Can we identify a CT‐based tissue window for thrombolysis without CTP?

Abstract

Our congratulations to Parsons and colleagues for their novel work on qualitative computed tomography perfusion (CTP) imaging using the Alberta Stroke Program Early CT Score (ASPECTS). 1 In patients presenting with acute anterior circulation stroke within 6 hours from symptom onset, the authors showed that scoring ASPECTS on CTP parameter images improves prediction of final infarct and outcome as compared with noncontrast CT (NCCT) or CT angiography source-images (CTA-SI). In addition, their work contributes to increasing evidence that infarct core as well as tissue at risk might be reliably identified by CTP (Murphy et al. personal communication). 2 However, we would like to discuss with the authors two issues that might contribute to further understanding and development of this technique.First, the authors did not mention which thresholds they used to determine abnormal ASPECTS regions on parametric maps for relative cerebral blood flow (rCBF), cerebral blood volume (rCBV), and mean transit time (rMTT). Depending on window and leveling, "dark blue or black" regions might represent different relative perfusion thresholds, for example, 0.2 versus 0.3 for rCBF. With this information lacking, results might be difficult to reproduce among different perfusion software and hardware manufacturers. Furthermore, it would be interesting to learn about incidence and tissue outcome of ASPECTS regions with reduced rCBF but increased rCBV.Second, based on our ASPECTS analysis in a similar cohort studying patients with middle cerebral artery occlusions within 6 hours from symptom onset, 3 we are currently testing the hypothesis that a favorable NCCT scan (ASPECTS Ն6) in the presence of intracranial arterial occlusion predicts benefit from thrombolysis in a greater than 3-hour time window. NCCT (or CTA-SI) ASPECTS estimates infarct core, and intracranial occlusion on CT angiography or transcranial Doppler sonography acts as surrogate marker for large perfusion deficit. This approach would be rapid and practical especially for stroke physicians lacking advanced CT technology. To learn more about the additional value of CTP compared with our approach, we would like to ask the authors the following: did a rCBV-ASPECTS Ͼ rCBF-ASPECTS mismatch occur in patients without demonstrable arterial occlusion? Which proportion of patients with and without ASPECTS Ն6 and proximal occlusion (M1 and ICA) showed this mismatch and how did this proportion differ from patients with distal (M2) occlusions?We are looking forward to studying more data on this important and evolving technology.

Use of Noncontrast Computed Tomography and Computed Tomographic Perfusion in Predicting Intracerebral Hemorrhage After Intravenous Alteplase Therapy

“…15 NCCT hypoattenuation degree is therefore a better marker of ischemia severity and time than hypoattenuation extent. [15][16][17] NCCT attenuation drops over time, whereas the infarct grows in size; this explains the modest correlation we see between NCCT hypoattenuation grade and ASPECTS at baseline. Our results support this premise and suggest that the contralateral white matter is a practical reference for physicians to assess degree of hypoattenuation and risk of PH with intravenous alteplase treatment.…”