• Grant Mair, Rüdiger von Kummer, Alessandro Adami, Philip M White, Matthew E Adams, Bernard Yan, Andrew M Demchuk, Andrew J Farrall, Robin J Sellar, Rajesh Ramaswamy, Daisy Mollison, Elena V Boyd, Mark A Rodrigues, Karim Samji, Andrew J Baird, Geoff Cohen, Eleni Sakka, Jeb Palmer, David Perry, Richard Lindley, Peter A G Sandercock, Joanna M Wardlaw, and IST-3 Collaborative Group.
• Division of Neuroimaging Sciences, University of Edinburgh, Western General Hospital, Crewe Road, Edinburgh, EH4 2XU, UK.
• Neuroradiology. 2015 Jan 1; 57 (1): 1-9.

IntroductionCT angiography (CTA) is often used for assessing patients with acute ischaemic stroke. Only limited observer reliability data exist. We tested inter- and intra-observer reliability for the assessment of CTA in acute ischaemic stroke.MethodsWe selected 15 cases from the Third International Stroke Trial (IST-3, ISRCTN25765518) with various degrees of arterial obstruction in different intracranial locations on CTA. To assess inter-observer reliability, seven members of the IST-3 expert image reading panel (>5 years experience reading CTA) and seven radiology trainees (<2 years experience) rated all 15 scans independently and blind to clinical data for: presence (versus absence) of any intracranial arterial abnormality (stenosis or occlusion), severity of arterial abnormality using relevant scales (IST-3 angiography score, Thrombolysis in Cerebral Infarction (TICI) score, Clot Burden Score), collateral supply and visibility of a perfusion defect on CTA source images (CTA-SI). Intra-observer reliability was assessed using independently repeated expert panel scan ratings. We assessed observer agreement with Krippendorff's-alpha (K-alpha).ResultsAmong experienced observers, inter-observer agreement was substantial for the identification of any angiographic abnormality (K-alpha = 0.70) and with an angiography assessment scale (K-alpha = 0.60-0.66). There was less agreement for grades of collateral supply (K-alpha = 0.56) or for identification of a perfusion defect on CTA-SI (K-alpha = 0.32). Radiology trainees performed as well as expert readers when additional training was undertaken (neuroradiology specialist trainees). Intra-observer agreement among experts provided similar results (K-alpha = 0.33-0.72).ConclusionFor most imaging characteristics assessed, CTA has moderate to substantial observer agreement in acute ischaemic stroke. Experienced readers and those with specialist training perform best.

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