Comment on "So you think you can DAS? A viewpoint on delay-and-sum beamforming" [Ultrasonics 111 (2021) 106309]

It is shown herein that Perrot et al., who reviewed delay-and-sum beamforming for ultrafast ultrasound imaging in [Ultrasonics 111 (2021) 106309], misinterpreted the purpose of dynamic receive apertures. Such apertures widen with the focal length as a function of a given f-number and improve the image quality by suppressing grating lobes. Perrot et al., however, attributed erroneously the image quality improvement to suppression of measurement noise and, in doing so, proposed a suboptimal method to determine an f-number.