X-ray interactions - Ch 12

attenuation

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

orbital electrons

Very high-energy photons can interact with

the nucleus

Radiation therapy photons are considered

very high-energy photons

Nucleus of an atom is

positively charged and contains protons and neutrons

Electrons are

negatively charged and fall in orbital paths around the nucleus

binding energy

the energy required to remove an electron from a shell. It's characteristic to a given shell and to a given atom.

K-shell electrons

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

Average body's K-shell binding energy

.5 keV

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
5. Photodisintegration

scattering

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 #.

Absorption

when a photon is absorbed, all of the energy of the photon is transferred to the matter and the photon no longer exists.

coherent scatter

most predominant in very low photon energy ranges

pair production and photodisintegration interactions

occur only at very high photon energy ranges

Photoelectric absorption

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

photoelectron

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 sig

Atoms of the body

are very low atomic number elements - binding energies of the K-shell electrons are very low.

characteristic photon

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

primary 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.

characteristic cascade

each shell electrons will fill lower shells with a corresponding emission of photons

secondary radiation

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 pho

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.

Coherent scatter

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 phot

2 types of coherent scattering

Thomson and Rayleigh. Both have the same basic interaction results.

Thomson scattering

involves a single electron in the interaction

Rayleigh scattering

involves all of the electrons of the atom in the interaction

Compton 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 ener

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 e

Compton effect formula

Ei = Es + Eb + Eke

Backscatter radiation

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

protective shielding

in the x-ray room

radiation fog

unwanted exposures caused by scattered photons.

Radiographic grids

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 ph

annihilation reaction

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 i

Photodisintegration

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 imagi

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

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 cau

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

incident photon

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

polyenergetic beam

provides grays

What causes fog

Compton scatter

fog

unwanted density on the film - it obscures detail

what is the primary source of a radiographer's radiation

Compton scatter

Matter

the IR, patient, glass envelope, dielectric oil - all are examples

diagnostic range

range of x-rays in the kev range.

transmitted photons

pass through without interaction - AKA remnant, exit, image-forming radiation - these produce the image

% of photons that actually make it to the IR

1-2%.

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

atomic mass

z number

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

centrifugal force

orbital electrons

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

tungsten

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

binding energy

what interaction provides the most patient dose

photoelectric absorption

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.