Diffusion and Perfusion MR Imaging of Acute Ischemic Stroke
Section snippets
Stroke pathophysiology
There are a variety of processes acting over different time periods that contribute to cell damage and death after the onset of ischemia and are often referred to as the ischemic cascade. Briefly, the occluded arterial vessel prevents blood flow, thereby limiting the delivery of oxygen and nutrients and causing excessive accumulation of metabolic by-products. In ischemic tissue, anaerobic metabolism replaces aerobic metabolism, producing lactate and tissue acidosis. Once energy stores are
Ischemic tissues descriptions
With advances in neuroimaging, tissue fate and tissue properties can be predicted; however, improved descriptions and interpretations are still desired, as well as definitive definitions that are not modality dependent or observer subjective. The following tissue descriptions are now commonly applied and used in clinical descriptions of ischemic stroke, although before being able to characterize tissue physiology with imaging, they had limited use. Perfusion and diffusion imaging provide the
Cerebral perfusion
There are varying degrees of ischemia depending on the part of the brain affected and the metabolic, functional, or morphologic definition of ischemia in use. Often with imaging, CBF indicating the volume of blood (in mL) passing through a mass of tissue (typically 100 g) over time (min) is used to describe and quantify cerebral perfusion.
The CBF in gray and white matter is approximately 65 mL min−1 (100 g)−1 and 20 mL min−1 (100 g)−1, respectively, according to positron emission tomography
Thrombolysis
Until 1995, there was no specialized treatment for ischemic stroke. In 1995, the National Institute of Neurologic Disease and Stroke (NINDS) tPA Stroke Trial10 demonstrated that thrombolysis with tissue plasminogen activator (tPA), when administered intravenously (IV) within 3 hours of stroke onset was effective in treating ischemic stroke. tPA is an endogenous enzyme that degrades blood clots; for stroke treatment, it is administered to degrade the thrombus that is occluding a cerebral artery,
Theory
The theoretical and technical aspects of diffusion MR imaging are explained in recent reviews.23, 24 Briefly, Brownian motion describes the random motion of particles in a fluid that occurs due to their thermal energy. In biologic tissues, diffusion becomes restricted by boundaries (such as membranes and organelles) and the diffusion distance decreases. Biologic tissues are complex because many of these boundaries are semi-permeable. The result is that in vivo diffusion is more limited than the
Theory
Perfusion imaging depicts blood flow at the microvascular level in the parenchyma. In stroke, perfusion-weighted imaging (PWI) is useful to identify tissue with reduced cerebral blood flow, thus it can theoretically be used to define the ischemic tissue exactly, including benign oligemia, the ischemic penumbra and the infarct. The most common MR technique to examine cerebral perfusion is dynamic susceptibility contrast (DSC) perfusion imaging. Arterial spin labeling is another perfusion
Mismatch Hypothesis
One of the most widely used concepts when using advanced MR techniques for acute ischemic stroke is the perfusion-diffusion mismatch theory (Figs. 1 and 5). The principle behind this hypothesis is that MR DWI and PWI can help identify a “tissue window” for thrombolysis rather than relying solely on time from onset to guide treatment. This hypothesis relies on information gained from both perfusion and diffusion imaging to predict which tissue has irreversible damage and which tissue is
Mild stroke and transient ischemic attacks
Diffusion and perfusion MR imaging may also be highly useful in the diagnosis and treatment options of mild strokes and transient ischemic attacks (TIAs). By classical definitions of TIA, it was assumed that there was no permanent cerebral damage; however, advances in neuroimaging, specifically diffusion imaging, have shown this to be incomplete or invalid (see Figs. 6 and 7). Approximately one third of patients with strokes/TIA that are too mild for thrombolytic treatment are dependent or dead
Stroke etiology
Determination of the etiology of the event is central to the management of stroke and TIA patients. Diffusion imaging improves stroke subtype diagnosis and the accuracy improves further when DWI is combined with MR angiography.86 Stroke subtype is important, as it will alter the long-term care of the patient, including secondary stroke prevention, as well as assigning prognosis, identification of patients with increased risk of neurologic worsening, recurrent stroke, and medical complications.
Summary
Diffusion and perfusion MR imaging have proven to be highly useful in the clinical description and understanding of hyperacute ischemic stroke. In this article, we have given a brief overview of the basic concepts of diffusion and perfusion images and described some of the current challenges and limitations of these techniques as applied to acute cerebral ischemia. Diffusion imaging is becoming a clinical standard for the identification of the acute stroke lesion. Perfusion imaging is useful in
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A.D.H. received funding from the Natural Sciences and Engineering Research Council of Canada, the Informatics Circle of Research Excellence and Alberta Heritage Foundation for Medical Research (AHFMR). R.F. is a Canada Research Chair in Image Science and an AHFMR Senior Medical Scholar. Operational support for our stroke imaging program is provided by the Canadian Institutes for Health Research and the Heart and Stroke Foundation of Alberta.