CHAPTER 4 RADIATION ATTENUATION

Transcription

1 HDR202 PHYSICS FOR RADIOGRAPHERS 2 CHAPTER 4 RADIATION ATTENUATION PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES

2 Learning Objectives At the end of the lesson, the student should be able to:- Define the scattering and absorption. Describe the probability of occurrence of interactions. Explain the photon energy, atomic number, K-edge, density, and thickness of attenuator. Explain the x-ray interaction with matters. Explain the particles and photons. Explain the direction and energy of scattered radiation. Explain the inverse square law. Slide 2 of 38

3 List of Contents 4.1 Scattering and Absorption 4.2 Probability of occurrence of interaction 4.3 Photon energy, atomic number, k-edge, density, and thickness of attenuator 4.4 X-ray interaction with matter 4.5 Particles and Photons 4.6 Direction and Energy of Scattered Radiation 4.7 Inverse Square Law Slide 3 of 38

4 Definition Scattering The diversion of radiation (thermal, electromagnetic, or nuclear) from its original path as a result of interaction or collisions with atoms, molecules, or larger particles in the atmosphere or other media between the source of the radiation (e.g., a nuclear explosion) and a point at some distance away. As a result of scattering, radiation (especially gamma rays and neutrons) will be received at such a point from many directions instead of only from the direction of the source. Slide 4 of 38

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6 Definition Attenuation The process by which radiation losses the energy as it travels through matter and interacts with it. Beam attenuation is the basis of the contrast observed in all X-ray based imaging methods. Absorption The process by which radiation losses the intensity as it passes through a material medium by conversion of the energy of the radiation to an equivalent amount of energy appearing within the medium; the radiant energy is converted into heat or some other form of molecular energy. Slide 6 of 38

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8 X-ray Interaction with Matter There are 5 interactions with matter:- 1. Coherent Scattering 2. Photodisintegration 3. Pair Production 4. Compton Effect or scattering 5. Photoelectric Effect Slide 8 of 38

9 a) Coherent Scattering A term sometimes used for Rayleigh scattering and Thomson scattering. They are both examples of coherent scattering, in which the incident photon undergoes a change in direction without a change in wavelength. Notice in this diagram that the direction of the photon is changed but the wavelength remains the same. Slide 9 of 38

10 b) Photodisintegration Collision of a high energy photon with an atomic nucleus. The photon is completely absorbed in the process, and a neutron, proton, or alpha particle is ejected from the excited nucleus. Need at least 10 MeV for photodisintegration. This is more energy than a normal x-ray. This diagram shows an x-ray interacting with the nucleus of an atom and expelling a piece of the nucleus. Slide 10 of 38

11 c) Pair Production The process in which a high-energy photon is completely transformed into an electron and a positron. Thus, this is a process whereby energy is transformed into matter. It occurs only in the vicinity of atoms which act as a sort of "catalyst". Since according to Einstein's theory of relativity, the energy (E) and the mass (m) are proportional to each other with the constant of proportionality being the square of the velocity of light (c), and the resting masses of electron and positron are 511 kev each, the minimum photon energy required for pair production to occur is MeV. The inverse reaction to pair production is the annihilation reaction. This is also more energy than normal x-ray. Slide 11 of 38

12 d) Compton Scattering During Compton scattering, a photon impinges on an electron in matter, and in this process transfers part of its energy to it. The excited electron is termed a Compton electron and is ejected or moved into an excited atomic state, while due to the law of conservation of energy the photon energy is reduced. This diagram shows a photon interacting with an electron, ejecting it and giving some of its energy to the electron. The photon is scattered by an angle, luckily we do not have to calculate the angle, and the wavelength is changed. Slide 12 of 38

13 e) Photoelectric Effect The effect discovered by Einstein (for which he received the Nobel prize in 1921) in which a photon transfers its entire energy to an electron in the material on which it impinges. The electron thereby acquires enough energy either to free itself from the material to which it is bound or to be elevated into the conduction band of a semiconductor or insulator (solid). This diagram shows a photon interacting with an electron and giving all of its energy to the electron. The electron is then ejected from the atom. Slide 13 of 38

14 Series of Photoelectric Effect Slide 14 of 38

15 The probability of photoelectric effect:- The probability of the photoelectric effect is inversely proportional to the cube of the x-ray energy. Slide 15 of 38

16 The probability of photoelectric effect is directly proportional to the cube of the atomic number of the absorbing material. Effective Atomic Numbers Types of Substance Effective Atomic Number Human Tissue Fat 6.3 Soft Tissue 7.4 Lung 7.4 Bone 13.8 Contrast Material Air 7.6 Iodine 53 Barium 56 Other Concrete 17 Molybdenum 42 Tungsten 74 Lead 82 Slide 16 of 38

17 Features of Photoelectric Effect Most likely to occur a) With inner-shell electrons b) With tightly bound electrons c) When x-ray energy is just higher than electron-binding energy As x-ray energy increases As atomic number of absorber increases As mass density of absorber increases a) Increased penetration through tissue without interaction b) Less photoelectric effect relative to Compton effect c) Reduced absolute photoelectric effect Increases proportionately with the cube of the atomic number Proportional increase in photoelectric absorption Slide 17 of 38

18 Differential Absorption Differential Absorption occurs because of Compton scattering, photoelectric effect, and x-rays transmitted through the patient. Slide 18 of 38

19 Radiopaque - or opaque, the relative capacity of matter to obstruct the transmission of radiant energy. When x-rays are obstructed the film is light, for example from bone. Radiolucent - or nonopaque, being permeable to radiation or penetrable by X- rays. The opposite term is radiopaque. When x-rays are not obstructed the film is dark. The difference between the radiopaque and the radiolucent areas of the body give the contrast or differential absorption. Differential absorption increases as the kvp is reduced. Slide 19 of 38

20 Dependence on Atomic Number Slide 20 of 38

21 Dependence on Mass Density Mass density is the mass per unit volume of a substance. The interaction between x-rays and tissue is proportional to the mass density of the tissue. Mass Density of Materials in Radiology Substance Human Tissue Mass Density Lung 320 Fat 910 Soft tissue, muscle 1000 Bone 1850 Contrast Material Air 1.3 Barium 3500 Iodine 4930 Other Calcium 1550 Concrete 2350 Molybdenum 10,200 Lead 11,350 Rhenium 12,500 Tungsten 19,300 Slide 21 of 38

22 Characteristics of Differential Absorption As x-ray energy increases As tissue atomic number increases As tissue mass density increases a) Fewer Compton interactions b) Many fewer photoelectric interactions c) More transmission through tissue a) No change in Compton interactions b) Many more photoelectric interactions c) Less x-ray transmission a) Proportional increase in Compton interactions b) Proportional increase in photoelectric interaction c) Proportional reduction in x-ray transmission Slide 22 of 38

23 Exponential Attenuation Attenuation - process by which radiation loses power as it travels through matter and interacts with it. Beam attenuation is the basis of the contrast observed in all X-ray based imaging methods. Slide 23 of 38

24 K-Edge K-edge describes a sudden increase in the attenuation coefficient of photons occurring at a photon energy just above the binding energy of the K shell electron of the atoms interacting with the photons. The sudden increase in attenuation is due to photoelectric absorption of the photons. For this interaction to occur, the photons must have more energy than the binding energy of the K shell electrons. A photon having an energy just above the binding energy of the electron is therefore more likely to be absorbed than a photon having an energy just below this binding energy. Slide 24 of 38

25 Particles Particles are the one that makes the atom and usually it is called subatomic particles. The atom is sometimes also called particles. The three main subatomic particles that form an atom are protons, neutrons, and electrons. Slide 25 of 38

26 Photons A photon is a discrete bundle (or quantum) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum, have a constant speed of light to all observers, at the vacuum speed of light (more commonly just called the speed of light) of c = x 108 m/s. Slide 26 of 38

27 Basic Properties of Photons move at a constant velocity, c = x 108 m/s (i.e. "the speed of light"), in free space. have zero mass and rest energy. carry energy and momentum. can be destroyed/created when radiation is absorbed/emitted. can have particle-like interactions (i.e. collisions) with electrons and other particles, such as in the Compton effect. Slide 27 of 38

28 Inverse Square Law an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity. generally applies when some force, energy, or other conserved quantity is radiated outward radially in three-dimensional space from a point source. Slide 28 of 38

29 Inverse Square Law It can be applied in (for examples):- I. Gravitational II. Electrostatics III. Electromagnetic Radiation (e.g. x-ray, gamma ray, etc) The equation is:- Where, I is Intensity P is Point of Source A is Area Slide 29 of 38

30 End of Lecture Thank You Slide 30 of 38