Hi,
The delays at T/R is used for steering and focusing the acoustic beam. This delays depend on mechanical dimensions of a single piezotransducer, number of transducers involved in acoustic wave excitation, shape of a probe (linear array, curved array), required direction and focal point of the ultrasound wave. Generaly, it is required that the signal acoustic signal traveling from every single transducer should arrive at the same time to the desired point. Some time ago I have developed a simple software for calculating the delays and visualizing the acoustic beam. You can download it from here:
http://www20.zippyshare.com/v/85159749/file.html
Just UNRAR this two files (using WINRAR - http://www.rarlab.com/download.htm) into any folder and run DELAY.EXE
Good Luck!!