X-ray interactions - Ch 12

when x-ray beam passes thru matter, this is the reduction in the # of x-ray photons in the beam, and subsequent energy loss
x-ray photons interacting with matter and losing energy through these interactions – usually an orbital electron is what the photons interact with.
What causes attenuation
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The whole atom, an orbital electron or directly with the nucleus, depending on the energy of the photon
What can x-ray photons interact with
the whole atom
Low-energy atoms most likely interact with
orbital electrons
intermediate-energy photons generally interact with
the nucleus
Very high-energy photons can interact with
very high-energy photons
Radiation therapy photons are considered
positively charged and contains protons and neutrons
Nucleus of an atom is
negatively charged and fall in orbital paths around the nucleus
Electrons are
the energy required to remove an electron from a shell. It’s characteristic to a given shell and to a given atom.
binding energy
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.

K-shell electrons
.5 keV
Average body’s K-shell binding energy
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.
Electrons further from the nucleus
K, L, M, N, O, P, AND Q. – therefore, K-Shell electrons possess less total energy than outer shell electrons.
Shells go in this order, closest to nucleus
it will release energy equal to the difference b/w the binding energies of the two shells.
When an outer shell electron moves into an inner shell
1. photoelectric absorption
2. Coherent scattering
3. Compton scattering
4. Pair production
5. Photodisintegration
5 basic interactions b/w x-rays and matter
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
coherent scatter
occur only at very high photon energy ranges
pair production and photodisintegration interactions
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.

Photoelectric absorption
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.

are very low atomic number elements – binding energies of the K-shell electrons are very low.
Atoms of the body
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
characteristic photon
energy created at the x-ray target
primary radiation
from shell to shell until the atom returns to a normal state & is no longer a positive ion.
electron transfer continues
each shell electrons will fill lower shells with a corresponding emission of photons
characteristic cascade
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.
secondary radiation
secondary radiation energies are significantly higher for these elements.
Iodine and barium
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.
3 rules that govern a photoelectric interaction
the chance of a photoelectric interaction decreases dramatically – inversely proportional. Photoelectric effect = 1/energy3 – Important for establishing appropriate technical factors for specific body tissues
As photon energy 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.
Probability of a photoelectric interaction increases dramatically as the atomic number increases
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.
Coherent scatter
Thomson and Rayleigh. Both have the same basic interaction results.
2 types of coherent scattering
involves a single electron in the interaction
Thomson scattering
involves all of the electrons of the atom in the interaction
Rayleigh scattering
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 scattering
The dislodged electron – available as a free electron to fill a shell “hole” created by another ionizing interaction.
Compton or recoil electron
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 scattered photon
Ei = Es + Eb + Eke
Compton effect formula
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.
Backscatter radiation
in the x-ray room
protective shielding
unwanted exposures caused by scattered photons.
radiation fog
devices designed to remove unwanted scatter and to improve radiographic image quality.
Radiographic grids
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.
pair production interaction
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.
annihilation reaction
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.
photoelectric absorption and Compton scattering
The 2 interactions that tech need to consider
attenuated. Only a small % of the photons exit to create the image.
Most of the x-ray beam is
the total number of photons which are transmitted without interactions increases – therefore the probability of photoelectric and Compton interactions decreases with increasing kVp.
As kVp increases
decreases with increased kVp – less absorption
The % of photoelectric interactions
increases with increased kVp – more scatter
The % of Compton interactions
predominant interaction through most of the diagnostic x-ray range.
Compton scattering is the
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
Photoelectric interactions predominate in
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.
When the photoelectric effect is more prevalent
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.
As the % of photoelectric interactions increases
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.
When Compton interactions prevail
Centrifugal force and the attractive electrostatic force./
what is responsible for maintaining the electron’s position and motion in its orbit
the number of protons and electrons is equal
In a neutral atom
different number of orbital electrons
Each element has a
so does the binding energy of a given electron, due to the increase in the positive charge in the nucleus
As the number of electrons and protons increases
are bound more tightly than the electrons of lower atomic number elements
electrons of high atomic number elements
keVs – kiloelectron volts
binding energy of an electron is measured in
requires much more energy than would be necessary to remove a k-shell electron from an atom of hydrogen, carbon or oxygen.
to remove the K-shell electron from an atom of tungsten or lead
the atom is said to be ionized
When an electron is removed from an atom
the x-ray energy must be greater than the binding energy of the electron
in order for ionization to occur
the photon from the focal spot of the anode
incident photon
transmission and photoelectric absorption. Not Compton scatter.
an image is formed between
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
Compton scatter will always
provides grays
polyenergetic beam
Compton scatter
What causes fog
unwanted density on the film – it obscures detail
Compton scatter
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.
diagnostic range
pass through without interaction – AKA remnant, exit, image-forming radiation – these produce the image
transmitted photons
% of photons that actually make it to the IR
the number of protons in the nucleus
what determines what an element is
they add mass to the nucleus
what purpose do neutrons have
atomic mass
protons + neutrons
the atomic # of an element
z number
the number of protons in the nucleus – how positive the nucleus is
atomic number represents
the energy of the photon and the type and volume of the material (characteristics of the atom(s))
what determines what interaction will occur
much bigger than the electron
in relation to the electron, the neutron is
centrifugal force
what holds electrons in their orbits
are held in orbit around the nucleus by electrostatic forces
orbital electrons
binding energy – the energy required to remove an electron from a shell. It’s characteristic to a given shell and to a given atom.
force holding electron in orbit
highest binding energy and lowest total energy
The K-shell has the
lowest binding energy and the highest total energy
The Q-shell has the
inversely related to binding energy
total energy is
so does the binding energy.
as the number of protons increase
require the greatest energy to remove them from orbit
electrons in the K-shell
because it has a higher atomic number, so it absorbs more photons
why do we use lead
what is the focal spot made of
the energy of a photon has to be slightly greater than the binding energy of the K-shell electron
In order to remove a K-shell electron
binding energy
no two elements have the same
photoelectric absorption
what interaction provides the most patient dose
at the focal spot
primary radiation exists
film will have a high radiographic contrast
When photoelectric absorption is prevalent interaction
with low kVp techniques and decreases as kVp increases
photoelectric absorption is more likely to occur
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)
Higher atomic number elements are more likely
film will have a low radiographic contrast
when Compton is the prevalent interaction
It means the unique way (characteristic) an atom’s electrons will drop into lower shells in a photoelectric absorption – characteristic to each element
what is meant by a “characteristic cascade”
Compton scatter is the predominant interaction.
Throughout the diagnostic range
percentages of interactions that will occur
kVp increases have an effect on the
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 increases, 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.
as kVp decreases, what is the result

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