After someone has had a stroke or tia can you tell from an mri or ct scan? also how long after one of these do post symptoms occur?

Variable, see below: Transient ischemic attack is brief focal neurological event, lasting less than 20 minutes, with usually full return to baseline. Stroke permanent neurological deficit, which may improve a bit over weeks to months. Tia often not seen on either scan. First few hrs, cat may miss stroke, but MRI software will likely pickup quite early.

Not typically: Sen on any scan...Symptoms by definition last less than 24 hours.

Yes and no: A definite stroke or cerebral infarction is visible on brain MRI right away as changes on the dwi sequence will be seen. A TIA can occur and by definition no true brain infarction occurs so in those studies will be normal. The diagnosis of TIA is based on history of a sudden neurologic event that completely resolves within 24 hours. Reversible ischemic neurologic deficit is another term.