Efficient dynamic focus control for three-dimensional imaging using two-dimensional arrays.

Dynamic receive focusing in ultrasonic array imaging involves extensive real-time computations and data communication. Particularly for three-dimensional imaging, using fully sampled, two-dimensional arrays, implementation of dynamic focusing can be extremely complicated because of the large channel count. In this paper, an efficient dynamic focus control scheme for a delay-and-sum-based beamformer is proposed. The scheme simplifies dynamic focus control by exploiting the range-dependent characteristics of the focusing delay. Specifically, the overall delay is divided into a range-independent steering term and a range-dependent focusing term. Because the focusing term is inversely proportional to range, approximation can be made to simplify dynamic focus control significantly at the price of minimal degradation in focusing quality at shallow depths. In addition, the aperture growth controlled by a constant f/number can also be utilized to devise a non-uniform quantization scheme for the focusing delay values. Efficacy of the proposed scheme is demonstrated using simulated beam plots of a fully sampled, two-dimensional array. Design procedures are also described in detail in this paper. One design example shows that, with the proposed dynamic focus control scheme, a 4096-element array only requires 227 independent controllers for the range-dependent focusing term. Moreover, only 28 non-uniform quantization levels are required to achieve the same focusing quality as that of a conventional scheme with 784 uniform quantization levels. The beam plots of a fully sampled array show that sidelobes are slightly increased below the -30 dB level for imaging depths less than 3 cm. At greater depths, there is no observable degradation.