People often do not realize that they are having a stroke until it is too late. However, because they are already in the hospital, patients who suffer a stroke around the time of surgery usually have more time to receive thrombolysis, a procedure that can diagnose and eliminate blood clots that cause strokes. To determine whether patients would benefit from thrombolysis, CT and MRI techniques can be used.Dr. Pablo R. Castillo of the University of Minnesota in Minneapolis and his colleagues at Mayo Clinic in Jacksonville, Fla., reviewed brain imaging techniques that can diagnose and allow for treatment of strokes around the time of surgery.The reviewers wrote that noncontrast CT imaging is the best method for imaging strokes because it scans patients in only five minutes, is available in most hospitals, can accurately detect hemorrhaging and can examine patients who should not undergo MRI. Such patients include those with pacemakers, invasive monitoring devices or mechanical ventilators.Contrast multimodal CT can provide additional information. It can show the core -- an area of severely restricted blood flow -- and the ischemic penumbra, an area of mildly or moderately restricted blood flow. Much of the tissue within the ischemic penumbra can be saved with intervention.However, MRI can depict conditions that CT cannot, including pituitary hemorrhage, small abnormalities in white matter and early-stage cancer. Diffusion-weighted MRI can detect areas of restricted blood flow within minutes of the onset of a stroke. By combining data from diffusion-weighted and perfusion MRI, the ischemic penumbra can be identified. Magnetic resonance angiography can show large blood vessels well.The reviewers noted that improvements in MRI could make it more applicable for analyzing stroke patients. Ultrafast imaging sequences might provide the desired speed and accuracy, and open MRI may enable scanning of obese and claustrophobic patients. (Neurologic Clinics, November 2006, pp. 807-820).