Role of Diffusion and Perfusion MRI in Selecting Patients for Reperfusion Therapies
Section snippets
Diffusion MRI
In a normal state random diffusion of water molecules occurs across cell membranes. Tissue damage and ischemia begin to occur at a cerebral blood flow value of approximately 30 mL per 100 g per minute. Severe ischemia of brain tissue induces the rapid dysfunction of cellular metabolism and ion exchange pumps responsible for adenosine triphosphate (ATP) metabolism. This dysfunction causes a massive shift of water from the extracellular to the intracellular compartment, leading to cytotoxic edema
Perfusion MRI
Bolus tracking perfusion imaging using a paramagnetic contrast agent (eg, gadolinium) is the most commonly used MRI technique for assessing cerebral perfusion. The delay in gadolinium arrival to the vascular bed and the intensity of the signal change induced by its passage within capillaries can be used to generate a time/concentration curve. Various maps and parameters can be generated from these data, including cerebral blood flow (CBF), cerebral blood volume (CBV), and time-based factors
Evolution of mismatch definitions and concepts
The initial MRI concept of an acute ischemic brain lesion involved a central core outlined by DWI surrounded by a single circumferential region of hypoperfusion defined by PWI. However, recent studies have shown that this is rarely present in patients with untreated acute stroke imaged within 3 to 6 hours after symptom onset. In most of the patients enrolled in DEFUSE, more than 50% of the acute DWI volume did not have a superimposed PWI lesion, suggesting that partial reperfusion had occurred
DEFUSE
In this trial, the investigators used open-label intravenous rt-PA to treat 74 patients with ischemic stroke who presented between 3 and 6 hours after the onset of symptoms.29 MRI was performed immediately before treatment and then repeated 3 to 6 hours after treatment and again at 3 months. Patients were not selected based on the MRI findings, but the primary hypothesis postulated that prespecified MRI profiles would identify patients with a robust clinical response (National Institutes of
Future directions
Ongoing studies are investigating the use of automated MRI software to identify patients who benefit from various reperfusion strategies. Two of these studies involve endovascular recanalization techniques: MR and Recanalization of Stroke Clots Using Embolectomy (MR RESCUE) uses an algorithm that takes multiple DWI and PWI parameters into account to identify a penumbral pattern before randomization to treatment with one of the Merci clot retrieval devices (Concentric Medical, Mountain View, CA,
Summary
Results of several recent trials support the role of acute MRI for guiding emergency management of stroke. Patients with a malignant MRI pattern may be poor candidates for reperfusion therapies, yet may benefit from hemicraniectomy. Post hoc analyses of recent studies suggest that PWI techniques that use a threshold to exclude benign oligemia from penumbra, and DWI techniques that use ADC thresholds to exclude reversible DWI lesions to distinguish the ischemic core form penumbra, seem to
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