Introduction

The target for most therapeutic interventions for focal ischemia should be ischemic tissue that can respond to treatment and is not irreversibly injured. Such tissue will be defined as potentially salvageable ischemic tissue and must be distinguished from non-salvageable ischemic tissue that has evolved to a status at which recovery is no longer possible. Characterization of potentially reversible vs. irreversible ischemic tissue is based on the ischemic penumbra hypothesis. Ideally, before any aggressive therapeutic approach (i.e., thrombolysis) is undertaken, four important questions concerning the individual stroke situation should be addressed using only one optimal diagnostic imaging procedure:

1. Does the patient have acute cerebral ischemia or is another underlying pathology responsible for the stroke symptoms (e.g., intracerebral hemorrhage, tumor)?

2. Is there already an area of irreversibly damaged ischemic tissue and what is the size of this infarct core?

3. Is there a tissue of risk (“penumbra”) that can be preserved from damage by therapeutic intervention, and what is the size of this area?

4. Is the vessel that is responsible for the ischemia still occluded or has there been a pontaneous recanalization?

The ideal imaging modality will be able to address all of these questions within an acceptable amount of time before a specific treatment is begun.

Diffusion-weighted imaging

Since the description of early findings in acute experimental ischemic stroke with diffusion-weighted imaging (DWI), it has been predicted that this technique might become an important tool for the identification of very early ischemic injury in patients.