Absorption of X-Rays
Absorption of X-rays refers to the process in which X-ray photons transfer their energy to matter, leading to the removal of the X-ray photons from the beam. This can occur through photoelectric absorption, Compton scattering, or pair production. The likelihood of each type of absorption depends on the energy of the X-rays and the atomic structure of the absorbing material.
8 Key excerpts on "Absorption of X-Rays"
eBook - ePub- Suzanne Easton(Author)
- 2012(Publication Date)
- Wiley-Blackwell(Publisher)
...These electrons will interact with neighbouring atoms through ionisation and excitation. This will happen until all the kinetic energy is lost. A photoelectron or Compton electron will produce several hundred ionisations within a fraction of a millimetre. This process may result in biological effects to the cells. Absorption During absorption, the energy from the X-ray photon is transferred to the atoms of the absorber. As more energy is absorbed, the number of X-ray photons passing through the matter and forming the image on the film decreases. This absorption can be as a result of the energy of the X-ray photons or the atomic number of the matter. During absorption, the photoelectric effect will take place (Figure 7.3). Figure 7.3 The differences in absorption caused by tissue and material type. Attenuation Attenuation is the reduction in intensity of the X-ray beam as it passes through matter. This can be due to absorption or scatter or a combination of both. Every type of matter has a different attenuation coefficient. This is the effectiveness of the matter to attenuate or absorb the radiation passing through. Every X-ray will have a fixed range of attenuation through tissue; this will depend on the atomic number of the atoms involved and the density of the atoms within the tissue. Attenuation is the sum of scattered and absorbed X-ray photons. Attenuation = Absorption + Scatter The effects of ionising radiation on the body Ionising radiation can originate from the X-ray tube or in the form of gamma rays. This radiation damages cells and is not selective of cell type. The effects are more obvious in the immature or rapidly dividing cells. The amount of cell damage is determined by the amount of dose, the type of cell and its maturity. If the cell is still being formed when the dose is received, then the DNA may mutate...
eBook - ePubPractical Radiotherapy
Physics and Equipment
- Pam Cherry, Angela M. Duxbury, Pam Cherry, Angela M. Duxbury(Authors)
- 2019(Publication Date)
- Wiley-Blackwell(Publisher)
...CHAPTER 5 X‐ray Interactions with Matter Kathryn Cooke Aim The aim of this chapter is to outline the principles of the interaction process that occur when ionising radiation interacts with a medium in the energy ranges relevant to radiotherapy. 5.1 Introduction There are several interaction processes that occur when ionising radiation interacts with matter. These depend on the nature and energy of the primary radiation beam and the structure of the medium through which the radiation beam passes. It is beyond the scope of this chapter to describe all possible interaction processes, so only the three that occur within the X‐ray energy ranges utilised in radiotherapy are presented later: photoelectric process, Compton scatter, and pair production. 5.1.1 Attenuation It is important to understand how the intensity of the X‐ray beam is affected when ionising radiation interacts with a medium. When a beam of X‐rays traverses matter, there is a reduction in the intensity of that beam. Intensity is defined as the rate of flow of photon energy through a unit area lying at right angles to the path of the beam. This reduction in intensity is referred to as attenuation and involves a process of absorption, scattering, or a combination of both. Absorption is the transference of energy from the primary X‐ray beam to the atoms of the medium through which the X‐ray beam is passing. It is defined as the energy deposited in the material per unit mass. The transferred energy is converted into kinetic energy of the electrons within the medium, enabling them to move through the medium. The electrons may interact with other atoms in the medium causing ionisation and excitation (see Chapters 2 and 3). This results in the chemical and biological changes important to radiotherapy. Scattering occurs following a collision interaction between the primary X‐ray beam and the atoms of the medium through which the X‐ray beam is passing...
eBook - ePub- Euclid Seeram(Author)
- 2019(Publication Date)
- Wiley-Blackwell(Publisher)
...Among these, the photoelectric absorption and Compton scattering are the most important attenuation mechanisms in diagnostic radiology. Therefore, only these two are presented in this chapter. The others do not occur with any noticeable significance in diagnostic radiology. The physics of the interactions will not be described but, rather, a basic overview is presented to demonstrate how these two interactions relate to dose to the patient, as well as beam energy. Photoelectric Absorption Photoelectric absorption (also referred to as the photoelectric effect) is shown in Figure 2.6. The following points are noteworthy: The incident photon interacts with an inner‐shell (K or L) electron (tightly bound). The photon is completely absorbed and the electron is ejected from the atom. This electron is called a photoelectron. The absorption of the incident photons in this interaction increases the dose to the patient as they do not pass through the patient and reach the image receptor. The probability that the photoelectric effect will occur depends on the photon energy (E) and the atomic number (Z) of the absorbing material. The probability is directly proportional to Z 3 and inversely proportional to E 3. At low beam energies (low‐kV techniques) the photoelectric effect predominates and the patient dose increases. However, image contrast is greater in materials with high atomic numbers. Figure 2.6 Photoelectric absorption is an attenuation mechanism whereby an incident photon interacts with inner‐shell electrons, ejecting them from the atom. See text for further explanation. Compton Scattering Another X‐ray interaction that is significant in diagnostic radiology is Compton scattering, as shown in Figure 2.7...
eBook - ePub- Loredana Marcu, Eva Bezak, Barry Allen(Authors)
- 2012(Publication Date)
- CSIRO PUBLISHING(Publisher)
...Very soft X-rays are not useful for the majority of applications and are therefore for practical purposes filtered out of the spectrum (absorbed by a layer of material placed in its path). Filtering is the removal of soft radiation by absorption in the glass envelope of the tube, metal filter (Al, Cu) and the anode material itself. The bremsstrahlung rate of electron energy loss is inversely proportional to the square of electron mass (strictly speaking, other charged particles will also undergo bremsstrahlung processes, but due to their large mass, the effect is negligible) and to the square of the atomic number of the nucleus involved in the interaction: 1.3 INTERACTION OF X-RAYS WITH MATTER 1.3.1 Linear attenuation coefficient When a beam of X-ray photons passes through an object, interactions occur that result in a decrease of the number of transmitted (non-interacting) photons. There are a number of factors that can affect the attenuation of X-ray photons. Consider a thin slab of uniform material with thickness dx as shown in Figure 1.5. Figure 1.5. Schematic showing X-ray photons impinging on a thin slab of material. Some X-rays will interact inside the material and only N photons will pass through. If N X-rays are incident on the material and have a probability of interaction m per meter, then the change in the number of photons which have not interacted, dN is given by: After integration over the total thickness of the slab, T, we get: where N is the number of photons which emerge from the slab without having interacted in the slab and N 0 is the number of photons entering the slab. The coefficient µ is referred to as the linear attenuation coefficient and the equation (1.7) is known as the Lambert-Beers law. It is common to define the units of the linear attenuation coefficient as cm -1, assuming the thickness of the material is measured in cm...
eBook - ePubX-Ray Imaging
Fundamentals, Industrial Techniques and Applications
- Harry E. Martz, Clint M. Logan, Daniel J. Schneberk, Peter J. Shull(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
...These photons move away from the site of the primary interaction. Scattering processes can involve local heat deposition (a recoil electron in a Compton event), but the distinguishing attribute of scattering is that a photon survives with a new direction. Coherent scatter is another attenuation mechanism but is very small and not discussed here. FIGURE 5.3 Just as with other electromagnetic radiation, x-rays, in addition to absorption, can be (a) reflected and (b) refracted. θ i is the angle of incidence, θ c is the critical angle, η is the index of refraction, δ is the phase shift, and β is the attenuation term of the refractive index. When the incident or primary photon energy exceeds 1.02 MeV, another interaction, called pair production, becomes energetically possible. In this interaction, the x-ray or γ-ray disappears, and an electron–positron pair is produced (see Figure 5.9). The positron promptly annihilates producing two 511 keV annihilation photons that are collinear and departing the annihilation site in opposite directions (see Section 4.2.3, Annihilation Radiation Generation). 5.2.1.1 Absorption (Photoelectric Absorption) Photoelectric absorption is dominant at low x-ray and γ-ray energies (see Figure 5.10). In photoelectric absorption, photons are absorbed and removed from the beam, as shown in Figure 5.7. It arises by the interaction of the photon with a tightly bound atomic electron. The energy of the incident photon is transferred to the electron, which is ejected from the atom. The energy of the incident photon, E p 0, is given by E p 0 = h υ 0, (5.1) FIGURE 5.4 Just as with other electromagnetic radiation, x-rays, in addition to absorption, can be diffracted (a) by the atoms in a crystal lattice or (b) by an aperture...
- Rachel A. Powsner, Matthew R. Palmer, Edward R. Powsner(Authors)
- 2013(Publication Date)
- Wiley-Blackwell(Publisher)
...This process is called the photoelectric effect and the ejected electron is called a photoelectron (Figure 2.4). This electron leaves the atom with an energy equal to the energy of the incident gamma ray diminished by the binding energy of the electron. An outer-shell electron then fills the inner-shell vacancy, and the excess energy is emitted as an X-ray: Figure 2.4 Photoelectric effect. Table 2.1 lists the predominant photon interactions in some common materials. Table 2.1 Predominant photon interactions in common materials at diagnostic energies Attenuation of Photons in Matter As a result of the interactions between photons and matter, the intensity of the beam (stream of photons), that is, the number of photons remaining in the beam, decreases as the beam passes through matter (Figure 2.5). This loss of photons is called attenuation ; the matter through which the beam passes is referred to as the attenuator. Specifically, attenuation is the ratio of the intensity at the point where the beam exits the attenuator, I out, to the intensity it had where it entered, I in. The attenuation is an exponential function of the thickness x of the attenuator in centimeters. The fact that the function is exponential can be understood to mean that if half of the beam is lost in traversing the first centimeter of material, half of the remainder will be lost traversing the next centimeter, and so on. This resembles the exponential manner in which radioactivity decays with time. Expressed symbolically, Figure 2.5 Attenuation. where μ, the linear attenuation coefficient, is a property of the attenuator. When, as is usually the case, thickness is given in centimeters, the linear attenuation coefficient is expressed as “per centimeter” or “cm −1.” As might be expected, the linear attenuation coefficient is greater for dense tissue such as bone than for soft tissue such as fat...
eBook - ePubModern Diagnostic X-Ray Sources
Technology, Manufacturing, Reliability
- Rolf Behling(Author)
- 2021(Publication Date)
- CRC Press(Publisher)
...Chapter 3 The interaction of X-ray with matter Interaction of X-ray photons with human tissue is a great source of information. Contrast bolus in cerebral vessels helps differentiate ischemic from hemorrhagic stroke and direct the therapy. Understanding the fingerprints of X-ray attenuation, scattering, and refraction is essential to gain the most information from the lowest dose of ionizing radiation. (Courtesy of Philips.) The explosion of scientific and clinical works immediately after Conrad Roentgen’s discovery was an indicator for the huge lack of knowledge about the interior morphology of patients. “More light” were supposedly Johann Wolfgang von Goethe’s last words. This is what Roentgen has generated. By the same medium of the invisible light, anatomic, and functional information is transmitted. The major interactions of X-ray photons with matter will be discussed in the following section to get a glimpse of the information, which can be expected by measuring them. X-ray photons interact with tissue, fat, bone, water, or air. They are either extinguished or scattered out of the initial direction, or generate secondary photons depending on the elementary composition of the object and the material density. In all these cases, photons disappear from the primary beam. Usually, only a small fraction in the order of a percent will reach the detector. Refraction of X-rays is very small compared to visible light for photons of the relevant energies. Differentiation between direct radiation, which originates in the focal spot, and indirect radiation, which originates elsewhere, is therefore the key for the interpretation of the spatial pattern of X-ray flux sensed by the detector. Although X-ray phase shifts are comparatively large refraction and phase shift of X-rays have not (yet?) been utilized in day-to-day clinical routine...
eBook - ePubMethods and Applications of Statistics in Clinical Trials, Volume 2
Planning, Analysis, and Inferential Methods
- Narayanaswamy Balakrishnan, N. Balakrishnan(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
...Only a few photons are emitted with the full 100 keV of energy of the incident electrons, and the vast majority are radiated away with far less. The measured spectrum for a real X-ray tube differs from this straight-line, curve B, For reasons soon to be discussed, the rate of X-ray absorption by materials in creases rapidly the lower their energy. This means that as newly minted X rays emerge from (sometimes deep) within the target itself, pass through the glass window of the X-ray tube, and even through intentionally placed filters (usually aluminum), C, the lower-energy ones are preferentially removed. This is good, because these low- energy X rays do not penetrate far through tissues, and therefore cannot contribute to image formation, but they will deposit dose, and there is a very slight chance that this will lead to adverse health effects. The most energetic characteristic X rays photons for tungsten, incidentally, lie in the range from 58 to 69 keV. The X rays emerging from a tube pass through an adjustable beam collimator (Figure 7), again. The result of all this is that a nearly uniform, rectangular beam of X rays of appropriate dimensions exits the tube via its window and heads off toward, say, a patient’s hand. 14.2.3 Exponential Attenuation of a Narrow Monochromatic Beam by a Homogeneous Medium Electromagnetic radiation can interact with matter by various mechanisms. The relative probability that a photon will collide with an atomic electron depends on the energy of the photons and on the nature of the material; physicists have expended much of their efforts over the past century in learning about those processes. The attenuation of a narrow X-ray beam in passing through soft tissues, bone, or IR materials, in particular, is easily studied by experiment (Figure 10a)...
