Purpose: Perfusion-based patient selection facilitates late-time window therapy in patients with acute ischemic stroke (AIS). Endovascular trials have emphasized anterior circulation (predominately ICA and MCA) large vessel occlusion (LVO), and generalizability to posterior circulation LVO may not be straightforward due to relative delays in anatomic territories of the posterior circulation, the propensity for anatomical variation and asymmetries in the posterior circulation, and the delay sensitivity inherent to deconvolved time-to-maximum (Tmax) parametric maps. Despite the widespread adoption of delay-insensitive deconvolution algorithms, which mitigate cerebral blood flow (CBF) errors relating to arterial input function (AIF) selection, the inherent arrival time sensitivity of tissue Tmax would be anticipated to vary significantly as a function of AIF, for example, between the anterior and posterior circulations. We therefore sought to determine how vary AIF affects posterior circulation Tmax. 

Materials and Methods: Patients undergoing CTP and CTA for presumed AIS were retrospectively identified by radiologic and clinical informatics query. Cases with final clinical or imaging evidence for acute stroke, or other acute intracranial pathology were excluded. CTP were analyzed using RAPID (iSchemaView, Menlo Park, CA) using automated AIF detection. AIF were then manually enforced so perfusion maps for the right and left MCA, ACA, and basilar AIF locations were generated. Tmax parametric maps were constrained into pre-defined vascular and anatomic territories of interest using ITK-SNAP. Median values for territorial average Tmax values were compared using the Wilcoxon signed-rank test. 

Results: Seventy-one cases (mean age 64+/-19 years) met inclusion between 1/2018 and 3/2019. With automated AIF selected, cerebellar hemisphere and brainstem vascular territories exhibited greater Tmax delays when an anterior circulation automated AIF was selected compared with a posterior circulation AIF (p<0.05). When AIF was then manually enforced on the right and left MCA or ICA, ACA, and basilar artery for each patient, median territorial Tmax was significantly lower when AIF was placed on the basilar artery compared with either the right or left MCA/ICA and ACA for all vascular territories (p<0.001).   

Conclusions: Territorial average Tmax values are significantly affected by AIF selection and AIF-based cutoff criteria may therefore be necessary in the setting of associated benign oligemic delays.

How varying arterial input function location affects automated CT perfusion posterior circulation Tmax values

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