While TIA has historically been defined as lasting less than 24 hours, recent data clearly demonstrates that ischemic attack lasting longer than one hour is often associated with actual brain infarction. Most TIA's last less than 5 minutes. Evidence of acute infarction can be identified by MRI in up to 50% of patients who meet the 24 hour criteria for TIA. Diffusion MRI in patients with transient ischemic attacks. Kidwell CS; Alger JR; Di Salle F; Starkman S; Villablanca P; Bentson J; Saver JL. Stroke 1999, Jun;30(6):1174-80. Transient ischemic attack--proposal for a new definition. Albers GW; Caplan LR; Easton JD; Fayad PB; Mohr JP; Saver JL; Sherman DG. New England Journal of Medicine 2002, Nov 21;347(21):1713-6.