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Similar to X ray attenuation- Attenuation is the reduction in the intensity of an x-ray beam as it traverses matter by either the absorption or deflection of photons from the beam.pptx
X ray attenuation- Attenuation is the reduction in the intensity of an x-ray beam as it traverses matter by either the absorption or deflection of photons from the beam.pptx
- 1. ATTENUATION (PART-1) PRESENTOR :Dr. Harshit (JR 1) Moderator : Dr. Likitha (JR 2) Date : 01/03/2024
- 2. CONTENTS • Definition • MonochromaticRadiation • Attenuation Coefficients • Relationship between: Density & Atomic number Density & Electrons per gram Atomic number & Electrons per gram Effects of energy & Atomic number • Factors affecting attenuation
- 3. • Quantity andquality are two terms used to express the characteristics of an x-ray beam. • Quantity refers to the number of photons in the beam, and quality refers to their energies. • The intensity of a beam is the product of the number and energy of the photons, so it depends on both quantity and quality. • Attenuation is the reduction in the intensity of an x-ray beam as it traverses matter by either the absorption or deflection of photons from the beam. • Attenuation depends on both the quantity and quality of the photons in a beam.
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- 5. • The qualityof monochromatic radiation does not change as it passes through an absorber. • Mainly deals with low energy photons (20- 80 kev), primary photons that have only one interaction. • A 50% reduction in the number of photons is the 50 % reduction in the intensity of the beam. • When the number of transmitted photons and the absorber thickness are plotted on a linear graph, it results in a curved line.
- 7. EXPONENTIAL ATTENUATION • When thenumber of photons remaining in the beam decreases by the same per-centage with each increment of absorber, as with monochromatic radiation, the attenuation is called exponential.
- 8. ATTENUATION COEFFICIENTS Anattenuation coefficient is a measure of the quantity of radiation attenuated by a given thickness of an absorber. Name is determined by the units used to measure the thickness of the absorber. Two types : • Linear attenuation coefficient • Mass attenuation coefficient
- 9. Linear Attenuation Coefficient •It is a quantitative measurement of attenuation per centimeter of an absorber, so it tells us how much attenuation we can expect from a certain thickness of tissue. • Because we measure our patients in centimeters, it is a practical and useful attenuation coefficient. • Its symbol is the Greek letter µ. • The unit of the linear attenuation coefficient is per centimeter, usually written cm-1 .
- 10. • The linearattenuation coefficient (µ) is used for monochromatic radiation. • It is specific both for : The energy of the x-ray beam : Type of absorber. • Water, fat, bone, and air all have different linear attenuation coefficients. • The size of the coefficient changes as the energy of the x-ray beam changes. When the energy of the radiation is increased the number of x rays that are attenuated decreases, and so does the linear attenuation coefficient.
- 11.
- 12. • The half-valuelayer is the absorber thickness required to reduce the intensity of the original beam by one half. • It is a common method for expressing the quality of an x-ray beam. • A beam with a high half value layer is a more penetrating beam than one with a low half-value layer. • The product of the linear attenuation coefficient and half-value layer is equal to 0.693.
- 13. Mass Attenuation Coefficient •The mass attenuation coefficient (also known as the mass absorption coefficient) is a constant describing the fraction of photons removed from a monochromatic x-ray beam by a homogeneous absorber per unit mass. • Mass attenuation coefficient is used to quantitate the attenuation of materials independent of their physical state. • For example water, ice, and water vapor, the three physical states of H2O, all have the same attenuation coefficient. • It is obtained by dividing the linear attenuation coefficient(µ) by the density (p), and is expressed in cm2 /g.
- 14. Absorber in cm Linear attenuation coefficientin cm-1 Linear Attenuation coefficient Absorber in g/cm2 . Mass Attenuation Coefficient in cm2 /g. Mass Attenuation Coefficient
- 16. • A gramof water and a gram of water vapor both absorb the same amount of radiation • But we don’t deal with 1 gm/cm2 of patient instead we measure patient’s thickness in cm. • So linear attenuation coefficient is more important in diagnostic radiology.
- 17. Density and AtomicNumber • In general, elements with high atomic numbers are denser than elements with low atomic numbers, but there are exceptions. • There is no relationship between atomic number and density when different physical states of matter are involved. • Water has an effective atomic number of 7.4 regardless of its state (ice, liquid, or vapor), but its density is different in each of these three forms.
- 18. Density and electronsper gram • Because density depends on volume (weight per unit volume), there is no relationship between density and electrons per gram. • A gram of water has the same number of electrons, regardless of whether they are compressed together in a 1-cm cube as a liquid, or spread out over 1670 cm3 as a vapor. Atomic Number and Electrons per Gram • Electrons per gram is a function of the neutrons in an atom. • If there are no neutrons, there will be 6.0 x 1023 electrons/g in all materials. • Hydrogen has twice as many electrons as any other element. • Elements with low atomic numbers have more electrons per gram than those with high atomic numbers.
- 19. Effects of Energyand Atomic Number • Energy and atomic number, in combination, determine the percentage of each type of basic interaction, so in this sense their effects on attenuation are inseparable. • As the radiation energy increases, the percentage of photoelectric reactions decreases for water and bone; as the atomic number increases, the percentage of photoelectric reactions increases. • With extremely low energy radiation (20 keV), photoelectric attenuation predominates, regardless of the atomic number of the absorber.
- 20. • As theradiation energy is increased, Compton scattering becomes more important until eventually it replaces the photoelectric effect. • The linear attenuation coefficient is the sum of the contributions from coherent scattering, photoelectric reactions, and Compton scattering: • With high atomic number absorbers, transmission may decrease with increasing beam energy. • It occurs because there is an abrupt change in the likelihood of a photoelectric reaction as the radiation energy reaches the binding energy of an inner shell electron.
- 21.
- 22. 1. Energy ofradiation • The percentage of transmitted photons increases as the energy of the beam increases means the attenuation decreases.
- 23. 2. Atomic number •As Atomic number increases transmission decreases. • This occurs because the likelihood of photoelectric reaction increases with increase in atomic number. • A photon cannot eject an electron unless it has more energy than the electron's binding energy. • Thus, a lower energy photon is more likely to be transmitted than a higher energy photon, provided one has slightly less and the other slightly more energy than the binding energy of the electron.
- 24. K edge • Asudden change in transmission occurs at 88 keV, which is the binding energy of the K-shell electron. • This is called the K edge.
- 25. • Gram forgram tin is a better absorber of x rays than lead between 29 and 88 keV. • Because tin attenuates more radiation per unit weight than lead, it has recently come into use for this purpose. • A lighter tin apron gives the same protection as a standard lead apron.
- 26. 3. Effect ofdensity • Tissue density is one of the most important factors in x-ray attenuation. • Density determines the number of electrons present in a given thickness, so it determines the tissue's stopping power. • If the density of a material is doubled, attenuation doubles.
- 27. 4. Effect ofElectrons per Gram • The number of Compton reactions depends on the number of electrons in a given thickness. • Absorbers with many electrons are more impervious to radiation than absorbers with few electrons. • The number of electrons is usually expressed in the unit e/g (a mass unit) rather than e/cm3 (a volume unit). • By multiplying electrons per gram and density, we get electrons per cubic centimeter:
- 28. • The numberof electrons per gram can be calculated by the equation. • The high atomic number elements have about 20% fewer electrons per gram than the low atomic number elements.
- 29. References: • Curry TS,Dowdey JE, Murry RE. Christensen’s physics of diagnostic radiology. 4th ed. Philadelphia, PA: Lea & Febiger; 1990. • Rock P, Vajuhudeen Z. Mass attenuation coefficient. In: Radiopaedia.org. Radiopaedia.org; 2020.
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Editor's Notes
- #4 A beam of 1000 photons is mono directed at a water phantom. The intensity of the beam is decreased to 800 photons by the first centimeter of water, which is an attenuation of20%. The second centimeter of water decreases the intensity to 640 photons, which is 20% less than had passed through the first centimeter. With each succeeding centimeter of water, 20% of the remaining photons are removed from the beam.
- #6 When the number of transmitted photons and absorber thickness are plotted on linear graph paper, a curved line results (Fig. 5-2A). The initial portion of the curve is steep, because more photons are removed from the beam by the first few centimeters of absorber. After the beam has passed through many centimeters of water, only a few photons remain. Although each centimeter continues to remove 20% of the photons, the total numbers are small, and the end of the curve is almost flat.
- #7 The same numbers plot a straight line on semilogarithmic graph paper (Fig.52B). When the number of photons remaining in the beam decreases by the same per-centage with each increment of absorber, as with monochromatic radiation, the attenuation is called exponential. Exponential functions plot a straight line on semilogarithmic graph paper.
- #14 Figure 5-3 illustrates the meaning of a gram per square centimeter by comparing aluminum and water. The density of water is 1 g/cm3, so a square centimeter of water must be 1 cm thick to weigh 1 g. The density of aluminum is 2.7 g/cm3, so a square centimeter of aluminum only has to be 0.37 cm thick to weigh 1 g. The arithmetic is simple. The thickness is merely the reciprocal of the density, or 1 divided by the density. A square centimeter of water 1 cm thick and a square centimeter of aluminum 0.37 cm thick both contain 1 g/cm2. The unit of the mass attenuation coefficient is per g/cm2. It can be expressed in several ways: per g/cm2 or 11g/cm2 or cm2/ g; usually it is written cm2/g.
- #16 The values in the illustration are for a 50-keV monochromatic beam. The linear attenuation coefficient for water is 0.214 cm-1, and a 1-cm thickness of water absorbs 20% of the incidence beam. The same thickness of ice absorbs 18.5%, because ice is a little less dense than water. Water vapor has very little density , and a 1-cm thickness absorbs almost nothing. Density has a profound effect on x-ray attenuation at all energy levels. The mass attenuation coefficient for water in Figure 5-4 is 0.214 cm2/g, which is the same as the linear attenuation coefficient. They must be the same, because the density of water is 1 g/cm3, and the mass attenuation coefficient is obtained by dividing the linear attenuation coefficient by the density. The mass attenuation coefficient is the same for water, ice, and water vapor, which is logical because 1 g of all three has exactly the same amount of mass. The thickness of 1 g/cm2 of water is 1 cm, ice 1.09 em, and water vapor 1670 cm. These thicknesses will all attenuate the same amount of x ray (i.e., 20%) because they all contain the same amount of mass. As y ou can see in Figure 5-4, the mass attenuation coefficient is independent of the density of the absorber. The coefficients for water, ice, and water vapor are all the same, but their densities vary considerably.
- #19 Whenever a factor is expressed in the units per gram, the concept of volume is eliminated.
- #22 One involves the nature of the radiation, and three involve the composition of the matter:
- #23 With low energy radiation (20 keV), most of the interactions are photoelectric, and few photons are transmitted. As the energy of the radiation increases, photoelectic attenuation becomes less important, and completely ceases at 100 keV. Even when all attenuation is from Compton scattering, the percentage of transmitted photons continues to increase as the radiation energy increases. A larger percentage of photons is transmitted with a 150-keV than with a 100-keV beam.
- #25 Table 5-4 shows the percentage transmission of monochromatic radiation through 1 mm of lead.
- #26 Figure 5-5 shows a plot of the mass attenuation coefficients of tin and lead over the range of energies used in diagnostic radiology. Remember, the higher the attenuation coefficient, the lower the number of transmitted photons.