[aesh2Xa1]

Yes, I think it is like saying that, but I also think that OP was considering targets less than 4mm in size. The focus of the FUS wave being no smaller than 4mm, very small targets would be larger than the smallest focus.

It's like saying "how can a syringe extract the cytoplasm of a cell?" if you take OP's angle.

[polishdude20]

Ah I guess in the case of ultrasound, the thing you're trying to do is not nudge or move something, you're literally trying to "ablate" or hit it and only it. That's more like pressing a tiny button with your finger and trying not to touch the outside of the button.