What is Classical/Coherent interaction with matter?
When X ray hits outer electron and bounces off
- Very Low energy= No ionization or significant contribution to image
- Minimal dose to patient
What is Compton Scatter?
X ray hits outer electrons and knocks them loose (ionization)–> photo released
- Increases dose to patient
- Increases Noise/Fog in the image
What determines the probability of Compton Scatter?
- Depends on the Density of the material
- NOT dependent on the Z
What is the probability of Photoelectric Effect occuring?
- Occurs when the incident energy is close (but higher) than the inner shell binding energy
- Equals 1/Incident Energy Photon Cubed (Larger Energy= less PE Effect)
- Equals Z Cubed
Why is Photoelectric effect so important?
Helps develop contrast in images
What has the highest atomic number in the body?
Calcium
What is the K edge?
Where the most Photoelectric effects will occur
K edge should be picked approximately 1/3 to 1/2 the KVP
How do you increase absorption?
Increase the Photoelectric effect
- KVP closer to K Edge (Energy of Beam)
- Higher Z (atomic number)
- More Tissue Density
What is linear attenuation?
Fraction of attenuated photons per unit thickness
-Different for ice, water and vapor
What is mass attenuation?
Fraction of attenuated photons per mass (g)
-Same for Ice, water an vapor
How do linear attenuation and HVL relate?
They are inversely related
- Increased Energy= Increased HVL= Lower LAC
- Increased Density= Lower HVL=Higher LAC
