Histotripsy achieves precise tissue destruction because cavitation is a threshold phenomenon: below a certain pressure level nothing happens, and above it tissue is damaged, with no gradient in between. This contrasts with heating based methods like HIFU or radiation, which deliver continuous, diffusing doses that make it hard to damage one spot without affecting neighboring tissue. Because the damage is mechanical rather than thermal, it depends on a tissue's mechanical strength, and structures like major blood vessels, nerves, and bile ducts are naturally resistant, letting histotripsy destroy tumors sandwiched between vessels while leaving those vessels intact.
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Thermal and radiation treatments are described as having a dose gradient that spreads through tissue via diffusion and perfusion, reducing targeting accuracy compared to histotripsy's sharp threshold effect.
671 words · auto-generated from the episode video
23:51What happens is that ultrasound is a mechanical wave. So it propagates by vibrating particles in the media, and when you actually compress the particles, it creates a positive pressure. Then when you kind of pull particles apart, it creates an active pressure. And if you imagine there's a balloon and you want to make the balloon to grow, what you do is that you want to make the pressure outside actually lower than the inside of the balloon so that you can overcome the tension of the balloon. So the nanometer gas pockets, what happens is the surface tension is 200 to 300 times of the atmosphere if it's in a water molecule environment, right?
24:36And we as humans, 70% of us are just water-based. And so most tissues are water-based, then the rest will be fat and you have bone, right? So if you have this water-based tissue and the gas pockets in there is 200 to 300 times of the atmosphere as a surface tension, then supposedly what you need to do is within really short duration instantaneously make the pressure outside and lower than the inside, meaning that I need to achieve 200 to 300 times of the negative pressure
25:17within microseconds region. Now I can explode that nanometer gas pockets to really large, right? And that's the trick. So we're producing a pulse wave with 200 to 300 times the negative pressure within microseconds, expand that nanometer gas pockets to from nanometer to 100 micro, then you take that pressure away, then we will just collapse. So don't keep it going literally microsecond, expand, then take it away, then let it collapse, then that is almost like a micro, micro bone, right? So you will produce this mechanical stress to the cells when the bubble expand, it
26:03crush the cells, squeeze it, when you collapse, you pull cells apart. So it's mechanically pulling cells apart. And because the water molecule surface tension is so consistent, therefore it's a threshold, you have to cross that threshold with your ultrasound pressure, then you generate bubbles, and if it's lower than the threshold, you don't generate. So even though the ultrasound focus for transducer typically is like a couple millimeters, if you make it so that your region that exceeding the pressure threshold, the limit to generate cavitation, and that's the region that
26:45measure. So that's the region where you can generate cavitation and it can be even less than millimeters, right? And beating the diffractional limit and the lower you don't have any activity, which is really cool and very helpful because if you think about using heating as a mechanism, heating is always heating, you can have high heating, low heating, so higher energy give you a high heating, lower energy give you low heating, then there's a linear thing and there's a perfusion too. Yeah, there's diffusion from... Yeah, so your accuracy is low because you can't just say, hey, I heat this region, then everywhere else I don't heat, it's a continuous
27:26thing, you're drinking everything. Radiation is the same thing, you have radiation, high dose, low dose in the beam pathway. So, but with histotripsy, the beautiful thing about here is that it's a threshold phenomenon, so either within the focus exceeding that threshold, you have cavitation, you have damage, and outside you have no damage. So really it's a very highly accurate treatment and because of threshold phenomenon. On top of that, it's a mechanical damage, so it really determined by the mechanical property of target tissue and naturally our major blood
28:09vessels, nerves, bile ducts, those structures have high mechanical strength and more resistant to mechanical damage. So we actually have been able to preserve those critical structures because they are naturally designed to resist positive blood flow, that kind of thing. So we are able to go in, treat tumor without damaging the vessel and treating inoperable, surgically inoperable tumors sandwiched between two vessels where vessels remain intact. It's really incredible.