Fig. 3.

Illustration of the limitation of the DAS beamformer in minimizing artifacts and noises, as well as the effectiveness of CF in compensating for this limitation. (a) Detected PA signals before DAS. The blue dashed lines indicate the adjacent signals involved in the summing process. Points 1, 2, and 3 are the representative pixels with PA signals labeled as PA1, PA2, and PA3, respectively. (b) Beamformed PA signals after DAS. For the main lobes (such as point 2), the PA signal after SAFT becomes the sum of PA1, PA2, and PA3. For the artifacts (such as points 1 or 3), the PA signal after SAFT becomes the sum of the original signal and the adjacent noises (i.e., PA1+ noises, PA3+ noises). (c) Beamformed PA signals after DAS+CF. The side lobes are effectively suppressed. (d) Case of the main lobes (such as point 2), in which the delayed signals are highly in-phase (coherent). (e) Case of the side lobes and background noises (such as points 1 or 3), in which the delayed signals are generally out-of-phase (incoherent).