Diffusion-Weighted Imaging Part I: Does a Positive DWI Guarantee Permanent Damage?
Abstracts & Commentary
Sources: Li F, et al. Transient and permanent resolution of ischemic lesions on diffusion-weighted imaging after brief periods of focal ischemia in rats: Correlation with histopathology. Stroke 2000;31:946-954; Kidwell CS, et al. Thrombolytic reversal of acute human cerebral ischemic injury shown by diffusion/perfusion magnetic resonance imaging. Ann Neurol 2000;47:462-469.
Data from animal experiments and observations in humans suggest that diffusion-weighted imaging (DWI) positivity is synonymous with irreversible neuronal energy failure. According to this view, a DWI lesion would inevitably go on to manifest as stroke in all cases. The important work by Li and colleagues and Kidwell and associates suggests that this may not be the case. As these studies suggest, further understanding of pathology at the tissue level is provided by perfusion-weighted imaging (PWI). Using bolus gadolinium enhancement, PWI measures the total volume of tissue being starved of blood flow and therefore at risk for infarction. Mismatch between the DWI and PWI allows measurement of the "tissue at stake," identifying tissue potentially salvageable with thrombolytic therapy. It is possible that in well-selected patients with such a mismatch there may be therapeutic benefit from aggressive treatment long after the conventional time frame for IV or intra-arterial thrombolysis. The ticking of the "tissue clock" may be more important than the "time clock," as clinicians make often-hurried decisions about these potentially risky therapies.
Li et al subjected 16 rats to 10 or 30 minutes of focal cerebral ischemia followed by reperfusion. The animals were imaged with DWI/PWI at regular intervals over the next 72 hours. Histopathological study followed. During occlusion, both groups showed abnormal DWI and PWI in the territory at risk. Following occlusion, during reperfusion, DWI and PWI normalized in both groups. However, at 12-hour follow-up, while the 10-minute group remained normal, new secondary abnormalities on DWI (as well at T2-weighted images) reappeared in the 30-minute group. Histopathological damage was demonstrated in both groups. This damage included the 10-minute ischemia group despite these animals being fully normal by imaging. Though less in degree, this damage suggests that reversibility of DWI lesions does not necessarily indicate a normal histological outcome.
Kidwell et al report similar observations in human cases of acute ischemic stroke. They demonstrate that with successful thrombolysis and recanalization of the occluded artery, DWI abnormalities may be reversed (mean DWI lesion volume at baseline was 23 cc and decreased to 10 cc after thrombolysis). However, in three of six patients at day 7, a secondary increase in DWI lesion volume was observed. PWI was performed in four patients, all showing complete resolution of the perfusion deficit with thrombolysis. —azs & ayeesha kamal, md (Dr. Kamal is Chief Resident of Neurology at New York Presbyterian Hospital.)