X-Radiation Interaction With Matter Chapter 3 PDF

Chapter 3 Interaction of X-Radiation with Matter  Interaction of X-Radiation with Matter  The processes of interaction between radiation and matter are emphasized because a basic understanding of the subject is necessary to select technical exposure factors...

Chapter 3 Interaction of X-Radiation with Matter  Interaction of X-Radiation with Matter  The processes of interaction between radiation and matter are emphasized because a basic understanding of the subject is necessary to select technical exposure factors such as  Peak kilovoltage (kVp)  Milliampere-seconds (mAs)  kVp  Peak kilovoltage (kVp) is the highest energy level of photons in the x-ray beam, equal to the highest voltage established across the x-ray tube.  Controls the quality, or penetrating power, of the photons in the x-ray beam and to some degree, also affects the quantity, or number of photons, in the beam.  mAs  Milliampere-seconds (mAs) is the product of milliamperes (mA) which is electron tube current and the amount of time in seconds that the x-ray tube is activated.  mA × s = mAs  Control of X-ray Beam Quality and Patient Dose  Radiographer  Selects technical exposure factors that control beam quality and quantity  Is actually responsible for the dose the patient receives during an imaging procedure With a suitable understanding of technical exposure factors, radiographers can select appropriate techniques that can minimize the dose to the patient while producing optimal-quality images.  Significance of X-ray Absorption in Biologic Tissue ( 1of 2)  X-rays are carriers of manmade electromagnetic energy.  If x-rays enter a material such as human tissue, they may 1. Interact with the atoms of the biologic material in the patient and be absorbed 2. Interact with the atoms of the biologic material and be scattered, causing some indirect transmission 3. Pass through without interaction Significance of X-ray Absorption in Biologic Tissue (2 of 2)  If interaction occurs, electromagnetic energy is transferred from the x-rays to the atom’s of the patient’s biologic tissue.  Process is called absorption.  The amount of energy per unit mass is absorbed dose (D).  Without absorption and the differences in the absorption properties of various body structures, it would not be possible to produce diagnostically useful images in which different anatomic structures could be perceived and distinguished. X-ray Beam Production and Energy  Production of primary radiation  A diagnostic x-ray beam is produced when a stream of very energetic electrons bombards a positively charged target in a highly evacuated glass tube.  Target (anode) composition used in general radiography  Tungsten (a metal)  Tungsten rhenium (a metal alloy)  Tungsten and tungsten rhenium are used as target materials because:  High melting points  High atomic numbers  Production of Primary Radiation  Primary radiation is the x-ray photon beam that emerges from the x-ray tube and is directed toward the image receptor.  Energy of Photons in a Diagnostic X-ray Beam  Not all photons in a diagnostic x-ray beam have the same energy.  The most energetic photons in the beam can have no more energy than the electrons that bombard the target.  The energy of the electrons inside the x-ray tube is specified in terms of electrical voltage applied across the tube.  In diagnostic radiology, the voltage is expressed in thousands of volts, or kilovolts (kV).  Because the voltage across the tube fluctuates, it is usually characterized by the kilovolt peak value (kVp).  Attenuation  When an x-ray beam passes through a patient, it goes through a process called attenuation.  Direct and indirect transmission of x-ray photons  Absorption vs. Scatter  Absorbed: x-ray photons that interact with the atoms of a patient such that they give up all of their energy and cease to exist  Scatter: x-ray photons that interact with the atoms of the patient, but only surrender part of their energy.  The continue to exist but emerge from the interaction at a different angle  Attenuation vs. Transmission  Attenuation: photons that have undergone either absorption or scatter and do not strike the image receptor.  Transmission: photons that strike the image receptor are transmitted.  Direct Transmission vs. Indirect Transmission  Direct transmission: Some primary photons will traverse the patient without interacting and reach the image receptor.  Indirect transmission: Other primary photons can undergo Compton and/or coherent interactions and as a result may be scattered or deflected with a potential loss of energy.  Primary, Exit, and Attenuated Photons  Fig. 3.3  Primary, exit, and attenuated photons (photons 1, 2, 3, and 4) are photons that emerge from the x-ray source.  Exit, or image-formation, photons (photons 1 and 2) are photons that pass through the patient being radiographed and reach the radiographic image receptor.  Attenuated photons (photons 3 and 4) are photons that have interacted with atoms of the patient’s biologic tissue and have been scattered or absorbed such that they do not reach the radiographic image receptor.  Probability of Photon Interaction with Matter  Interaction of photons with biologic matter is random.  It is impossible to predict with certainty what will happen to a single photon when it enters human tissue.  See Table 3.1 demonstrates the factors that influence the probability of interactions in matter.  Processes of Interaction  Five types of interaction between X-radiation and matter are possible.  Only two of the five types of interactions between X- radiation and matter are important in diagnostic radiology.  Table 3.1 Presents an overview of the interactions.  Coherent Scattering  A relatively simple process that results in no loss of energy as x-rays scatter.  It occurs with low-energy photons, typically less than 10 keV.  Because the wavelengths of both incident and scattered waves are the same, no net energy has been absorbed by the atom (see Appendix E in textbook).  Rayleigh and Thompson scattering play essentially no role in radiography.  Coherent Scattering (Cont.) Figure 3-5. Coherent Scattering. The incoming low-energy x-ray photon interacts with an atom and transfers its energy by causing some or all of the electrons of the atom to momentarily vibrate. The electrons then radiate energy in the form of electromagnetic waves. These waves nondestructively combine with one another to form a scattered wave, which represents the scattered photon. Its wavelength and energy, or penetrating power, are the same as those of the incident photon. Generally, the emitted photon may change in direction less than 20 degrees with respect to the direction of the original photon. (Wavelength is the distance from one crest to the next.)  Photoelectric Absorption  Diagnostic radiology energy range: 23 to 150 kVp  This is the most important mode of interaction between x-ray photons and the atoms of the patient’s body for producing useful images.  Process of Photoelectric Absorption (1 of 2) Figure 3-6. Photoelectric absorption. (A) On encountering an inner-shell electron in the K or L shells, the incoming x-ray photon surrenders all its energy to the electron, and the photon ceases to exist. (B) The atom responds by ejecting the electron, called a photoelectron, from its inner shell, thus creating a vacancy in that shell. (C) To fill the opening, an electron from an outer shell drops down to the vacated inner shell by releasing energy in the form of a characteristic photon. Then, to fill the new vacancy in the outer shell, another electron from the shell next farthest out drops down and another characteristic photon is emitted, and so on until the atom regains electrical equilibrium. There is also some probability that instead of a characteristic photon, an Auger electron will be ejected.  Process of Photoelectric Absorption (2 of 2)  Auger effect (pronounced awzhay)  Discovered by Pierre Victor Auger in 1925  Produces an Auger electron  Is a radiationless effect  By-products of Photoelectric Absorption  Photoelectrons  Characteristic x-ray photons  Probability of Occurrence of Photoelectric Absorption  Depends on  Energy (E) of the incident x-ray photons  Atomic number (Z) of the atoms comprising the irradiated object  Increases markedly as the  E of the incident photon decreases  Z of irradiated atom increases  Mass Density and Effective Atomic Number of Different Body Structures  Influences attenuation Figure 3-7. (A) Equal thickness of bone and soft tissue are shown here. The bone absorbs nine times as many photons as the soft tissue. A factor of 2 is the result of bone’s being approximately twice as dense as soft tissue. A factor of 4.5 is the result of the higher atomic number of bone compared with that of soft tissue. Both factors together result in 2 × 4.5 = 9 times more absorption in this sample of bone.  Body Part Thickness  The thickness factor is approximately linear. Figure 3-7. (B) In the example shown here, the soft tissue is twice as thick as the bone. This thickness difference approximately cancels out the density difference between the bone and soft tissue. However, because the difference in atomic number still exists, in this example the bone would absorb 4.5 times as many photons as the soft tissue. Effects of Attenuation on Radiographic Images  The less a structure attenuates radiation, the darker the image will be and vice versa.  An image must have a sufficient amount of variation in densities to clearly visualize anatomic structures of interest.  Impact of Photoelectric Absorption on Radiographic Contrast  The greater the difference in the amount of photoelectric absorption, the greater the contrast in the radiographic image will be between adjacent structures of differing atomic numbers.  As absorption increases, so does the potential for biologic damage.  To ensure both radiographic image quality and patient safety, choose the highest-energy x-ray beam that permits adequate radiographic contrast for computed radiography, digital radiography, or conventional radiography.  Use of Contrast Media to Ensure Visualization of Anatomic Structures (1 of 2)  If tissues or structures are similar in Z, and mass density must be distinguished, use of appropriate contrast media may be needed to ensure visualization of those tissues or structures in the radiographic image.  Use of positive contrast medium  Use of negative contrast medium Use of Contrast Media to Ensure Visualization of Anatomic Structures (2 of 2) Figure 3-10. (A) Anteroposterior (AP) projection of an abdomen without the aid of a positive contrast medium. Parts of the urinary system other than the kidneys, which have their own unique density, are not radiographically demonstrated. (B) AP projection of the abdomen after the intravenous injection of an appropriate positive contrast medium that permits visualization of the entire urinary system, thus allowing each contrast-filled structure to be distinguished. (A, From Ballinger PW, Frank ED: Merrill’s Atlas of Radiographic Positions and Radiologic Procedures, ed 10, St Louis, 2003, Mosby. B, From Frank ED, Long BW, Smith BS: Merrill’s Atlas of Radiographic Positioning & Procedures, ed 12, St. Louis, 2012, Mosby.)  Compton Scattering Responsible for most of the scattered radiation produced during radiologic procedures. Figure 3-12. Compton scattering is responsible for most of the scattered radiation produced during a radiologic procedure.  Process of Compton Scattering  Incoming x-ray photon interacts with a loosely bound outer electron of an atom.  The incoming x-ray photon surrenders a portion of its energy, ionizing the atom. Figure 3-11.  Pair Production  Occurs at an energy level of at least 1.022 million electron volts (MeV).  Beyond diagnostic energy range.  Process of Pair Production  Incoming x-ray photon interacts with the electric field surrounding the nucleus of an atom of irradiation tissue and disappears.  The energy of the photon is absorbed and transformed into a negatron and a positron. Figure 3- 14. Use of Annihilation Radiation in Positron Emission Tomography (PET)  Source of positrons  Process of positron decay  Formation of annihilation photons  Examples of unstable nuclei used in PET scanning  Photodisintegration  An interaction that occurs at more than 10 MeV in high-energy radiation therapy treatment machines Figure 3-15. Photodisintegration. An incoming high- Process of energy photon collides with the nucleus of the atom of  the irradiated object and absorbs all the photon’s photodisintegration energy. This energy excess in the nucleus creates an instability that is usually alleviated by the emission of a neutron. In addition, if sufficient energy is absorbed by the nucleus, other types of emissions will be possible, such as a proton or proton–neutron combination (deuteron), or even an alpha particle.  The End  Chapter Summary  General Discussion Questions  Review Questions