Q. A 72 year-old man presents with his wife to the ER. She reports that as of 90 minutes ago, he lost the ability to speak and the ability to understand what his wife is saying. As well, he has developed right facial weakness and right side of body weakness. He has no prior history of similar episodes. His past medical history is significant for hypertension and atrial fibrillation. She does not know his complete list of medications but does report that he was taken off of a "blood thinner" three days ago by his dentist for an upcoming root canal. Vital signs are normal. Brief neurological exam demonstrates that he is unable to speak or follow commands. You can get the patient to follow tactile commands and mimic and he appears to have a right facial droop with preserved ability to wrinkle his forehead, preserved visual fields, right upper extremity demonstrates flaccid weakness, right lower extremity has mild weakness. Cardiopulmonary exam demonstrates an irregularly irregular rhythm. Noncontrast head CT is normal. Which of the following best explains why the head CT is normal?