Quiz No. 3
What are the three possible outcomes of x-rays interacting with matter?
What is coherent scattering?
Coherent scattering occurs when an x-ray photon interacts with the entire electron cloud of an atom, and the electron cloud then reemits a photon of equal energy but off in a different direction.
What is compton scattering?
Compton scattering occurs when an x-ray photon interacts with a loosely bound outer shell electron and ejects it, therefore ionising the electron. The electron is called a Compton or recoil electron. A new photon of longer wavelength and lower energy is created, which can move off in any direction, even in the direction of the incident photon, known as backscatter. The energy of the incident photon is equal to the output energy, which is shared between the recoil electron kinetic energy, the electron binding energy and the scattered photon. However, because Compton scattering involves loosely bound outer shell electrons, the electron binding energy is virtually negligible. In all cases, the energy of the scatter photon is greater than the kinetic energy of the recoil electron, although the energy of the scattered photon depends on the angle of scatter. If it moves off at 0º to the incident photon, minimal energy is transferred to the recoil electron. However, if it moves off at 180º to the incident photon, maximum energy is transferred to the recoil electron. The scattered photon is absorbed within a few millimetres of human tissue.
The probability of compton scattering occurring is proportional to electron density and inversely proportional to photon energy. This means as photon energy increases, the chance of compton scattering occurring decreases. As electron densities are essentially the same for all materials (except hydrogen), the chance of compton scattering occurring for all materials is extremely similar.
What are the differences between Coherent scattering and Compton scattering?
In coherent scattering, the photon interacts with the entire electron cloud, whereas compton scattering only interacts with a single loosely bound outer shell electron. In coherent scattering, the scatter photon always moves off in the same direction as the incident photon. In compton scattering, this can occur, but the scatter can also move off up to 180º from the direction of the scatter photon.
What is photoelectric absorption?
Photoelectric absorption occurs when an incident photon interacts with an inner shell electron, giving up all of its energy to the electron. The electron is then ejected and the atom is ionised. This creates a vacancy in the shell which a higher shell electron falls into, losing energy in the form of secondary radiation (characteristic radiation). The energy before an interaction must be equal to the energy after the interaction, meaning that the energy of the incident photon must be equal to the kinetic energy of the electron and the electron binding energy.
The probability of photoelectric absorption is proportional to atomic number cubed and inversely proportional to photon energy cubed. This means that the probability of photoelectric absorption occurring decreases much more rapidly than compton scattering.
What are the differences between Compton scattering and photoelectric absorption?
In compton scattering, the incident photon interacts with a loosely bound outer shell electron and ejects it, sharing the incident photon energy with the produced scatter photon. The electron binding energy is negligible. In photoelectric absorption, the incident photon gives up all of its energy to an inner shell electron and ejects it. There is no scatter radiation produced, although secondary characteristic radiation is produced. This means the energy of the incident photon is shared between the electron binding energy and the kinetic energy of the recoil electron. The probability of Compton scattering occurring is equal to electron density divided by photon energy. The probability of photoelectric absorption occurring is equal to atomic number cubed divided by photon energy cubed. This means that the probability of photoelectric absorption decreases much more rapidly than compton scattering, meaning the relative amount of compton scatter increases with increasing photon energy.
Define the following terms:
- Transmission
- Absorption
- Scattering
What are the 5 ways photons may interact with matter?
- Coherent scattering
- Compton scattering
- Photoelectric absorption
- Pair production
- Photodisintegration
What is coherent scattering?
Coherent scattering occurs when an x-ray photon interacts with the entire electron cloud of an atom, and the electron cloud then reemits a photon of equal energy but off in a different direction.
What is compton scattering?
Compton scattering occurs when an x-ray photon interacts with a loosely bound outer shell electron and ejects it, therefore ionising the electron. The electron is called a Compton or recoil electron. A new photon of longer wavelength and lower energy is created, which can move off in any direction, even in the direction of the incident photon, known as backscatter. The energy of the incident photon is equal to the output energy, which is shared between the recoil electron kinetic energy, the electron binding energy and the scattered photon. However, because Compton scattering involves loosely bound outer shell electrons, the electron binding energy is virtually negligible. In all cases, the energy of the scatter photon is greater than the kinetic energy of the recoil electron, although the energy of the scattered photon depends on the angle of scatter. If it moves off at 0º to the incident photon, minimal energy is transferred to the recoil electron. However, if it moves off at 180º to the incident photon, maximum energy is transferred to the recoil electron. The scattered photon is absorbed within a few millimetres of human tissue.
The probability of compton scattering occurring is proportional to electron density and inversely proportional to photon energy. This means as photon energy increases, the chance of compton scattering occurring decreases. As electron densities are essentially the same for all materials (except hydrogen), the chance of compton scattering occurring for all materials is extremely similar.
What are the differences between Coherent scattering and Compton scattering?
In coherent scattering, the photon interacts with the entire electron cloud, whereas compton scattering only interacts with a single loosely bound outer shell electron. In coherent scattering, the scatter photon always moves off in the same direction as the incident photon. In compton scattering, this can occur, but the scatter can also move off up to 180º from the direction of the scatter photon.
What is photoelectric absorption?
Photoelectric absorption occurs when an incident photon interacts with an inner shell electron, giving up all of its energy to the electron. The electron is then ejected and the atom is ionised. This creates a vacancy in the shell which a higher shell electron falls into, losing energy in the form of secondary radiation (characteristic radiation). The energy before an interaction must be equal to the energy after the interaction, meaning that the energy of the incident photon must be equal to the kinetic energy of the electron and the electron binding energy.
The probability of photoelectric absorption is proportional to atomic number cubed and inversely proportional to photon energy cubed. This means that the probability of photoelectric absorption occurring decreases much more rapidly than compton scattering.
What are the differences between Compton scattering and photoelectric absorption?
In compton scattering, the incident photon interacts with a loosely bound outer shell electron and ejects it, sharing the incident photon energy with the produced scatter photon. The electron binding energy is negligible. In photoelectric absorption, the incident photon gives up all of its energy to an inner shell electron and ejects it. There is no scatter radiation produced, although secondary characteristic radiation is produced. This means the energy of the incident photon is shared between the electron binding energy and the kinetic energy of the recoil electron. The probability of Compton scattering occurring is equal to electron density divided by photon energy. The probability of photoelectric absorption occurring is equal to atomic number cubed divided by photon energy cubed. This means that the probability of photoelectric absorption decreases much more rapidly than compton scattering, meaning the relative amount of compton scatter increases with increasing photon energy.
Define the following terms:
- Phosphor material: the material in the fluorescent layer in an intensifying screen which converts x-ray photons into visible light, must have high atomic number, high conversion efficiency, appropriate spectral emission and produce minimal phosphorescence/afterglow
- Conversion efficiency: the number of visible light photons produced per x-ray photon in an intensifying screen
- Screen speed: a relative number that describes how efficiently light photons are produced from x-ray photons in the intensifying screen, 100 to 1200 range, uses the intensification factor (exposure without screen divided by exposure with screen)
- Spatial resolution: how small a body part can be imaged, measured in line pairs per millimetre and can range from 8-15lp/mm
- Luminescence: the ability of an object to emit light in response to excitation
- Image noise: appears as a speckled background, occurs when high kVp and fast screens are used
What is pair production?
Pair production occurs when a high energy photon (1022keV) interacts with the electric field of a nucleus and causes the emission of an electron and a positron, each of 511keV. The positron moves through tissue gradually losing its energy until it combines with an electron and by pair annihilation they annihilate each other (opposite process). Two gamma photons of 511keV each speed off in opposite directions.
What is photo disintegration?
Photodisintegration is when an extremely high photon (10MeV) interacts with the nucleus of an atom and causes it to emit a neutron, proton or another nuclear fragment.
Describe the structure and function of the layers of an intensifying screen.
- The base is made of polyester and provides mechanical protection and is chemically inert.
- The reflective layer is made of magnesium oxide or titanium dioxide and makes sure that any visible light photons produced by the fluorescent layer that travel back towards the base are reflected down to the film.
- The fluorescent layer is made of a phosphor material (rare earth element) which converts x-rays to visible light, which the film is more sensitive to.
- The protective coating is made of cellulose acetate and is transparent for the visible light photons to travel through from the fluorescent layer to the film.
What are the two types of luminescence?
- Fluorescence is the instantaneous light emission within a short period of time
- Phosphorescence is delayed light emission
What are the desirable characteristics of a phosphor material?
- High atomic number to increase the chance of interaction between the material and x-rays
- High conversion efficiency to increase the number of visible light photons produced per x-ray photon (1000 for a 50keV x-ray)
- Appropriate spectral emission to emit light in a range that the film is sensitive to
- Produce minimal afterglow/phosphorescence
What three factors determine the amount of scatter radiation reaching the film and how do these affect it?
- kVp increases the relative amount of compton scatter compared to photoelectric absorption, meaning the amount of scatter radiation increases
- X-ray field size increases the amount of compton scatter because there are more atoms, increasing the chance of compton scatter occurring
- Tissue thickness also increases the amount of comtpon scattering, because there are more atoms thereby increasing the chance of compton scattering occurring
Comments
Post a Comment