Acute cerebral perfusion imaging should be part of the routine evaluation of patients with acute ischemic stroke secondary to large vessel occlusion presenting beyond 6 hours from time last known well, and may help identify those who may benefit from endovascular perfusion therapy. However, it is important to understand the limitations of automated CT perfusion (CTP), which are detailed in a review published in Neurology.

The clinical benefit of endovascular reperfusion therapy was impressive in 2 randomized trials (DAWN and DEFUSE 3) for patients with acute ischemic stroke with larger vessel occlusion who presented beyond 6 hours from last known well and were deemed to be good candidates based on cerebral perfusion imaging. The capability to perform and interpret perfusion imaging, either CTP or magnetic resonance perfusion-weighted imaging, should be available in every center treating patients with acute stroke.

A few commercially available, automated software packages, such as RAPID automated perfusion postprocessing software, can detect the presence of irreversible brain tissue injury, based on cerebral blood flow compared with that seen in contralateral perfused tissue by CTP or magnetic resonance perfusion, and assist in selecting appropriate candidates for endovascular therapy.

Simply put, CTP consists of a sequence of images obtained during the wash-in and the wash-out of an intravenous iodinated contrast agent. This is followed by transferring the source images to the workstation with the dedicated postprocessing software, which provides calculation of hemodynamic parameters. The automated assessment overcomes possible observer variability associated with subjective inspection.

There are disagreements about the quantitative thresholds between the ischemic core (the critically hypoperfused tissue with low cerebral blood flow) and the ischemic penumbra (the potentially salvageable tissue that may recover if reperfusion is achieved), and different software packages recommend slightly different thresholds. The automated software RAPID generated specific thresholds to identify good candidates for delayed endovascular: relative cerebral blood flow <30% and Tmax (time from the start of the scan until the maximum intensity of contrast bolus arrives at each voxel) >6 seconds.

Although perfusion imaging can be very valuable, there are several diagnostic pitfalls and challenges, and the review details the common technical and clinical pitfalls, along with practical pearls.

Technical pitfalls refer to the image quality from a technical standpoint and include several important challenges:

Clinical challenges to consider in routine practice:

Magnetic resonance perfusion is less quantitative compared with CTP and requires the use of echo planar imaging, which can have multiple artifacts. Furthermore, MRI is not as commonly available in urgent cases.

Lead author Achala Vagal, MD, professor of radiology at the University of Cincinnati Medical Center, Ohio, told Neurology Advisor, CTP is an important tool in the triage and treatment decision making in acute ischemic stroke. However, like any other technique, it is imperative to understand its limitations, especially when these decisions have to be made quickly. She goes on to add that automated CTP output maps are now available within minutes on our PACS, phones and emails; all the more reason to be cognizant of the pitfalls leading to inaccurate estimations of core and penumbra. A quick checklist can be very helpful.

Reference

Vagal A, Wintermark M, Nael K, et al. Automated CT perfusion imaging for acute ischemic stroke: pearls and pitfalls for real-world use. Neurology. 2019;93(20):888-898

Link:
Clinical Pearls and Pitfalls of Automated CT Perfusion for Acute Ischemic Stroke - Neurology Advisor