Filtration

Filtration

process using absorbing materials like aluminum to remove low energy photons

Characteristics of filtration

hardens beam/makes it stronger, improves contrast, reduces scatter, reduces entrance skin exposure (ESE)

Beam hardening

happens when low energy x-ray photons are absorbed as they pass through a material (patient)

characteristics of beam hardening

the average beam energy is increased as low energy photons are removed, harder beam = greater penetrability, more consistent exposure across tissues

To increase beam hardening

use filtration

filtration and beam hardening help..

reduce patient dose (reduces photoelectric effect) and improve image contrast consistently

No filtration =

Higher ESE (entrance skin exposure) which may cause artifacts -> mainly a problem in CT (can lead to a misdiagnosis)

Filtration =

Lower ESE (entrance skin exposure), more consistent images

Artifact

Unnatural or incorrect visual element or an image that does not correspond to the actual anatomy of the patient, often caused by technical issues, patient factors or external objects (Piercings, bra strap, chain)

Half Value Layer

Refers to the thickness of aluminum needed to reduce beam intensity by half of its original value

What does Half Value Layer measure?

beam quality / beam penetrability

How does filtration affect HVL?

Filtration increases HVL

Importance of HVL

Radiation protection and medical imaging

Radiation protection

determines how much shielding is needed to protect against harmful radiation

Medical imaging

Used to optimize image quality and minimize patient exposure in X-rays and CT scans

Inherent filtration

Built into xray tube and housing and removes very low energy photons EX: tube glass, oil, window

Added filtration

Placed by manufacturer or technologist and Improves beam quality and protects patient EX: aluminum sheet, copper + aluminum (compound filter), compensating filter 

Inherent filtration + added filtration =

Total filtration

Compound filters

layered materials for spectrum cleanup

Compensating filters

Shaped filters to equalize exposure across anatomy

What is the purpose of compensating filters?

to improve image detail by equalizing exposure across variable anatomy thickness

Why is HVL important?

Measures beam quality (penetrating power), Indicates how effective filtration is, ensures patient protection by removing low energy photons, used for equipment performance testing

Added filtration = 

increased HVL (removes soft,lower energy photons)

Remove filtration =

decreased HVL (softer beam, low energy photons pass)