Evolution of apparent diffusion coefficient, diffusion-weighted, and T2-weighted signal intensity of acute stroke

Abstract

Background and purpose: Serial study of such MR parameters as diffusion-weighted imaging (DWI), apparent diffusion coefficient (ADC), ADC with fluid-attenuated inversion recovery (ADC(FLAIR)), and T2-weighted imaging may provide information on the pathophysiological mechanisms of acute ischemic stroke. Our goals were to establish the natural evolution of MR signal intensity characteristics of acute ischemic lesions and to assess the potential of using specific MR parameters to estimate lesion age.

Methods: Five serial echo-planar DWI studies with and without an inversion recovery pulse were performed in 27 patients with acute stroke. The following lesion characteristics were studied: 1) conventional ADC (ADC(CONV)); 2) ADC(FLAIR); 3) DWI signal intensity (SI(DWI)); 4) T2-weighted signal intensity (SI(T2)), and 5) FLAIR signal intensity (SI(FLAIR)).

Results: The lesion ADC(CONV) gradually increased from low values during the first week to pseudonormal during the second week to supranormal thereafter. The lesion ADC(FLAIR) showed the same pattern of evolution but with lower absolute values. A low ADC value indicated, with good sensitivity (88%) and specificity (90%), that a lesion was less than 10 days old. All signal intensities remained high throughout follow-up. SI(DWI) showed no significant change during the first week but decreased thereafter. SI(T2) initially increased, decreased slightly during week 2, and again increased after 14 days. SI(FLAIR) showed the same initial increase as the SI(T2) but remained relatively stable thereafter.

Conclusion: Our findings further clarify the time course of stroke evolution on MR parameters and indicate that the ADC map may be useful for estimating lesion age. Application of an inversion recovery pulse results in lower, potentially more accurate, absolute ADC values.

fig 1.

MR images (6000/210/1) at five subsequent time points in a 67-year-old woman with left hand weakness, left facial droop, and slurred speech. This example shows the typical evolution of the SI_T2 (top row), SI_FLAIR (second row), SI_DWI (third row), ADC_CONV (fourth row), and ADC_FLAIR (bottom row) of an acute ischemic lesion (right hemisphere). On the ADC maps, the lesion is hypointense up to day 7 and hyperintense at 27 days, making it possible to differentiate the acute from the chronic lesion. On the DWI images, the lesion is hyperintense at all time points so that, by visual inspection, the signal intensity of the lesion on the acute scan cannot be differentiated from that on the chronic scan. On the T2-weighted and FLAIR images, lesion signal intensity increases up to day 4 and remains high thereafter. Note that at 27 days the lesion is more identifiable on the ADC_FLAIR map than on the ADC_CONV map

fig 2.

Average relative lesion ADC values and signal intensities with 95% confidence intervals during five time intervals after onset of stroke symptoms. A, rADC_CONV, rADC_FLAIR, and rSI_DWI. B, rSI_T2 and rSI_FLAIR (see Table 2 for the number of lesions included at each time interval).