chp 11 filtration

filtration or “hardening of the beam” is the process of

eliminating undesirable low energy x-ray photons by the insertion of absorbing materials into the primary beam

filtration removes

low-energy or soft photons

soft x rays or low energy x rays are mostly absorbed by

first few cms of tissue but the soft x rays don’t contribute to image

soft xrays increase what

patient dose

filter is

anything that. selectivity absorbs photons from the x ray beam

filter is placed

between tube and patient

most common filter material

aluminum measured in mm

filtration is expressed in

HVL or half-value layer

HVL or half-value layer is the

amount of absorbing material that will reduce the intensity of
the primary beam to ½ its original value

An x-ray tube that operates above 130 kVp must have minimum

3.5 mm Al/Eq

An x-ray tube above 150 kVp must have minimum

4.1 mm Al/Eq

Mammography machines use below 50 kVp and would operate on

.3 mm Al/Eq

2 types of filtration are

inherent and added

inherent filtration is and its AI/Eq 

Built into the x-ray tube and housing and .5 to 1mm

examples of inherent filtration

Glass envelope of the tube, Insulating oil surrounding the tube, Dielectric oil, Glass window of tube

added filtration is

Any filtration that occurs outside the x-ray tube and above the image receptor

low atomic number functions well as

low energy absorber

total filtration=

inherent + added filtration

below 50 kvp is what total filtration

.5mm

50-70 kvp is what total filtration

1.5mm

above 70 kvp is what total filtration

2.5mm

compound filters is when

Use 2 or more materials to complement one another in their absorbing abilities

compound filters also called

k-edge filters

compound filters contain

one layer Al and one of copper

filter with highest and lowest atomic number are placed

highest- closest to the tube lowest number- is closest to patient

compensating filters are specially designed filter to

even out odd shaped parts or uneven densities of the body

compensating filter are shaped 

thick to thin so that x-rays will hit the thinner end of the part and pass through more easily than the thicker areas

three most common compensating filters

wedge, trough, boomerang

wedge used for

feet hands and t-spine

trough or double wedge used for

chest

filters effect on technique

density ir exposure- decrease

contrast- decrease

spatial resolution- no effect

distortion- no effect