The hosts explain how researchers gained precise spatial control over ultrasound-induced cavitation, a technique called microtripsy that uses very short ultrasound pulses. Because sound has an inherent wavelength that limits how tightly it can be focused, they explain that cavitation probability rises as a nonlinear, switch-like function of pressure rather than a gradual one. This lets researchers control the size of the bubble cloud, and therefore the area of tissue destruction, by tuning how loud (high-pressure) the ultrasound pulse is rather than by trying to focus it more tightly.
- 01
The hosts reference a specific paper describing histotripsy 'beyond the intrinsic cavitation threshold' using very short pulses, which they term microtripsy.
- 02
They cite a plot from that paper showing cavitation probability against pressure as a sharp on/off switch rather than a linear ramp.
- 03
They mention the technique being tested using red blood cell phantoms and in canine tissue to visualize how cavitation size changes with pressure.
- 04
They use an analogy to Mount Kilimanjaro, where only the icy summit sits above a threshold, to illustrate how raising overall wave amplitude changes only the area exceeding the cavitation threshold.
- 05
They compare the sound wavelength limit to the diffraction limit of light discussed elsewhere on the podcast.
1,274 words · auto-generated from the episode video
13:42instantiate this cavitation process. >> Yeah. Now I need to localize this. >> Yeah. We now we need controllability. >> Right. We need really precise controllability for this to be a clinical application. So that's where we get into the next paper. This is um a paper again in it e explore hystoripsy beyond the intrinsic cavitation threshold >> using al very short ultrasound pulses. So they call it microy micro. Now, um here's the idea. So, like you're using sound, right? Um and sound has a particular wavelength, right? Even this ultrasound is going to have a particular wavelength, but my
14:24target region could be much smaller >> than this wavelength. >> It's a question about resolution >> at the end of the day, right? like I've let's say I've got a focused region and I want to I I want to put a bunch of pressure amplitude using the sound into this area. Um it's largest near the center and then it falls off, right? So there's going to be a large area where I could produce bubbles and it becomes a stochastic process. >> So how do I control where the bubbles happen and where they don't? I need a very nice turning on and turning off mechanism. >> That's also specific again to an XYZ
15:05position in 3D space. >> Yeah. Yeah. But but here's the idea. The only thing I can focus really is the sound waves. And the sound waves have an inherent blurriness, >> right? >> So So how do I how do I actually control where the bubbles are happening? Well, it turns out cavitation naturally is a nonlinear switch. Ah, >> okay. And that's what this this paper is about that. So on the X here's we've got we've got a plot from that paper and I thought this was very very cool. So on the x-axis is your pressure. >> Mhm. >> Okay, this is the pressure that you're creating using your ultrasound and on the y-axis is the cavitation probability.
15:45Notice it is not a linear function. Yeah, >> it's not like at 10 megapascals there's like 20% probability, at 20 there's 40, at 30 there's 60 and it just linearly increases. No, there is a regime >> where it switches on and off. >> Okay. So, what you can do, you can't control the spatial extent, >> okay, >> of your sound waves, but you can control how loud they are. >> Mhm. >> Okay. And if you can control how loud they are, imagine a wave, right, that's massive. Even like take the extreme case of a wave that has a crest that is a meter big. But if I can control this the the amplitude of this wave very
16:26precisely, how loud this wave is, the the the spots, even though the wavelength of my sound wave is like a meter long, the spots where um the pressure goes above a certain threshold can be as small as I want it to be. So this is a degree of freedom where we have a level of precision and you basically use that as part of your first point of control. >> Yeah. Because you can't control the wavelength of the sound in some sense, right? Like you're limited by the technology of our time and by the physics of the thing. Like I mean if you make the sound wave too short then the bubble >> is not going to happen anymore, right? And like the focusing capability is not going to happen anymore. So there's level precision where we can know that we can create this height of the wave
17:09>> and that height of the wave is a a factor in this cavitation process. That is a good control to have. >> Exactly. And the height of the wave is the x-axis on this plot, right? And so >> suppose I make the suppose the threshold is exactly I don't know uh over here it seems like to be 25 megapascals, right? >> So if the if the entire amplitude of the wave is only let's say 26 or 27 there's going to be a very tiny region >> that goes above that 20 26 right 27. If on the other hand I want to target a very large area. I make the the sound super loud like 40 and then most of the sound wave is going to go above that threshold. You see? So I'm controlling
17:49the space based on the loudness of the ultrasound. >> So if you want to make the speaker go to 11, >> you're going to take care of everything. But if you only make it the speaker go to four. >> Yeah. Then >> it's it's a smaller area. >> It's a smaller area. And they show this with the next figure that I have. Um here they they've got um they tested this idea with red blood cell phantoms where you can clear the clearing can make the damage visible and this is again in canine tissues at lower pressures. So that's on the left hand side. >> At lower pressures, the size of the cavitation is very small. >> Mhm. >> But all I'm doing now is increasing the pressure. And as I increase the pressure
18:29and I'm focusing on this region, I get >> I can make the size of my cavitation much larger where the bubbles are happening are now much larger. And that is actually how I control localization. >> So this is interesting. So I think part of what we're saying is the knob we have that we can twist in this context is do we want small bubble or big bubble and depending on the use case you can have multiple small bubble multiple big bubble but the point is the the the there's a dial for the size of the cavitation I'm using bubble as >> layman's term >> no but there is it's a bubble yeah >> um and that and we have very the the ability to have sort of fine control
19:11over that size is is an is an important factor in again having functional control over whatever outcome we want to implement this cavitation towards. Exactly. It's like um it reminds me of like the the mountain Kilamanjaro. >> Mhm. >> Kilimanjaro is a massive mountain. Okay. The size of like like states in the United States, right? Several European countries is a single mountain. But the amount of ice on that mountain is only at the summit. >> Mhm. >> Right. And so, um, you're not breaking defraction. There's a diffraction limit similar to the diffraction limits that we've talked about on this podcast with light, right? Where it's like I can't localize the light beam to a certain small amount because of the wavelength
19:52of light. Well, in this case, similarly, the wavelength of sound prevents me from localizing the sound in a certain spot. But I can make it loud and I can control the loudness, the height of Kilimanjaro, for example. And if only the tip is above that threshold, then I've I can make stuff, you know, small enough for my for my liking. I thought that was that was pretty cool. >> No, that's quite nice. That's quite nice. >> Yeah. So So that was that was that that um >> second paper. >> Second paper. Now um our attention has been very local, right? Like where are the bubbles? Which cells are being broken up? Um now let's talk about what
HistotripsySpaceSoft Matter Physics