Comparison of virtual source synthetic aperture beamforming with an element-based model.

Ultrasound beamforming relies on models of propagation to convert samples of the backscattered field through time into spatial image samples. The most common model is straight-line propagation of a focused wave, assuming a narrow steered and focused beam that propagates along a selected direction. The reconstructed image suffers from defocusing, reduced signal-to-noise ratio (SNR), and contrast loss away from the focus. "Virtual source" methods coherently combine the recorded data from multiple transmissions to form a synthetic transmit focus by making geometric assumptions about the transmissions. These also include diverging waves (virtual source behind the array) and plane waves (virtual source at infinity). Retrospective encoding for conventional ultrasound sequences (REFoCUS) beamforming has been proposed to instead model transmission as the superposition of the responses of individual transmit elements on the transducer array and to efficiently estimate the "complete data set"-individual element transmit and receive responses. In addition to isolating individual element contributions, the result of this unifying framework is a high-SNR, two-way focused image from focused plane wave or diverging transmissions. No significant differences were observed for either SNR or image quality measured by contrast-to-noise ratio between the appropriate virtual source method and REFoCUS beamforming in simulation and experimental imaging.