CT Perfusion Imaging to Predict Vasospasm in Subarachnoid Hemorrhage

Rupture of a brain aneurysm results in a type of bleeding into the brain called subarachnoid hemorrhage (SAH). This is a substantial cause of morbidity and mortality world-wide: even with the best possible care, up to half of patients die and many are left disabled. Sometimes this is because the immediate brain damage from the bleed is very severe. However, many patients who seem to be doing well at first go on to develop something called "vasospasm": narrowing of large arteries in the brain. This results in the brain not getting enough blood, and the patient can suffer a stroke as a result. The lack of blood to the brain is called delayed cerebral ischemia (DCI), and is the major secondary cause of stroke and death in patients who survived the initial aneurysm rupture.

Vasospasm can be seen on angiograms (blood vessel imaging) in about two-thirds of SAH patients, and causes neurological symptoms of DCI in half of those patients. It usually develops about a week or so after SAH. Early diagnosis and treatment of cerebral vasospasm and delayed cerebral ischemia (DCI) reduces morbidity and mortality in SAH patients.

CT perfusion imaging is able to identify areas of reduced cerebral blood flow that are at risk of DCI and stroke. This technique is able to measure cerebral blood flow and blood volume (CBF, CBV), and calculates two other measures of cerebral perfusion: time to peak intensity (TTP) and mean transit time (MTT). Taken together, these provide information about the amount of blood the brain is receiving and whether the brain is compensating for any reduction in flow. This is helpful because seeing vasospasm on imaging is not enough to predict brain ischemia and stroke; the additional physiological information provided by the perfusion assessment strengthens the diagnosis of DCI. Early evidence suggests that CT perfusion is a fast, accurate, inexpensive and non-invasive method of brain imaging to identify patients with DCI after SAH, and guide appropriate therapy.

If we were able to successfully predict which patients are at high risk of DCI and/or stroke, we could intervene early. This could improve patient outcomes, while potentially allowing better use of limited intensive care and nursing resources. Other studies have shown that patients with DCI had altered perfusion prior to developing clinical symptoms, but we do not know whether we can use CT perfusion to predict patients at risk, or which CT perfusion measurements (e.g., CBF or TTP) are most useful.

The radiation risk associated with CT perfusion imaging is negligible, due to advances in CT technology which allow for routine simultaneous CT perfusion and CT angiogram (CTA) without a significant increase in radiation dose over conventional head CT-CTA alone. Judicious use of CT perfusion, through a structured algorithm that includes routine perfusion scans at admission and during the peak risk period for vasospasm, may actually decrease cumulative radiation dose for patients with SAH, by ruling out perfusion deficit and brain ischemia as a cause of decreased neurological function and limiting repeated angiograms.