attenuation and interactions

Question 1

Explain what is meant by the term linear attenuation coefficient

Answer 1

Total linear coefficient is the fraction of x-rays removed from the beam per unit of thickness of the particular attenuating material

Question 2

Fully explain what is meant by the term half-value thickness or half-value layer

Answer 2

The half value thickness is the thickness of material that will transmit one half of the intensity incident radiation.
As an example lets say that at a certain x-ray energy the HVT in a given material is 5 mm i.e. 5 mm of this material will reduce the intensity of the beam by 50%. A further 5 mm will reduce it to 25% etc.

Question 3

Explain what is meant by absorption and scattering of a photon

Answer 3

I get scatter feed and loses a bit of energy and goes off at an angle it can go anywhere
2 is total absorption
3 Some are unattenuated and reach the detector and get a negative image

Question 4

types of scatter and absorption

Answer 4

Elastic coherent scattering
The incident photon will interact with the outer photons of any atoms it interacts with and it will just get deflected
A scattering process
Prodeomeinelaty forward direction scatter
Occurs mainly at low energies
The probability of this interaction occurring is inversely proportional to the incident exergy and proportional to the square of the atomic number

Compton scatter
The x rays photon interacts with the outer shell of any atoms it interacts with and can easily know them out and the x ray photon lose a bit of energy and it carries on. The angle of deflection determines the amount of energy lost.
Compton scatter
A mixture of scattering and absorption
Scatters in all directions
The probability of this interaction occurring is independent of Z but inversely proportional to incident photon energy so as the energy gets really high we start to lose the Compton interactions

Photoelectric absorption
An absorption process
Predominant at low energies
Leads to the characteristic x rays and auger electrons
The probability of this interaction occurring is inversely proportional to the incident energy cubed and proportional to the atomic number cubed

Question 5

importance of Attenuation

Answer 5

Importance of attenuation
photoelectric dominates at low energies
Compton scattering dominates over wide range of energies
Compton scattering is almost the same in all attenuating materials

Question 6

Compton process and its problems

Answer 6

Results in scatter
Reduces radiographic contrast
Increases dose to the patient
Increases dose to staff