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6: Filters and Contrast Materials

6: Filters and Contrast Materials. Filtration is the process of shaping the x-ray beam spectrum. This has the following effects: 1: to increase the ratio of photons useful in imaging ie reduce scattered radiation 2: to remove low energy photons which cause superficial burns

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6: Filters and Contrast Materials

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  1. 6: Filters and Contrast Materials Filtration is the process of shaping the x-ray beam spectrum. This has the following effects: 1: to increase the ratio of photons useful in imaging ie reduce scattered radiation 2: to remove low energy photons which cause superficial burns 3: to reduce the overall radiation dose received by the patient

  2. 1 Inherent Filtration This results from absorption of x-rays as they pass through the tube and its housing. The materials responsible include: !         glass envelope !         cooling/ insulating oil !         tube window 2 Added Filtration Added filtration results from material placed in the beam to shape the x-ray energy spectrum. Aluminium ( atomic number 13) filters low energy x-rays Copper ( atomic number 29) absorbs higher energies

  3. Mixed Filters • Filters are made of aluminium together with another metal. • The higher atomic number material always faces the x-ray tube. • Heavy metals produce most attenuation • Heavy metals produces characteristic radiation • The aluminum backing absorbs characteristic radiation • from the copper at about 8 keV. x-ray tube copper aluminium

  4. Operating kVp Filtration < 50 kVp 0.5 mm Al 50kVp > V > 70 kVp 1.5 mm Al >70 kVp 2.5 mm Al Reduction of Skin dose All x-ray tubes operate with at least 0.5mm equivalent of aluminium. The NCRP recommend that the following filtration is used to reduce does and skin damage.

  5. Useful Filters !Wedge filters used to obtain films of uniform density across organs which rapidly vary in thickness. It may be used to reduce the heel effect. Heavy Metal Filters used in conjunction with radio-opaque dyes eg Barium utilise the rapid changes in attenuation near the K-edge of absorption spectrum.

  6. Filtration of X-raySpectrum Filter transmission X-ray spectrum

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