Ch. 9 Interactions Between X-Rays, Gamma Rays And Matter

Front 1

What is meant by attenuation?

Back 1

Attenuation is the reduction in the intensity of an X-ray beam as it traverses matter by the absorption or scattering of photons from the beam.

Front 2

What is the linear attenuation coefficient (µ)?

Back 2

The fraction of incident photons removed from the beam per unit thickness of material (cm–1).
It is the sum of individual µ for each type of interation (classical and compton scatter, PE).

µ applies to monochromatic x-rays only.

Front 3

Give an equation for attenuation by matter.

Back 3

-∆N = µN ∆x

∆N = number primary x-rays removed from the primary beam of intensity N by thickness of material ∆x.

Decrease in beam intensity ∞
Number of photons in the beam (intensity)
Thickness of the material

This is integrated to form the exponential law of linear attenuation:
N = No exp (-µ x)

N = number of remaining photons
No = number of incident photons at depth x.

Or, by taking natural logarithm of this equation - to give the equation for a straight line:

ln N = ln No - µx
N plotted against x
Gradient µ

Front 4

What is the mass attenuation coefficient?

Back 4

µ depends on the medium (density).
The mass attenuation coefficient is µ standardised for density.

= µ / density.

Therefore water in 3 states has the same mass attenuation coefficient.

Front 5

What is the half value layer (HVL).

Back 5

Defined as the thickness of material needed to reduce the intensity by a factor of 2.

For monochromatic radiation:

HVL = 0.693 / µ

Front 6

Describe the attenuation of polychromatic radiation.

Back 6

X-ray tubes produce polychromatic radiation (spectrum of photon energies from 0 to maximum determined by kVp).
Mean photon energy is typically 1/3-1/2 of maximum (although also depends on filtration).

The HVL is the thickness of some standard material required to reduce the air kerma of the beam to half its original value.

However, polychromatic radiation QUANTITY and QUALITY changes when it passes through an absorber - low energy photons absorbed preferentially - mean energy of beam increases (beam hardening).
Therefore simple exponential law of attenuation does not apply.

Front 7

What are the different types of photon interaction?

Back 7

3 principle types of photon interaction in the range of photon energies used in diagnostic imaging:
1. Coherent scattering.
2. Compton scattering.
3. Photoelectric absorption.

4. Pair production.

It is the probability of a given reaction that is most important.