Interactions With Matter

Question 1

What is photoelectric effect dependent on?

Answer 1

Atomic number

Question 2

Probability of photoelectric event

Answer 2

Proportional to 1/keV3

Question 3

What materials best for absorption in photoelectric effect?

Answer 3

Denser tissues have better absorption as more E hit image receptors
Lead is a good materials as it absorbs protons
Lower kV more likely to give PE effect

Question 4

Intensifying screens

Answer 4

Found in EO cassettes

When exposed to Xrays screens emit XS energy as light. Light effects film emulsion and produces the image

Question 5

What is the Compton effect?

Answer 5

X-ray absorption and scattering process
Predominates with high E photons
Incoming X-ray photon interacts with free/loosely bound outer shell E of tissue atom
Outer shell E ejected taking some energy from incoming photon (absorption) RECOIL E always travels forwards - acts as Compton or PE
Remaining incoming photon energy scattered, higher energy - higher scatter. Travels in any direction, escapes tissues or more Compton/PE

Question 6

Is Compton good diagnostically?

Answer 6

No. Due to scatter not good discrimination on tissues therefore not good diagnostically.

Question 7

Is Compton dependent on atomic no?

Answer 7

No. Proportion of Compton interactions higher with higher energy photons.

Question 8

What does direct damage do to the body?

Answer 8

X-ray photon/high energy electron breaks nucleus bonds = defective chromosomes
Fail to pass on Info
Replicate abnormally
Cause cell death
Cause temporary damage

Question 9

Direct damage on somatic and stem cells.

Answer 9

Somatic - radiation induced malignancy

Stem cell - congenital abnormality

Question 10

Indirect damage

Answer 10

Ionisation of intracellular H2O Gives hydrogen peroxide

Cell poison damages DNA/RNA

Question 11

Biological effects of radiation

Answer 11

Stochastic
Cancer induction / heritable effects

Tissue reactions (deterministic) 
Early and late

Question 12

Tissue reactions

Answer 12

Occur from specific high dose
Have a threshold dose
Severity of effect proportional to dose

Dentistry dose too low to have effect

Question 13

Tissue reaction high radiation effects on oral cavity

Answer 13

Mucositis
Loss in taste
Dry mouth 
Radiation caries
Tooth defects if irradiated whilst teeth developing

Question 14

Stochastic effects

Answer 14

No known threshold dose - no safe dose
Each IR exposure caries risk
Most important in diagnostic medicine
Size of exposure to IR does not effect severity of damage, only probability of it occurring.

Question 15

Stochastic heritable effects

Answer 15

Damage spree/egg DNA
Congenital abnormality
No threshold dose
Risk estimates based on animals
Dose of 0.5-1.0 sV doubles rates of spontaneous mutation

Question 16

Photoelectric effect process

Answer 16

Pure X-ray absorption - contributes to patient dose
Incoming X-ray P interacts with bound inner shell electron of tissue atom
Inner shell E ejected (now called photoelectron) with high energy into tissue for more interactions
X-ray P disappears after giving up all energy - pure absorption
Vacancy filled by E from outer shells, XS energy - heat and light
Atomic stability via capture of free E
Ejected photoelectron ejects other E when passing through tissue