FILTERS:

 average energy  = is the sum of the units of energy of each photon divided by 2

 intensity  = is the amount of energy in the beam (which is the sum of the amount of photons in the beam and the amount of units each type of photon is)

• when a beam is filtered the energy of the beam is increased there for the beam becomes more penetrating  (the average energy)
• also the intensity of the beam is decreased   (therefore the beam on time must be increased to keep the dose the same) (in diagnostic radiology the mas must be increased to keep the film density the same)

Graph of Intensity vs Photon Energy for different Filters:

THEOREUS FILTER:

Combination filter

• Al filter  (aluminim): removes the low energy characteristic photons from the beam (placed closest to the patient)
• Note : the characteristic radiation from Aluminum is 1.5 Kev
• Cu filter (copper):  added in the middle to reduce low energy characteristic photons from the Sn filter
• Note: charactersistic radiation from Copper is 9 Kev
• Sn Filter (tin): removes the low energy characteristic photons from the tungsten target (placed closest to source of radiation)
• Note: characteristic radiation from Tin is 29 Kev

Combination Filters are used to obtain the maximum amount of Hardeness with the least amount of reduction in Intensity

HALF VALUE LAYERS

 HVL - the amount of material that is required to reduce the intensity of an X-ray beam to half.

• adding a HVL reduces the intensity of the beam by one half
• increases the average energy of the beam / makes the beam more penetrating, BOT must be increased to achieve the same dose (depending on the amount of photons that got throught the HVL)
• adding more layers to beam each concurrent layer must be thicker because the average energy of the beam is increasing due to the filter (polyenergetic beams only)
• by adding HVL to a beam the hardness of the beam is increased and intensity is decreased

Was used to describe the Hardness of the beam (how penetrating the beam is)

diagnostic x-rays beams have a HVL of : .5mm Al   (inherent filtration from the glass window and target)

for Monoenergetic Beams – the 1st HVL is the same thickness as the 2nd HVL

• adding filters does not change the hardness of the beam it will just reduce the intensity

for Polyenergetic Beams – the 1st HVL is smaller than the 2nd HVL

• you will reach a point were it will behave like a monoenergetic beam
• Adding filters will increase the Hardness and reduce the Intensity

SUMMARY :