Was about time I got some serious litterature review :)

• http://sci-hub.io/10.1109/ULTSYM.2015.0517 for a full fledge Smartphone Probe =)
• Microcontroller USB interfacing with MATLAB GUI for low cost medical ultrasound scanners

Some others

• Integrating hardware and software for the development of microcontroller-based systems
• IDEA: Design of a USB based multichannel, low cost data acquisition system using PIC microcontroller