Magnetic Resonance Diffusion-Weighted Imaging: Sensitivity and Apparent Diffusion Constant in Stroke

Summary

Magnetic resonance diffusion-weighted imaging (MR-DWI) is sensitive to the diffusibility of water and may offer characterization and anatomical localization of stroke leading to early tailored therapeutic intervention. We compared DWI, the apparent diffusion constant (ADC), and autoradiographic cerebral blood flow (CBF) in a model of focal cerebral ischemia in the rat.

Sprague-Dawley rats were embolized with a single silicone cylinder injected into the internal carotid artery. Both common carotids were permanently ligated. The animals were anesthetized (isoflurane in O₂), and paralyzed (gallamine). MR-DWI were obtained with a GE 4.7 T magnet (TE = 3 s, TR = 80 msec, b = 2393 · l(h³ mm²/ s, slice thickness 3 mm). DWI and CBF autoradiograms were compared visually. ADC was assessed in various regions, including ischemic cortex and a region homologous to ischemic cortex. Imaging times from stroke onset were 50 ± 6 min (mean ± SEM) for DWI, 185 ± 17 min for a second DWI. CBFwas determined at 258 ± 15 min.

The specificity was 100% at both 50 min and 185 min, indicating that there were no false positives; in 3 animals ischemia was not present. However, the sensitivity analysis indicated that early DWI yields some false negatives: at 50 min the sensitivity was 60%. We attribute our result of low early sensitivity to small infarcts in relation to the slice thickness. Later, at 185 min, sensitivity was 100%. The first ADCs were higher than the second ADC values in ischemic cortex.

For infarcts larger than the slice thickness, early MR-DWI is highly sensitive for imaging evolving ischemia. Unlike other imaging methodologies (e.g. T₂ or CT Scan), MR-DWI will contribute to the diagnosis and treatment of early stages of cerebral ischemia.