  Delay-and-sum (DAS) is the most widespread digital beamformer in
high-frame-rate ultrasound imaging. Its implementation is simple and compatible
with real-time applications. In this viewpoint article, we describe the
fundamentals of DAS beamforming. The underlying theory and numerical approach
are detailed so that users can be aware of its functioning and limitations. In
particular, we discuss the importance of the f-number and speed of sound on
image quality, and propose one solution to set their values from a physical
viewpoint. We suggest determining the f-number from the directivity of the
transducer elements and the speed of sound from the phase dispersion of the
delayed signals. Simplified Matlab codes are provided for the sake of clarity
and openness. The effect of the f-number and speed of sound on the lateral
resolution and contrast-to-noise ratio was investigated in vitro and in vivo.
If not properly preset, these two factors had a substantial negative impact on
standard metrics of image quality (namely CNR and FWHM). When beamforming with
DAS in vitro or in vivo, it is recommended to optimize these parameters in
order to use it wisely and prevent image degradation.
