Chapter 7 Lecture Notes

X-Ray Production

 

Conditions for X-Ray Production

 

Electron Target Interaction

Kinetic Energy - Kinetic energy is energy possessed by a body by virtue of its motion. The kinetic energy of a body is equal to the amount of work needed to establish its velocity and rotation, starting from rest. The formula for calculating the kinetic energy of an object is:

kinetic energy

KE is the kinetic energy, m is the mass in kilograms, and v is velocity or speed in m/s. Kinetic Energy is measured in joules.

 

• To raise the kinetic energy of the electrons is to raise the kVp.

• When the electron kinetic energy goes up the intensity (quantity) and the energy (quality) go up.

Kiloelectronvolts are equivalent to joules. When we look at the energy for x-rays keV is a more convienent measurement. The conversion from keV to J is:

keV to Joules

 

Example 1

How much energy does an electron have that has a speed of speed?

First we need to use the kinetic energy formula above. The mass of an electron is mass of electron. This formula gives the answer in joules.

ex1a

Convert the joules to kiloelectronVolts -

ex1b

 

 

Target Interactions

Heat Production - The high energy electrons that interact with the anode create mostly heat. When they excite an outer shell electron, the electron will not be ionized but will give off radiation in the infared range. This is heat.

99% of the kinetic energy of the projectile electrons is converted into heat.

Efficiency of x-ray production is increased by increasing the kVp.

kVp
Converted to x-rays
60 kV

0.5%

100 kV 1%
20 MV 70%

 

Bremsstrahlung Interactions

Bremsstrahlung Radiation - Bremsstrahlung, also called general or white radiation, an electromagnetic radiation generated when negatively charged electrons in motion are deflected by positively charged atomic nuclei. The deflection of the electrons involves loss of kinetic energy (velocity). This energy loss is promptly emitted as electromagnetic radiation (X-rays) called bremsstrahlung, a German word meaning "braking radiation" (referring to the deceleration of electrons as the cause of the radiation). Bremsstrahlung is the main source of X-rays produced by diagnostic X-ray tubes.

brems

In this diagram, the interaction on the left does not result in as high energy x-ray photon as the interaction on the right. This is because the change in direction is not as great as the right hand diagram. The greater the change in direction the more loss of energy and therefore the higher energy photon. The frequency is higher in a higher energy x-ray photon. If you remember Planck's equation from previous chapters (E = hf) then the higher the frequency the higher the energy.

brems

This diagram also shows the different interactions that give different energy x-ray photons.

brems

And finally a third and probably redundent diagram showing Bremsstrahlung X-rays.

 

 

Characteristic Interactions

Characteristic x-rays - Characteristic x-rays are emitted from heavy elements when their electrons make transitions between the lower atomic energy levels.

characteristic x-rays

Characteristic X-Rays are the x-rays that are the most useful for diagnostic imaging. They occur when a high energy electron ionizes an inner shell (K Shell) electron and then that hole is filled by an outer shell electron. This is illustrated above.

tungsten

When a K shell electron is ejected from a tungsten atom the hole needs to be filled. If it is filled with an L shell electron the energy of the photon emitted when the hole is filled will be about 59 keV, (70 keV – 11 keV) shown above in the binding energy graph. If, however, the hole is filled with an M shell electron the energy of the photon emitted will be about 68 keV, (70 keV – 2 keV).

characteristic x-rays

This diagram shows some different examples of electrons filling a hole in the K shell. Each example will have a different photon energy and thus a different x-ray energy.

 

Characteristic X-Rays of Tungsten
Electron Transition from Shell (keV)
Characteristic
L-shell
M-Shell
N-Shell
O-Shell
P-Shell
Energy of X-Ray
K
57.4
66.7
68.9
69.4
69.5
70
L
9.3
11.5
12.0
12.1
12
M
2.2
2.7
2.8
3
N
0.52
0.6
0.6
O
0.08
0.1

The chart above shows the different energy levels that can occur when electrons fill lower level spaces. For example if an L shell fills a K shell then the photon will have about 57.4 keV of energy. Or if a P shell electron fills an M shell the photon will have 2.8 keV of energy. The nice thing is that you need only remember the last column of 70, 12, 3, 0.6, 01.

 

Examples

1. A K-shell electron is removed from a tungsten atom and is replaced by an L-shell electron. What is the energy of the characteristic x-ray that is emitted?

Solution: From the table above the ionization energy of a K-shell electron of Tungsten is 70 keV and the ionization energy of a L-shell electron of Tungsten is 12 keV therefore, the difference of these two energies will be the energy given off.

70 keV – 12 keV = 58 keV

 

2. A K-shell electron is removed from a tungsten atom and is replaced by an O-shell electron. What is the energy of the characteristic x-ray that is emitted?

Solution: 69.4 keV

 

 

interactions

This diagram shows some possible interactions of electrons with the anode. If you look at the bottom (6) electron it might interact with the first atom and create a Bremsstrahlung X-Ray. Then it goes on and interacts with another atom and ionize the inner K shell electron which is filled and creates a Characteristic X-Ray.

 

 

Emission Spectrum

Characteristic X-Rays have discrete energy values, so their graph are vertical lines. While Bremsstrahlung X-Rays are continuous and can have any energy level up to the maximum that the x-ray system is set to.

 

x-ray emissions

This diagram shows the Bremsstrahlung X-Ray and the spikes for the Characteristic X-Rays. From the graph it looks like the kVp was set to 80 kVp.

 

Remember from previous chapters that the energy of an x-ray is proportional to the frequency of the x-ray and Planck's Constant.

planck

Therefore, the higher the frequency the higher the energy of the photon. The higher the energy of the photon the better the penetration of the x-ray and the higher quality.

 

Also,

wave length

which means that the frequency and wavelength are inversely proportional to each other. So that if the frequency increases the wavelength decreases. So, for high energy x-rays the frequency is high and the wavelength is short.

 

 

Factors Affecting the X-Ray Emission Spectrum

Quality of X-Rays - radiation quality, the penetrating power of an X-ray beam which is determined by the kVp and employed filtration. The penetrating power may be measured by the half value layer.

Quantity of X-Rays - the amount of x-rays is influenced by many factors. It is the area under the curve for X-Ray emission spectrum. The larger the area the more x-rays.

The chart below shows some of the factors that can affect the size and shape of the X-Ray Emission Spectrum. The higher the Emission Spectrum the more x-rays there are and the more the peak is to the right the better the quality of the x-rays (energy).

Factors Affecting Size and Relative Position of X-Ray Emission Spectrum
Factor Effect
Tube Current Amplitude of Spectrum
Tube Voltage Amplitude and position
Added Filtration Amplitude, most effective at low energy
Target Material Amplitude of spectrum, position of line spectrum
Voltage Waveform Amplitude, most effective at high energy

 

Four Principal Factors Influencing the Shape of an X-Ray Emission Spectrum

1. Kinetic Energy of the Projectile Electrons

2. Thickness of the target

3. Low-energy x-rays absorbed by the target

4. External filtration - absorbs low-energy

 

Now lets look at some of the factors a little closer so that we can see how each affects the quality and quantity of the x-rays.

Effect of mA and mAs

A change in mA or mAs results in a proportional change in the amplitude of the x-ray emission spectrum at all energies. Looking at this diagram if you have a setting for 200 mA then switch to 400 mA the area under the curve doubles. Also, notice that the highest part of the graph does not move left or right, but stays in the same relative area.

mA change

 

Effect of kVp

A change in voltage peak affects both the amplitude and the position of the x-ray emission spectrum. When you change the kVp the quantity and quality of the x-rays increases. A few things to notice about the diagram below are:
1. the peak (highest part) of the graph moves to the right as kVp goes up, (quality goes up).
2. the peak goes up, (quantity goes up).
3. the characteristic x-rays do not occur at kVp lower than 70 keV.

kVp change

 

A change in kVp has no effect on the position of the discrete x-ray emission spectrum.

In the diagnostic range, a 15% increase in kVp is equivalent to doubling the mAs. This is the 15% rule that you need to know for x-rays.

 

Effect of Added Filtration

The overall result of added filtration is an increase in the average energy of the x-ray beam with an accompanying reduction in x-ray quantity. When filtration is added the quality goes up and the quantity goes down. The next chapter will be about filtration and you will also get filtration help in your exposure classes.

filtration graph

 

Effects of Target Material

Increasing target atomic number increases the efficiency of x-ray production and the energy of characteristic and bremsstrahlung x-rays.

 

Effect of Voltage Waveform

When three-phase and high frequency power sources are used the ripple is much smaller so that at the same kVp there is a 12% increase in x-rays or almost a doubling of mAs compared to a single-phase power source.

phase type change

 

Now to summarize:

Changes in X-Ray Beam Quality and Quantity
An increase in: Results in:
Current (mAs) Increase in quantity
Voltage (kVp) Increase in quantity and quality
Added Filtration decrease in quantity, increase in quality
Increased target Z increase in quantity and quality
Increased voltage ripple decrease in quantity and quality

 

The End