1. Resident Physics Lectures (year 1)
Christensen, Chapter 6
Filters
George David
Associate Professor
Department of Radiology
Medical College of Georgia

2. kVp (as set on generator)
Energy Spectrum
X-ray beams from tubes
Polychromatic
Brehmstrahlung
Characteristic
spectrum of energies from 0 – kVp set on generator
average beam energy
1/3 to 1/2 of peak (kVp)
kVp (as set on generator)

3. Unfiltered Beams
most energy deposited in first few centimeters of tissue
lowest energy photons selectively removed
energy of low energy photons
contributes to dose
does not contribute to image
photons don’t reach image receptor
Patient
Image Receptor

4. Filtration Ideal absorption characteristics Function
absorbs all low energy radiation
absorbs no high energy radiation
Function
shape beam’s energy spectrum
selectively attenuate low energy photons
less low energy radiation incident on patient
energy deposited in filter, not in patient

5. Filtration Locations x-ray tube and housing
inherent filtration
metal sheets placed in beam path
placed between tube and collimator or in collimator
Usually aluminum
added filtration
collimator mirror
table (for under-table tube fluoro)
Filter
Tabletop
Light
Lamp
Tabletop
X-Rays

6. Inherent Filtration Absorption of x-rays by tube glass insert
insulating oil
housing window
Insulating Oil
Insert
Glass Window

7. Added Filtration
Filtration intentionally added by placing metal plate in collimator
Added Filter

8. Filters: The Good & Bad Most commonly aluminum Disadvantages Advantage
reduces beam intensity
increases
tube loading
exposure time
patient motion
Advantage
gross reduction in patient dose
80% typical

9. Filtration vs. HVL Filtration Half Value Layer (HVL)
equivalent absorber present in beam
Half Value Layer (HVL)
amount of absorber (aluminum) which reduces beam intensity by exactly 50%
Easy to measure

10. Half Value Layer (HVL) Depends upon Minimum HVL regulated by law kVp
kVp HVL
(mm Al)
Depends upon
kVp
waveform (single/three phase)
inherent & added filtration
Minimum HVL regulated by law
Georgia State Rules &
Regulations for X-Ray