What is X-ray, Scatter Radiation and the Absorption of 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. Compared to the visible wavelength of light, the shorter wavelength of X-rays enables them to penetrate matter. 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 X-ray or 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).

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Photoelectric Absorption

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.

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Compton Scattering

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.

 

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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.

Rayleigh Scattering

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.

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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

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Absorption of X-rays

The increase in Kev will increase the penetration of materials and lower the wavelength of the X-rays. The shorter wavelength increases penetration of materials. Increased Energy, (E) results in an increase in penetration.  

Energy(E)

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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)

Density (ρ)

The Thickness (t) of a material decreases radio-translucence. Thickness also includes distance through the material. 

Thickness (t)

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Absorption of X-rays at Different Intensities

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.

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                                      50ua                                                                100ua                                                            150ua                                   

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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

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Inverse Square Law

The inverse square law can be defined easily by comparing the spread of photons at different distances from the focal spot. 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. This is a law of science that is caused by geometric dilution of a point source in three dimensional space.

Inverse-square law - Wikipedia

The divergence of a vector field which is the resultant of radial inverse-square law fields with respect to one or more sources is everywhere proportional to the strength of the local sources, and hence zero outside sources. Newton's law of universal gravitation follows an inverse-square law, as do the effects of electricmagneticlightsound, and radiation phenomena.

230Kv Step wedge

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What are Contrast Materials?

Contrast materials are used to improve image contrast and they are also called contrast agents or contrast media. The human body is close to the radiation mass attenuation coefficient or density of water except for bone tissue. In medical imaging modalities, contrast agents are used to improve the generally homogeneous images of internal structures inside of the body produced by x-rayscomputed tomography (CT), magnetic resonance (MR) imaging, and ultrasound. They are substances that temporarily change the way x-rays or other imaging tools interact with the body tissue. For instance, when a contrast agent is introduced into the blood stream the cardiologist can visualize the tissue of the blood vessels to identify blockage, aneurisms and other abnormalities because the radiation absorption factor is temporarily changed. The goal is to improve the visibility of specific organs, blood vessels or tissues, contrast materials help physicians diagnose medical conditions. X-ray CT systems for non-destructive testing generally do not use contrast agents because the materials are can not absorb the contrast agent. However material penetrants are often used for delamination and planar cracks to enhance the contrast in the detection of such defects. Silver nitrate, zinc iodide, chloroform and diiodomethane are a few of the penetrants used. Choice of the penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed. Diiodomethane has high opacity, ease of penetration, and ease of removal and fast evaporation. There are other contrast enhancements that are developed that do not require a contrast medium such as Talbot-Lau Interferometry. Additionally, new techniques such as dual energy image subtraction can be used to provide more separation between elements that have different absorption characteristics through out a range of X-ray energies.

Half Value Layer (HVL)

The half value layer or HVL is used to test the hardness of an X-ray beam. Each material has a specific thickness where the X-ray energy is cut in half. This is call the HVL of a material. However, not only thickness and density of a material determines the HVL but also the energy of the x-ray beam. In medical X-ray imaging testing the HVL is important because the patient should get the minimum dose of low energy radiation possible with the best image. Aluminum filters are inserted in the X-ray beam path to  attenuate the radiation to one half. There are specific guidelines set forth by safety code 35 showing the proper HVL at each energy. In medical systems with rotating anodes, typically tungsten deposits on the tube port which hardens the beam so your HVL increases over time. For industrial X-ray and CT the HVL can be used to determine proper technique for different materials. For linear accelerators in the MeV range the Tenth Value Layer our TVL is used due to the higher energy measurements. When filtration is added to the tube port, the level of soft radiation is reduced thus providing less incidence of scatter radiation and a cleaner image.