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.

Purpose of Filtration

Absorb low energy x-ray photons, thus reducing patient dose. Also, hardens the beam/ improves quality.

X-ray Frequency and Energy: Direct relationship

Higher frequency means higher energy.

Low Frequency means lower energy.

Wavelengths and frequency are

inversely porportional

What are the 2 Beam Attenuation/ Absorption

Attenuation and Remnant beam

Attenuation

Reduction in the number of x-ray photons in beam as it passes through matter. Loss of energy.

Remnant beam

Portion that exits patient to strike image receptor

Filtration Hardens The Beam and Increases average energy of beam by

-improves beam quality

-Improves beam penetrability

What Changes shape of the emission spectrum?

Fewer total number of photons in beam

Harder beam = ____energy = Higher _____ = higher ____ ___ = greater ________

Harder beam = higher energy = Higher quality = higher average kVp = greater penetrability

what are three factors that harden the beam?

kVp
Generator type
filtration

kVp

kilovolt peak operator sets at console

Generator type

High frequency generator produces a harder beam

Filtration

absorbs low energy photons to increase average energy and reduce total photon quantity in the X-ray beam, improving image quality and reducing patient dose.

The average photon energy in the beam =

the beam quality increases

The total number of photons in the beam =

the beam quantity decreases

How do they measure filtration?

Aluminum equivalency (Al/eq)

What is the standard filtering material?

Aluminum

what are the two types of filtration?

Inherent an added filtration

Inherent Filtration

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

Added Filtration

Any filtration that occurs outside of the x-ray tube and housing. (Mirror, Collimator, thin sheet of aluminum)

Tungsten Vaporization

acts like inherent filtration, decreases tube efficiency

According to the National Council on Radiation Protection and Measurement (NCRP)

They recommend 2.5mm Al/eq that operates above 70 kV

Al filters absorb a significant % of low energy rays while allowing majority in the DX range to pass

% of photons attenuated decreases as kV increases

Any added filtration absorbs SOME of the useful primary beam, in this case you would…

raise technique when filters are used to compensate for the reduction in exposure.

For HVL what is the best method for describing x-ray penetrability

Beam quality
Beam hardness

If HVL is adequate

total filtration in the xray tube is adequate to protect the patient from unnecessary radiation.

Tube operating above 80kVp require…

HVL of 2.3mm Al/eq

What are the minimum Half Value Layer requirements for xray systems in the US (title 21 of 21 CFR 1020.30) operating range under 51:

operating range under 51:

30 - 0.3 mm Al/eq
40 - 0.4 mm Al/eq

50 - 0.5 mm Al/eq

What are the minimum Half Value Layer requirements for xray systems in the US (title 21 of 21 CFR 1020.30) operating range 51 to 70:

operating range 51 to 70:

51 - 1.2 mm Al/eq

60 - 1.3 mm Al/eq
70 - 1.5 mm Al/eq

What are the minimum Half Value Layer requirements for xray systems in the US (title 21 of 21 CFR 1020.30) operating range above 70:

operating range above 70:

71 - 2.1 mm Al/eq
80 - 2.3 mm Al/eq

90 - 2.5 mm Al/eq
100 - 2.7 mm Al/eq

110 - 3.0 mm Al/eq

120 - 3.2 mm Al/eq

130 - 3.5 mm Al/eq

140 - 3.8 mm Al/eq

150 - 4.1 mm Al/eq

CALCULATION: 4mm of Al reduces an exposure from 5mR to 2.5 mR. What is he half-value layer of this equipment?

4mm of Al

A 100mR source has a half-value layer of 2mm Al. How many mm are required to reduce the exposure to 25mR?

4 mm of Al

what are three types of filters?

compensating

underpart

compound

What type of energy is responsible for patient dose?

low energy photons that will be absorbed by patient

Original beam intensity is 300mR.
How many half-value layers are required to reduce beam to 75mR?

2 HVL