What are X-rays?
X-radiation or X-rays
X-rays are a form of electromagnetic radiation. The energy of all waves determines their wavelength. Visible light has a wavelength that is at a lower frequency than X-rays. The shorter wavelength of X-rays enables them to penetrate mater. The shorter the wavelength, the higher the higher the penetration into matter. Most commonly X-rays are used in Medicine to pass through the body and provide an image because the frequency of the wavelength does not vary but the different tissues in the body absorb X-radiation differently. Other types of electromagnetic radiation are radio waves, microwaves, infrared, visible light, ultraviolet, and gamma rays. The types of radiation are distinguished by the amount of energy carried by the individual photons. All electromagnetic radiation consists of photons, which are individual packets of energy. For example, a household light bulb emits about 1021 photons of light (non-ionizing radiation) per second. The energy carried by individual photons, which is measured in electron volts (eV), is related to the frequency of the radiation. It has been proven that X-rays are produced as a byproduct in cathode ray tubes, valve tubes and other high voltage devices. However, the X-ray Tube is manufactured to create the highest level of X-rays with the lowest heat.
X-ray interaction with Matter, Scatter Radiation
For radiographic studies and imaging with CT systems, the mass attenuation coefficient of a type and size of material characterizes how easily it can be penetrated by electromagnetic radiation such as X-rays. There are four possible interactions between x-rays and matter. 1) Photoelectric absorption 2) Compton scattering 3) Rayleigh scattering 4) Pair production (only important in high energy regions).
A low energy photon interacts with the inner shell electron in an atom when the energy of the photon is equal to or just greater than the binding energy of the electron in its shell and the electron is tightly bound. Due to the collision the electron is removed from the shell. The removed electron is then called photo electron. The photon is completely absorbed during the process. The energy of the Photo electron is the initial energy of the photon minus the binding energy of the electron. An outer shell electron fills the vacancy in the inner shell to stabilize the atom. The energy which is lost by this electron as it drops to the inner shell is emitted as characteristic radiation. To learn more see Auger electron.
Compton scattering is the scattering of a photon by a charged particle (electron). It results in a decrease of energy and an increase in wavelength ( λCompton > λPhoton ). Part of the energy is transferred in the recoiling electron (Compton Electron). Can be described as inelastic scattering.
This is the most common photon interaction with matter. Additionally, it is also the least desirable photon interaction. The X-ray photon enters an atom in a part or material and the X-ray photon energy is partially absorbed by a loosely bound outer shell electron. This crash results in the electron being knocked out of its shell orbit. This is a Compton electron. The rest of the photon energy immediately exits the atom as a scattered photon. It has less energy than the original photon and it's going in a different direction or scatters somewhat randomly. Compton scattering is a photon with an electron and a scattered photon out. Photoelectric absorption takes place in an inner shell electron and it results in only an ionized electron but there is no scattered photon. Rayleigh scattering is also different. This occurs when the incident photon interacts with the entire atom the energy, the photon is temporarily absorbed, and then released as a scattered photon.
The two main areas we should be considering are radiation safety and image quality. Compton scattering affects both areas and none of these effects are good effects. We need to take scattering into account with the design of the cabinet and bunker and shield completely around areas that will be susceptible to direct beam as well as scatter radiation. Remember that Compton scattering does result in ionization. This means it could injure an operator if they are not correctly shielded. Compton scattering blurs imaging details because it lowers image contrast with randomly directed photons. At higher energies the combination of photoelectric absorption and transmission creates what would otherwise be a high contrast, high quality image. There's clearly visible differences in image quality when any kind of scattering especially Compton scattering adds random noise to the image.
Compton scattering occurs when there is more matter. In other words more matter equals more scatter. For example, part thickness. With more matter in the way more photons interact by Compton scattering. The same idea applies to part density. This refers to the amount of matter packed into the imaging area. The material overall density and geometry creates scatter. Collimating the beam is quite important to achieving a good quality X-ray. If we decrease the collimation, or use a larger field size, there's more matter being exposed therefore more scatter and lower contrast. One more factor that influences the amount of Compton scattering taking place is the beam energy, which of course, is controlled by the KeV and in some cases MeV. If we increase the energy or KeV, this proportionally increases the amount of scatter and therefore decreases the total image contrast. The ideal situation would be to have as little scatter radiation as possible.
Ways to decrease scatter include collimation, beam filtering, digital imaging averaging, positioning of the part, energy, and part thickness. All of these factors will never be perfectly set because they interact with each other. However, it is important to understand how to optimize the total image so that the best result is achieved.
The x-ray photon interacts with the whole atom. There is no change of initial energy ( ERayleigh = EPhoton ) and no change of initial wavelength ( λRayleigh = λPhoton ). The Rayleigh Scattering is a minor contributor to the absorption coefficient and scattering is mainly in the forward direction.
Photon Mass Attenuation Coefficients
As you can see on the graph to the right, the combined attenuation effect is different over the range of energy. Included in this graph for Iron is all of the interactions we see when X-rays hit Iron. For more data concerning the X-ray Mass Attenuation Coefficients including Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients from 1 keV to 20 MeV for Elements Z = 1 to 92 and 48 Additional Substances of Dosimetric Interest go to: https://www.nist.gov/pml/x-ray-mass-attenuation-coefficients
Absorption of X-rays
The increase in keV will increase the penetration of materials and lower the wavelength of the X-rays. Increase in Energy, (E) equals an increase in penetration.
The higher the Atomic Number (Z) of a material the lower the penetrability of X-rays. Lead has a Z of 82 and is used to shield X-rays because it is radio-opaque, while Carbon has a Z of 6 and is used in many cases for X-ray fixtures and table tops because of its radio-translucence.
Density, is a quantitative expression of the amount of mass contained per unit volume. The higher the Density (ρ) the higher the opacity.
Atomic Number (z)
The Thickness (t) of a material decreases radio-translucence. Thickness also includes distance through the material.
To explore in depth how the equation is derived go to the following: https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuationCoef.htm#:~:text=The%20Linear%20Attenuation%20Coefficient%20%28%C2%B5%29%20The%20linear%20attenuation,or%20scattered%20per%20unit%20thickness%20of%20the%20absorber.
Absorption of X-rays at Different Intensities
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Absorption of X-rays at Different Wavelengths
The two step wedge images below show the effects of higher Energy (E) hence shorter wavelength and more penetration. With a higher current a better contrast can be achieved. With higher voltages the majority of occurring thicknesses during the CT-scan or digital image can be displayed, but the contrast decreases.
150Kv Step wedge
230Kv Step wedge
Inverse Square Law
With a greater distance from S, the X-Ray photons spreads to a larger surface. The same square (A) gets less radiation quantity and the amount of radiation drops with the square of the distance.
For the doubled distance (2r) only a quarter (1/4) of the radiation is reached. For the tripled distance (3r) only a ninth (1/9) of the radiation is reached.
In X-ray Tube Theory you will learn about how to increase the number of photons at a given energy. For now we will simply state that as tube current (measured in micro amps using a micro-focus tube or in milliamps when using a fractional focus tube) increases, there is an increase in x-ray beam photons or intensity. A step wedge phantom is a block that contains several different thicknesses of material. Notice the step wedge images below. As the current increases the number of X-ray photons increase to give you a higher contrast or difference in shades of gray between each step. When using film we describe this difference using a film density scale from 0 to 3 as shown to the right. With digital imaging we use a gray scale coincident with the total bit depth of a given system so 0 to 8 bit is 00000000 to 11111111 in binary and 0 to 255 shades of gray in decimal where 16 bit the unsigned range is 0 through 65,535 shades of gray. We will discuss digital imaging in later segments. For now we will note the differences in the images below when current is increased thereby increasing the intensity of the X-ray beam. Note the increase in contrast.