Quantitative diffusion-weighted MR imaging in transient ischemic attacks

Abstract

Background and purpose: The risk of stroke after a transient ischemic attack (TIA) is high. Appropriately directed therapies may reduce this risk. However, sensitive means of detecting the presence of subtle neuronal ischemia are lacking. We investigated the potential use of quantitative diffusion-weighted (DW) MR imaging in the detection of deficits produced by transient cerebral ischemia.

Methods: Twenty-eight patients who came to the stroke service from the emergency room of a tertiary teaching hospital with the final diagnosis of transient cerebral ischemia underwent conventional MR imaging, MR angiography, and DW MR imaging within 24 hours of presentation. Fifteen patients had normal conventional DW images confirmed by a staff neuroradiologist and neurologist. For these patients, absolute quantitative diffusion values were subsequently calculated for the clinically relevant brain region and were compared with the values calculated for the corresponding contralateral unaffected brain region. Thirteen patients had conventional DW images positive for lesions and were not studied.

Results: Quantitative DW imaging enabled detection of abnormal decreases (9-26%, P <.05) in the diffusion constant in brain regions suspected to be clinically involved by ischemia, when compared with the contralateral clinically unaffected brain tissue as well as with two other internal controls.

Conclusion: Quantitative DW imaging depicts diffusion deficit in patients with TIA. Quantitative DW imaging may have better sensitivity compared with conventional DW imaging in detecting transient cerebral ischemia.

Fig 1.

A, Quantitative D_av map shows ROIs (boxes) in the location of clinically suspected region of brain (right motor cortex) and contralateral control site. These two measurements were used to calculate the ratio m1. B, Quantitative D_av map shows regions used for measurements made on right and left thalami (boxes) for calculating the control ratio of c1. C, Mean value of the brain tissue diffusion constant (BD_av) and width of the distribution (σ) were determined from the distribution analysis map by using a three-compartmental model (see Methods). This value was used in determining the ratios m3 and c3. Data are shown as circles, and the fit is shown as a line.

Fig 2.

Four measured ratios (m1, m2, c1, c2) are shown for all 12 patients. TIA-affected regions are compared with corresponding contralateral brain regions (m1) and to thalamic controls (m2) not affected by TIA (solid symbols). The resultant ratios are consistently below unity and are around 0.82. However, when regions of brain not affected by the TIA are compared (c1, c2), no interhemisperic difference is noted and the ratios approach unity (open symbols).

Fig 3.

Patient with left hand and arm weakness (patient 8). A and B, DW image (A) and T2-weighted image (B) of an axial section through the motor cortex do not show any abnormality. C, Quantitative D_av map shows a dark region (arrow) in the right motor cortex.