1. 2.4.2 interaction of x-rays with matter

2. The Interaction of X-rays With Matter
X-ray source
Bone
Image on a photographic plate - film

3. Medical Physics
The reverse contrast image is formed because the bone absorbs the x-rays so that the film cannot be exposed in that area.
Clearly absorption is important. It depends on two things
1 the material doing the absorbing
2 the nature of the x-rays
There are 4 processes by which absorption can occur. You should fully understand all of them (but only need the last 3!).

4. 1 Simple (or elastic) Scatter
Low energy photons (less than the ionisation energy)
Deflected by atoms without losing energy
No absorption so doesn’t leave a clear shadow.
Scatter so does blur the image - not helpful.

5. Photoelectric Effect
Incident Photon gives up all energy
Energy used to eject a electron (called a photoelectron)
Produces an excellent shadow BUT
Another electron will fall down to fill its place
Emitting another photon of lower energy.
High Energy
X-ray
Photon
Ejected
electron
Outer electron “falls” in releasing energy as a low energy photon.

6.  IT IS THE DOMINANT PROCESS USED FOR DIAGNOSIS.
Photoelectric Effect - continued
Absorption is proportional to Z3
What is Z?
Proton Number
For bone Z 14 and for soft tissue Z  7
bone absorbs 143 / 73 (= 8) times better than soft tissue and we get good contrast.
 IT IS THE DOMINANT PROCESS USED FOR DIAGNOSIS.
Photoelectric absorption decreases markedly as x-ray energy increases.

7. Compton Scattering
Independent of Z.
So what will the contrast be like?
Does not produce good contrast shadows.
Used in therapy as absorbed equally well by all material in the beam.
High energy photon ejects inner electron (recoil electron of low energy).
Low energy x-ray is scattered off the atom.

8. Note that it looks as if the Photon has scattered off the electron.

9. Pair production
Occurs only with very high energy x-rays photons.
One photon will turn into an electron and its antiparticle - a positron (this is the same in all respects as an electron but is positively charged).
The process is only significant at > 5MeV
Can only happen in a strong electric field so occurs near the nucleus of an atom.

10. Pair production - continued

11. 4 What does absorption of x-rays mean?
Significant attenuation of beam intensity
How many absorption processes are there? 4
Which form of absorption is dominant in x-ray imagining?
Photoelectric effect
Which form is most dominant in x-ray therapy?
Compton Scattering
What is meant by:
Imaging?
Therapy?
Diagnosis?
Obtaining an image (picture)
Treating an complaint (frequently cancer)
Finding out what is wrong with the patient

12. Photon Energy
Absorption
Which line represents which type of absorption?
Simple scatter - reduces contrast
Photoelectric effect - good contrast between bone & tissue
Pair production - good for therapy
Compton effect - reduces contrast