- Aim To test the performance of computed tomography angiography “source images” (CTA-SI) versus unenhanced CT (NCCT) for stroke detection and extent using the Alberta Stroke Programme Early CT Score (ASPECTS), and examine the effect of experience and clinical history. Materials and methods Studies of 23 consecutive patients presenting within 4.5 h were analysed by three reviewers of varying experience. Each reviewer, blinded to clinical information reviewed a random order of NCCT and CTA-SI and documented side of infarct and the ASPECTS. The readings were repeated for CTA-SI with and without clinical information. Performance measures and observer agreement were calculated. Applying an ASPECTS threshold of ≤ 7, the number of patients misclassified was determined. Results CTA-SI improved trainee accuracy by 9%, but had little impact on more experienced readers. The accuracy and sensitivity of stroke extent assessment was increased for all readers, but was greatest for the trainee (17% and 12%, respectively). Clinical history contributed little to CTA-SI accuracy. Observer agreement was higher for CTA-SI. NCCT could have resulted in the misclassification of more patients than CTA-SI. Conclusion CTA-SI are an important adjunct in acute stroke assessment, improving stroke extent determination for all readers irrespective of level of experience. In addition less experienced readers may benefit from CTA-SI for detection of presence of strokes. CTA-SI performance appears independent of clinical history. CTA-SI resulted in fewer misclassified patients if an ASPECTS threshold of ≤ 7 is considered.