X-ray Tube Physics, [Bremsstralung and Characteristic Illustrated for Rad Techs]

X-ray tube physics is the same for the tubes using in X-ray exams all use the same physical principles including: Bremsstralung and Characteristic (https://howradiologyworks.com/xraygen to get a self check study guide on x-ray generation).

chapters:
00:00 Intro
00:43 X-Ray tube
02:05 X-Ray Generation
02:32 Bremsstralung radiation
04:17 Characteristic radiation
05:18 X-ray production
06:24 Outro

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 ‘breaking’).

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.

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.

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.