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X-ray interactions - Ch 12
Terms in this set (107)
when x-ray beam passes thru matter, this is the reduction in the # of x-ray photons in the beam, and subsequent energy loss
What causes attenuation
x-ray photons interacting with matter and losing energy through these interactions - usually an orbital electron is what the photons interact with.
What can x-ray photons interact with
The whole atom, an orbital electron or directly with the nucleus, depending on the energy of the photon
Low-energy atoms most likely interact with
the whole atom
intermediate-energy photons generally interact with
Very high-energy photons can interact with
Radiation therapy photons are considered
very high-energy photons
Nucleus of an atom is
positively charged and contains protons and neutrons
negatively charged and fall in orbital paths around the nucleus
the energy required to remove an electron from a shell. It's characteristic to a given shell and to a given atom.
possess the highest binding energy for a given atom and binding energies decrease progressively for successive shells. More tightly bound to the nucleus in high atomic number elements. The higher the number of the element, the more energy will be required to remove a K-shell electron from the atom.
They have the lowest energy total with the highest binding energy. Each successive shell, total electron energies increase and binding energies decrease.
Average body's K-shell binding energy
Electrons further from the nucleus
are not bound as tightly and need less energy to remove them from their orbit. The further an electron is from the nucleus, the higher the total energy of the electron will be.
Shells go in this order, closest to nucleus
K, L, M, N, O, P, AND Q. - therefore, K-Shell electrons possess less total energy than outer shell electrons.
When an outer shell electron moves into an inner shell
it will release energy equal to the difference b/w the binding energies of the two shells.
5 basic interactions b/w x-rays and matter
1. photoelectric absorption
2. Coherent scattering
3. Compton scattering
4. Pair production
when x-ray photons interact and change direction or are absorbed by the atom. Photons still exist but have less energy. They will continue on until their energy is absorbed or scatters again. # of interactions depends on the incident energy and atomic #.
when a photon is absorbed, all of the energy of the photon is transferred to the matter and the photon no longer exists.
most predominant in very low photon energy ranges
pair production and photodisintegration interactions
occur only at very high photon energy ranges
results when an x-ray photon interacts with an inner-shell electron. The incident photon possesses a slightly higher energy than the binding energy of the electrons in the inner (K or L) shells. The incident photon ejects the electron from its inner shell and is totally absorbed in the interaction.
The results is an ionized atom due to the missing inner-shell electron and an ejected electron. The incident must have a slightly greater energy than that of the binding energy of the electron for the interaction to occur.
The body has elements of low atomic number, so binding energies of the K-shell electrons are very low.
The ionized atom is unstable with an inner-shell electron missing. Instantly filled by an electron from the L-shell or M-shell (uncommon) or a free electron. Releases energy in the form of a characteristic photon AKA secondary radiation.
an ejected electron - travels with kinetic energy equal to the difference b/w the incident photon and the binding energy of the inner-shell electron. Ei= Eb + Eke.
is matter. It will not travel far. Absorbed within 1-2 mm in soft tissue, but this is a significant way in which x-ray energy can create biological changes.
Atoms of the body
are very low atomic number elements - binding energies of the K-shell electrons are very low.
In a photoelectric absoprtion interaction, when electrons transfer from outer to inner shells to fill vacancies - it has excess energy to release - in the form of secondary radiation
energy created at the x-ray target
electron transfer continues
from shell to shell until the atom returns to a normal state & is no longer a positive ion.
each shell electrons will fill lower shells with a corresponding emission of photons
AKA characteristic photon - energy produced is very low. Produced in photoelectric absorption interactions when electrons of outer shells drop to lower shells - When the electrons drop, they have excess energy to release.
Iodine and barium
secondary radiation energies are significantly higher for these elements.
3 rules that govern a photoelectric interaction
1. The incident x-ray photon energy must be greater than the binding energy of the inner-shell electron.
2. A photoelectric interaction is more likely to occur when the x-ray photon energy and the electron binding energy are nearer to one another
3. A photoelectric interaction is more likely to occur with an electron which is more tightly bound in its orbit. Most interactions will occur with the K-shell electron with low atomic number elements. Probability of a photoelectric interaction increases dramatically as the atomic number increases. It's for this reason that radiography is so useful in demonstrating the bones of the body.
As photon energy increases
the chance of a photoelectric interaction decreases dramatically - inversely proportional. Photoelectric effect = 1/energy3 - Important for establishing appropriate technical factors for specific body tissues
Probability of a photoelectric interaction increases dramatically as the atomic number increases
approximately proportional to the third power of the atomic number (photoelectric effect = atomic #3. It's for this reason that radiography is so useful in demonstrating the bones of the body.
interaction which occurs b/w very low-energy x-ray photons and matter. "Classical" or "unmodified" scatter. When the very low-energy photon interacts with the electron in an atom, it may cause the electron to vibrate at the same freq. as the incident photon. It immediately releases this excess energy by producing a secondary photon which has the same energy and wavelength but travels in a direction different. The atom is not ionized in the process. Occurs in very low x-ray energy ranges, outside the usual range for diagnostic imaging. Some reaches the IR and has little significance.
2 types of coherent scattering
Thomson and Rayleigh. Both have the same basic interaction results.
involves a single electron in the interaction
involves all of the electrons of the atom in the interaction
occurs when an incident x-ray photon interacts with a loosely bound outer-shell electron, removes the electron from the shell and then proceeds in a different direction as a scattered photon. Produces the Compton effect. Part of the incident photon's energy removes the outer-shell electron and imparts kinetic energy to it.
The incident photon energy is divided b/w the ejected electron and the scattered photon.
Compton or recoil electron
The dislodged electron - available as a free electron to fill a shell "hole" created by another ionizing interaction.
Compton scattered photon
The photon that exits the atom in a different direction. Possesses less energy than the incident photon and has lower freq and wavelength. Retains most of the energy because little energy is needed to eject an outer-shell electron due to its low binding energy. Will interact until absorbed photoelectrically.
The energy retained depends on the initial energy of the photon and angle of deflection. If 0°, no energy is transferred. 180° - more energy is imparted to the recoil electron and less remains with the scattered photon.
These photons can create a radiation hazard and impair image quality.
It's the primary cause of occupational radiation exposure to the radiographer and is primary reason for wearing protective devices.
Compton effect formula
Ei = Es + Eb + Eke
when a scattered photon is deflected back toward the source - it travels in the direction opposite to the incident photon. Most go in a more forward direction, especially when photon energy increases. For this reason, scatter has a serious impact on image quality.
in the x-ray room
unwanted exposures caused by scattered photons.
devices designed to remove unwanted scatter and to improve radiographic image quality.
pair production interaction
the energy of the x-ray photon is converted to matter in the form of two electrons. Needs a very high-energy photon with an energy of at least 1.02 mEv. (energy equivalent of the mass of one electron at rest is equal to .51 MeV). A high energy incident photon comes close to the strong nuclear field and loses all its energy. This energy is used to create a PAIR of electrons, one negative (negatron), and one positive (positron). The negatron is quickly absorbed by the other nearby atoms becuz it is negative. A positron is volatile. It combines with a negative electron nearly instantaneously. When the 2 particles combine, they disappear and give rise to 2 photons moving in opposite directions, each with .51 MeV. (annihilation reaction). Doesn't become significant until 10 MeV, therefore it is not in the diagnostic x-ray imaging range.
reaction during a pair production interaction - matter is being converted back to energy. A positron is volatile. It combines with a negative electron nearly instantaneously. When the 2 particles combine, they disappear and give rist to 2 photons moving in opposite directions, each with .51 MeV.
an interaction b/w an extremely high-energy photon above 10 MeV and the nucleus. A high-energy photon strikes the nucleus and all of its energy is absorbed by the nucleus, exciting it. The nucleus emits a nuclear fragment. Not relevant to diagnostic imaging.
The 2 interactions that tech need to consider
photoelectric absorption and Compton scattering
Most of the x-ray beam is
attenuated. Only a small % of the photons exit to create the image.
As kVp increases
the total number of photons which are transmitted without interactions increases - therefore the probability of photoelectric and Compton interactions decreases with increasing kVp.
The % of photoelectric interactions
decreases with increased kVp - less absorption
The % of Compton interactions
increases with increased kVp - more scatter
Compton scattering is the
predominant interaction through most of the diagnostic x-ray range.
Photoelectric interactions predominate in
two circumstances: 1. in the lower-energy ranges (25-56 kev) produced by 40-70 kVp and 2. when high atomic number elements are introduced, such as CM like iodine and barium. They absorb a greater % of the photons through photoelectric interactions - thus creating a visible radiograph
When the photoelectric effect is more prevalent
the resulting radiographic image will possess high contrast (lots of differences b/w densities). It is the result of the complete absorption of the incident photons without the creation of undesirable scatter to fog the image. Use low kVp/high mAs.
As the % of photoelectric interactions increases
so does the absorption of radiation by the patient. It increases the likelihood of biological effects. So high-contrast, low kVp, high mAs techniques tend to result in higher pt doses.
When Compton interactions prevail
the resulting radiographic image has lower contrast - small differences b/w densities, with more gray shades. Created by high kVp and low mAs because Compton interactions predominate as kVp increases. Scatter from Compton interactions is a significant cause of the lower-contrast images. However, low-contrast, high kVp/low mAs techniques reduce pt dose.
what is responsible for maintaining the electron's position and motion in its orbit
Centrifugal force and the attractive electrostatic force./
In a neutral atom
the number of protons and electrons is equal
Each element has a
different number of orbital electrons
As the number of electrons and protons increases
so does the binding energy of a given electron, due to the increase in the positive charge in the nucleus
electrons of high atomic number elements
are bound more tightly than the electrons of lower atomic number elements
binding energy of an electron is measured in
keVs - kiloelectron volts
to remove the K-shell electron from an atom of tungsten or lead
requires much more energy than would be necessary to remove a k-shell electron from an atom of hydrogen, carbon or oxygen.
When an electron is removed from an atom
the atom is said to be ionized
in order for ionization to occur
the x-ray energy must be greater than the binding energy of the electron
the photon from the focal spot of the anode
an image is formed between
transmission and photoelectric absorption. Not Compton scatter.
Compton scatter will always
occur. But it is not useful. It shows up as a grainy look on the film - fog. Compton scatter obscures detail. It is random in direction. It is the primary source of the rad tech's radiation
What causes fog
unwanted density on the film - it obscures detail
what is the primary source of a radiographer's radiation
the IR, patient, glass envelope, dielectric oil - all are examples
range of x-rays in the kev range.
pass through without interaction - AKA remnant, exit, image-forming radiation - these produce the image
% of photons that actually make it to the IR
what determines what an element is
the number of protons in the nucleus
what purpose do neutrons have
they add mass to the nucleus
protons + neutrons
the atomic # of an element
atomic number represents
the number of protons in the nucleus - how positive the nucleus is
what determines what interaction will occur
the energy of the photon and the type and volume of the material (characteristics of the atom(s))
in relation to the electron, the neutron is
much bigger than the electron
what holds electrons in their orbits
are held in orbit around the nucleus by electrostatic forces
force holding electron in orbit
binding energy - the energy required to remove an electron from a shell. It's characteristic to a given shell and to a given atom.
The K-shell has the
highest binding energy and lowest total energy
The Q-shell has the
lowest binding energy and the highest total energy
total energy is
inversely related to binding energy
as the number of protons increase
so does the binding energy.
electrons in the K-shell
require the greatest energy to remove them from orbit
why do we use lead
because it has a higher atomic number, so it absorbs more photons
what is the focal spot made of
In order to remove a K-shell electron
the energy of a photon has to be slightly greater than the binding energy of the K-shell electron
no two elements have the same
what interaction provides the most patient dose
primary radiation exists
at the focal spot
When photoelectric absorption is prevalent interaction
film will have a high radiographic contrast
photoelectric absorption is more likely to occur
with low kVp techniques and decreases as kVp increases
Higher atomic number elements are more likely
to have photoelectric absorption - because their atomic numbers are higher, they have greater binding energies (barium, lead, bone - all have higher atomic number than soft tissue, so they absorb more photons)
when Compton is the prevalent interaction
film will have a low radiographic contrast
what is meant by a "characteristic cascade"
It means the unique way (characteristic) an atom's electrons will drop into lower shells in a photoelectric absorption - characteristic to each element
Throughout the diagnostic range
Compton scatter is the predominant interaction.
kVp increases have an effect on the
percentages of interactions that will occur
as kVp increases, what is the result
Compton Scatter increases (increased % of scatter), fog occurs, and a lower contrast film results. Photoelectric absorption decreases due to the increased force of the photons pushing thru.
as kVp decreases, what is the result
Photoelectric absorption increases (more photons being absorbed by material because the photons have a lower energy and are not pushing right thru), and a higher contrast film results.
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