Recently manufacturers have started to produce full 2D arrays in which there are almost as many elements in the Z axis as there are along the length of the array. For example, Philips has an X6-1 Matrix transducer that contains over 9,000 elements!
The advantage of these new transducers is that not only can they be electronically focused in the Z axis, they can also be electronically steered in the Z axis. This permits the real-time acquisition of data from 2 planes of section and gives the transducer the capability of collecting all the data needed to produce a real-time 3D display.
Check out this video of the Philips XL14-3 matrix transducer.