A 28-year-old man presents to the emergency department after being struck on the head. He had a brief loss of consciousness at the scene and now has a headache and vomiting. On arrival, his Glasgow Coma Scale score is 15, there are no focal neurological deficits and no signs of skull fracture. He is not anticoagulated. According to the Canadian CT Head Rule, which of the following is an indication for CT imaging?