Hello, I found the following image in a video about ultrasound imaging physics

and I would like to ask some questions:
1- What is the physical signification of negative (below 0) echos?
2-In the rectification process, the negative signals are ignored or the rectification is full wave?

 

With no context, heading or labels, the diagram becomes just an abstract picture. With no reference to the source of the video, my answer to your questions is "I don't know".

 

In the left of the figure, the diagram shows the signal coming form one elements of the probe, the second similar diagram shows the signal coming from another elements, the third diagram shows the rectified signal (as an example) from one of the previous signal, and finally the points represent grey levels in B-Mode imaging.
The video link is:

With no context, heading or labels, the diagram becomes just an abstract picture. With no reference to the source of the video, my answer to your questions is "I don't know".

 

In the left of the figure, the diagram shows the signal coming form one elements of the probe, the second similar diagram shows the signal coming from another elements, the third diagram shows the rectified signal (as an example) from one of the previous signal, and finally the points represent grey levels in B-Mode imaging.
The video link is: