X-Ray Interaction with Matter Flashcards
What are the 3 ways X-ray photons interact with matter?
Transmission (passes through matter unchanged)
Absorption (stopped by the matter)
Scatter (changes direction)
What is X-ray beam intensity proportional to?
Energy & number of photons
–energy can be affected by potential difference across X-ray tube
–number of photons in X-ray beam primarily affected by current in the filament
What is attenuation?
Different tissues/materials have varying degrees of attenuation, which determines how many photons reach the receptor
–so attenuation INDIRECTLY leads to radiographic image
What are the different types of attenuation?
What colour do these present as on the X-ray?
Minimal attenuation- BLACK e.g. the cheek
Partial attenuation- GREY e.g. bone and teeth
Complete attenuation- WHITE e.g. restorations
What is the Photoelectric Effect?
What photons is this more likely to occur with?
Photon in X-ray beam interacts with inner shell electron in subject, resulting in the absorption of the photon and creation of a photoelectron
Occurs when energy of incoming photon is equal to, or just greater than binding energy of inner shell electron
The X-ray photon has lost all of its energy and therefore disappears
Prevents X-rays reaching receptor which leads to a lighter area on the images
This effect occurs more with LOWER ENERGY PHOTONS as human tissues have relatively low binding energies
How can you reduce the Photoelectric Effect?
Increasing the kV of the X-ray unit, will reduce the attenuation of the x-ray beam and you will have less photoelectric effect occurring
Why do things like gold and lead have good attenuation?
They have good attenuation as they have high values of cubed atomic numbers and a huge number of photoelectric effect
What is the Compton Effect?
Photon in X-ray beam interacts with outer shell electron in subject, which results in partial absorption & scattering of the photon and creation of a recoil electron
Occurs when the energy of incoming photon is much greater than binding energy of electron
Predominates with higher energy photons & outer shell electrons
What is a Recoil Electron?
Recoil electrons can ionise (&potentially damage) adjacent tissues
What are the 2 main directions of scatter?
What energy photons cause these?
Higher energy photons are deflected more forward
–forward scatter
Lower energy photons are deflected more backward
–back scatter
What is the effect of back scatter on the radiographic images?
They will not reach the receptor so do not affect the image
What is the effect of forward scatter on radiographic images?
Photons scattered slightly obliquely forwards may still reach the receptor but will interact with the wrong area causing darkening of the image in the wrong place
Results in fogging of the image- which reduces image contrast/quality
What is the effect of lowering kV on X-ray unit?
Lower X-ray tube potential difference
Overall lower energy photons produced
Increase of photoelectric effect interactions
Increase of contrast between tissues with different atomic number
Increased dose absorbed by patient
What is the effect of raising kV on X-ray unit?
Higher X-ray tube potential difference
Overall higher energy photons produced
Lower photoelectric effect interactions (& increased forward scatter)
Lower dose absorbed by patient
Lower contrast between tissues with different atomic number
