A 51-year-old woman on apixaban for a recent DVT had a fall onto a bed of sharp rocks. She landed on her right palm and forearm. EMS was called and she was brought to the ED with a report of “a lot of blood at the scene.” On exam, she has a blood-soaked dressing applied to her right wrist and hand. Which of the following can be used to reliably rule out the possibility of significant vascular injury?