Separable 2D direct-sampled in-phase quadrature beamforming for greatly increased computational efficiency

Two dimensional arrays present a significant engineering challenge due to high channel count and data rates. In addition, for 2D arrays in handheld devices, power consumption is also very important in order to achieve multi-hour battery life. Previous work indicates that a 2D beamforming operation can be decomposed into two successive 1-D focusing operations for computationally efficient volume imaging. In this paper, we analyze the application of this method to a power-efficient 2D C-mode beamforming method using phase rotation focusing, known as direct-sampled in-phase quadrature (DSIQ). We show that the separable approach applied to DSIQ does not discernably degrade imaging performance in most practical conditions. Using this method increases C-mode frame rates in a handheld device 17-fold from 11.4 Hz to 193 Hz. The energy cost of beamforming a frame of data falls from 107.2 mJ to 6.2 mJ. When this power efficiency is allied with the low-duty cycle strategy used by the DSIQ front-end ASICs, a very power efficient system is achieved, capable of beamforming a frame of data from 3600 channels, including front-end power, using <; 10mJ of energy. This level of energy efficiency is essential to enabling multi-hour battery life in handheld ultrasound systems with 2D transducer arrays.