Ultrafast magnetic resonance imaging: diffusion and perfusion

Echo-planar magnetic resonance imaging (MRI) can be used to measure apparent diffusion coefficients noninvasively in vivo, with scan times of 150 milliseconds or less, and to assess early ischemic effects in the feline experimental model, which has an occluded middle cerebral artery (MCA). The apparent diffusion coefficient in ischemic regions, which are identified later from vital staining, is significantly decreased from normal values within 1 hour after the MCA becomes occluded. A series of 10 echo-planar images that are progressively diffusion-weighted can be collected in 1 minute (effective TR of 6 seconds). Semilogarithmic plots of image intensity versus gradient strength factors (b values) were linear. Collecting sequential gradient echo-planar images during the passage of a bolus of contrast medium is also useful when assessing perfusion or vascular integrity before and during ischemic episodes. After intravenous injection of dysprosium-diethylene triamine penta-acetic acid-bis(methylamide), typical signal losses of 40% to 80% were observed and were dose-dependent. Areas of possible ischemia identified from diffusion-weighted images did not lose signal intensity with the use of contrast medium and were seen as regions of relative hyperintensity, clearly discernible from normally perfused tissues.