Thanks for sharing this! Linked it on the website.
96 karma · joined February 13, 2019
I had went down a huge rabbit hole to find the source for the 22 dB/cm/MHz paper that everyone quoted. People reference the Diagnostic Ultrasound textbook; that textbook references an old nondigital ultrasound reference book, which finally references a Fry paper from 1977 [1].
It's funny because the Fry paper never explicitly mentions the number 22 dB/cm/MHz. But there was one figure (Figure 12), where if you fit a line through the data in that figure, you get a slope of 23.5 dB/cm/MHz.
Here's a spreadsheet of me fitting the data: https://docs.google.com/spreadsheets/d/1a70svm-zrzp1SQT5v2m_...
But yes, you're totally right that even the Fry paper Figure 12 was only up to 2 MHz, so it's totally not fair to extrapolate to 10 MHz.
The method we’re proposing would have mm resolution.
> Is it fair to say that their claims about spatial resolution being >>> existing EEG options are jumping the gun? If I understand correctly, you need to be targeting individual 1mm^2 regions with individual acoustic lenses, which means 17,000 channels would required 17,000 separate, uniquely-tuned ultrasound emitters, yes? Even if that's possible without messing up the data (the MHz range is big, but is it that big?) it seems like a trivial impossibility to fit that in one headset -- even the standard 32-64 EEG channels alone seem like a long shot. But maybe I'm overly cynical, or one emitter could be used to usefully excite multiple regions at once?
Since the system is linear, you could use a single probe to focus at multiple spots. Each focus would be at a slightly different modulation frequency.
> Another oddity in that paper is that it reads like we're trying to find persistent signals in the brain, like a needle in a haystack, whereas my understanding was that the field is moving decisively towards tracking signal changes over time in a given region. Is my intuition correct that accounting for a moving target would add considerable complexity to this approach?
This method would indeed let you track signals that change over time. Lock-in-amplifiers can output time-varying signals.
Here’s why: https://x.com/raffi_hotter/status/1795910298936705098
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