In this AJNR Fellows' Journal Club article summary, Dr. Francis Deng and Dr. Amir Khadivi discuss the article by Proner et al., "Impact of Clinical and Radiologic Factors on CTP Timing in Acute Ischemic Stroke." They discuss the authors' findings that cardiac arrhythmias and older age are independent predictors of nondiagnostic CTP exams. Specifically, these factors often lead to the truncation of reference vessel time-attenuation curves that fail to reach equilibrium within a 45-second acquisition window.