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
What effect does added filtration have?
reduces patient dose by removing low energy photons which contribute to patient dose but no to the image
Filtration shifts the continuous spectrum to…
the right
What are K-edge filters?
They remove both the high and low energy photons but
are relatively transparent to photons with energies just
below the K-edge energy for the filter material
Common in mammography
