Overestimation of ischaemic core on computed tomography perfusion

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Álvaro García-Tornel Garcia-Camba discusses his recent publication in Stroke, which aimed at evaluating the influence of time and collateral status on ischaemic core overestimation. This was a retrospective, single-centre study including patients with anterior circulation large-vessel stroke that achieved reperfusion after endovascular treatment. 

Current volumetric estimation of ischaemic core on computed tomography perfusion (CTP) in patients with large vessel stroke is based on fixed thresholds derived from studies that compared baseline CTP to follow-up diffusion-weighted imaging (DWI) magnetic resonance imaging (MRI). While this approach might be practical for daily treatment decisions, it also carries a major limitation that is related to an inherent assumption that has been widely accepted. The assumption is that, for any given patient with a large vessel stroke, the accuracy of currently used thresholds for ischaemic core estimation (relative reduction of cerebral blood flow below <30% compared to contralateral hemisphere) is independent of patients’ baseline characteristics, including time from symptom onset to imaging, imaging-to-reperfusion, collateral status, occlusion location, glycaemia, etc.

Previous studies have shown that the general rule which points to CTP-ischaemic core (Regional cerebral blood flow [rCBF] <30%) as a predictor of minimal final infarct volume is, in fact, overestimating the final infarct in a non-negligible number of patients. Our study focused on predictors of ischaemic core overestimation, rather than on core underestimation, or on the magnitude of both types of error, for one reason: if we consider CTP derived thresholds, the biomarker to assess the real core at the point that image is performed, it should be impossible (the ground truth) for the core to decrease over time. Juxtaposed to this, patients in which follow-up infarct is larger than estimated infarct, two hypotheses should be considered: core has been underestimated by a CTP technical pitfall (real underestimation) and/or infarct has increased its size until the point that is larger than estimated infarct (infarct growth).

The main finding of our study is that the odds of ischaemic core overestimation on CTP is influenced by collateral circulation. In particular, a poor collateral status, determined by CTP-derived hypoperfusion intensity ratio (HIR) ≥0.4 in patients imaged within four hours from onset increases the chances of overestimation. While previous evidence supports that CTP thresholds are time-dependent, there is a lack of evidence regarding how cerebral haemodynamic state does influence the interpretation of perfusion maps. Patients with poor collateral status, leading to rapid conversion from tissue at risk to infarcted tissue, are those who show larger ischaemic core on admission CTP but also those in which core prediction may be most misleading. In our study, severe hypoperfusion influence on core overestimation differed according to onset-to-imaging time, with a stronger size of effect in patients imaged early after onset.

Perfusion imaging has been widely promoted as the tool to shift acute stroke management from a time-based to a tissue-based paradigm. An important limitation in perfusion imaging interpretation is that volumetric calculation of CTP-derived core is forecasted based on hypoperfusion severity rather than real tissue fate. Our results emphasise that collateral circulation status should also be considered when forecasting final infarct on admission CTP maps.

We consider that major limitations of our study are related to its retrospective design, the fact that it is a single-centre study, and that a selection bias cannot be excluded, as some patients with a large core on baseline imaging, either in non-contrast CT or CTP, were excluded from treatment. Moreover, as mentioned before, all findings of our study are conditioned to the assumption that reperfusion is achieved, a variable that is unknown at the time of imaging.

Álvaro García-Tornel Garcia-Camba practices at the Stroke Unit, Neurology Department, Hospital Universitari Vall d’Hebron, Barcelona, Spain.

The author has no disclosures.


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