PRINCIPLES: Filtration

Added Filtration

Any filtration that occurs outside of the x-ray tube and housing.

Al/eq

aluminum equivalent- the absorption property of a material is expressed as how well the material absorbs x-ray photons as compared to how well aluminum absorbs them.

Collimator

the mirror of the collimator provides approximately 1mm Al/eq filtration

Compensating Filter

an absorber that compensates for unequal absorption of the subject, used to even out unequal densities resulting from unequal body part thickness.

Compound Filter

uses two or more materials that complement one another in their absorbing abilities. Each layer absorbs the characteristic photons created by previous layer.

Filter

any material designed to effectively absorb photons from the x-ray beam. (purpose is to reduce low energies and reduce patient dose)

Filtration

The process of eliminating undesirable low energy x-ray photons by inserting an absorbing material into the primary beam.

Half-Value Layer

That amount of absorbing material that will reduce beam intensity to half its original amount. Code of federal regulations establishes requirements for equipment HVL.

Inherent Filtration

Filtration as a result of the composition of the tube and housing, typically amounts to 0.5 to 1.0mm of Al/eq with most coming from the tube window itself

Thoraeus Filter

Compound filter used in radiation therapy which combines tin, copper and aluminum, in that order

Total Filtration

The sum of inherent and added filtration of the tube. National Council on radiation protection requires that equipment operating about 70 kVp must have a minimum filtration of 2.5mm Al/eq

Trough Filter

compensating filter most commonly used to visualize structures of the mediastinum without overexposing the lungs

Wedge Filter

Compensating filter shaped in a wedge, used to compensate for unequal thickness of the anatomy of interest. (spine, foot, thigh)

Filtration greater than 3mm is typically…

not used because the reduction to entrance skin dose does not warrant the tube loading increase. (beneficial effect is diminished)

Attenuation

Reduction in the number of x-ray photons in beam as it passes through matter, % achieved by specific amounts of filtration at various kV levels

What is the primary purpose of a filter?

Absorb low energy x-ray photons

List two ways the emission spectrum is affected by adding filtration

Changes shape of emission spectrum (amplitude decreases) and increases average energy of beam.

The use of filtration is associated with 

Increase in beam quality
decrease in patient dose
decrease in low energy wave length 

What components make up Inherent Filtration?

the x-ray tube envelope, tube window, and dielectric oil. The tube window itself is the primary contributor to inherent filtration.

How much filtration is required of equipment operating above 70kVp?

2.5mm Al/eq

How is filtration expressed in unit?

Al/eq

On the filtration emission spectrum, which curve would represent the beam with more filtration?

the curve that represents the beam with MORE filtration would have a lower amplitude (fewer photons overall) and its peak would be shifted to the right (indicating a higher average energy) compared to a beam with less filtration.

On the filtration emission spectrum, which curve would represent the beam with higher average energy?

the curve that would represent the beam with higher average energy is the one whose peak is shifted to the right.

On the filtration emission spectrum, which curve would represent the “harder“ beam?

a curve with its peak shifted to the right indicates higher average energy, signifying a harder beam.