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X RAY ATTENUATION AND DIFFERENT TYPES OF FILTERS
- 1. X RAY ATTENUATIONAND FILTERS
- 2. ATTENUATION • Reduction inthe intensity of an x ray beam as it traverses matter either by absorption or deflection of photons from the beam. • Depends on the both quantity and quality of photons in the beam.
- 3. • Characteristics ofan x ray beam • Quantity Quality No of photons in x ray beam Energy of an X ray beam Intensity of x ray beam = Quantity x Quality
- 4. Monochromatic radiation • Havingphotons of only one type of wavelength or energy. • The quality of monochromatic radiation does not change as it passes through the absorber. • A 50% reduction in the number of photons is a 50% reduction in the intensity of the beam. • When the number of transmitted photons and absorber thickness are plotted on a linear graph paper, it results in a curved line.
- 5. POLYCHROMATIC RADIATION • Morecomplex than the attenuation of monochromatic radiation. • Contains a whole spectrum of photons of varying energies. • In general, the mean energy of polychromatic radiation is between one third and one half of its peak energy.
- 6. Exponential attenuation • Whenthe number of photons in the beam decrease by the same percentage with increment of the absorber. • As seen in the monochromatic radiation – Plots a straight line on a semi logarithmic graph.
- 7. Attenuation Coefficients • Measureof the quantity of radiation attenuated by given thickness of a absorber. • Name is determined by the units used to measure the thickness of the absorber • Linear attenuation coefficient. • Mass attenuation coefficient.
- 8. LINEAR ATTENUATION COEFFICIENT •Most important for diagnostic radiology. • Quantitative measurement of attenuation per centimeter of the absorber, • Is for monochromatic radiation and is specific for both the energy of the x ray beam and type of the absorber. • When the energy of the radiation is increased, the number of x rays that are attenuated decreases, and so does the linear attenuation co efficient.
- 9. MASS ATTENUATION EFFECT •Quantitates the attenuation of materials independent of their physical state. • Obtained by dividing the linear attenuation coefficient by the density. • MASS ATTENUATION COEFFICIENT: • Unit – g/cm2 • Mass attenuation coefficient is independent of the density of the absorber.
- 10. HALF VALUE LAYER •Absorber thickness required to reduce the intensity of the original beam by one half. • Common method for expressing the quality of an x ray beam. • A beam with high HVL is a more penetrating beam than one with low HVL.
- 11. FACTORS AFFECTING ATTENUATION •1) Energy of the radiation • 2) Density • 3) Atomic number • 4)Electrons per gram
- 13. • Increasing theradiation energy increases the number of transmitted .photons ( and decreases attenuation), while increasing the density, atomic number, or electrons per gram of the absorber decreases the number of transmitted photons (and increases attenuation)
- 14. • As polychromaticradiation passes through the absorber • Transmitted photons undergo a change in both quantity and quality • Number of photons decrease because some are deflected and absorbed out of the beam • Quantity also changes because the lower energy photons are readily attenuated than the higher energy photons.
- 15. SCATTER RADIATION • Theprimary radiation passes through the patient unchanged or is completely removed from the useful beam. • The primary radiation caries the x ray image. • Scatter radiation has nothing worthwhile to offer. Scatter radiation from film quality and contributes no useful information.
- 16. FACTORS AFFECTING SCATTERINGRADIATION • Kilo voltage • Part thickness • Field size
- 17. FIELD SIZE • Mostimportant factor in the production of scatter radiation. • A small x ray field( usually called a narrow beam) irradiates only a small volume of tissue, so it generates only a small number of scattered photons. • Most of them miss the film because they have a large angle of escape. • As the x ray field is enlarged, the quantity of scatter radiation increases rapidly at first and then gradually tapers off until it reaches a plateau.
- 18. PART THICKNESS • Thetotal number of photons keeps increasing as the part becomes thicker, but photons originating in the upper layers of the patient do not have sufficient energy to reach the film.
- 19. KILO VOLTAGE • Inthe low energy range – extremely little scatter radiation is produced. • As the radiation energy increases – so does the scatter radiation. • Unlike field size and part thickness, the plateau is not as well defined. • This is due to the increasing beam energy that causes more photons scatter in the forward direction allowing them to penetrate greater thickness of tissue to reach the film.
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- 21. • It isa thin metal sheet placed between x ray tube window and patient to remove low energy photons. • The process of removing low energy x ray photon is called filtration.(beam hardening). • FUNCTION: • 1)Remove low energy/ soft x ray photon • 2) Reduces patient radiation dose.
- 22. • X raybeam filtered at three different levels: • 1) The x ray tube and its housing ( Inherent filtration) • 2) A thin sheet of metal is placed between x ray tube and patient (Added filtration) • 3)Patient itself
- 23. Types of Filtration: •Basically they are 2 types • 1)Inherent filtration • 2) Added filtration
- 24. Inherent Filtration • Filtrationresulting from absorption of x ray photons as it pass through the x ray tube and its housing is called Inherent filteration. • Inherent filter material are: • 1) Glass envelope enclosing • 2) Insulating oil surrounding the tube • 3) Window in the X ray tube housing • It is between 0.5- 1 mm aluminum equivalent.
- 25. • Filtration increasesthe mean energy of x ray beam so decreases tissue contrast. • In mammography x ray tube the exit port of the glass envelops is replaced with beryllium (At no 4) as it is more transparent to low energy radiation then glass. The radiation from these type of x ray tube has minimum inherent filtration.
- 26. ADDED FILTRATION • Itmeans a metal sheet is placed between patient and x ray tube window. Selection of metal depends on atomic number. Aluminum and copper is general selected for diagnostic radiology. Material used are: 1) Aluminum = It has atomic number 13 It is an excellent filter material for low energy radiation. 2)Copper = It has atomic number 29 It is a better filter for high energy radiation.
- 27. • Copper neverused alone as filter material. It is always used in combination with aluminum as a compound filter. • Advantages of Al and Cu as filter materials: • 1) Low At no there fore excellent material for absorbing low energy radiation. • 2)Low in weight there fore make the x ray tube lighter in weight and easy handling.
- 28. COMPOUND FILTER: • Acompound filter is consist of two or more layers of different metals • The layer is so arranged that higher atomic number element faces the x ray tube and lower atomic number element face towards patients. • Example: • 1) copper+ aluminum • 2)Tin+ Copper+Aluminum( Thoraeus Filter)
- 29. • Total filtration=Inherent filtration+ Added filtration Operating Kvp Total Filtration Below 50 Kvp 0.5 mm aluminum 50 – 70 Kvp 1.5 mm aluminum Above 70 Kvp 2.5 mm aluminum
- 30. • Wedge Filter:It is used occasionally to get film of uniform density when the part be examine have variation in thickness from one side of the field to the other • It is shaped like a wedge. • It is also called Compensation filter • It is often used in Lower limb angiograghy.
- 31. • Heavy metalfilter: These filter make use of K absorption edge of elements with atomic number greater than 60 Offers advantage imaging barium and iodine The purpose of heavy metal filter is to produces x ray beam that has high no of x ray photon in a specific range that is useful in diagnostic radiology. ADVANTAGES: 1) Enhance image contast 2) Reduce patient dose 3) Increase tube Loading
- 32. Molybdenum filter • Aspecial application of K-edge filter is use of molybdenum filter with molybdenum target x ray tube use in mammography.
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