X-ray Interaction With Matter Flashcards
How does the beam leave X-ray unit
Focused stream of x-ray photons
Travelling in same direction
Low energy photons removed by aluminium filtration
beam energies up to 70keV (continuous interaction)
Spikes around 59keV and 67keV (characteristic interaction)
Photons travel at speed of light
3 types of X-ray photon interaction
Transmission- passes through matter (tissue) unaltered
Absorption- stopped by matter
Scattered (and absorbed)- changes direction, then slightly absorbed, photon continues in new direction…
Name for absorbed and scattered and absorbed photons
Attenuation (reduction in intensity of X-ray beam)
What is X-ray beam intensity
Quantity of photon energy passing through a cross-sectional area of the beam per unit time
Degrees of attenuation and corresponding colour on radiograph
Minimal attenuation- black
Partial attenuation- grey
Complete attenuation- white
Two types of attenuation
Photoelectric effect- complete absorption
Compton effect- partial absorption and scatter
REMEMBER THESE ARE IN TISSUES NOT IN X-RAY TUBE
Photoelectric effect
Photon in X-ray beam interacts with INNER shell electron in SUBJECT (e.g. carbon atom in tooth)
Results in absorption of photon
… Creation of photoelectron, which is released
Therefore space in shell, outer shell electron drops down
This energy is released as light
Key points of photoelectric effect
Typically happens with low energy photons
Photon overcomes binding energy, causing release of photoelectron
Photoelectron can ionise and damage adjacent tissues
Vacancy in inner shell causes cascade which produces light photons
Why does the photoelectric effect lead to lighter areas on radiographic image
Absorption prevents the X-ray photons reaching the receptor
What is photoelectric effect proportional to
The atomic number of the atom (being hit by xray photons) CUBED (Z^3)
Compton effect
Photon interacts with OUTER SHELL electron in SUBJECT
Results in partial absorption
Then scattering
Compton effect main points
High energy photons and outer shell electrons (loosely bound)
photon energy transferred to electron, which is ejected
Photon loses energy and changes direction (scattered)
Energy of photon and direction of scatter relationship
High energy photon-> forward scatter ( majority)
Low energy photon-> back scatter
THIS IS REASON FOR CONTROLLED AREA
Effects of scatter on radiographic image
Fogging of image, reduces contrast and quality (photons scattered and hitting wrong site of receptor)
Darkening of image in wrong place (photon scattered and does not hit receptor)
Continuous and characteristic radiation interactions
Vs
Photoelectric and Compton effects
CCRI
In xray tube
Electrons interacting with tungsten atoms
Leads to production of X-ray photons
PCE
Occur in patient/receptor/shielding
Xray photons interacting with atoms
Leads to attenuation of X-ray beam
