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X-Ray Machines

What, exactly, is an x-ray machine?
An x-ray machine is just one example of a radiation-generating device, sometimes called an “RGD”.

Why are x-ray machines RGDs?
Simply because they generate ionizing radiation when they are in operation. For your information, many other types of devices are also categorized as RGD’s even if they are used for a different purpose.

What kinds of devices?
Well, if we look to the U.S. Department of Energy (DOE) in their Radiation-Generating Devices Guide, DOE G 441.1-5 (April 15, 1999), which is used to implement the requirements of Title 10 of the Code of Federal Regulations Part 835 or 10 CFR Part 835 (“Occupational Radiation Protection”), we find that a variety of devices are classified as RGD’s. These include sealed photon- or neutron-emitting radiation sources; x-ray producing radiography equipment; research and analytical x-ray or electron beam machines; sealed radioactive sources used as irradiators; particle accelerators; neutron generators; Van de Graff generators; electromagnetic pulse generators (limited to those that produce ionizing radiation); electron microscopes; electron arc welders; microwave cavities that produce x-rays incidentally, and cabinet x-ray machines used for security applications. While not mentioned in the guide, devices that produce x-rays for use in the health care industry would also apply.

Wow. That’s an impressive list!
Yes, it is. The important point is that an RGD can be either a sealed radiation source that emits ionizing radiation, or a device that produces x-rays incidental to its operation. X-ray machines fall in the latter category.

Where can I find x-ray machines?
Almost anywhere in both commercial and government settings, and in industrial and medical applications.

What are some examples of x-ray machine applications?
If you want an excellent historical example of such a device, look no further than “shoe-fitting fluoroscopes”. These are x-ray emitting devices that were used several decades ago to provide a radiograph of the bones of the feet to assist with correct shoe size selection .

Decades ago? What happened?
Well, the idea did seem like a good one at the time . . . a more high-tech way of ensuring a good shoe fit than just pushing down out the outside edges of a new pair of shoes. It seemed especially elegant for children, who can be even harder to fit than adults. However, the radiation protection industry quickly realized that the benefits of these devices did not outweigh the risks of the exposure, especially when it became clear that children and teenagers were so enamored with these devices that they spent quite a bit of time observing the bones of their feet. As you can imagine, they received multiple radiation exposures. So we only saw shoe fitting machines in the stores for a short period of time.

How high was the radiation exposure the children and teenagers received?
From what we know of the machines in use at that time, individual exposures could have ranged from a few hundred millirem when everything was working correctly, to several thousand millirem per hour if the housing of the machine “leaked”. By the way, when we are talking about x-rays, a “millirem” is approximately equal to a “millirad”.

Interesting, but that was yesteryear. What about now? Are there newer applications of x-ray machines?
Yes there are. There is a particular type of screening device known generically as a “cabinet x-ray machine”, that you might find interesting. We call them that because the unit is designed to enclose and shield the x-ray-generating components from those standing in its vicinity.

Where can I find some of these?
Perhaps the best example today is that of the airport baggage inspection systems every flying member of the public is well acquainted with. Using similar equipment, many facilities in this country also utilize mail/package scanning devices which employ x-rays as the specific type of radiation produced while the device is operating. In both cases, the energy of the x-rays produced by the scanner is relatively high so that they can penetrate a dense object such as a suitcase or mail parcel and provide an image of its contents in order to detect any suspicious materials.

How interesting. Are there any other examples?
X-ray machines are also commonly used for radiography, which means the examination or inspection of the structure of materials by non-destructive means. In this case, “non destructive” means the passage of x-rays through the material does not affect the material in any way.

Any more?
Of course there are medical applications . . . perhaps the most widespread use of x-ray machines.

What exactly is an x-ray?
X-rays are a type of electromagnetic radiation. Other types include radio waves, microwaves, infrared, visible light, ultraviolet, and gamma rays. The various types of radiation are distinguished by the amount of energy carried by the individual photons.

What do you mean by that?
All electromagnetic radiations consist of pure energy in the form of photons, which are often referred to as “individual packets” of energy. For example, a household light bulb emits about 100,000,000,000,000,000,000 photons of light each second. However, this light is non-ionizing. Nonetheless, just like ionizing radiation, we can measure the energy carried by the individual photons. These are typically reported in units of “electron volts” or “eV”.

Would you give me a definition of an electron volt?
Sure. It simply means the amount of energy required to move one electron through a potential difference of one volt.

You know, I have heard that gamma rays are electromagnetic radiation, just like x-rays. Are they the same thing?
They are quite similar. X-rays and gamma rays both ionize atoms. The energy necessary to cause an ionization of a nearby atom varies with the type of atom – such as 34 eV for atoms in air and 25 eV for atoms in tissue – but is generally in the range of several eV. A 140 to 160 kiloelectron electron volt (keV) x-ray machine, which is the typical energy in a package scanner, produces thousands of ion pairs.

Then how do x-rays and gamma rays differ?
One distinction between them is their origin. Gamma rays originate within the nucleus of an unstable atom, whereas x-rays originate either outside the nucleus from electronic transitions between energy levels, or from “free” electrons decelerating in the vicinity of atoms. In addition, gamma rays often have more energy than x-rays. Typically, gamma ray energies are in the “millions of electron volts” or “MeV” range, while x-ray photons are more often in the keV range. Although there are exceptions in both cases.

What is an x-ray machine fundamentally supposed to do?
An x-ray machine is designed to provide a flow of electrons with enough intensity to produce an x-ray beam. However, in order for that machine to be useful, we must be able to control the quantity (number) and energy of the x-rays produced.

How do we do that?
Historically, x-ray devices have been in existence for about 100 years. Even the very old ones share the same basic components as the more modern units. These include a source of electrons, an electrical potential difference to accelerate the electrons, and an anode, or “target” for the accelerated electrons to strike.

Let’s talk more about the source of electrons first, okay?
Sure. Usually the source of electrons in an x-ray machine is a thin wire filament (cathode) from which electrons are emitted when it is heated by a large electrical current. Controlling the current through the filament then becomes a way of controlling the number of electrons available for acceleration. And controlling the number of electrons has a direct effect on the number of x-ray photons emitted from the anode!

How is the tube current controlled?
There usually is some sort of an adjustment knob, dial, or digital mechanism on a control panel that is called a “milliampere (mA) control”. If it’s been a while since you learned this in the classroom, electrical current is measured in units of amperes (A) or milliamperes (mA). In some machine designs, this particular control may be fixed and therefore not adjustable by the operator.

Okay. I understand the source of electrons, but you also said we needed something called the potential difference. What is that?
The electrical potential difference between the cathode and the anode/target is the force that accelerates the electrons. The larger the potential difference, the more kinetic energy, or energy of motion, the electrons will acquire. The potential difference is measured in units of kilovolts (kV), while the energy of the electrons is measured in units of kiloelectron volts (keV). The potential difference across the x-ray tube is selected at the x-ray machine control panel using something called a “kV control”, or “KVP”.

What happens then?
After acquiring kinetic energy by moving through the electrical potential difference between the cathode and the anode, the accelerated electrons then strike the anode, or target. The target may consist of various materials, depending on the purpose and design of the x-ray tube.

Is x-ray production affected by the target composition?
Absolutely. X-ray production is most “efficient” in materials with high atomic numbers such as tungsten, which is the target element used in most x-ray devices.

How are all of these parts contained?
The cathode and anode of the x-ray tube are enclosed in an evacuated glass tube or envelope. It is necessary to maintain a vacuum within the tube in order to ensure that the accelerated electrons will interact in the target, and not with gas molecules between the filament and target. The anode is usually encased in copper, which serves to dissipate the large amount of heat produced when electrons strike the anode. In many x-ray machines, the anode also rotates at a high speed, which increases the area of bombardment and therefore is also useful in dissipating heat. The x-ray tube housing is an insulated metal casing around the glass envelope that provides both electrical and radiation shielding for the x-ray tube itself. Oil is often used between the x-ray tube and the protective housing to provide insulation and cooling. The housing will intercept most of the x-rays produced in the target that are not part of the “useful” beam.

Where do the electrons interact in the target?
Accelerated electrons either interact with the nucleus of the target atoms, or with their orbital electrons. The result is either ionization, excitation, or bremsstrahlung. Two of these mechanisms, ionization and bremsstrahlung, result in the production of x-ray photons. Excitation interactions result in the production of heat in the target.

I’ve read about “ionization” in a previous webpage chapter, but please describe it again in the context of this topic.
When an accelerated electron collides with the orbital electrons of a target atom, it can dislodge or “eject” (ionize) the orbital electron from its shell. For this to occur, a minimum or “threshold” energy is required. When the orbital electron is removed, it leaves a vacancy that is filled by an electron from a higher energy level. As this electron moves down to fill the vacancy, the difference in energy is emitted as an x-ray photon. This radiation is known as characteristic radiation.

Why so?
Because its energy is “characteristic” of the two electron energy levels involved in the transition, which in turn are characteristic of the atomic element of the target material.

Where do the x-rays go once they are produced?
X-rays are produced in all directions in the target. However, only x-rays oriented toward the exit port, or window, will comprise the so-called “useful” beam.

Now what about excitation?
In contrast to ionization, an accelerated electron may not have enough energy to eject an orbital electron from the shell of a target atom. In this case, the electron may simply transfer some kinetic energy to the orbital electron without ejecting it from the atom. This transfer of kinetic energy results in heating of the target. No radiation is produced!

How much heat are we talking about?
You may be interested in knowing that on the order of 95 to 99 percent of the kinetic energy acquired by the accelerated electrons is converted into thermal energy. Only one to five percent of the kinetic energy is converted into electromagnetic energy in the form of x-ray photons. So an x-ray machine is an amazing, but inefficient, device!!

I’m definitely going to need a brief tutorial on “Bremsstrahlung Radiation”.
No problem. First, keep in mind the energy of the x-ray photons coming out of the x-ray machine is of interest to the users of the machine. The typical energy “spectrum” from an x-ray machine consists of the characteristic x-rays from the target, which have discrete energies, and the bremsstrahlung photons which have a whole range of energies.

What an interesting term!
“Bremsstrahlung” is a German word for “braking radiation”. When accelerated electrons come near the large, positively charged nucleus of a target atom, they change direction. The change in energy resulting from this directional change or “deceleration”is emitted as a bremsstrahlung photon. The maximum energy depends on the potential difference across the tube. For a typical x-ray machine, the bremsstrahlung photons far outnumber the characteristic x-rays.

Do the bremsstrahlung photons have a single energy?
No. It is the potential difference (kV) across the tube that determines the amount of kinetic energy the electrons acquire as they are accelerated across the x-ray tube. The potential difference also determines the maximum energy bremsstrahlung photons that can be produced. No photon can be created with an energy greater than that of the electrons. The bremsstrahlung photons will have a whole spectrum of energies while the characteristic radiation produced will always have discrete energy values depending on the element, and the electron energy shells involved in the interaction. In short, the x-ray beam that emerges from an x-ray tube has a spectrum of photon energies determined by several factors.

Can an x-ray tube be used indefinitely?
Unfortunately, no. The tube will fail at some point.

The most common reason for x-ray tube failure is the filament breaking, similar to what happens in a burned out light bulb. Usually the entire tube insert is replaced when that occurs.

What else can happen?
The other most common cause of tube failure is a heat-damaged target or anode. Excessive heat to the target will cause small areas of the target to melt, resulting in “pitting”. A pitted target reduces the x-ray output of that particular tube due to absorption of photons in the pits and can also severely affect the image quality. It is this latter reason that usually results in a decision to replace the tube.

So it’s inevitable?
Yes. In all machines, the x-ray tube output will tend to decrease over time as the tube ages. As the tube ages, metal evaporated from the filament tends to deposit or plate out on the inside of the glass envelope. This thin metal deposit absorbs low energy x-rays and tends to reduce the x-ray output. This metal deposit can actually be seen on older tubes that are removed from the units.

Let’s go back to talking about the different types of x-ray units out there.
Okay.  As we stated previously, x-ray machines are used in both industrial and medical applications. Medical machines are fairly standardized in appearance, and in the way they are installed. However, that is not true of x-ray machines used for industrial applications. In the latter case, x-ray machines may be fixed installations, mobile units, or completely enclosed cabinet systems.

I’m interested in the hazards associated with the industrial devices.
The major hazard from all x-ray machines is the external dose hazard to machine operators and other people in the vicinity. In the case of cabinet systems, this is primarily in the form of scattered radiation from the components and any contents (i.e., samples, luggage, etc.) and is typically fairly low. However, no one should ever be exposed to the primary, or useful beam. Exposure to leakage radiation (from the housing) and scatter radiation can be reduced by employing appropriate radiological controls.

Since there can be an external hazard from these devices, is some thought put into how they are manufactured?
You bet there is! Manufacturing requirements for these devices are found in the federal Food and Drug Administration’s (FDA ) Title 21 of the Code of Federal Regulations, Subchapter J, “Radiological Health”, Part 1020, Performance Standards for Ionizing Radiation Emitting Products.

What does the FDA require?
Well, as you can imagine, lots of things! If we focus on those requirements for cabinet x-ray systems found in 21CFR1020.40, perhaps the most important one is that the exposure rate cannot exceed 0.5 milliroentgen per hour (mR/hr) at 5 centimeters (cm) from any accessible surface.

How does the manufacturer accomplish this?
The reasons the manufacturer is able to achieve such low external surface exposure rates are because they have installed extensive shielding within the unit, they employ a low potential difference and beam current in the generation of x-rays, and they maximize the use of intensifying screens to improve the visual image.

The FDA also requires the use of physical components (“active” controls) to reduce the radiation hazard.

Like what?
Like equipping the device with interlocks to prevent access to the x-ray beam.

How do the states get involved?
Because of the potential hazards, individual states require first that the device be “registered”. Once that is done, other requirements tailored to the individual state kick in.

Can you give me an example?
Absolutely. Requirements for annual inspections of the unit to verify radiation emissions, preparation and implementation of a radiation safety control program, and use of individual external monitoring devices, as warranted. There are other requirements as well.

This all sounds fine, but I still have concerns. What about the warning sign you often see that says, “Do not put any part of body in the machine; x-ray hazard.”?
The signs on airport scanners are there for your protection. While we can certainly understand that a warning sign on an airport x-ray unit may be unsettling, they are intended to warn you about the fact that x-rays are in use. However, the design of the units affords quite a bit of protection . . . even if someone does something really stupid like reaching in to remove a piece of luggage.

Ok, then, what if I place my carry-on luggage on the x-ray inspection conveyer and after I pass through the metal detector to pick up my bag, it gets stuck in the machine. I reach in the conveyer with both hands, and pull my bag the rest of the way through. What about the exposure to my hands?
Well, of course, we know you would never do that in real-life. You would wait for the operator to retrieve your stuck bag . . . right? But in case you did “go it alone”, your potential for exposure is still minimal. From a radiological perspective, you don’t need to worry about this scenario.

The beam from the x-ray generator is highly focused, or collimated. This means that it strikes the luggage only and is not allowed to escape from the confines of the cabinet. And, because of the position of the x-ray generator within the cabinet, it is unlikely you could have put your hand directly within the x-ray beam by simply reaching from outside of the conveyor.

But if I did do such a thing, how much radiation could I have received?
The National Council on Radiation Protection and Measurements (NCRP) in Report 95, Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources (1987), estimates that the exposure to a typical traveler with two bags of luggage to be scanned is less than 0.002 millirem. If you read the Radioactivity Basics chapter entitled “It’s Everywhere”, you will see that these very low levels of radiation dose are not distinguishable from the background dose we receive every day of our lives.

How interesting.
If you think that is interesting, get a load of this. The federal and state standards for radiation exposure are based upon limiting exposure to the blood-forming organs, meaning the organs located in the trunk, in the arms above the elbows, and in the legs above the knee. An encounter with an x-ray scanner at an airport may have resulted in a minuscule, if any, exposure to your extremities, meaning your arm or hand below the elbow. The extremities tolerate much higher radiation doses than the remainder of the body with even lesser effects. And, just so you can put some numbers into perspective, remember that there have never been any demonstrable radiation-related health effects for acute (all at once) whole-body doses of 20,000 millirem.

It sounds like the radiation issue is not a big deal in this case.
That’s right! However, while the x-rays being produced in the scanner do not pose much of a risk, the risk of getting caught in a moving conveyor belt should not be overlooked. Physical damage to the baggage or worse – a person’s arm – are much more probable than any radiation-related injuries from placing one’s hands inside the unit.

Is it possible to sterilize food using an x-ray machine?
No! Because the radiation exposure rates generated within cabinet x-ray machines are so low, it is not possible to sterilize or harm the food you eat in any way. Considerably greater doses are required to sterilize food, a practice increasingly employed within the U.S. and in other countries for public health purposes. And food sterilization units, by the way, utilize gamma radiation, not x-rays. And the procedures followed include strict radiation safety and quality control safeguards that are simply not applicable to x-ray units such as cabinet x-ray machines.

Well, I can now relate to the cabinet inspection units, but I would appreciate another industrial application.
How about this one. Characteristic x-rays that result from ionization of atoms can be used to identify atoms, since the characteristic x-rays will have energies that are unique to that element. This forms the basis for x-ray fluorescence spectroscopy.

Well that’s interesting. How does it work?
A sample to be analyzed is irradiated by a beam of high-intensity x-rays. The x-rays ionize atoms in the sample, which emit characteristic x-rays when the electron shell vacancies created by ionization are filled. The characteristic x-ray emissions can be evaluated and the element identified in fairly short order.

Like what kind of element?
How about gold! If you take your gold-colored ring and have it irradiated, the characteristic x-rays can be identified and verification provided as to whether it is made of “real” gold.

Would that make my ring radioactive?
Absolutely not. Remember, this is an irradiation process. The ring will not be contaminated, so after the experiment, it can be placed right back on your finger. No waiting period required.

Is there another industrial example?
Let’s talk about the principle of x-ray diffraction. When x-rays are scattered by a crystalline solid, they are scattered from the different atoms, but only in certain directions. This technique is used for crystal structure research.

Tell me more.
The primary beam and the diffracted beams are very small and well collimated. In some types of diffraction equipment, the sample cannot be enclosed in a structure. The primary beam is controlled by a shutter that opens and closes.

Are there hazards with these devices?
The major hazard associated with diffraction units is intense, localized exposure from the primary beam to the hands or eyes. This might occur during sample changing or beam alignment procedures with the shutter inadvertently open. The primary beam is very small, but can generate extremely high exposure rates. Even short exposures of the hands and fingers might result in severe injury, and potential loss of fingers.

Let’s talk a bit now about medical applications of x-rays.
Certainly. Many individuals have had occasion for one reason or another to receive x-rays as a commonly-employed form of diagnostic radiology. Radiology, in general, refers to imaging techniques allowing doctors to see inside a patient’s body.

I know I have.
So have I. Medical x-ray devices in use today localize the beam to the specific area of interest. While there would be some radiation scattered to other parts of the body, it would be minimal, resulting in a very low “whole body” dose from each procedure.

What technical resources are available if I want to learn more about medical x-ray machines and exposures to patients?
The NCRP has a number of references issued in the form of reports. These include NCRP Report No.100 entitled “Exposure of the U.S. Population from Diagnostic Medical Radiation”; NCRP Report No. 54, “Medical Radiation Exposure of Pregnant and Potentially Pregnant Women”, NCRP Report No. 85, “Mammography – A Users Guide”; NCRP Report No.99, “Quality Assurance for Diagnostic Imaging Equipment”, and NCRP Report No. 102, “Medical X-ray Electron Beam and Gamma-Ray Protection for Energies up to 50 MeV (Equipment Design, Performance and Use)”.

Where can I find these?
You can find these documents in any technical library, or you may order them directly from the NCRP. There is a link to the NCRP web site in the Plexus-NSD web page, under the “Links” section.

Are there any other references?
Yes indeed. There are a couple of classical textbooks entitled “Medical Radiation Physics” by William R. Hendee and “The Physics of Radiology”, by Harold Johns and John Cunningham. Be forewarned this is not light reading! These should also be available in any technical or university library.

What about internet resources?
Try searching the web for specific types of diagnosis and treatment using x-ray machines. For example, there may be some useful information included in pages devoted to mammography techniques. Each breast center performing mammography must be certified by the FDA to meet the minimum standards under the Mammography Quality Standards Act (MQSA). Additional certifications, which are not required but are recommended, can be obtained through the American College of Radiology (ACR) and, in very limited (to date) states in this country.

For further information, you might try these additional reference sources. First, visit the FDA’s web site at http://www.fda.gov and click on “Medical Devices/Radiological Health”. Then review their “Mammography Quality and Radiation Programs” section for information of interest to you. The American Association of Physicists in Medicine (AAPM) has a website at http://www.aapm.org and contains links to medical physics sites. The ACR also has a website at http://www.acr.org. As a final internet source, Idaho State University maintains a web site at http://www.physics.isu.edu/radinf. This site contains an extensive amount of radiation-related information. Once at the site, click on “Medical Radiation Sources and Related Information”. This will take you to a discussion of radiology and general information on mammography.

NCRP Report No. 93, “Ionizing Radiation Exposure of the Population of the United States”, reports that an average exposure to a patient from a typical chest x-ray in 1980 was less than 6 millirem per examination. Improvements in film speed, collimating the beam and filtering of low energy x-rays that cannot penetrate the body have reduced this exposure level even further since that time. However, the benefits of any diagnostic radiology procedure by far outweigh what little, if any radiation-related risk there might be.

We certainly have covered a lot in this chapter?
Yes we have! You now know that x-ray machines constitute one particular type of radiation generating device used in industrial and medical applications. And we also discussed a variety of topics such as the properties of x-rays, x-ray machine design and function, radiation hazards, and federal and state requirements in the use and control of these devices.

It sounds like we should be grateful for this technology.
You are absolutely right. X-ray devices used in industrial applications provide us with great benefit, yet they pose no radiological health threat as long as prudent radiological safety practices, such as not misusing the device, placing your body or a portion of your body purposely inside the unit or in the line of sight of the x-ray beam while it is operating, and regulatory requirements are implemented. Medical x-ray devices clearly benefit humankind by making diagnosis of injuries and disease infinitely easier. The radiation exposures in this instance are definitely worthwhile when “risk” versus “benefit” considerations are evaluated.

So I should not refuse dental or medical x-rays?
No. And let me tell you why. The type and number of x-ray procedures you have as part of a diagnostic or treatment protocol is determined by your physician. As you know, these professionals take ethical vows to administer only those procedures that are deemed necessary to diagnose the patient’s medical situation. The benefits obtained by ordering those procedures by far outweigh the radiological risks associated with them. Can you imagine what your risk would be if your doctor did not have access to or did not use these valuable diagnostic tools? Diseases and adverse medical conditions that are readily treatable when diagnosed reasonably early in their progress could escalate quickly, jeopardizing your health . . . or worse. When one reaches this stage, conventional treatment modalities are often ineffective.

I can see that. But what about the risks associated with the exposure itself?
Generally speaking, the amount of radiation exposure you receive during each of your medical or dental procedures is trivial in light of the amount needed to result in demonstrable radiation-related health effects. There has never been a demonstrable health effect associated with radiation exposures of 10,000 millirem or less. And even for acute exposures between 10,000 and 50,000 millirem, the effects are typically small and reversible. By acute, I mean instantaneously, not over a one-year period. As the exposure duration increases, the body’s ability to repair increases, raising the threshold at which effects can be seen. It takes very high radiation exposures, much higher than the types you have received to date and expect to receive in the future, for there to be any increased risk of effects (i.e., cancer) above the normal population incidence.

Then I should never question an x-ray procedure when prescribed?
I didn’t say that. As a patient, you should exert your rights whenever possible. Your doctor knows exactly how much radiation exposure you have received, and how much they anticipate you will receive in the future as they follow your case. Consequently, you should always feel free to pose questions to these health care professionals. They are the only ones that know your specific course of treatment and are thus prepared to not only provide you with the answers that you seek, but to put everything into perspective in light of your specific medical condition. As with any medically invasive procedure, you are entitled to answers to all of your questions. Furthermore, a dialogue of this nature will be helpful in easing your anxieties.

Okay. Ask questions, but don’t refuse the procedures, right?
We are really only cautioning you not to avoid future radiology procedures for medical or dental diagnosis and treatment simply because of a fear of possible radiation-related health effects. It would be much riskier for you to follow this path than it would be to follow that laid out by your health care professionals who have your well-being in mind. The bottom line is that the benefits of x-ray machines in medicine and industry far outweigh their negligible risks. When used by skilled professionals, and when simple precautions are taken, we can all feel good about the fact that we have this amazing technology at our disposal.