I hope this diagram helps explain why the surgery could nt be done. Dr. j said there were two main reasons why the ultrasound could ot penetrate the skull and reach the thalamus. First, the outer layer of Josh's skull is unusually thick (no jokes, please). Second, the inner layer, which resembles Swiss cheese, is also unusually thick.

The ultrasound ended up "bouncing around" in the soft inner layer, instead of  going through it. The ideal temperature for the ultrasound to work is 55 C, but Dr. J could not get it above  42  because of the skull issues. So, that's the science in a nutshell.