Plane Wave Imaging Methods for Ultrafast Ultrasounds

Ultrafast ultrasound is an imaging technique intended to replace conventional ultrasound scanning when very high temporal resolutions are needed, e.g. to detect movements or to reduce motion artefacts. Traditionally, echography consists in conveying focused ultrasound waves line by line and in juxtaposing the received backscattered echoes from each line. In the last decade, the development of parallel computing, especially in terms of GPUs (graphics processing units), paved the way to the definition of a new method for ultrasound imaging, namely the plane wave transmission. Thanks to this new technique, it is possible to insonify the entire image area in a single transmission, rather than using a number of focused lines. Plane wave imaging greatly decreases the time requested to perform a scan and allows to increase the frame rate by two orders of magnitude, reaching frequencies of thousands of Hz. Disadvantage of this technique is the absence of the focus, which allows to concentrate the energy in specific regions to improve the spatial resolution. As a consequence, even though the frame rate largely increases, the image quality worsens. Several studies have been carried out to restore the image quality while using this new imaging technique. A notorious attempt by Montaldo et.al. explored the use of a series of plane wave transmissions, each with a different steering angle, to be subsequently beamformed and coherently compounded into one single frame, in order to virtually and dynamically recreate focusing points. Nevertheless, according to this technique, a large number of plane waves is required to perfectly restore the image quality. Even though in this way the maximum frame rate decreases, the advantage of plane wave imaging still persists, as images obtained through this method, with comparable quality to those obtained with conventional ultrasounds, can still be acquired with higher frame rates than traditional ones. Later on, other studies based on the Montaldo method directed their research towards higher quality ultrafast images. Two techniques in particular are reported and compared in this work: multiplane waves imaging and cascaded dual-polarity waves imaging. Their objective is to find a way of amplifying the transmitted signal amplitude, to improve the image quality, without causing any damage to the fragile transducers or any bioeffect risk to the imaged area. This work aims at identifying how to implement these techniques in an ultrasound simulator and under which conditions one is suggested over the others.