Which of these areas can an infant have an ultrasound but an adult cannot?

Because bones are natural impedance providers to ultrasound, taking an ultrasound of any organ that is covered or surrounded by bones is impossible or extremely difficult.

 The bones of an infant are soft and do not provide much resistance to the passage of the ultrasound waves. 

Thus, an ultrasound of the brain can be performed on an infant but not on an adult.