Some transducers (or just one, depending on design choices) are excited. The excitement produces a spherical wave, whose echoes are collected. Then, a second group of transducers is excited, the spherical wave emitted and then the echoes collected. This process is done by the number of times you want to generate single LRIs. The emissions drawn in the previous scheme are then collected and compounded together.
Due to the nature of this process, the delay of emission can be computed and every emission can be focused in both transmission and reception (the usage of the virtual sources is explained later). The dynamic focusing in the reception is obtained by combining all the LRIs because the origin of the wave is know as well as the transducer(s), which generates it. Thus, the path Rx-Tx can be calculated. For this reason, due the sequential emission technique of SA, the insonification area created is focussed on the target:
Every line represents the investigation of subgroup of transducer(s), “rectified” in the process of reception due to the sequential mode of emissions.
On contrary, PW excites all the elements, as shown in figure 2. The insonification area cannot consequently be focused. Hence, it must be computed during the process of Beamforming. To eliminate the differences, perform the following three operations:
- Compute a delay for the reception for the transducers per line.
- Focus the plane wave emitted on the direction of propagation (the cartesian system of the probe is explained in upcoming sections).
- Modifying the formulation of the virtual sources to fit a different type of emission.
Performing this three operations rectifies the insonification by aligning a plane wave emitted (which should covers the overall area of transducers) and obtains the same result as the SA.