Imaging with unfocused regions of focused ultrasound beams.

This article gives an analytical, computational, and experimental treatment of the spatial resolution encoded in unfocused regions of focused ultrasound beams. This topic is important in diagnostic ultrasound since ultrasound array systems are limited to a single transmit focal point per acoustic transmission, hence there is a loss of spatial resolution away from the transmit focus, even with the use of dynamic receive focusing. It is demonstrated that the spatial bandwidth of a Gaussian-apodized beam is approximately constant with depth, which means that there is just as much encoded spatial resolution away from the transmit focus as there is in the focal region. The practical application of this principle is discussed, an algorithm for retrospectively focusing signals from unfocused regions of fixed-focus beams is presented, and a quantitative comparison between the authors' methods and dynamic-receive beamforming is provided.