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Comment by blensor | Hacker News Reader

I have no idea how hackernews always reads my mind about this kind of things but I started thinking about this topic due to a personal interest a few days ago.

And my question is. Can the ultrasound probe be made of off-the shelf parts. I think I understand why it's not possible with electromagnetic parts alone (kind of a speaker) they just can't vibrate fast enough to reach the Mhz frequency range.

But would it be possible to use a regular crystal oscillator they should be cheap, can easily be found for Mhz frequencies and are basically the same technology as the PZTs used in ultrasound probes. Just remove the casing and excite them, would that work?

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I'd been thinking about this technology recently as well, but hadn't gotten any further than perusing data sheets a little.

My impression is that quartz crystals as found in oscillators are so brittle that it's hard to use them for things like this; I guess PZT (lead zirconium titanate) is more robust somehow? There are also polymer-based piezo materials, like PVDF.

Thanks for that explanation. What would happen with the quartz crystal. Would it just not emit the sound or would it crack under the vibrations?

And if it can't withstand the vibrations it generates, how far off would it be from the required energy (many orders of magnitude?)

Another potential issue is that ultrasound machines send out an impulse instead of a single frequency. So maybe the quartz crystal oscillators aren't good at producing a short high-bandwidth signal?

That being said, there's a type of radar system called "stepped frequency continuous wave" radar that uses a bunch of single frequency transmissions instead of an impulse transmission. The basic idea is that instead of using a high-bandwidth transceiver to send and receive the impulse, you can use a low-bandwidth transceiver to send and receive tones, and then hop this transceiver over the large bandwidth to get a high-resolution image (the tradeoff being that it takes longer to acquire an image). Since ultrasound is basically radar, I'd imagine this technique could be used for ultrasound too.

But wouldn't a lower frequency be at the cost of spatial resolution due to the longer wavelength?

The spatial resolution depends on the bandwidth of your transmission. An infinite-bandwidth signal is a delta in time, and gives infinite precision. Any finite bandwidth impulse will be a sinc in time, with temporal width proportional to the inverse of the bandwidth. So a higher bandwidth impulse will be shorter in time, which would give better temporal resolution.

Typical radar works by sending and receiving a high-bandwidth impulse, which requires high-bandwidth transceivers. Let's say that transmission occupies frequencies between f0 and f1. SFCW radar works by sending a bunch of individual low-bandwidth transmissions between f0 and f1. So each transmission is small, but together they occupy the same f0 to f1 bandwidth. Assuming the environment didn't change in the time it took to send all of those low-bandwidth transmissions, you've effectively simulated a high-bandwidth impulse using a bunch of low-bandwidth impulses. So the spatial resolution will be the same.

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