Chapter 11 - Filtration PowerPoint

What is a filter?

a device placed at the x-ray port to absorb low energy (20 kev or less) radiation that does not contribute to the diagnostic value

#1 reason for using a filter?

decrease patient dose

#2 reason for using a filter?

increase quality of x-ray beam

What is inherent filtration?

x-ray tube and housing + insulation oil

What is added filtration?

• metal sheets placed in beam path

• Collimator mirrors

Where is added filtration placed?

between x-ray tube and collimator

How much filtration does inherent filtration typically offer?

0.5 mm AL/EQ

How much filtration does the metal sheet from added filtration typically offer?

1.0 mm AL/EQ

What is the metal sheet from added filtration typically made of?

Aluminum

How much total filtration should tubes operating above 70 kVp have?

2.5 mm AL/EQ

Total filtration =

inherent + added

Inherent filtration can be increased how?

vaporization

• tungsten from anode collecting on bottom of glass envelope

A way to describe filtration

hardening the beam

What is HVL used for?

a way to measure filtration

Definition of Half Value Layer (HVL)

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

If an absorbing material has what thickness, it’s considered to be an HVL?

2.00 mm AL.EQ.

How much does the HVL reduce the intensity of an x-ray beam?

50%

What are the two most common types of compensating filters?

• wedge type

• trough filter (double wedge)

What are wedge type filters commonly used for?

foot/femur

What are trough (double wedge) filters commonly used for?

chest

What purpose were compensating filters designed for?

solve problem involving unequal part thickness

What is a common use for a boomerang compensating filter?

shoulder