Stroke affects over 125,000 people each year in the UK and leaves at least 50% disabled. Treatment of stroke caused by a blockage in a blood vessel (ischaemic stroke), with clotbusting drugs improves the chances of good recovery, but must be given within 4.5 hours of onset. Currently only a small proportion of patients who arrive in hospital within 4.5 hours are treated. This is largely due to uncertainty about diagnosis and concerns about risk of bleeding associated with clotbusting medication. Patients with mild or improving symptoms in particular are often not treated because of uncertainty about relative risks and benefits. However, around one third of these patients go on to be significantly disabled. Routine CT scanning often does not show abnormalities in acute stroke (which take hours to become easily visible), and cannot show the extent or severity of blood flow changes in ischemic stroke.

We wish to investigate the value of additional CT scanning that gives information on the blood vessels (angiography, CTA) and blood flow to the brain (perfusion, CTP) by undertaking a randomised trial. Extra scans are done in the same scanner and involve some extra radiation, injections of a contrast dye, and some extra time to acquire process and interpret. The extra scans may allow better treatment decisions for patients by increasing diagnostic certainty and by better assessment of stroke severity. However, we do not know whether the potential gains from better selection justify the resources and potential treatment delays that are involved. We will investigate whether the proportion of patients given clotbusting drugs differs between the two scanning protocols; and whether the outcomes differ, using standard measures of disability. We will also investigate whether use of different scanner manufacturers' software affect interpretation of scans.

Inclusion Criteria:

• Clinical diagnosis of stroke eligible for IV rtPA according to current guidelines
• Informed consent
• Male or nonpregnant female ≥18 years of age
• Within 4.5 hours of onset as defined by time since last known well

Exclusion Criteria:

• Contraindications to thrombolytic drug treatment for stroke
• Pregnancy
• Known impaired renal function precluding contrast CT
• Known allergy to CT contrast agents
• Severe concurrent medical condition that would prevent participation in study procedures (e.g. cardiac failure with severe pulmonary oedema) or with life expectancy ≤ 3 months.