Diffusion tensor imaging may help the determination of time at onset in cerebral ischaemia

The apparent diffusion coefficient (ADC) and anisotropy (eg, fractional anisotropy (FA)) of ischaemic tissue evolve over time. A reduction in diffusivity (ie, lambda(2) and lambda(3)) is an important marker for characterising hyperacute-stage infarction, as these parameters may reflect axonal membrane status. The study examines whether transverse diffusivity could be useful in assessing white matter infarcts of various ages.

Diffusion tensor imaging data from 44 adult patients (34 men, 10 women, aged 46 to 89 years, mean = 70.3) with acute white matter infarction (1-168 h) of the internal capsule were analysed. Relative eigenvalues were calculated as: (lambda(ipsi)-lambda(contra))/lambda(contra). Lesions were classified based on theoretically expected evolution of diffusivity over time as follows: stage I, FA higher than the contralesional region of interest (ROI); stage II, diffusivity lower than the contralesional side for all eigenvalues; stage III, one of two transverse eigenvalues (lambda(2) or lambda(3)) higher than the contralesional ROI.

Stage I infarcts (n = 5) were found primarily within 24 h of the onset of symptoms, with one case found on the third day. Stage II infarcts were found most commonly within 24 h (n = 18), and fewer after 24 h. After the first day, the ratio of stage III infarcts increased significantly. Thus, diffusivity-based classification of white matter infarcts seems to show a chronological trend.

Diffusion anisotropy may be useful for defining the biological tissue clock of white matter infarctions.