3D ultrasound is becoming common for non-invasive medical imaging because of its accuracy, safety, and ease of use. However, the extreme computational requirements (and associated power requirements) of image formation for a large 3D system have, to date, precluded hand-held 3D-capable devices. Sonic Millip3De is a recently proposed hardware design that leverages modern computer architecture techniques, such as 3D die stacking, massive parallelism, and streaming data flow, to enable high-resolution synthetic aperture 3D ultrasound imaging in a single, low-power chip. In this paper, we enhance Sonic Millip3De with a new virtual source firing sequence and dynamic receive focusing scheme to optimize receive apertures in multiple depth focal zones. These enhancements further reduce power requirements while maintaining image quality over a large depth range. We present image quality analysis using Field II simulations of cysts in tissue at varying depths to show that our methods do not degrade CNR relative to an ideal system with no power constraints. Then, using RTL-level design for an industrial 45nm ASIC process, we demonstrate 3D synthetic aperture with 120×88 transducer array within a 15W full-system power budget (400x less than a conventional DSP solution). We project that continued semicondutor scaling will enable a sub-5W power budget in 16nm technology.