High-b-value diffusion-weighted MR imaging of hyperacute ischemic stroke at 1.5T.

Background and Purpose: Diffusion-weighted (DW) imaging at b = 2000 s/mm(2) offers theoretical advantages over DW imaging at b = 1000 s/mm(2) for detection of hyperacute ischemic stroke. The purpose of this study was to determine whether b = 2000 images are better than b = 1000 images for detecting and estimating the extent of diffusion change within 6 hours after stroke onset.
Methods: We compared DW images obtained with a b value of 1000 s/mm(2) (TR/TE/NEX, 7500/71/1) with those obtained with a b value of 2000 s/mm(2) (TR/TE/NEX, 7500/83/2) in 94 patients examined within 6 hours of clinically suspicious hyperacute ischemic stroke (57 men, 37 women; mean age +/- SD, 62 years +/- 8; age range, 47-80 years; mean time interval +/- SD, 206 +/- 90 min). Three observers performed qualitative analysis of DW images and reached a consensus about lesion conspicuity, lesion extent, and image artifact. In the quantitative analysis of 34 patients with lesions in the territory of the middle cerebral artery, the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and volume of ischemic lesion were measured and findings of the b = 1000 and b = 2000 images were compared.
Results: The sensitivity and specificity of b = 1000 and b = 2000 images were calculated as 94% (80/85)/ 100% (9/9) and 98% (83/85)/ 100% (9/9), respectively, relative to the presence or absence of infarction on the follow-up T2-weighted images. In three patients, hyperintense lesions were depicted only on b = 2000 images. On qualitative analysis, lesions were more conspicuous and larger on b = 2000 images in 23 and 11 patients, respectively. On quantitative analysis, as the b value increased, the mean lesion volume increased by 47% (22.1 +/- 27.9 mL at b = 1000 s/mm(2) versus 32.5 +/- 36.5 mL at b = 2000 s/mm(2), P < .001, n = 34). As the b value increased, mean SNR decreased both in the lesion and in the contralateral normal area by 17% and 28%, respectively, but the mean CNR increased by 23% (8.7 +/- 6.4 at b = 1000 s/mm(2) versus 10.7 +/- 6.5 at b = 2000 s/mm(2), P < .001, n = 34).
Conclusion: DW images acquired with a b value of 2000 s/mm(2) were better than DW images acquired with a b value of 1000 s/mm(2) for the detection and estimation of the extent of diffusion change in patients examined within 6 hours of ischemic stroke onset.