetic signal and numerous ‘background’ factors are often overlooked or ignored. A better understanding of the mechanisms underlying the changes in DWI is crucial to elucidate what is really observed. Starting with the basic MRI physical principles, we consider some factors like: changes of pH, temperature, insufficiency of energy, changes of activity of Na+/K+-ATPase, role of aquaporins and glucose transporters, and glutamate excitotoxicity, which contribute to the ‘diffusion’ observed in DWI.

Keywords: Diffusion weighted imaging; MRI; Brain stroke; Water restriction; Cytotoxic edema; Einstein-smoluchowski equation