Using MRI to detect stroke after cardiac surgery

Stroke and brain damage can occur as a result of coronary artery bypass grafting, a procedure commonly performed worldwide. Dr. Megan C. Leary and Dr. Louis R. Caplan of Beth Israel Deaconess Medical Center in Boston have reviewed the efficacy of MRI for detecting stroke immediately after this operation. They state that neuroimaging is necessary both for confirming diagnosis and for distinguishing ischemic stroke from hemorrhagic stroke.

They note that, although for years noncontrast CT has been the standard procedure for initial evaluation of stroke, more information is provided by multimodal MRI using diffusion- and perfusion-weighted imaging and gradient-recalled echo. They imply that CT likely is preferred because it is perceived as a more rapid imaging method; however, abbreviated multimodal MRI protocols for stroke can be performed in less than five minutes, and a comprehensive protocol can take only 10 to 15 minutes.

Because MRI involves magnetic fields, patients with metal medical devices have not been considered good candidates for the procedure. Interestingly, the reviewers note that a current European clinical trial is re-evaluating whether patients with pacemakers can undergo MRI.

The authors cite numerous studies demonstrating the effectiveness of diffusion-weighted MRI for detecting stroke immediately after cardiac surgery. They state that the technique yields the most reliable estimate of the area of the tissue most damaged by stroke, or the ischemic core, and that it can identify damaged tissue that might be missed by other imaging methods. Importantly, they report that it provides a clear prognosis and that it could be useful for evaluating neuroprotective agents.

The authors note that perfusion-weighted MRI also can detect ischemia reliably; however, it needs further evaluation to determine its effectiveness for detecting ischemia after cardiac surgery in particular. Gradient-recalled echo is useful for identifying intracerebral hemorrhage and microscopic bleeding, they said.

The reviewers concluded that multimodal MRI is crucial for evaluating stroke directly after cardiac surgery. (Nature Clinical Practice Cardiovascular Medicine, July 2007, pp. 379-388.)