Filtration

What is the purpose of filtration?

to absorb low energy x-ray photons (thus reducing patient dose)

hardens the beam/improves beam quality

What effect does filtration have on the beam?

hardens the beam and improves beam quality

High x-ray frequency has ___ energy, ___ wavelength, and ___ penetrability

high energy

short wavelength

high penetrability

Low x-ray frequency has ___ energy, ___ wavelength, and ___ penetrability

low energy

long wavelength

low penetrability

Do low or high energy x-rays add useful information?

high (low energy x-rays just deliver more dose to patient)

Define attenuation

reduction in the number of x-ray photons in beam as it passes through matter

Does attenuation result in a gain or loss of energy?

loss

Define remnant beam

portion of beam that exits patient to strike the IR

What is another term for remnant beam?

exit beam

When filtration is applied and beam is hardened, how is the emission spectrum effected?

• average energy is increased (shifts curve →)

• # of photons in beam is decreased (amplitude of curve decreases)

What are the 3 factors that harden the beam? *IMPORTANT*

1. kVp

2. generator type (high efficiency generator produces a harder beam)

3. filtration (absorbs low energy photons to increase average energy and decrease photon quantity)

Define filter

any material designed to selectively absorb photons from x-ray beam

What is the standard filtering material?

Aluminum

What unit is filtration expressed in?

aluminum equivalency (Al/eq)

Define inherent filtration

permanently fixed filtration

What are the 3 kinds of inherent filtration, and which is the most important?

1. tube envelope

2. tube window

3. dielectric oil

Define added filtration

filtration outside of the tube

What are the 3 kinds of added filtration, and which is the most important?

1. thin sheet of aluminum

2. mirror w/ silver coating

3. collimator

Tungsten vaporization acts like ___ filtration

inherent

Total filtration = ___ + ___

inherent + added

What is the Al/eq range for inherent filtration?

0.5-1.0 mm Al/eq

What is the Al/eq for the collimator?

1.0 mm Al/eq

What is the Al/eq for the mirror?

1.0 mm Al/eq

What is the Al/eq range for added filtration?

1.0-2.5 mm Al/eq

The NCRP recommends ___ mm Al for equipment that operates above 70 kV

2.5

% of photons attenuated decreases as kV ___

increases

Should you raise or lower technique when filters are used?

raise (even with raised technique, patient dose is reduced)

Define Half-Value Layer

amount of absorbing material that reduces intensity of beam to ½ of its original value

Tubes operating above 80 kVp require an HVL of ___ (per the code of Federal Regulations)

2.3 mm Al/eq

If 4mm Al reduces an exposure from 5 mR to 2.5 mR, what is the HVL?

4 mm

If a 100 mR source has a HVL of 2mm Al, how many mm are required to reduce the exposure to 25 mR?

4mm

Explain compensating filters

• filters added to the outside of the collimator head

• evens out the beam intensity over a body part with unequal thickness (for a more uniform exposure)

• made of aluminum or clear lead

What is a wedge filter used for?

foot or femur

How do you place a wedge filter?

the thicker end of the filter goes over the thinner body part

What is a trough filter used for?

mediastinum and lung

What is a ferlic filter used for?

shoot through hip and c-spine (swimmers)

Explain the boomerang filter (underpart)

• compensates for variations in tissue density

• placed beneath the patient

• can produce artifacts

• used for shoulders

• does not decrease pt. dose

  • absorbs beam after it travels thru pt.

Define compound filters

filters with 2+ materials with complimentary absorbing properties

In compound filters, each layer absorbs characteristic radiation produced by ___

the previous layer

What is another term for a compound filter?

K-edge filter

Are compound filters used in diagnostic x-ray?

no

The Thoraeus filter is used in ___

radiation therapy

What 3 layers is the Thoraeus filter made up of?

tin, copper, aluminum

What is the atomic # of Tin?

50

What is the atomic # of Copper?

29

What is the atomic # of Aluminum?

13

What type of filter is shown in the image?

wedge filter

What type of filter is shown in the image?

trough filter

What type of filter is shown in the image?

ferlic filter

What type of filter is shown in the image?

boomerang filter