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Fireside Chat: Q&A with the Xperts

In this Fireside Chat with the Xperts, Dr. Glen Thomas and Dr. Bill Cardoso take time to address some of the questions that have come up during earlier presentations.  We got some great questions from viewers, and the doctors really put the “X” in Xperts when tackling them.  Questions covered this week included:

  1. What are the differences between open and closed X-ray tubes?
  2. How are reflective and transmissive sources different?
  3. How does geometric magnification work?
  4. What is the difference between kV and mA?

If you are operating an X-ray system, considering how a system might suit a particular application, or just intensely curious about how X-ray machines work, this presentation will scratch your itch. Register for upcoming Fireside Chats with the Xperts and view our archives here.

 

Transcript:

David Kruidhof:

Morning, everyone. Thanks for joining us this week for another. Fireside Chat with the Xperts. Today, we’re doing a Q&A, that we’ve had several questions over the last few weeks on our topics that we’ve covered, we weren’t able to get all of the questions, so we just want to take a break for going into these new topics and answer some of those for you. So we have today, Dr. Bill Cardoso, our CEO, and Dr. Glen Thomas, our VP of Technology. So we’re pulling out the big guns, lots of knowledge here, lots of experience with X-ray. So we’ll be diving into these questions for you.

Dr. Bill Cardoso:

He was talking about you, Glen, when he said big guns, by the way.

Dr. Glen Thomas:

Yeah. Okay. Yep. We’ll see. We’ll see how stumped we can get. Sounds good.

Dr. Bill Cardoso:

Should have called the segment, “Stump the Chumps.” That’d be funny.

Dr. Glen Thomas:

There you go.

David Kruidhof:

So our first question, what are the differences between open and closed X-ray tubes?

Dr. Glen Thomas:

Open and closed X-ray tubes are, that’s a consistent question that we get on a regular basis. You have to go back to the basics of why an open X-ray tube was designed, and it all starts with the physics of X-ray, right? We can bend the rules, but the rules are always going to be there. And the rule is, you can have one watt of power per micron of focal spot size. Focal spot size gives us the resolution, because it’s a pin source, right? It’s a point source, just like a flashlight or even the sun. It’s a point source. So that is the point that the X-ray, or the electron beam hits the target anode and reflects.

Dr. Glen Thomas:

So with that one watt per micron limitation, in the late sixties, they were running into some problems in industry that we were looking, trying to see through jet engine impellers and jet engine components, right? And we needed the resolution to see some of those welds. The problem was that to get past that one watt per micron of focal spot law, essentially, we would have to open up the focal spot, to the point where you were less resolution on your X-ray tube than the requirement of the application, right?

Dr. Glen Thomas:

So some really smart guys got together and designed an open X-ray tube, which is essentially, all X-ray tubes have consumables. The consumable is the filament, which is where we burn the electrons off of the filament, just like a tungsten light bulb. And then we have the anode, which is the point that we have the electron beam coming from the filament, hitting the anode, producing photons. You have about a 3,850 degree temperature at that point. And that’s the main reason why we can’t exceed that wattage, because it will start melting holes in the tungsten. So essentially, to be able to pull that off reliably, pull that heat off, there’s 98% of X-ray energy that’s produced during making an X Ray image is heat. So we had to pull that heat off very quickly.

Dr. Glen Thomas:

To get around that concept, they decided that it would be a good idea to make those consumables replaceable by the operator. There are some other schemes in medical applications that we do the same concept as far as saving the focal spot, and being able to apply a lot more power to the focal spot. But that’s just, again, another way to trick physics, right? So the concept there was, we could take an image and we can take a very high power image, say 10 watts per micron, and we can intentionally destroy the X-ray tube. We can destroy the filament and we can destroy the anode.

Dr. Glen Thomas:

So an open X-ray tube is designed to be field repairable by a fairly experienced or trained technician. You’ll get a lot of rhetoric about, “Oh, it’ll last forever,” and you just got to change the filament and you just have to change the anode. It’s an easy task if you have the training to do so. So essentially this X-ray tube was designed that would vacuum itself down every time you turn the tube on. So you would take an image, you would get a two minute shot, you would just totally destroy the X-ray tube, but they were accomplishing what was needed. And what the requirement was, we need high resolution images of very heavy, dense components. So that was a success.

Dr. Glen Thomas:

Open X-ray tubes have a lot of ancillary sub components that no one ever seems to mention. They say, “Oh yeah, this tube will last forever. You can change the filament and rotate the anode. Right?” What they don’t tell you is you have a turbo molecular pump, you have a roughing pump, you have a high voltage component that’s almost, in some cases, larger than the cabinet itself, as well as a cooling system to cool the tube, right? Because if we’re pulling out all of that heat and we’re intentionally creating a lot of heat, you can literally destroy the X-ray tube housing. You can melt it. So we still have to pull off a real large amount of heat fairly quickly.

Dr. Glen Thomas:

So, in a closed X-ray tube is essentially that. It’s an X-ray tube that’s vacuumed down during manufacturer to about 10 torr, as far as pressure. The X-ray tube has a fixed element, it has a fixed anode, as well as, in most cases, a lot of focusing grids, or essentially magnetics that focus that electron beam to a really fine point. Service wise, there are no end user serviceable parts. In open X-ray tube, you have the filament, you have the anode, you have O-rings, you have greasing of the high voltage components like the wells for the high voltage cables. So there’s all kinds of different parts that have to be maintained.

Dr. Glen Thomas:

Closed X-ray tube, it either works or it doesn’t, there’s not really much that’s a user serviceable issue, but they are highly reliable and they are somewhat maintenance free. We get a lot of complaints or questions that says, “Well, I have to replace the whole tube when it breaks.” Yes. Closed X-ray tube, you do have to replace the complete housing, the complete tube, the high voltage, say in five to seven years, 10 years, depends on your usage, because if you remember, every X-ray tube has consumable components. Every time you create X-rays, you’re pulling electrons off of that tungsten, just like a light bulb in your house, and you turn on the tungsten light bulb.

Dr. Glen Thomas:

So eventually the X-ray tube, the closed X-ray tube, will die or will become unusable due to the pitting on the anode itself. So you get a really bad X-ray image, and it happens so slowly, over time, that you don’t even realize it. If you walked in, the operators using this machine, and unless he’s using IRQ or image quality indicators, he really has no clue that it’s degraded to the point that it has. And you walk in as a service guy, or even a sales guy, and you look at it and you’re like, “Wow, what’s up with that?” And they’re like, “What?” So, but over a five to seven year period, you will probably have to replace the closed tube.

Dr. Glen Thomas:

Yes, they are fairly expensive. Could cost tens of thousands of dollars, depending on the tube. But when you look at the individual maintenance, and downtime, and the expertise that it takes to service an open X-ray tube, the costs are pretty close to the same, right? So you pay a lot more money up front for the ability to have a lot more service issues with an open X-ray tube, and a closed X-ray tube, you pay a fair amount of money, but you have maintenance free for five to seven years. Right?

Dr. Glen Thomas:

One of the major disadvantages of that X-ray tube is, if you don’t really know what you’re doing, and you get some dust in there, so in a dusty environment or production environment, one speck of dust would be enough to destroy an open X-ray tube. Problems that you have with open X-ray tubes is when you, if you get a fingerprint, or you get dust, and you fire that thing up, you’re going to create carbon tracks. That X-ray electron beam is going to find a different place to land, other than the anode that you expect. And when it does that, it could pit the interior of the X-ray tube to the point where it’s now unusable, you have to buy a new one. A single fingerprint inside of an open X-ray tube will destroy the tube forever. It’s impossible to fix that. The oils in your skin will create an arc, and you will have a major issue.

Dr. Glen Thomas:

The only advantages of an open X-ray tube, a recent, in the last few years is, when you use it in electronics, you can get high magnifications. And the main reason for that is we can direct that electron beam to, say, the window of the X-ray source. Typically we would use an anode made out of tungsten, and that anode is embedded in a copper substrate, a bar. And essentially that bar is grounded and it has a heat sink on the end of it that’s immersed in high voltage oil, right? When you look at the open X-ray tube, we can actually aim that electron beam at the window, and we can use a diamond, and we can use some other exotic windows, that create the X-ray photons at the window. And what that does is that allows for extreme magnification on the X-ray source. So in applications like electronics, the only reason to go with an open X-ray tube would be if you needed extreme magnifications.

Dr. Glen Thomas:

And being a capitalist world, closed X-ray tube manufacturers have made some really unbelievable X-ray tubes in the last few years that rival the magnification of an open X-ray tube, in a closed tube package. So you get to all of the advantages of an open X-ray tube for a general operator with electronics, you’re never going to overdrive the X-ray tube intentionally. So you get all of the advantages of an open X-ray tube, as far as magnification, and you get it in a nice compact package, which results in a smaller cabinet in the X-ray, and a small overall footprint of the X-ray system itself, because you don’t have the external coolers, the external high voltage requirements.

Dr. Glen Thomas:

So it’s just, open X-ray tubes have their place. But in my opinion, open X-ray tubes really don’t have a place in electronics unless you’re looking at high magnifications, and we’re eliminating that as well.

Dr. Bill Cardoso:

Yeah, I think that’s been a huge improvement, or development, or the past few years is that transmissive closed tube source that’s being… And just for some background, on a closed tube, or even in an open tube, we have two different processes to generate X-rays, right. You can have your target, your anode, it can be on an angle like this one, pretend my phone’s a target here. And you have what’s called an electron gun, or the filament, that’s going to warm up and generate free electrons. Those electrons get accelerated and when they hit the target, you have this shower of X-ray photons getting out of the source, right? So X-rays get generated here, they go super fast, hit the target, and there’s something called bremsstrahlung radiation. So the electoral is moved out of the shell, and then when it goes back to its shell, it emits this X-ray photon. So that’s called reflective tube, because you can see it reflects, there’s a reflective motion to generate X-rays.

Dr. Bill Cardoso:

And then what Glen was talking about is the transmissive. So the window that provides that vacuum seal of the tube, so this is the outside world, this is inside the tube. Instead of reflecting, what you do is you put the target at the window and you shoot the electron beams right at the window, and on the other side you have X-ray photons coming out.

Dr. Bill Cardoso:

So what does it matter? Right? Because this method is really, really, really hard to make. The reflective method, it’s been around since the late 1800s. That’s what Röntgen was playing with when he discovered X-rays in the first place, right? This, the advantage of reflective, right? It’s been around for a while, because here in the back of the anode, you’re usually going to see a chunk of copper, a chunk of metal, to cool off this side of the X-ray tube. Why? Because as you have, as Glenn was saying, right, you have a lot of electrons hitting this target. It’s going to warm up, right? It’s going to create a lot of heat, and you have to dissipate the heat. If you don’t, you end up putting a pin, a hole on the target, and that hole is going to screw up with your resolution. Why? Because out of this target, you want the X-ray’s to get out of here in a perfect cone, every line going like this, going like that. But perfect, so that target ideally is a point in space.

Dr. Bill Cardoso:

If you start warming up because of the temperature, the stress of the material of the target, what happens is that there’s not a point anymore. Now it became an area, and in that area is going to happen, is this electron comes this way, one X-ray photon on goes that way, the other one goes that way, and as a result, you end up with a fuzzy image. So the edges of the image are not as sharp as they could be. This is the difference between mini focus or micro-focus or a nano focus X-ray resource, the smaller the target, the better the resolution, the lower the power that you can run. Like Glenn said, if you want to blast a lot of power, you’re not going to have a tiny target anymore. Right? It’s going to warm up and make your resolution worse.

Dr. Bill Cardoso:

So there’s a lot of heat here, right? So if you take this target and you put at the window, you want this window to be as thin as possible. If you have a thick window, a block of copper to cool off the target, you’re not going to have an X-rays coming out, right. They’re going to be shielded by the window itself. So you need thermal dissipation, right? A low impedance thermal path from the target to your cooling, heat sink. And at the same time it has to be really thin, ideally, no matter at all. So that is a big challenge that X-ray source companies have been working with the past few years. And diamond is one of the materials that fits the profile of being good heat dissipation, it’s very strong and you can make it very thin, right? So it doesn’t shield the X-rays that you’re generating.

Dr. Bill Cardoso:

And so what does it matter? What does it matter if it’s a reflective source or a transmissive source, right? Why go through the pains of making a transmissive source with this exotic window? Because, can you imagine, you have to hold vacuum here, right? So you have the box of the X-ray source, that’s my little window that this target, and at the same time it has to hold vacuum. What happens when things warm up? Expand? What happens when they cool off? They contract, right? So this has to keep vacuum, at the same time that it has to stay stationary in one place, because you don’t want your target doing this. Right. And it has to hold vacuum while expanding and contracting, because of the heat that’s being generated here. So not an easy mechanical object to build.

Dr. Bill Cardoso:

It matters because of magnification. Right? Glenn also mentioned magnification. Magnification, if you imagine you have your source here, okay, on my window, the sample is going to be sitting on top of the source. Then I have my sample, and then the sensor is going to be on the other side of the sample, and the X-ray image is going to be the shadow of my sample on to the sensor. The magnification is going to be, so M for magnification is going to be one plus the distance between the sample and the sensor, is the numerator. And the denominator is distance between the sample and the source. So what does that mean? So it’s one plus this distance, divided by this distance, what does it mean? It means that to magnify, I have to bring my sample as close as possible, right? As close as possible to my source.

Dr. Bill Cardoso:

Now, if my source is like this, and that point is several, maybe a half an inch, an inch away from where the window is, that limits how much I can magnify my image, right. Now, if my target is now 300 microns behind a window, or half a millimeter, right, now I can bring my sample, place right on top of the source and achieve very, very high degrees of magnification. So this transmissive closed tubes have really allowed us to provide the best of both worlds. Really high mag, because a transmissive source, and at same time, a closed tube. So we can offer a low maintenance X-ray source.

Dr. Bill Cardoso:

And so nowadays you could ask us, when do we use one or the other? Because we’ve used both, and open tubes and closed tubes, they have their place in X-ray applications. For the high mag, in the past, we used an open tube, now we’re using this transmissive closed tubes because they’re just easier to use, and we don’t have to be worrying about the two vacuum pumps and a high voltage block, and a cooling system for the tubes, it’s much simpler, more compact and make things much easier for the user. And we leave the open source nowadays for the high power stuff, right. But when you get to the 225, 300 kVs and up, we’re going to be talking about an open tube application, because at that level or that amount of power, there’s no diamond enough today that can pull that kind of power. Now we’re talking, you’re going to be discussing rotating anodes, we’re going to be talking about a chiller that can pull quite a bit of heat out of the source.

David Kruidhof:

All right, thank you, gentlemen. That was a lot about reflective or transmissive as well.

Dr. Bill Cardoso:

Well that was the, we were summarizing, right, Glen?

Dr. Glen Thomas:

Exactly. We had at least three questions there.

David Kruidhof:

Great.

Dr. Glen Thomas:

That’s good.

David Kruidhof:

All right. That’s good. Well, we got eight minutes, so I’ll give you a simpler one.

Dr. Bill Cardoso:

It’s got to be a yes or no question, otherwise it’s going to take more than eight minutes.

David Kruidhof:

What’s the difference, practically speaking, right? What’s the difference between kV and mA, kilovolts and amps, milli amps, micro amps, in a source? What does that practically mean, when a customer is designing a system?

Dr. Glen Thomas:

So that’s another one that could be a long conversation, right?

David Kruidhof:

Don’t make this one long, Glen.

Dr. Bill Cardoso:

Yes. There is a difference.

David Kruidhof:

There is a difference. Done.

Dr. Glen Thomas:

So if we look at kV and mA, and an easy way for a layman, without getting into the physics, is to think of a garden hose, right? Consider the X-ray tube, the same as a garden hose, where we’re projecting photons, and in this case, where our water hose is projecting water, right? So if you look at the kV, the kV would be the pressure, the amount of pressure that pushes the water, or the photons out of the hose, right? So the kV gives you the ability to penetrate items. The higher the kV, the better, and the faster the photons are going to be able to traverse through the sample. So kV is the penetration, that’s the power behind the electrons and the photons.

Dr. Glen Thomas:

mA, if you think of the water hose analogy, the mA would be the diameter of the hose, the amount of water, or in an X-ray tube, the amount of photons that make up the X-ray beam. Because we could essentially have an X-ray beam that’s 150 kV and one photon, right? You will get a small blip on some screen somewhere, but you won’t be able to create an X-ray image. To create a usable X-ray image, you need to have the proper amount of penetration, which is the kV, to be able to penetrate the part, and you need enough photons to actually create an image. If you think of that, you think of a dot matrix printer, right? The lack of photons would be the same as if you had a dot matrix printer that was putting out, or even worse yet the nine pin printer of the eighties, right? You’ll get a definite-

David Kruidhof:

Maybe he doesn’t know what that is, Glenn.

Dr. Glen Thomas:

You’ll get a definite degradation of the image. So the mA gives you more photon statistics, the ability to light up the image detector, whether it’s a film, digital detector, image intensifier, regardless, the mA gives you more photon statistics. It puts more photons on the image. And then it goes back to that physics concept, as we still have a wattage issue. Right? So if we have, typically in an electronics X-ray system, pretty standard is nine watts of power, right? So that gives us the ability just to do a hundred, let’s say 90 kV at 0.2 mA right, is a good number. 0.256 mA. So we use very low mA. So we have the ability to maintain that focal spot.

Dr. Glen Thomas:

A lot of X-ray tubes are iso watt, which means that they are limited in wattage. You can’t overdrive the X Ray tube. If you crank up the kV to a real high point, the X-ray system will drive down the mA, to maintain a stable wattage that stays within that safety threshold of the X-ray source and the X-ray tube. So in a lot of cases, you’re not using the full kV of an X-ray source. Your optimum kV is somewhere in between zero and the full, maximum power. So with that, you get a happy medium in kV, which is the ability to penetrate the part. Because if you over penetrate the part, you start losing artifacts, or you start losing details in the actual X-ray image. So there’s a fine point where kV and mA make a difference. If you over penetrate the part, and you don’t have enough kV, you could still get an image, but it’s going to be a very grainy image. It’s going to be featureless for a matter.

Dr. Glen Thomas:

So what you want to do is have a happy medium of kV and mA. And through algorithms, back in the day, you had to be fairly fluent in X-ray to understand how much kV and mA. A practitioner like an RT in a medical industry, they’re really good at knowing what their kV and mA is going to come out to. They make those determinations fairly quickly, because their samples change. They may be doing an elbow or wrist, but each individual has a different density, so they will make those changes on the fly. And they’re pretty highly trained at being able to make those judgments.

Dr. Glen Thomas:

Used to be X-ray, industrial X-ray, was the same thing. Back in the day, when it was all film, and you only had one shot one and chance. So you would climb up on a bridge, or you would climb up on a tower, essentially they would stack the film and they would run a multiple, so they would stack the film and they would do one X-ray image at, say, one mA, 10 seconds at a hundred kV. And then they would pull one off the film, and since film is a cumulative, they would continue to add time and kV and mA adjustments to get a bunch of exposures, right? So they would have a lot of different exposures in this one shot, and they were up in the air. So they would come down, and then they would pick the best image out of, say, 10, right? So that kV and mA adjustment is somewhat tricky.

Dr. Glen Thomas:

And one thing that we’ve done to eliminate some of those issues for an operator is the wise software that we have. It adjusts kV and mA, and it adjusts brightness and contrast, and has some algorithms that will give you the ability to make those changes seamless. The software looks at the exposures and looks at the gray scale values based on the histograms, and will make those adjustments for you. So all you really need to know if you’re an operator is the basics of what they do. And the main reason for that is that you get a general idea of what kV you need to not over penetrate the part, and you let the X-ray system and the software make those other decisions as far as how much brightness, contrast, exposure rates and the mA that’s needed to accomplish the goal.

David Kruidhof:

Nice, thanks, Glen. Right on time. It’s 10:30 on the dot. You must be watching the clock.

Dr. Glen Thomas:

Well, we can keep going.

Dr. Bill Cardoso:

He’s done it before, David.

David Kruidhof:

Well, we have a long list of questions. So we’re going to continue this next week. I want to keep these to half an hour for you guys, as we said we would. So if you have any other questions that came up today, or questions you just want answered in general, if you’re watching this on YouTube later, you can leave a comment below, or you’re welcome to email us info@creativeelectron.com. Thanks, Glen. Appreciate it. Thank you, Bill for your time.

Dr. Bill Cardoso:

Thank you, guys.

David Kruidhof:

And please join us again next week.

Dr. Glen Thomas:

Thanks. Take care.

David Kruidhof:

Bye.

Speaker 5:

Creative Electron

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