X-Ray Attenuation Flashcards
What is attenuation?
The reduction of the intensity of an X-ray beam as it
traverses matter
How much of the primary beam reaches the image receptor?
Only around 3%
What does the transmitted intensity of the beam depend on?
on the thickness and type of material
What is the linear attenuation coefficient?
The fractional change in X-ray intensity per unit
thickness of the attenuating material
What factors affect atteniation?
- Beam energy
- Intensity (no. of photons)
- Density & thickness of material
- Atomic no of the material
Attenuation follows a _____ rule
Exponential rule, it is therefore non linear
Attenuation is a result of a combination of…
Absorption and scattering
The penetrating ability of the x-ray beam depends on…
the energy present in the beam (quality)
What is used to describe the penetrating power of an x-ray beam?
HVL (Half Value Layer)
What is the Half Value layer described as?
as the thickness of a standard
material (usually aluminium) that reduces the beam
intensity by one half
How can HVL be measured?
using an ionisation
chamber and a set of known thickness aluminium filters
Must be done under “narrow field conditions” to avoid
the effect of scatter
___ changes alongside HVL
kV
By measuring the HVL…
we can get an estimate of the quality of the x-ray beam
What is beam hardening?
As a polyenergetic beam passes through a material the low energy photons are attenuated more rapidly than the high energy photons and so the effective energy of the beam increases
The x-ray beam is filtered by:
- Inherent filtration within the tube
- Added filtration
- The patient
