This is cool! I've got a personal project in the works to build an ultrasound machine just to learn how it works. There's a lot of electronics design, PCB design, FPGA work etc that in learning. The probes themselves are used medical grade ones from ebay.

The hard part is that there can be over hundreds of piezo elements each needing to receive on their own channel. That means lots of expensive ADCs. Not to mention either having many pulser circuits or muxing a pulser to many elements.