Principals of radiographic imaging ch 11: Filtration

process of eliminating undesirable low-energy x-ray photons

filtration

filtration permits the radiographer to shape the phton emission spectrum into a

more useful beam

filtration removes

low-energy soft photons

primary reason for filtration

eliminate photons that increase radiation dose but do not enhance radiographic image

at 20 KeV ____ percent of the incident photons will penetrate 1 cm of soft tissue, but __________ percent will penetrate 15 cm

45%

0.0006%

at 50 KeV _____ percent of incident photons will penetrate 15 cm

3.5%

significant soft tissue penetration occurs between ______ and ______ KeV

30

40

any material designed to selectively absorb photons from the x-ray beam is called a

filter

filtration is typically added between the

source and patient

the most common filter

aluminum

other filtration materials

glass

oil

copper

tin

filtration is expressed in

Al/Eq

Al/Eq means

thickness of aluminum equivalency

half-value layer is another way to express

filtration

is the amount of absorbing material that will reduce the intensity of the primary beam to one-half its original value

half-value layer

who specifies the minimum HVLs for all diagnostic x-ray tubes

federal government

if the HVL is at the appropriate level, the total filtration in the x-ray tubes is

adequate to protect the patient from unnecessary radiation

filtration is either

inherent

added

filtration that is a result of the composition of the tube and housing is called

inherent filtration

what contributes to inherent filtration

glass envelope

dielectric oil

glass winde

typical x-ray tube might have a total inherent filtration of

0.5-1 mm Al/Eq

most inherent filtation comes from the

window

mammographic tubes target material

molybdenum

mammographic tubes window material

beryllium

Beryllium can reduce the inherent filtration to

0.1 mm Al/Eq

as tubes age, they become

gassy

pit anode

coating of vaporized material

when tubes age and become gassy, their

efficiency is reduced

any filtration that occurs outside the tube and housing and before the image receptor is considered

added filtration

functions very well as a low-energy absorber

aluminum

what device adds filtation

collimator

collimators average _____- most of which comes from the silver on the mirror situated in the beam

1 mm Al/Eq

uses 2 or more materials that complement one another in their absorbing abilities

compound filter

most compound filters are constructed so that each layer absorbs the

characteristic photons

compound filter is referred to as a

k-edge

compound filters place what closest to the tube

the highest-atomic-number material

compound filters place what closest to the patient

lowest-atomic-number material

the final layer in a compound filter is usually

aluminum

aluminum atomic number

13

what functions well for slightly higher energies

copper

copper filters should be at least ______ thick and backed with a minimum of _____ of ________

0.25 mm

1 mm

aluminum

there has been renewed interest in copper filters for digital diagnostic imaging, especially with

pediatric patients

cooper decreases entrance surface dose between

25% and 44%

a good example of a compound filter is the

Thoreaus filter

thoraeus filter combines

tin

copper

aluminum

is usually designed to solve a problem involving unequal subject densities

compensating filter

compensating filters can be made of

aluminum

leaded plastic (clearPb)

saline solution bags

two most popular compensating filters

wedge and trough

the thicker portion of the filter are match to the

less dense patient body part

a wedge filter can be useful for proceures on the

T spine

feet

lower extremities

filter designed to be placed under the patient

boomerang filter

compensating filters may be objected against because the

cast artifacts

is equal to the sum of inherent and added filtration and does not include any compound or compensating filters

total filtration

the percentage of photons attenuated decreases as

photon energy increases

aluminum filters of __________ absorb significant percentages of photons below the diagnostic range while permitting the vast majority to pass

1-3mm

who recommend minimum total flirtation levels for diagnostic radiography

NCRP

recommended minimum total filtration levels below 50 kVp

0.5 mm Aluminum

recommended minimum total filtration levels 50 - 70 kVp

1.5 mm aluminum

recommended minimum total filtration levels above 70 kVp

2.5 mm aluminum

when filtration is increases

technical factors must be increases

why must techical factors increase when filtration is increased

to maintain the same IR exposure

beyond 3.0 mm/Al filtration, a point of

diminishing returns is reached

the reduction in ___________- does not warrant the tube loading increase

entrance skin exposure (ESE)