I saw the "crescent" sign: Waxing clues and waning lumen.

The "crescent" sign is characterized by a crescent-shaped rim of hyperintensity on T1-weighted fat-saturated magnetic resonance imaging (MRI), surrounding a narrowed arterial lumen. It represents an intramural hematoma, most often seen in cases of arterial dissection. Although the differential diagnosis for neck pain and headache is broad, the "crescent" sign in the appropriate clinical and radiologic setting is highly suggestive of an internal carotid artery (ICA) dissection. In this case study, we present a 58-year-old man with a history of hypertension and hyperlipidemia who presented with acute onset headache and neck pain. Imaging revealed classic features of left ICA dissection, including vessel wall irregularity on computed tomography angiography (CTA) and the crescent sign on MRI.