Interactions Of X-rays With Matter Flashcards
What happens to radiation as it passes through the body?
It undergoes different amounts of attenuation (absorption)
What is an x-ray image?
A visual display in the difference of x-ray attenuation at different positions
What does it mean if an x-ray image is black?
It means that more radiation has reached the detector
What does it mean if an x-ray image is white?
It means that no/less radiation has reached the detector
What are the 2 steps in primary radiation?
Transmission
Absorption
What will all x-ray either be?
(3)
Transmission
Absorption
Scattered
What does is mean if an x-ray undergoes transmission?
It goes straight through the target material
What happens when an x-ray undergoes absorption?
It’s absorbed by the target material
What happens when an x-ray undergoes scatter?
The radiation either goes straight through the patient or reaches another point in the detector
What does scatter result in?
Blurring of the x-ray image
When does attenuation occur?
When there’s a sharp decrease in the intensity of the radiation, which slows down as the thickness of the material increases
Why does the intensity of the radiation decrease as the thickness of the material increases?
Because the radiation is coming into contact with more of the material
What does the linear attenuation coefficient depend on?
(3)
The energy of the radiation
The density of the material passing through
The type of material passing through
What does half value layer (HVL) mean?
For a given material, certain energy is needed to get half of the intensity that we started with
What does the half value layer allow us to do?
Make comparisons with other materials
What is attenuation caused by?
Scattering
Absorption
What are the 2 types of scattering?
Elastic/coherent
Inelastic/compton
What are the 2 types of absorption?
Photoelectric effect
Pair production
What happens in coherent/elastic scattering?
(9)
The incident electron interacts with the target material
This gives energy to the atom
The photon is absorbed
The atom gains energy and becomes excited
The atom wants to get rid of the additional energy
In order to do this, it releases the photon in another direction than it entered
Most photons are absorbed by the patient
The patient receives radiation
The atom becomes stable
Does ionisation occur in coherent/elastic scattering?
No
What happens in Compton/inelastic scattering?
(7)
The photon enters the atom
The photon interacts with an electron
Some of the electrons scatter the photons
The photons are released in another direction
This gives energy to the electron
The atom becomes ionisied
The electron becomes a free electron
When are we less likely to see Compton/inelastic scattering?
When we have higher energy photons
When are we more likely to see Compton/inelastic scattering?
When we have lower energy photons
Which type of scattering happens more regularly?
Compton/inelastic scattering
Where does Compton scattering occur?
In all tissues and bones
What negative things does Compton scattering do?
(2)
It contributes to the noise of the image
It causes radiation exposure to people nearby
What causes radiation to go in all directions?
Compton scattering
Why do we have to shield from radiation?
Because Compton scattering causes radiation to go in all directions
What happens in the photoelectric effect?
(8)
Photons enter the atom
Energy is given to electrons
Electrons get ionised
Electrons leave the atom
This causes a vacancy
The vacancy can be filled by another electron
This causes multiple characteristic x-rays
This increases the radiation dose to the patient IF we have the energy to overcome the binding energy
In order for the photoelectric effect to produce characteristic x-rays and increase the radiation dose, do photons have to have more or less energy than the binding energy?
Photons must have more energy than the binding energy
What does the photoelectric effect allow for the attenuation of the patient?
It gives us the difference in the attenuations of a patient
What happens in pair production?
(4)
The photon interacts directly with the nucleus
This causes an electron-positron pair to be created near the nucleus
The positron later gets annihilated by another electron
This creates two 511 keV photons IF the x-rays are above 1.02 MeV
The photons go in opposite directions
Does pair production occur in diagnostic x-ray imaging?
No
What is pair production essential for?
PET imaging
Which causes of attenuation impacts image quality?
(2)
Inelastic/compton (scattering)
Photoelectric effect (absorption)
How does Compton scattering differ for all materials?
It doesn’t- it’s similar for all materials
Different attenuation results in … differences
In contrast differences
What are the uses of highly attenuating materials?
(2)
Lead- small amounts are highly attenuating and can be used for radiation safety purposes
Contrast- it enhances attenuation to highlight something in an image
What are contrast agent?
Substances that increase the contrast of structures in the body
What does the introduction of contrast increase?
(2)
It increases attenuation
It increases the contrast
When is the Compton effect more likely?
(2)
At lower atomic numbers
At higher energies
What does intensity mean?
The amount of radiation passing through an area
What are the key properties of x-ray attenuation?
(2)
Linear attenuation coefficient
Thickness of the material
What is the binding energy?
The energy that every electron has to hold them in place in the shell
What is the K edge?
The sudden increase in energy when the photon energy exceeds a binding shell threshold
What is a positron?
A positively charged subatomic particle that has the same mass and charge as an electron
