3D ultrasound, an emerging medical imaging technique that is presently only used in hospitals, has the potential to enable breakthrough telemedicine applications, provided that its cost and power dissipation can be minimized. In this paper, we present an FPGA architecture suitable for a portable medical 3D ultrasound device. We show an optimized design for the digital part of the imager, including the delay calculation block, which is its most critical part. Our computationally efficient approach requires a single FPGA for 3D imaging, which is unprecedented. The design is scalable; a configuration supporting a 32x32-channel probe, which enables high-quality imaging, consumes only about 5W.