Isn't this like saying how can we move something one millimeter if our fingers are bigger than one millimeter?

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.

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.

More like saying “how can we draw a 1mm dot with a pencil bigger than 1mm”.

Though not a perfect analogy as you can do stuff with interference patterns to get sub-single wavelength resolution IIUC, I think this is used in silicon wafer photolithography.

> My understanding is that sound with a frequency greater than about 150Khz dissipates after passing through 5cms of air; it must dissipate faster in flesh.

Flesh is basically water. Water transmits sound extremely well.

Ultrasonic imaging typically uses frequencies in the low MHz. Like 1-10 MHz.

> Ultrasonic imaging typically uses frequencies in the low MHz. Like 1-10 MHz.

Oh, thank you! So the dissipation is low (because it's water), and the theoretical resolution is 500x greater than my ignorant estimate. I now consider myself better-informed.

[Edit] Now I guess I'm off to see if I can find out what a MHz-grade ultrasound transducer looks like...

All I know is they charge a lot for them. Ultrasound imaging at home would be amazing, but the transducers are hella expensive.

You can actually get quite cheap ones now. They make integrated scanners that work with a phone app. Easily under £1k second hand. Maybe close to that new if you shop around.

I wouldn't recommend it though. The only reason we use ultrasound imaging is because it's cheap, easy and completely harmless. As an actual imaging method it's terrible. There's so much speckle you can barely see anything, except in some situations like pregnancy where you have a convenient bag of water around the thing you're looking it.

There's a reason ultrasonographers are well paid - it's really really hard to read an ultrasound.

> At 300KHz, the sound has a wavelength of 1mm

And that’s in air. In water (which is more like what most bodily tissues are made of) the speed of sound is almost five times higher so the wavelength is five times longer.