X-ray tubes used in diagnostic X-ray exams all use the same physical principles including: thermionic emission (boiling off electrons), accelerating electrons by a kiloVoltage potential (kVp), and two physical interactions contribute to x-ray generation: bremstraalung (braking radiation), and characteristic radiation (peaks in energy spectrum). The output x-ray distribution as a function of energy is termed the x-ray spectrum and is dependent on the kVp (tube potential) and mA (tube current).
In this post we will cover these topics, and if there is one that you are most interest in feel free to select that one from the table of contents to skip straight there.
As an x-ray technologist or radiographer you use x-ray systems of different types all the time. It is important to understand how the x-rays are made so that you have a firm understanding of the technical parameters that can be adjusted on the system.
We start with a high level illustration of x-ray generation and then go into the details in the subsequent sections.
While the generation of x-rays may seem like magic it is a very well understood process that we will describe below. First starting with an overview of the components of an x-ray tube and then describing the physical interactions that result in the generation of x-rays.
X-Ray Tube Overview
Within an x-ray tube, electrons are accelerated quickly into a large piece of heavy metal such as Tungsten. We will cover below why that works as a method to generate x-rays.
Source of Electrons
The first thing that we need is a source of the electrons. To generate the electrons a coiled wire (a.ka. a filament) is heated until it gets hot enough that electrons boil off. The source of electrons is also called the cathode of the x-ray circuit.
This is happening inside of a glass tube where the air has been mostly all removed, i.e. a vacuum tube.
The area on the piece of heavy metal where the electrons are being directed is called the target.
Acceleration of Electrons
The x-rays will migrate from the filament and will be attracted to something that is more positively charged, since opposite charges attract. This is accomplished by establishing a potential difference between the cathode and the anode so that the electrons will be pulled by the electric potential from the cathode to the anode, (i.e. pulled from the filament to the heavy metal target).
In any battery the cathode is negatively charged and the anode is positively charged and collects the electrons. The same terminology is also used in x-ray tubes. The coiled wire (filament) is also called the cathode of the x-ray circuit.
Electrons flow from this cathode to positively charged anode and bombard the heavy metal. The electrical potential across this area, created by the oppositely charged cathode and anode, is called the kVp (kiloVolt peak).
If you are familiar with a car battery that typically has a difference of 12.6 Volts between the cathode and the anode.
In a diagnostic x-ray tube the potential difference is usually 30,000-150,000 Volts. That is why we usually use kiloVolts (1 kV=1000V). So a typical x-ray tube usually runs from 30 kVp to 150 kVp.
If you aren’t very familiar with batteries one analogy that you can use is that a battery is like a waterfall. In a waterfall the water runs down due to the pull of gravity.
In the x-ray tube the electrons flow from the cathode to the anode because of the tube potential (kVp). The tube potential in an x-ray tube is analogous to the height of a waterfall.
While kVp measures the potential difference between cathode and anode, the tube current (mA) is a measure of the number of electrons, flowing from the cathode (filament) to the anode (heavy metal target).
In the waterfall analogy the mA can be compared with volume of water flowing over a waterfall in a given period of time.
Focusing of electrons
As you can see in the the Figure of the x-ray tube the electrons leave the filament and are pulled toward the target by the kVp (tube potential). Additionally, there is a grid plate that can be used to steer the electron beam. Namely, changes in the electric field can help to keep the focal spot small while enabling a significant flux of electrons from the filament.
There are even fancier designs now in the state of the art x-ray tubes which use two sets of magnets to steer the electron beam toward the target. This can be used along with a novel design for the cathode which is a flat emitter rather than a coiled wire. The flat emitter enables a larger surface area for the electrons to boil off, which enables higher mA at low kVps. One commercial example of a state of the art tube which uses magnetic steering is the Quantix tube. But if you are just learning x-ray generation, you should know that a grid plate is typically used to steer the x-ray beam on the vast majority of x-ray tubes.
Deceleration of electrons
When the fast moving electrons hit the heavy metal they will undergo rapid deceleration in the target material. The electrons have a lot of energy as they are incident on the target and 99% of that energy is deposited as heat in the target as the electrons slow down in the heavy metal.
However, about 1% of the energy of the electron beam will be transfer to the production of x-rays. We will discuss the mechanisms below but the physical effect responsible for most of the x-ray generation is called braking radiation (Bremsstralung in German).
During this process of electrons bombarding the target, the target material will get very hot where the electrons hit. To prevent the target material from melting most x-ray tubes have a rotating shaft so that the electrons hit different parts of the rotating target. If the target material rotates quickly the electrons will be incident on a track (or a circular region on the target material).
In order for the tube to be rotating quickly so that the heat can be spread out ball bearings are typically used to reduce friction, and in state of the art designs a liquid metal bearing can be used to increase the lifetime of the rotating bearing.
Rad Take-home Point:
In a diagnostic x-ray tube the x-rays are generated by accelerating electrons from the cathode to the anode where they quickly decelerate in a heavy metal and x-rays are generated.
Overview Video
In the next couple sections we describe the physical mechanism for x-rays being generated, when electrons are accelerated into a heavy metal. Understanding a little about these two interactions will help you have a clear picture how the shape of the x-ray spectrum is generated.
Bremsstrahlung X-Rays (Braking Radiation)
When electrons come out from the cathode, they are bombarded at a heavy metal such as Tungsten.
The heavy metal will have a large nucleus. As the electrons from the cathode come very close to the nucleus they can be rapidly decelerated.
When the electrons decelerate so quickly due to an interaction with the protons in the nucleus an x-ray photon is generated in order to conserve energy.
After interaction with the nucleus, the electron goes off in one direction while the newly generated x-ray photon goes off in an opposing direction (see Figure).
This process is called Bremsstrahlung radiation (this name comes from the German word for ‘braking’).
It is possible that electron may change its trajectory only slightly, which would generate a low energy x-ray photon.
It is also possible that electron deposits almost all its energy to newly created x-ray photon, generating a relatively higher energy x-ray photon.
The energies of the x-rays that are generated via Bremsstralung will be continuous and can have any energy from zero to the maximum energy deposited by the electron (determined by the kVp). There are more low energy x-ray photons generated and fewer high energy x-rays generated with Bremsstralung.
The vast majority of the x-rays produced from a diagnostic medical x-ray tube are from Bremsstralung radiation.
One interesting point to note is that this method for making x-rays is not very efficient. Most of the electrons just end up stopping in the anode (about 99% of the energy of the electrons) and deposit their energy as heat.
Characteristic X-Rays
Characteristic radiation takes place when the incoming electrons collide with the electrons within the heavy metal and knock-out the electrons from the electron shell.
When an inner shell electron gets knocked out by an incoming electron an electron from the neighboring shell will drop down to fill the vacancy left after the inner shell electron was knocked out.
Since there is an energy difference between the two electron shells an x-ray photon will be emitted with an energy that is exactly the difference in energy between the two electron shells (this preserves energy of the system).
After the neighboring electron drops to the electron shell where the electron was knocked out from, then there is a vacancy in the next outer electron shell. Another x-ray photon will be emitted with the same energy as the difference between these electron shells, and so on as the electrons transition from outer to inner shells.
The K shell electrons are more tightly bound, ie. they are in a more stable configuration than the L shell electrons. Likewise the L shell electrons are more tightly bound than the M shell electrons. The term that describes how tightly bound the electrons are is referred to as Binding Energy (BE).
The Energy of characteristic x-rays = BE K shell electrons – BE L shell electrons for transition from L shell to K shell. Likewise, the Energy of characteristic x-rays = BE L shell electrons – BE M shell electrons for transition from K shell to M shell.
In this Figure we show an example where an energetic electron coming from the cathode knocks out a K-shell electron, and electrons transition from (L->K), (K->M) and so on.
Unlike Bremsstralung the characteristic radiation only produces x-rays of a few energies corresponding the energy differences between the electron shells. This accounts for the spikes that you see when you look at an x-ray spectrum.
Rad Take-home Point: A secondary contribution to the x-rays generated in a diagnostic energy spectrum where all of the x-rays generated via characteristic radiation are at just a few energy levels, causing spikes in the x-ray spectrum.
Video for Bremsstrahlung & Characteristic X-Rays
X-Ray Spectrum
It would be nice and simple if all of the x-rays coming out of an x-ray tube had the same energy, (a so called mono-energetic x-ray beam).
But in reality x-rays coming out have a variety of energies. It is useful to examine the different energies in an x-ray beam. This is termed the x-ray spectrum and is essentially a plot of the number of x-rays for each given energy. The number of x-ray photons and the energy of the x-ray photons that come out determine how much radiation dose is used in a given exposure.
For each kVp setting the spectrum of x-ray photons generated by Bremsstrahlung interactions is approximately a linear function where is it less likely to have high energy x-rays and the highest energy x-ray is determined by the kVp.
In reality photons are filtered out as they leave the x-ray target, by the glass window of the x-ray tube and by additional pre-patient filtration. This filtration more strongly filters out the low energy photons as shown in the right portion of the figure on x-ray spectrum.
Finally, we consider the effect of characteristic radiation which adds the spikes or peaks to the x-ray spectrum. These peaks are determined by the target material used in the x-ray tube.
So even though the x-ray spectrum may look imposing and tricky to understand it is really the contribution of three things that give it it’s shape.
- Bremsstralung (Braking Radiation)
- Characteristic Radiation
- Filtration from the target material, exit window, etc
Rad Take-home Point: The majority of the contribution to the x-rays generated are from Bremsstralung radiation. Characteristic radiation adds the spikes at special energies, and the low energies are removed more by pre-patient filtration.
Effect of Technical Parameters (kVp and mA)
As discussed above the main technical parameters that can be changed when generating an x-ray exposure are the kVp and the mA. These each change the x-ray spectrum or x-ray distribution differently so it is important to keep the effect of each one in mind.
The kVp changes both the overall shape of the x-ray spectrum as well as how many photons are produced, as can be seen in the kVp Figure. In this example Figure we can see the effect on the x-ray spectrum of changing from 80 kVp to 140 kVp.
Important points to remember for the kVp dependence:
- The maximum x-ray energy is determined by the kVp
- The total dose of each exposure is strongly dependent on the kVp, Exposure~kVp2 (*a power law slightly higher than 2, but just remember this approximation)
- The kVp changes the overall shape of the x-ray spectrum, it does not just scale it.
The effect of the mA is more straightforward and hence typically the kVp is changed first and then the mA is then used to fine-tune the delivered x-ray exposure.
The relative number of photons at each energy bin stays the same when the mA is changed. This is to say that the shape of the x-ray spectrum stays the same when the mA is changed and it is just scaled up or down. This is called being directly or linearly dependent on the mA so when the mA is changed by a given amount the patient exposure is changed by the same amount, ie. If the mA is doubled the x-ray exposure to the patient is also doubled.
Also, just for completeness, we’ll mention the exposure time. When you do an exposure of the x-ray tube, you turn that x-ray tube on and you have the exposure on for a given amount of time. And if you leave the kV and the mA constant then the exposure is also linearly proportional to time, just like the patient exposure is linearly dependent on the mA. So, the longer that the x-ray tube is on, the more exposure and the more dose the patient will receive.
Rad Take-home Point: The kVp changes the average energy of the x-ray spectrum and the dose dependence goes like (kVp2 ), whereas the shape of the x-ray spectrum is not changed with mA but just scaled with mA.