Purpose: Temporal evolution of the water apparent diffusion coefficients (ADC) parallel (ADC∥) and perpendicular (ADC⊥) to the human white matter tract following ischemia has not been investigated systematically. We attempted to quantify the evolution of ADC∥ and ADC⊥ and examine whether it can be interpreted by a model of ischemic edema.
Methods: We retrospectively selected 53 patients with ischemic lesions involving the posterior limb of the internal capsule (PLIC) and placed regions of interest in the right and left PLIC on ADC maps. We performed regression analysis of lesion-to-contralateral ratios of ADC∥ and ADC⊥ against the time (t=1-1600 h) from onset. We then fitted the estimated time courses of ADC∥ and ADC⊥ obtained from the analysis to a model of nerve tissue composed of cylinders (axons) and spheres corresponding to isotropic structures, particularly focal cytoplasmic swellings of glial cells and axons seen in ischemic white matter.
Results: The evolution of ADC⊥ and ADC∥ differed. The estimated time course of ADC∥ in μm²•ms^-1 was 0.64+0.88 exp (-0.24t) for 1<t<54 h and 0.00059t+0.61 for t≥54 h (contralateral normal value, 1.52). That of ADC⊥ was 0.19-0.063 exp (-0.24t) for 1<t<54 h and 0.00040t+0.17 for t≥54 h (normal value 0.22). The model fitted to these values showed that the volume of the cylinders decreased, that of the spheres increased, and extracellular volume changed little from one hour to approximately one day after stroke onset.
Conclusion: In the human PLIC, ADC∥ continued to decrease from one hour to a few days after stroke onset, and ADC⊥ tended to increase. The temporal evolution could be interpreted by progression of the focal cytoplasmic swelling of glial cells and axons previously observed in animal studies.