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Registered Member #3255
Joined: Wed Sept 29 2010, 01:37AM
Location: Pittsburgh, PA
Posts: 2
I’m new to the forum so a little about myself, I’m a 3rd year electrical engineering student, a ham radio operator, and love crazy projects.
I’m looking for some input on planning a dental x-ray based radio-graphing setup. My goal is to be able to take x-rays of relatively small objects, not larger than 6 inches. I plan on purchasing a self contained dental head unit which has specs of 70Kv, 10ma, 110v. It seems like the most efficient solution since I’m more interested in the radiographs than the design of a high voltage supply (although that does sound fun, I doubt I could make it anywhere near as compact).
A word about safety, as seems customary I will elaborate on my plans for executing this project safely: I chose a dental system since they’re well contained, simple to set up, and fairly cheap. As far as I can tell, all dental tubes have sensible outputs with an Al filter and copious shielding to attenuate and directionalize the output. The enclosure I had in mind was a “wine†cellar in my basement, an 8X12’ room which has cement walls/floor on 5 sides plus a heavy steel door. I plan on triggering the tube with a closed loop wireless remote timer (2 atmegas, 2 zigbees, etc…) from very far away >100ft. I’m fairly handy at circuit/microcontroller design so this should only be a mild challenge. Built into the timer/remote I plan on implementing multiple interlocks and a max exposure time of 5 seconds.
For basic imaging, I have a Sony digital camera with a fast lens that I plan to aim at the phosphor screen of an x-ray cassette. The camera’s shutter will be triggered by the custom timer module.
As far as actual questions for the forum:
For radiation measurement, all I have is a Radex 1706 which “claims†to be able to measure x-rays but my research tells me that these GM tubes really only alert you to the presence of x-rays and are largely useless for quantitative dose measures. What would be the proper device to maximize safety and leak detection?
I chose the dental unit because it should be safer than energizing a bare tube. Is the emission of a dental unit at all predictable? Will I need more than cement and distance between me and the backscattered radiation?
Finally, what kind of image quality can I expect from a dental tube? Will 70Kv x-rays with a max exposure of 5 seconds yield usable penetration of common objects and/or circuit boards?
Thanks for taking the time to parse all this text. I’m really looking forward to a “peer review†of this plan.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
I’m looking for some input on planning a dental x-ray based radio-graphing setup. My goal is to be able to take x-rays of relatively small objects, not larger than 6 inches. I plan on purchasing a self contained dental head unit which has specs of 70Kv, 10ma, 110v. It seems like the most efficient solution since I’m more interested in the radiographs than the design of a high voltage supply (although that does sound fun, I doubt I could make it anywhere near as compact)
That's a good idea. Using a dental x-ray head saves you a lot of work, as building a suitable power supply for an x-ray tube can be a big project in itself. An additional advantage is that x-ray heads are internally shielded according to strict safety regulations, making shielding of the whole setup a lot simpler. In addition, complete x-ray heads tend to sell for a lot less than both x-ray tubes and x-ray transformers on eBay. Have you already bought the head? If not, I can give you a few tips on what to get and how much you should expect to pay for it.
A word about safety, as seems customary I will elaborate on my plans for executing this project safely: I chose a dental system since they’re well contained, simple to set up, and fairly cheap. As far as I can tell, all dental tubes have sensible outputs with an Al filter and copious shielding to attenuate and directionalize the output. The enclosure I had in mind was a “wine†cellar in my basement, an 8X12’ room which has cement walls/floor on 5 sides plus a heavy steel door. I plan on triggering the tube with a closed loop wireless remote timer (2 atmegas, 2 zigbees, etc…) from very far away >100ft. I’m fairly handy at circuit/microcontroller design so this should only be a mild challenge. Built into the timer/remote I plan on implementing multiple interlocks and a max exposure time of 5 seconds.
That's correct, dental x-ray heads are required by law to have 1.7mm of aluminium equivalent filtering if they operate at 70kVp or below, and 2.7mm if operating above 70kVp. They also have cast barium sulfate or lead oxide shields around the tube, for directionalizing the output and getting rid of most of the off-axis radiation. Your safety plan sounds good, it's probably as safe, if not safer than the x-ray rooms in dental clinics.
For basic imaging, I have a Sony digital camera with a fast lens that I plan to aim at the phosphor screen of an x-ray cassette. The camera’s shutter will be triggered by the custom timer module.
That should work well. There is actually a lot of difference between phosphor screens, in regards to resolution. For best results, you should get the screen from a "Fine" or 100 speed cassette, or even better, from a mammography cassette. Fast cassettes are normally around 1 line pair per millimeter at an MTF of 0.5 (the MTF curve of a screen describes the contrast vs. resolution function, but a google search should give you a lot of info on this subject, I'm simply using the resolution at an MTF of 0.5 to compare different types of screens). Medium tends to be around 1.5lp/mm, regular around 2lp/mm and Fine around 2.5lp/mm. Mammography screens tend to be between 4 and 5lp/mm, so significantly better. Brand names for mammography cassettes are Kodak Min-R, Agfa Mamoray and Fuji Mammo.
For radiation measurement, all I have is a Radex 1706 which “claims†to be able to measure x-rays but my research tells me that these GM tubes really only alert you to the presence of x-rays and are largely useless for quantitative dose measures. What would be the proper device to maximize safety and leak detection?
I found a datasheet on the Radex 1706, and it is apparently sensitive to x-rays in the range of 30keV to 3MeV. Since a lot of the x-rays from dental tubes have energies of lower than 30keV, I wouldn't really trust this unit for safety. Most common geiger counters tend to be very non-linear in their energy response at lower energies, so they are not good for quantitive measurements. Most of them are actually more sensitive at lower energies, so if you find one that registers down to 10keV, it should be good to check for leaks.
I chose the dental unit because it should be safer than energizing a bare tube. Is the emission of a dental unit at all predictable? Will I need more than cement and distance between me and the backscattered radiation?
Dental units are made according to regulations, and there are limits to how much off-axis radiation they can emit. I'm not sure what the limit is though, but a geiger counter will be able to tell you. As for the backscattered radiation, it's a very tough question to answer, especially without knowing how thick the concrete is, I would use a geiger counter sensitive to the energies in question. But I whould think that at 70kV or less on the tube, and a concrete wall and 100 feet of air between you and the backscattered radiation, it would be much less than background radiation.
Finally, what kind of image quality can I expect from a dental tube? Will 70Kv x-rays with a max exposure of 5 seconds yield usable penetration of common objects and/or circuit boards?
Dental tubes usually have focal spots of around 1mm, with some newer ones having smaller focal spots. This means that the fluorescent screen will be the limiting factor when it comes to resolution, unless the x-ray source is very close to the subject, or the fluorescent screen is a distance from the subject (magnification radiography). Here is an example of a radiograph done with a dental x-ray head and film, and here is an example with a dental x-ray head and mammography screen. The hard drive is a 2.5'' one.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
kanonic wrote ...
I plan on purchasing a self contained dental head unit which has specs of 70Kv, 10ma,
At a distance of 100cm, a tube with these inputs and an Al filter will deliver a dose rate of about 20 Gy/hr.
kanonic wrote ...
For basic imaging, I have a Sony digital camera with a fast lens that I plan to aim at the phosphor screen of an x-ray cassette.
Placing a digital camera in the beamline may damage it. Consider imaging the screen in a front surface mirror, so the camera can be at rightangles to - and out of - the beam.
kanonic wrote ...
For radiation measurement, all I have is a Radex 1706 which “claims†to be able to measure x-rays but my research tells me that these GM tubes really only alert you to the presence of x-rays and are largely useless for quantitative dose measures. What would be the proper device to maximize safety and leak detection?
GM tubes are unsuitable for X-ray measurement, and have no role in radiography. A calibrated ionization chamber remains the gold standard for X-ray measurement.
kanonic wrote ...
Is the emission of a dental unit at all predictable?
The emission of tubes still within their manufacturer's guarantee period is predictable. Used tubes may have pitted and cracked anodes, sputtered tungsten deposits on the glass that may eventually lead to failure, and so on.
kanonic wrote ...
Will I need more than cement and distance between me and the backscattered radiation?
Follow the ALARA principle, and always plan for the worst case scenario.
kanonic wrote ...
Finally, what kind of image quality can I expect from a dental tube?
As good as dentists get! A clapped out tube with a pitted, cracked anode like the lunar surface will no longer be capable of very high resolution because of the blurring of the focal spot. You can image your spot with a pin hole camera to verify its condition.
kanonic wrote ...
Will 70Kv x-rays with a max exposure of 5 seconds yield usable penetration of common objects and/or circuit boards?
Remember that only a very small percentage of the photons emitted by a tube run at 70kVp will have an energy of 70keV. Very roughly two thirds of the output will be in the lower one third - that is below ~23keV when you have 70kVp on the anode. These long wavelengths are the most dangerous because they are the most readily absorbed by the human body, yet they contribute little to the image. Aluminium filters are generally fitted as standard in dental tube heads to attenuate the softer rays in the interests of both safety and better image resolution.
But you will certainly be able to image circuit boards and so on.
For my own part, I would not consider having anything to do with such an unnecessarily powerful a device as a dental tube, Using XRD and XRF tubes with 15 - 30kV or so on the anode, and currents of a few hundred microamps, allows a dramatic reduction in shielding costs, as well as being much more interesting.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Proud Mary wrote ...
For my own part, I would not consider having anything to do with such an unnecessarily powerful a device as a dental tube, Using XRD and XRF tubes with 15 - 30kV or so on the anode, and currents of a few hundred microamps, allows a dramatic reduction in shielding costs, as well as being much more interesting.
I think the dental unit is probably perfectly fine. Those low-powered tubes can give you nasty skin burns because the lack of aluminium filters lets them emit lots of soft X-rays, so you have to do your own safety work when you get one. The dental head comes with the safety measures already in place, so all you need to do is not get any closer or use longer exposures than your dentist would. (they usually leave the room and work it with a fire button on the end of a long cable)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
Proud Mary wrote ...
For my own part, I would not consider having anything to do with such an unnecessarily powerful a device as a dental tube, Using XRD and XRF tubes with 15 - 30kV or so on the anode, and currents of a few hundred microamps, allows a dramatic reduction in shielding costs, as well as being much more interesting.
I think the dental unit is probably perfectly fine. Those low-powered tubes can give you nasty skin burns because the lack of aluminium filters lets them emit lots of soft X-rays, so you have to do your own safety work when you get one. The dental head comes with the safety measures already in place, so all you need to do is not get any closer or use longer exposures than your dentist would. (they usually leave the room and work it with a fire button on the end of a long cable)
Yes, that is fair comment, which is why I stressed that I was speaking 'for my own part' of my own personal safety preferences.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
The emission of tubes still within their manufacturer's guarantee period is predictable. Used tubes may have pitted and cracked anodes, sputtered tungsten deposits on the glass that may eventually lead to failure, and so on.
I can't claim to have the absolute knowledge in this area, but I have about ten dental tubes, some bought on eBay, and some I've taken out of dental x-ray heads myself. I've also seen several pictures on the web of other dental tubes from decommisioned x-ray heads, and the condition of the tubes is usually great. They aren't really run that hard in usual service, so even well-used units from the seventies have tubes with hardly browned glass. Most of my tubes are from the seventies, and none of them show visibly browned glass unless I view a piece of white paper through the glass. As for the anodes, the focal spot is usually visible as a slightly rougher texture on the tungsten anode, but nothing enough to drastically alter the tube characteristics. I've only seen cracked anodes and tungsten deposits on the glass in tubes that have been operated way outside of their ratings, either due to operator error or malfunctioning control gear.
Again, YMMV, as these are not absolute truths, just my observations.
Placing a digital camera in the beamline may damage it. Consider imaging the screen in a front surface mirror, so the camera can be at rightangles to - and out of - the beam.
This is true, thanks for mentioning it. The chance of actually killing the camera is small, but the radiation will most likely lead to a lot of noise in the form of hot (lit-up) pixels in the picture.
GM tubes are unsuitable for X-ray measurement, and have no role in radiography. A calibrated ionization chamber remains the gold standard for X-ray measurement.
A GM tube with a known energy response can be useful in looking for radiation leaks, as long as the tube is sensitive to the energies in question.
As good as dentists get! A clapped out tube with a pitted, cracked anode like the lunar surface will no longer be capable of very high resolution because of the blurring of the focal spot. You can image your spot with a pin hole camera to verify its condition.
This is a good idea, do you have any tips on making a good pinhole? I suppose the material would have to be quite radiopaque. Maybe a thin lead sheet with a needle hole in it.
For my own part, I would not consider having anything to do with such an unnecessarily powerful a device as a dental tube, Using XRD and XRF tubes with 15 - 30kV or so on the anode, and currents of a few hundred microamps, allows a dramatic reduction in shielding costs, as well as being much more interesting.
This is an interesting area of radiography as a hobby, but it has it's own dangers. XRD/XRF tubes have beryllium windows that let everything down to about 2keV pass, and this low energy radiation has serious potential of damaging the skin, even permanently removing it. I'm sure you know about this, just thought I'd mention it in case anyone else here wants to have a go with these tubes, and I haven't seen it brought up here before. Edit: Steve just beat me to it.
Also, I don't think dental tubes are overpowered. They may be if you want to radiograph small objects at close range, but if you're radiographing something more substantial, and using direct film exposure without intensifying screens, even doing it with a dental tube at full power can take minutes. With a lower powered tube, this would take hours, and this is often just not practical.
Grenadier wrote ...
I am surprised nobody said anything about a lead apron! I do agree too, using the whole head will definitely cut down on your worries.
I don't think a lead apron will do much of a difference when the head is behind a concrete wall 100 feet away and pointed in the other direction.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Anders M. wrote ...
This is a good idea, do you have any tips on making a good pinhole? I suppose the material would have to be quite radiopaque. Maybe a thin lead sheet with a needle hole in it.
An accurate, circular hole of between 0.01 - 0.03mm in a thin sheet of tungsten or lead foil will give good results where a simple pinhole is required.
The diagrams below show the templates of masks you can make or buy (£$$£) for testing the geometry and spacial resolution of the beam, from which conclusions about the state of the anode can be drawn.
Special gizmos are needed for centering the beam before the performance of these tests, but these can be made on the workbench.
Source:The Essential Physics of Medical Imaging. 2nd ed. Bushberg, JT et al, Lippincott, Williams & Wilkins, 2002.
GM tubes are unsuitable for X-ray measurement, and have no role in radiography. A calibrated ionization chamber remains the gold standard for X-ray measurement.
Proud Mary. Could you recommend any brands/suppliers. Also, what is your opinion on low dose (not CD) quartz fiber dosimeters.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
That's not a bad alternative, but there are cheaper options. This one for example, is a bit cheaper. I've bought stuff from this seller in the past with no trouble.
Edit: some more information.
As for the PCB oil, wikipedia says the use of PCB oils in transformers was banned in 1979, so anything made after this is pretty safe. I don't think PCB oil was commonly used in dental x-ray heads before this either, but I can't recall where I read this.
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Help planning an x-ray machine build
