Filtration

Define Filtration

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

Does filtration increase or decrease the average energy of the beam?

increases

Low energy x-rays are called _____ x-rays.

Most are absorbed in the ____ few cms of tissue.

They ____ contribute to anything to the image.

They only ____ patient dose.

Low energy x-rays are called soft x-rays.

Most are absorbed in the first few cms of tissue.

They do not contribute to anything to the image.

They only increase patient dose.

Explain “hardening the beam”

elimination of as many low-energy x-rays as possible is desirable

Anything that selectively absorbs x-rays is called a _______.

filter

Where is a filter usually placed?

between the tube and the patient

What is the most common filter material?

aluminum

What terms is ALL filtration expressed in?

• in terms of the thickness of aluminium equivalency (Al/Eq)

• half-value layer

Define the half-value layer (HVL) and give the units of measure

the amount of absorbing material that will reduce the intensity of the x-ray beam to ½ of its original value

units of measure are mm Al/Eq

Filtration is either _____ or _______.

inherent or added

Explain inherent filtration

• built into the x-ray tube and housing

• glass envelope of the tube

• insulating oil surrounding the tube

Total inherent filtration=

0.5 to 1 mm Al/Eq

Explain added filtration

• added outside the x-ray tube

• thin aluminium plates are placed in the collimator = 1 to 2 mm Al/Eq

• mirror and collimator (about 1 mm Al/Eq)

Total filtration =

inherent filtration + added filtration

What is the council’s name that gives recommendations for total filtration based on operating kVp?

NCRP (National Council on Radiation Protection)

NCRP Recommendations

below 50 kVp-

50 to 70 kVp-

above 70 kVp-

0.5 mm Al/Eq

1.5 mm Al/Eq

2.5 mm Al/Eq

Define compound filters

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

In relation to Compound Filters:

when x-rays hit a filter, there is some ________ x-rays produced

the second filter absorbs these x-rays _____ they hit the patient

characteristic x-rays

before

A compound filter composed of one layer of aluminium and one layer of copper can also be called a

K-edge filters

The filter with the ______ atomic number is placed closest to the tube and the ____ Z number is placed closest to the _____.

highest, lowest, patient

Define Compensating Filters

specially shaped pieces of aluminium that are used to compensate for uneven densities of a particular body part

What do compensating filters do and where are they usually attached?

• they even out the densities by absorbing more of the x-rays that would hit the thinner end of the part

• are usually attached to the bottom of the collimator

Examples of filters:

• wedge filter

• trough filter (chest)

• boomerang filter (shoulder/nasal bone)

• prism filter (calcaneus)

• lower extremity/scoliosis filter

• adjustable filters

• ferlic filter (swimmer’s/hip)