Diffusion-weighted MR imaging in the acute phase of transient ischemic attacks

Abstract

Background and purpose: Radiologic assessment of acute transient ischemic attacks (TIAs) has been handicapped by the low sensitivity of CT and conventional MR imaging for acute small-vessel infarction and the difficulty in differentiating between acute and chronic lesions by use of these methods. Our purpose was to evaluate the incidence of TIA-related infarction by using diffusion-weighted MR imaging to determine whether the presence of a diffusion imaging abnormality correlates with the duration of symptoms or cause of TIA.

Methods: We prospectively studied 58 consecutive patients with acute TIA by use of diffusion-weighted imaging. All MR imaging was performed with a 1.5-T whole-body system with 24-mT/m gradient strength and an echo-planar-capable receiver. All patients were imaged within 10 days of stroke onset.

Results: Thirty-nine patients (67%) manifested a diffusion imaging abnormality consistent with acute ischemia. Cortical lesions were identified in 54% of these patients; most of them associated with other acute ischemic lesions. Subcortical lesions were identified in 46%; most of them were isolated from other lesions. The mean duration of symptoms in patients with no TIA-related diffusion imaging abnormalities was 0.96 hours (median, 0.33 hours) compared with a mean of 6.85 hours (median, 1.53 hours) in patients with diffusion imaging abnormalities (P =.025, Mann-Whitney U test). This significant correlation between the duration of TIA symptoms and the presence of TIA-related abnormalities was lost when we excluded from the analysis patients whose symptoms lasted longer than 6 hours (P =.513, Mann-Whitney U test). No significant correlation was observed between the size of TIA-related lesions and the duration of symptoms or cause of TIA.

Conclusion: Two thirds of our TIA patients showed focal abnormalities indicative of acute ischemic lesions on diffusion-weighted images. This incidence is higher than that previously reported in the literature. The presence of such abnormalities increased with increasing total symptom duration, but this relation was not observed when only patients whose symptoms lasted less than 6 hours were considered. No significant correlation was observed between the cause and presence of TIA-related lesions on diffusion-weighted MR images. These TIA-related lesions are probably irreversible and may lead to subsequent infarct.

Fig 1.

Diffusion-weighted images obtained in a 75-year-old man with transient left paresis. A, Axial diffusion-weighted image depicts hyperintense signal indicative of acute infarction in the precentral gyrus of the right frontal lobe. B and C, Additional subsidiary lesions in the same arterial territory (middle cerebral artery) are shown at different levels (parietal and temporal cortex).

Fig 2.

Axial MR images obtained in a 40-year-old man with transient right hemiparesis 5 days after onset of symptoms. A–C, T2-weighted (A), fast-FLAIR (B), and diffusion-weighted (C) imaging all reveal a small, acute ischemic lesion in the thalamocapsular region equally well. Note absence of prior ischemic lesions in A and B that helped identify the TIA-related infarct.

Fig 3.

Plot of the relationship between TIA symptom duration and presence of TIA-related lesions on diffusion-weighted images. A significant statistical correlation existed between symptom duration and presence of TIA-related lesions (P = .025); the significance was lost when only patients with symptoms lasting less than 6 hours were included in the analysis (P = .513).

Fig 4.

Axial MR images obtained 4 hours after onset of symptoms in a 65-year-old woman with sudden-onset left-sided pure motor hemiparesis lasting 6 hours. A and B, Fast-FLAIR (A) does not show the clinically appropriate small lesion in the right insular cortex, which is well visualized on a diffusion-weighted (B) image.

Fig 5.

Axial MR images obtained in a 75-year-old man with transient left hemiparesis. A, Fast-FLAIR T2-weighted MR image shows multiple focal lesions in the subcortical white matter. It is not possible to discern whether any of the lesions are acute. B, Diffusion-weighted MR section obtained at the same level as A depicts a single focus of high signal intensity in the left centrum semiovale suggestive of acute infarction.

Fig 6.

Axial MR images obtained 3 days after symptom onset in a 75-year-old man with left hemiparesis that rapidly resolved within 4 hours of onset. A and B, Small cortical ischemic lesions in the right middle frontal gyrus are better visualized on a diffusion-weighted image than on a T2-weighted image. C, A subsequent infart was overlooked on this follow-up T2-weighted MR image obtained 4 months after A and B. D, A follow-up fast-FLAIR MR image obtained at the same time as C helps identify a small lesion at the same location as the prior ischemic lesions, indicating a chronic infarct.

Fig 7.

Axial MR images obtained in a 65-year-old man with aphasia that rapidly resolved within 3 hours of onset. A, A diffusion-weighted image obtained 2 days after symptom onset clearly depicts a small, acute cortical ischemic lesion in the opercula of the left frontal lobe. B, A follow-up fast-FLAIR MR image obtained 3 months later shows a small cortical lesion with high signal intensity at the same location as the lesion in A, indicating a chronic infarct. This subsequent infarct might have been overlooked had the reader not been aware of the location of the lesion on the initial diffusion-weighted image.

Fig 8.

Plot of the relationship between TIA cause and the presence of multiple TIA-related lesions on diffusion-weighted images. Although the small number of patients in some groups impeded correlation analysis, patients with large-vessel disease and significant stenosis showed more multiple lesions. Note.—SV indicates small-vessel disease; LV w SS, large-vessel disease with significant stenosis; LV wo SS, large-vessel disease without significant stenosis; CE, cardioembolism; and Unk/MTO, unknown or more than one mechanism.