Serial, Multimodality Imaging Plays Important Role for Ischemic Stroke

The ability to properly image the hemodynamics of stroke across the continuum from acute stroke to more chronic conditions such as intracranial atherosclerosis now exists and has demonstrated the importance of blood flow, according to a review article [Intervent Neurol. 2013;2:105-117].

During the past few years, it was recognized that cerebral ischemia is a dynamic process. Blood flow states evolve over prolonged time periods in individuals; therefore, every patient is different in terms of surviving ischemia. Imaging data at >1 time point may provide information regarding the underlying pathophysiology, and the use of serial imaging via such modalities as computed tomography (CT), magnetic resonance imaging (MRI), and angiography, and can illustrate how much ischemic injury has occurred. The effects of attempted therapeutic interventions can also be evaluated and the degree of reperfusion, if any, measured.

However, defining different time phases among individual patients is difficult. Time is generally used to mark the onset of symptoms, but the time when symptoms begin is almost never the same as the time of vascular occlusion. The use of imaging may allow identification of patients requiring early and aggressive treatment.

The amount of collateral flow and its capacity to compensate for hypoperfusion determines the time from vascular occlusion to symptom onset, the nature of symptoms, and when intervention may be necessary. As flow diminishes, even when the patient has been treated rapidly, reperfusion may not provide satisfactory results and may even have a negligible clinical impact. Therefore, addressing time alone may not be sufficient for achieving definitive reperfusion and good clinical outcomes.

Serial imaging allows the time curves of an individual patient to be differentiated while discerning collateral recruitment and subsequent fluctuations. This allows the relationship between changes in blood flow and tissue fate in a particular region to be seen, ranging from infarction to hemorrhage, or neither. Many clinicians today order serial imaging at baseline, after intervention, and again later in the course of the patient’s hospital stay.

Thrombolysis
Clinicians commonly try to extend the time window of thrombolysis, and it is now known that baseline imaging can predict response, as recent study results have suggested that time alone may not guarantee a favorable outcome. Patients with better collateral flow may survive longer without extensive injury, and these patients are likely to experience enhanced effects of reperfusion. Again, imaging may be important at very early stages to identify and possibly exclude cases that are likely to have poor flow and a poor response to revascularization.

Serial Imaging and Scoring
ASPECTS [Alberta Stroke Program Early CT Score] is a systematic approach that can be useful in grading severity of ischemia on CT/MRI, for monitoring serial changes, and to evaluate the impact of recanalization and reperfusion. Serial CT/MRI or perfusion imaging may chronicle the dynamics of territorial perfusion from acute to chronic phases after stroke, and may provide new insight on the dynamics of reperfusion and hemorrhagic transformation.

The TICI [Thrombolysis in Cerebral Infarction] score on angiography may be used to measure the amount of reperfusion in downstream tissue. But definitive or optimal reperfusion needs to be defined based on associated clinical outcomes. Although trials have attempted to define reperfusion, an optimal degree of TICI reperfusion has not necessarily been identified as of yet.

Using the techniques described, blood flow today can be examined from baseline imaging and through different combinations of therapy between neuroprotection and revascularization. Furthermore, what happens after revascularization and during the subsequent hospital stay can be accurately measured. Therefore, imaging plays a large role in the future of ischemic stroke treatment and clarifying the importance of flow.

Intracranial Atherosclerosis (ICAS)
As with acute ischemic stroke, there is homeostasis of flow between limited forward flow as measured by the TICI scale and retrograde collateral flow. Studies such as WASID [Warfarin-Aspirin Asymptomatic Intracranial Disease] and SAMMPRIS [Stenting and Aggressive Medical Management for Preventing Recurrent Stroke] showed that outcomes are strongly predicted based on collateral flow, beyond the degree of luminal stenosis. Advanced imaging techniques such as computational fluid dynamics can be useful in providing valuable information on hemodynamics and the impact of a stenosis or ICAS lesions.

Overall, the balance of antegrade flow and collateral perfusion may affect the likelihood of successful reperfusion, of infarction or ischemic evolution, the probability of hemorrhagic transformation, and most importantly, subsequent clinical outcome. The authors concluded that the use of multimodal imaging techniques may be able to reveal more about ischemic stroke and allow clinicians to better care for patients in both acute and chronic stages across different disorders with thromboembolic and atherosclerotic mechanisms.—Mary Mihalovic