A 72-year-old woman with atrial fibrillation (not on anticoagulation) presents with acute onset left hemiparesis and hemisensory loss. Non-contrast CT head performed at 6 hours post-symptom onset shows no acute intracranial hemorrhage. MRI brain with DWI sequences shows acute ischemic changes in the right middle cerebral artery territory. However, 48 hours later, repeat imaging shows hyperdensity in the same region with mass effect and clinical deterioration. Which imaging modality is most appropriate to characterize this hemorrhagic transformation and assess for thrombus extension?