S2 L5 Flashcards
3 types of X-ray interaction
- X-ray passes through without interacting
- X-ray completely stopped
- X-ray scattered from original path
X-ray interaction names
- Photoelectric absorption
- Compton scattering
- Coherent scattering
Define absorption
Transfer of energy from X-ray beam to medium
Describe photoelectric absorption
X-ray with energy higher then binding energy of inner electron interaction
All energy transferred to electron
Electron freed from shell
Vacant filled by higher shell -> repeat
Why is photoelectric absorption bad for patient dose
Free electron absorbed nearby
When vacancy filled make low energy photon also absorbed nearby
Why does photoelectric absorption make good images
Enhances natural contrast
No scatter
What energy effects probability of photoelectric absorption
Highest just above binding energy
Too low = 0
Too high = low due to inverse relationship
Probability of photoelectric effect equation
Approx equal to = pZ^3/E^3
Why are iodine and barium good contrast
K edge absorption within diagnostic range but different to each other
Describe Compton scattering
X ray interacts C with loosely bound outer shell electron
Electron absorbs some energy and is deflected with X-ray
Angle depends on how much energy absorbed
What affects probability of Compton scattering
Total number of electrons in absorber
Density
Number of electrons per gram
Incident photon energy
Why is Compton scattering bad for imaging
Arrives to detector from all directions which decreases contrast
How does Compton scattering cause dose issues
Scatter travels all directions
Absorbed by staff
Absorbed by patient anatomy not in primary beam
Which interaction dominates at high photon energy
Compton scattering
