Which findings on diffusion-weighted imaging (DWI) suggest acute stroke?

Updated: Dec 19, 2018
  • Author: Souvik Sen, MD, MPH, MS, FAHA; Chief Editor: Helmi L Lutsep, MD  more...
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Answer

Answer

Cytotoxic edema appears following sodium/potassium pump failure, which results from energy metabolism failure due to ischemic insult; this occurs within minutes of the onset of ischemia and produces an increase in brain-tissue water of up to 3-5%. Reduction in intracellular and extracellular water molecule movement is the presumed explanation for the drop in ADC values.

The diffusion of water molecules is guarded by biologic barriers in the brain tissue (eg, cell membranes and cellular organelles). The behavior of water molecules is not symmetrical and may show uneven distribution of the ADC when measured in one direction; this uneven distribution may give a false impression of a lesion. ADC values are measured in several directions (3, 6, or more), and ADC maps are created to produce a direction-insensitive measurement of the diffusion. When the ADC is measured in 6 or more directions, the diffusion motion of all the water molecules (ie, ADC tensor matrix) can be calculated to create what is called full-diffusion tensor mapping, which can also be used to visualize white-matter tracts.

Reduction in the ADC also occurs in other conditions, such as global ischemia, hypoglycemia, and status epilepticus; it should always be evaluated in relation to the clinical condition of the patient.

Human studies have demonstrated that damage in the areas showing decreased ADC levels is very rarely reversible (in contrast to that in animal models), although a few studies have indicated that intra-arterial thrombolysis may occasionally result in the disappearance of the diffusion defect.

The technique most commonly used to acquire DWI is an ultrafast one, echo-planar imaging (EPI); this technique decreases scanning time significantly and eliminates movement artifacts.

The acute drop in ADC is gradually normalized to baseline at 5-10 days after ischemia (pseudonormalization); it even exceeds normal levels as time passes, helping in some cases to differentiate acute, subacute, and chronic lesions.

DWI is very sensitive and relatively specific in detecting acute ischemic stroke. DWI findings have shown high levels of diagnostic accuracy; however, studies have demonstrated that small brainstem lacunar infarctions may escape detection. Normal DWI in patients with strokelike symptoms should trigger further investigation for a nonischemic cause of the symptoms.

DWI has been shown to reveal diffusion abnormalities in almost 50% of patients with clinically defined transient ischemic attacks (TIAs); it tends to be of higher yield at increasing time intervals from the onset of stroke symptoms.


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