What are the characteristic imaging findings of neonatal herpes simplex encephalitis on CT and ultrasound, and what structures are typically spared?

Early in the course of neonatal herpes simplex encephalitis, imaging may show diffuse brain swelling or bilateral patchy areas of decreased density on CT, and hypoechoic regions in the cerebral white matter and cortex on ultrasound. The basal ganglia, thalami, and posterior fossa structures are typically spared. Multicystic encephalomalacia can develop as a late sequela. Key diagnostic features include the presence of hemorrhagic necrosis and edema, while common pitfalls include mistaking these findings for hypoxic-ischemic injury or metabolic disorders.