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Ultrafast magnetic resonance imaging: diffusion and perfusion
M E Moseley1, R Sevick, M F Wendland
1Department of Radiology, University of California, San Francisco 94143.
Abstract:
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.
Insights
Rapid echo-planar magnetic resonance imaging (MRI) detects early stroke effects by measuring decreased apparent diffusion coefficients within 1 hour of middle cerebral artery occlusion in cats. Contrast-enhanced imaging further identifies ischemic areas.
Area of Science:
- Neuroimaging
- Medical Physics
- Experimental Neurology
Background:
- Echo-planar MRI enables rapid, noninvasive in vivo measurement of apparent diffusion coefficients.
- Early detection of ischemic effects is crucial for timely intervention in stroke.
- The feline middle cerebral artery occlusion model mimics human ischemic stroke.
Purpose of the Study:
- To evaluate the utility of echo-planar MRI for assessing early ischemic changes.
- To investigate the temporal dynamics of apparent diffusion coefficient changes post-MCA occlusion.
- To assess the role of contrast-enhanced MRI in characterizing ischemic regions.
Main Methods:
- Utilized rapid echo-planar MRI with diffusion-weighting (b values) to measure apparent diffusion coefficients.
- Induced middle cerebral artery occlusion in a feline model.
- Administered intravenous contrast agent (dysprosium-DTPA-BMA) to assess perfusion and vascular integrity.
- Correlated MRI findings with vital staining for ischemic region identification.
Main Results:
- Apparent diffusion coefficients significantly decreased in ischemic regions within 1 hour of MCA occlusion.
- Diffusion-weighted images showed decreased signal intensity in early ischemic areas.
- Contrast-enhanced MRI revealed relative hyperintensity in ischemic regions, indicating preserved vascular integrity or lack of contrast extravasation.
- Signal loss from contrast was dose-dependent (40-80%).
Conclusions:
- Echo-planar MRI is a rapid and effective tool for detecting early ischemic changes in vivo.
- Diffusion-weighted imaging can identify acute stroke within an hour of middle cerebral artery occlusion.
- Contrast-enhanced MRI complements diffusion imaging by highlighting ischemic areas as regions of relative hyperintensity.