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My dad requires many CT scans, and we're asking this out of curiosity. We're not promoting or affiliated the company.

Can someone please explain the para. below like I'm 5, in Simple English? The company's website uses even more abstruse parlance, and we don't understand anything!

At issue is its invention of a cold cathode X-ray tube for CAT scans. The current technology involves using a large amount of thermal energy to create an electron beam that forms the basis of the X-ray (hot cathode tube), which results in more than 99% of energy being lost as heat. Cold cathode tubes involve the application of an electric field to extract electrons from a metal to form the X-ray, dramatically increasing the device's energy efficiency.

  • I don’t see any obvious reason that cold cathode technology would make having lots of CT scans more risky or less risky. Is that what you’re asking? – G. Smith Dec 29 '20 at 05:40
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    @G.Smith It doesn't seem like they are asking about risk. They are probably just looking for some basic information on how electrons are initially generated for use in an accelerator. – kaylimekay Dec 29 '20 at 05:46
  • @ESL the general concepts behind thermal generation of electrons for applications like this can be found here https://en.wikipedia.org/wiki/Thermionic_emission – kaylimekay Dec 29 '20 at 05:47
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    Unstated is the assumption that reducing the energy consumption of a CAT scan is highly desirable. They probably hope people will think this will reduce the cost of a scan. I am sure the cost of the electricity is trivial compared to everything else involved in a CAT scan. – Ross Millikan Dec 29 '20 at 06:11
  • I just read the blurb. the technology they are pushing does not seem to hold any great advantage over the default i.e., thermionic emission-driven x-ray tubes. lots of snazzy blah-blah, but no fundamental breakthrough. – niels nielsen Dec 29 '20 at 07:20
  • As an electrical engineer I’d add that it’s not just about heat. The size of generator of the X-rays can be smaller for the focussed energy. Lower power consumption can lead to smaller equipment sizes, therefore portability etc – Chris Maxworthy Sep 14 '21 at 01:23

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Cold cathode emission of electrons is more efficient, but the effectiveness of the actual electron emitter is extremely sensitive upon the surface chemistry and sharpness of the tip from which the electrons are being pulled by the field.

As pointed out by others, the electricity cost to run a CT machine is small in comparison to the worth of the data it collects, and in any case the current required to heat the emitter in the x-ray tube is a small fraction of the total power consumption of the whole machine. So running an x-ray machine on cold cathode emission is not going to generate significant power savings.

Then there are other problems that the blurb you quoted does not address, as follows.

It takes a significant amount of x-ray flux to take CT scans, which for a hot-wire e-beam emitter means lots of current through the hot filament. And there's the problem: To get the same beam current off a cold emitter requires the same current to flow through the emitter tip, and the ohmic heating of the tip is capable of melting it into bluntness, at which point it stops emitting electrons because the electric field gradient at the tip is reduced when it gets rounded off. Preventing this is a hard problem.

Emitter tip degradation because of current heating is the core problem in implementing cold emission devices in general: they are very delicate devices. One current surge through their micro-spindt emitter array will fry the emitters into oblivion.

niels nielsen
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  • +1 This is the real answer. The advantage of cold cathodes is their brightness (i.e. more x-rays per angular spread), which is much higher than heated cathodes, but they have much lower overall current (i.e. less total number x-rays). I don't see how the invention in question is making use of the brightness vs current tradeoff though. My guess would be that CT scans are not in the right regime to make use of brightness compared to just having a higher total number of x-rays. – KF Gauss Feb 19 '21 at 08:29
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Atoms contain negative electrons and positive nuclei. These attract each other and stick together strongly. When you have an electric current, electrons flow in a wire.

To generate X_Rays, you need to get electrons out of a wire (the cathode) into a vacuum. You need to accelerate them to high speed and then slam them into a thin metal target (the anode). The sudden stop makes them give off X-Rays.

Getting them out of a wire can done be a couple ways.

  1. Normally there are just enough negative electrons to balance the positive nuclei. But it is possible to push extra electrons into a wire. If you push enough on, they repel so strongly they fly out of the wire. You see this as a spark when you have static electricity. Here the goal isn't to create a spark, but a steady stream of electrons.

  2. If you heat the wire containing extra electrons, the atoms vibrate harder. This can "shake loose" electrons, even if you don't have enough extra to create a spark.

The second way requires keeping the wire hot like a light bulb filament. It takes a lot of energy.

The first way doesn't require heating. I expect it requires higher voltages to push more electrons onto the cathode. But I don't really know the difficulties involved.

mmesser314
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To make X rays you want to liberate electrons from a surface which you can then accelerate through an electric field and send into a target that will generate the X-rays. The usual way to get the electrons is to heat a cathode so that some of the electrons "boil off". An alternative approach is to put a cathode into a strong enough electric field so electrons are pulled off by the field. This is called "cold cathode field emission" and is well known. It is true that heating the cathode takes a fair amount of energy, but it is an easy device to make. Cold cathode emission takes such a high field that the tolerances in making it are rather small. They seem to want you to infer that a cold cathode device will be less expensive to operate because less energy is wasted. I haven't looked it up, but I strongly suspect that the cost of energy for a CAT scan machine is trivial compared to all the other costs involved. That doesn't say that cold cathode emission is a bad idea for a CAT scan, but it suggests you should look elsewhere for a reason to build the machine differently from the ones that exist today.

Ross Millikan
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  • You're right about the power. A few watts is neither here nor there. The reason might be that in hot cathode tubes the current is controlled by filament temperature (probably because a grid would require too much voltage unless it is extremely close like those nano scale ones), and the filament is slow to respond. – Peter R. McMahon Aug 06 '21 at 00:56
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The cold cathode works like corona discharge. If you hold the point of a sewing needle towards a statically charged object in a dark room you will see a small glow off the end. It works for both polarities in air, but works better if the object is positive (point negative). You can check the polarity with a small neon lamp (the gas lights up around the negative electrode). If the point is small and sharp enough, the highly concentrated field at the tip will force electrons into a vacuum. To control the current, there is a plate with holes which acts like the grid in a picture tube (or a valve) and repels the electrons. With hot cathode tubes, they control the current with the cathode temperature. They don't use the anode voltage to control current because it determines the frequency and penetrating power of the X rays (high voltage high freq. X-rays for seeing the inside of bones, and lower voltage lower freq. to see soft tissue).Early tubes were cold cathode with a bit of gas in them. The trouble was, they couldn't control the current and anode voltage independently, so it was difficult to control the frequency of X-rays and dose. They had to do it by time, but with modern technology, they can emit electrons into a vacuum and use a grid to control the current. When operating, most of the heat is generated in the anode by the electrons hitting it, like in a valve. Only a small part of the energy is converted into X-rays.

Peter R. McMahon
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Like you are five years old?

We can create X-rays but the tools we have for that require a lot of heat. When we create a lot of heat, most of it isn't actually useful for creating an actual X-ray. So these guys are looking into a way of creating X-rays without requiring a lot of heat.

tripleee
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