Dealing acoustically with something different from the surrounding material (calcium deposits) is a lot easier than focusing on tissues with roughly the acousti

Dealing acoustically with something different from the surrounding material (calcium deposits) is a lot easier than focusing on tissues with roughly the acoustic properties of water among other tissues with the acoustic properties of water - in the former case the stuff can distinguish itself w.r.t. application of the waves, the latter requires focusing to come "from the outside."