This isn't a totally crazy concern: focused ultrasound (fUS) can be used to modulate brain activity non-invasively (or even make therapeutic lesions). However,

This isn't a totally crazy concern: focused ultrasound (fUS) can be used to modulate brain activity non-invasively (or even make therapeutic lesions). However, a toothbrush is almost certainly not going to do that. The skull's impedance is way too high for the dinky transducer in your toothbrush and even if it weren't, you need to do all sorts of clever corrections to focus the ultrasound on a particular spot.