[daemin]

I worked at a startup making a handheld ultrasound machine back over a decade ago. Even back then we could easily power the transducer using a mobile phone capacity battery, you just need to step up the voltage to around 100v so you could feed it through the transducer and you'd get a signal back.

Last I heard the place is still going although most of the people I worked with have left already.

EDIT: Back then we were eyeing up using MEMS technology instead, though I don't think anything happened with that.